Rules for Building and Classing Offshore Installations

.

Rules for Building and ClassingOffshore Installations

1997

American Bureau of ShippingIncorporated by Act of the Legislature ofthe State of New York 1862

Copyright © 1997American Bureau of ShippingTwo World Trade Center, 106th FloorNew York, NY 10048 USA

Special Committee on Offshore Installations

Contents

Introduction

Table 1 Division and Numbering of Rules

Part 1 Classification, Testing and Surveys

SECTION1 Scope and Conditions of Classification......................................................................................... 52 Surveys During Construction and Installations...........................................................................113 Surveys After Construction..........................................................................................................174 Definitions and Design Documentation.......................................................................................21

Part 2 Materials and Welding

SECTION1 Materials ........................................................................................................................................272 Welding and Fabrication...............................................................................................................33

Part 3 Design

SECTION1 Environmental Conditions ............................................................................................................412 Loads..............................................................................................................................................453 General Design Requirements......................................................................................................514 Steel Structures..............................................................................................................................555 Concrete Structures.......................................................................................................................596 Foundations ...................................................................................................................................697 Marine Operations .........................................................................................................................75

Part 4 Extension of Use and Reuse

SECTION1 Extension of Use ...........................................................................................................................792 Reuse..............................................................................................................................................81

APPENDIX A Material Selection ................................................................................................85APPENDIX B References by Organization.................................................................................87

Introduction

These Rules apply to the design, construction and installation of Offshore Installations defined in 1/4.1.7, aswell as the periodic surveys which are to be carried out after installation for maintenance of classification. Theserviceability of an installation is also addressed in these Rules, but only to the extent that the proper functioningof the structure or its components will affect safety. These Rules are also applicable to the Certification or Veri-fication of either the design, construction or installation of an offshore installation or any combination of them.ABS will Certify or Verify the design, construction or installation of an offshore installation when requested bythe owner of an installation or mandated by government regulations to verify compliance either to these Rules,set of specific requirements, national standards or other applicable industry standards. If ABS’s Certification orVerification of the offshore installation is in accordance with these Rules and covers the design, constructionand installation, then the offshore installation is also eligible for ABS Classification.

The Rules have been written for worldwide application and as such, the satisfaction of individual requirementsmay require comprehensive data, analyses and plans to demonstrate the adequacy of the structure. This instancemay arise for unique structural types or structures located in frontier areas, which are those characterized byrelatively great water depth or areas where little or no operating experience has been obtained. Conversely,many provisions of these Rules often can be satisfied merely on a comparative basis of local conditions or pastsuccessful practices. The Bureau acknowledges that a wide latitude exists as to the extent and type of documen-tation which is required for submission to satisfy these Rules. It is not the intention of these Rules to imposerequirements or practices in addition to those which have previously proven satisfactory in similar situations.

Where available, design requirements in these Rules have been posed in terms of existing methodologies andtheir attendant safety factors, load factors or permissible stresses which are deemed to provide an adequate levelof safety. Primarily, the Bureau’s use of such methods and limits in these Rules reflects what is considered to bethe current state of practice in offshore installation design. At the same time, it is acknowledged that new meth-ods of design and construction are constantly evolving along with new structural types, or new uses for estab-lished structural types and components. In recognition of these facts, the Rules specifically allow for such inno-vations. The application of these Rules by the Bureau will not seek to inhibit the use of any technological ap-proach which can be shown to produce an acceptable level of safety.

This Rule book supersedes and replaces the section 1 of “ABS Guide for Building and Classing Undersea Pipe-line Systems and Risers 1991” and all relevant cross references to “ABS Rules for Building and Classing Off-shore Installations 1983” in “ABS Guide for Building and Classing Facilities on Offshore Installations 1991.”

The new requirements become effective 19 May 1997.

Scope and Conditions of Classification Section 1

1997 Offshore Installations ABS1

Part 1 Classification, Testing and Survey



.



Table 1 Division and Numbering of Rules

Division NumberPart 1/Section 1/1Subsection 1/1.1Paragraph 1/1.1.1Subparagraph 1/1.1.1aItem 1/1.1.1a1Subitem 1/1.1.1a1a



.



PART 1SECTION 1 Scope and Conditions

of Classification

1/1.1 Classification

1/1.1.1 Process

The Classification process consists of a) the deve l-opment of Rules, Guides, Standards and other crite-ria for the design and construction of offshore in-stallations, materials, equipment and machinery,b) the review of design, and survey during and afterconstruction to verify compliance with such Rules,Guides, Standards or other criteria, c) the assignmentand registration of class when such compliance hasbeen verified and d) the issuance of a renewableclass certificate, with annual endorsements, valid forfive years.

The Rules and Standards are developed by Bu-reau staff and passed upon by committees made upof naval architects, marine engineers, shipbuilders,engine builders, steel makers and by other technical,operating and scientific personnel associated withthe worldwide maritime industry. Theoretical re-search and development, established engineeringdisciplines, as well as satisfactory service experienceare utilized in their development and promulgation.The Bureau and its committees can act only uponsuch theoretical and practical considerations in de-veloping Rules and Standards.

1/1.1.2 Certificates and Reports

a Plan review and surveys during and after con-struction are conducted by the Bureau to verify toitself and its committees that an offshore installa-tion structure, item of material, equipment or ma-chinery is in compliance with the Rules, Guides,Standards or other criteria of the Bureau and tothe satisfaction of the attending Surveyor. All re-ports and certificates are issued solely for the useof the Bureau, its committees, its clients and otherauthorized entities.

b The Bureau will release information from reportsand certificates to the Port State to assist in recti-fication of deficiencies during port state controlintervention. Such information includes text ofconditions of classification, survey due dates, andcertificate expiration dates. The owner will be ad-vised of any request and/or release of information.

c The Bureau will release certain information to theoffshore installation underwriters and P&I clubs

for underwriting purposes. Such information in-cludes text of overdue conditions of classification,survey due dates, and certificate expiration dates.The owners will be advised of any request and/orrelease of information. In the case of overdueconditions of classification, the owners will begiven the opportunity to verify the accuracy of theinformation prior to release.

1/1.1.3 Representations as to Classification

Classification is a representation by the Bureau as tothe structural and mechanical fitness for a particularuse or service in accordance with its Rules and Stan-dards. The Rules of the American Bureau of Shippingare not meant as a substitute for the independentjudgment of professional designers, naval architects,marine engineers, owners, operators, masters andcrew nor as a substitute for the quality control proce-dures of constructors, steel makers, suppliers, manu-facturers and vendors of marine structures, materials,machinery or equipment. The Bureau, being a techni-cal society, can only act through Surveyors or otherswho are believed by it to be skilled and competent.

The Bureau represents solely to the offshore in-stallation’s Owner or other client of the Bureau thatwhen assigning class it will use due diligence in thedevelopment of Rules, Guides and Standards, and inusing normally applied testing standards, proceduresand techniques as called for by the Rules, Guides,Standards or other criteria of the Bureau for the pur-pose of assigning and maintaining class. The Bureaufurther represents to the offshore installations Owneror other client of the Bureau that its certificates andreports evidence compliance only with one or moreof the Rules, Guides, Standards or other criteria ofthe Bureau in accordance with the terms of such cer-tificate or report. Under no circumstances whatso-ever are these representations to be deemed to relateto any third party.

1/1.1.4 Scope of Classification

Nothing contained in any certificate or report is to bedeemed to relieve any designer, builder, Owner,manufacturer, seller, supplier, repairer, operator,other entity or person of any warranty expressed orimplied. Any certificate or report evidences compli-



ance only with one or more of the Rules, Guides,Standards or other criteria of the American Bureauof Shipping and is issued solely for the use of theBureau, its committees, its clients or other authorizedentities. Nothing contained in any certificate, report,plan or document review or approval is to be deemedto be in any way a representation or statement be-yond those contained in 1/1.1.3. The validity, appli-cability and interpretation of any certificate, report,plan or document review or approval are governedby the Rules and Standards of the American Bureauof Shipping who shall remain the sole judge thereof.The Bureau is not responsible for the consequencesarising from the use by other parties of the Rules,Guides, Standards or other criteria of the AmericanBureau of Shipping, without review, plan approvaland survey by the Bureau.

The term “approved” shall be interpreted to meanthat the plans, reports or documents have been re-viewed for compliance with one or more of the Rules,Guides, Standards, or other criteria of the Bureau.

The Rules are published on the understandingthat responsibility for operation, reasonable handlingand loading, as well as for avoidance of distributionsof loads, which are likely to set up abnormally se-vere stresses in offshore installations, does not restupon the Committee.

1/1.2 Suspension and Cancellationof Class

1/1.2.1 Termination of Classification

The continuance of the Classification of any offshoreinstallation is conditional upon the Rule require-ments for periodical, damage and other surveys be-ing duly carried out. The Committee reserves theright to reconsider, withhold, suspend, or cancel theclass of any offshore installation or any part of themachinery for noncompliance with the Rules, for de-fects reported by the Surveyors which have not beenrectified in accordance with their recommendations,or for nonpayment of fees which are due on accountof classification and other surveys. Suspension orcancellation of class may take effect immediately orafter a specified period of time.

1/1.2.2 Notice of Surveys

It is the responsibility of the owner to ensure that allsurveys necessary for the maintenance of class arecarried out at the proper time. The Bureau will giveproper notice to an owner of upcoming surveys. Thismay be done by means of a letter, a quarterly vesselstatus or other communication. The non-receipt ofsuch notice, however, does not absolve the ownerfrom his responsibility to comply with survey re-quirements for maintenance of class.

1/1.2.3 Special Notations

If the survey requirements related to maintenance ofspecial notations are not carried out as required, thesuspension or cancellation may be limited to thosespecial notations only.

1/1.2.4 Suspension of Class Includes:

a Class is suspended for any use, operation, loadingcondition or other application of any offshore in-stallation for which it has not been approved andwhich affects or may affect classification or thestructural integrity, quality or fitness for a par-ticular use or service.

b If the periodical surveys required for maintenanceof class are not carried out by the due date and noRule allowed extension has been granted, classwill be suspended.

c If recommendations issued by the Surveyor arenot carried out within their due dates, class will besuspended.

d Class is suspended for any damage, failure, dete-rioration or repair that has not been completed asrecommended.

e If proposed repairs as referred to in 1/1.15.1have not been submitted to the Bureau andagreed upon prior to commencement, class maybe suspended.

1/1.2.5 Cancellation of Class

a If the circumstances leading to suspension ofclass are not corrected within the time specified,the offshore installations class will be canceled.

b An offshore installations class is canceled imme-diately when an offshore installation resumes op-eration without having completed recommenda-tions which were required to be dealt with beforeresuming operations.

1/1.3 Class Designation

1/1.3.1 Offshore Installations Built UnderSurvey

Offshore installations which have been built underthe supervision of the Surveyors of the Bureau to therequirements of these Rules or to their equivalent,where approved by the Classification Committee,will be classed and distinguished in the Record bythe symbols @ A1 Offshore Installation.

Offshore Installations which have been built tothe satisfaction of the Surveyors of the Bureau, to therequirements as contained in the “Guide for Buildingand Classing Facilities on Offshore Installations”and/or “Guide for Building and Classing UnderseaPipeline Systems and Risers”, and which are ap-proved by the Committee will be classed and distin-



guished in the Record by the symbols @ A1 OffshoreInstallation followed by the appropriate notation:

@ A1 Offshore Installation—HydrocarbonProcessing

@ A1 Offshore Installation—HydrocarbonProduction

@ A1 Offshore Installation—Electric Gener-ating Plant (electric generating plant—ex-port load)

@ A1 Offshore Installation—Undersea Pipe-line

@ A1 Offshore Installation—Chemical Proc-essing

@ A1 Offshore Installation—Metals/Ore Proc-essing

1/1.3.2 Offshore Installations Not Built Under

Survey

Offshore Installations which have not been built un-der the supervision of the Surveyors of the Bureau,but which are submitted for classification, will besubject to a special classification survey. Wherefound satisfactory, and thereafter approved by theClassification Committee, they will be classed anddistinguished in the Record in the manner as de-scribed as in 1/1.3.1 but the mark “@” signifying thesurvey during construction will be omitted.

1/1.3.4 Classification Data

Data on offshore installations will be published inthe Record as to the latitude and longitude of the lo-cation of the structure, structure type, structural di-mensions and the depth of water at the site.

1/1.5 Rules For Classification

1/1.5.1 Application of Rules

These Rules are applicable to offshore installationsas defined in 1/4.1.7 and are generally intended toremain at a particular site for support of offshore fa-cilities.

These Rules are applicable to those features ofthe system that are permanent in nature and can beverified by plan review, calculation, physical surveyor other appropriate means. Any statement in theRules regarding other features is to be considered asa guidance to the designer, builder, owner, et al.

1/1.5.2 Alternatives

The Committee is at all times ready to consider al-ternative arrangements and scantlings which can beshown, through either satisfactory service experienceor a systematic analysis based on sound engineeringprinciples, to meet the overall safety, serviceabilityand strength standards of these Rules. The Commit-

tee will consider special arrangements, or equipment,or machinery which can be shown to comply withstandards recognized in the country in which the off-shore installation is registered or built, provided theyare not less effective.

1/1.5.3 Novel Features

Offshore installations with novel features of designin regard to structural arrangements, machinery,equipment, etc., to which these Rules are not directlyapplicable, may be classed when approved by theCommittee on the basis that these Rules, insofar asapplicable, have been complied with and that specialconsideration has been given to the novel features,based on the best information available at the time.

1/1.5.4 Effective Date of Rule Change

a) Six Month Rule

Changes to these Rules are to become effectivesix (6) months from the date on which the Tech-nical Committee approves them. However, theBureau may bring into force individual changesbefore that date if necessary or appropriate.

b) Implementation of Rule Changes

In general, the Rules in effect will apply unlessapplication of new Rules before their effectivedate is specifically requested by the party sig-natory to the application for classification.Where designs for one or more offshore instal-lation comply with the Rules applicable at thetime of approval, no retroactive application oflater Rule changes to such offshore installationswill be required unless necessary or appropriate.

1/1.5.5 Other Conditions

The committee reserves the right to refuse classifi-cation of any offshore installation where items forwhich there are Rule requirements are found not inaccordance with the Rules.

1/1.7 Other Regulations

1/1.7.1 Governmental and Other Regulation

While these Rules cover the requirements for theclassification of new offshore installations, the at-tention of Owners, builders, and designers is directedto various governmental regulations which controlstructural, machinery, and electrical features par-ticularly in hazardous areas where gas may be pres-ent or accumulate. Other considerations may includethe arrangement and extent of watertight bulkheadsand decks, fire-retarding bulkheads, the acceptabilityof watertight and weathertight closures, ventilation,and means of escape.



1/1.7.2 Governmental Regulations

Where authorized by a government agency and uponrequest of the Owners of a classed offshore installa-tion or one intended to be classed, the Bureau willsurvey and certify a new or existing offshore instal-lation for compliance with particular regulations ofthat government on its behalf.

1/1.9 IACS Audit

The International Association of Classification So-cieties (IACS) conducts audits of process followedby all its member societies to assess the degree ofcompliance with the IACS Quality System Certifi-cation Scheme requirements. For this purpose, audi-tors from IACS may accompany ABS personnel atany stage of the classification or statutory workwhich may necessitate the auditors having access tothe offshore installation or access to the premises ofthe manufacturer or the fabricator.

In such instances, prior authorization for the audi-tor’s access will be sought by the local ABS office.

1/1.11 Plans and Design Datato be Submitted

1/1.11.1 Submission of Site Condition Reports

As required in subsequent sections of these Rules,site condition reports are to be submitted. The prin-cipal purpose of these reports is to demonstrate thatsite conditions have been evaluated in establishingdesign criteria. Among the items to be discussed are:

Environmental conditions of waves, winds, currents,tides, water depth, air and sea temperature and ice;

Seabed topography, stability, and pertinent geotech-nical data; Seismic conditions;

Where appropriate, data established for a previousinstallation in the vicinity of the installationproposed for classification may be utilized if ac-ceptable in the opinion of the Bureau.

1/1.11.2 Submission of Design Data andCalculations

Information is to be submitted for the offshore in-stallation which describes the methods of design andanalysis which were employed to establish its de-sign. The estimated design service life of an offshoreinstallation is also to be stated. Where model testingis used as a basis for a design, the applicability of thetest results will depend on the demonstration of theadequacy of the methods employed, including enu-meration of possible sources of error, limits of appli-cability, and methods of extrapolation to full scaledata. Preferably, procedures should be reviewed andagreed upon before model testing is done.

As required in subsequent sections, calculationsare to be submitted to demonstrate the sufficiency ofthe proposed design. Such calculations are to be pre-sented in a logical and well-referenced fashion em-ploying a consistent system of units. Where the cal-culations are in the form of computer analysis thesubmitted is to provide input and output data withcomputer generated plots for the structural model. Aprogram description (not listings), user manuals, andthe results of program verification sample problemsmay be required to be submitted.

1/1.11.3 Submission of Plans and Specifications

Plans or specifications depicting or describing the ar-rangements and details of the major items of the off-shore installation are to be submitted for review orapproval in a timely manner.

Where deemed appropriate, and when requestedby the Owner, a schedule for information submittaland plan approval can be jointly established by theOwner and the Bureau. This schedule, which the Bu-reau will adhere to as far as reasonably possible, is toreflect the construction schedule and the complexityof the platform as it affects the time required for re-view of the submitted data.

1/1.11.4 Information Memorandum

An information memorandum on the offshore in-stallation is to be prepared and submitted to the Bu-reau. The Bureau will review the contents of thememorandum to establish consistency with otherdata submitted for the purpose of obtaining classifi-cation. The Bureau will not review the contents ofthe memorandum for their accuracy or the featuresdescribed in the memorandum for their adequacy.

An information memorandum is to contain, asappropriate to the installation, the following:

Site plan indicating the general features at the siteand the exact location of the installation;

Environmental design criteria, including the recur-rence interval used to assess environmental phe-nomena (see 3/1.5.1);

Plans showing the general arrangement of the off-shore installation;

Description of the safety and protective systems pro-vided;

The number of personnel to be normally stationed atthe installation;

Listing of governmental authorities having cogni-zance over the installation;

Listing of any novel features;Brief description of any monitoring proposed for use

on the installation;Description of transportation and installation proce-

dures.



1/1.15 Conditions for Surveys afterConstructions

1/1.15.1 Damage, Failure and Repair

a) Examination and Repair Damage to off-shore installation structure, machinery, equipment,or which affects or may affect classification, is to besubmitted by the Owners or their representatives forexamination by the Surveyor, at the first opportunity.All repairs found necessary by the Surveyor are to becarried out to his satisfaction.

b) Representation Nothing contained in thissection or in a rule or regulation of any governmentor other administration, or the issuance of any reportor certificate pursuant to this section or such a rule orregulation, is to be deemed to enlarge upon the rep-resentations expressed in 1/1.1.1 through 1/1.1.4thereof and the issuance and use of any such reportsor certificates are to be governed in all respects by1/1.1.1 through 1/1.1.4 thereof.

1/1.15.2 Notification and Availability for Survey

The Surveyors are to have access to classed offshoreinstallations at all reasonable times. For the purpose ofSurveyor monitoring, monitoring surveyors shall alsohave access to classed offshore installations at all rea-sonable times. Such access may include attendance atthe same time as the assigned Surveyor or during asubsequent visit without the assigned Surveyor.

The offshore installations Surveyors are to un-dertake all surveys on classed offshore installationsupon request, with adequate notification, of theOwners or their representatives and are to reportthereon to the Committee. Should the Surveyors findoccasion during any survey to recommend repairs orfurther examination, notification is to be given im-mediately to the Owners or their representatives inorder that appropriate action may be taken.

The Surveyors are to avail themselves of everyconvenient opportunity for carrying out periodicalsurveys in conjunction with surveys of damages andrepairs in order to avoid duplication of work.

1/1.15.3 Attendance at Port State Request

It is recognized that port State authorities legallymay have access to an installation. In cooperationwith port States, ABS Surveyors will attend on boarda classed installation when so requested by a portState, and upon concurrence of the offshore installa-tions owner, will carry out a survey in order to fa-cilitate the rectification of reported deficiencies orother discrepancies that affect or may affect classifi-cation. ABS Surveyors will also cooperate with PortStates by providing inspectors with background in-

formation, if requested. Such information includestext of conditions of class, survey due dates, andcertificate expiration dates.

Where appropriate, the offshore installationsflag state will be notified of such attendance and sur-vey.

1/1.17 Fees

Fees in accordance with normal ABS schedules willbe charged for all services rendered by the Bureau.Expenses incurred by the Bureau in connection withthese services will be charged in addition to the fees.Fees and expenses will be billed to the party re-questing that particular service.

1/1.19 Disagreement

1/1.19.1 Rules

Any disagreement regarding either the proper inter-pretation of the Rules, or translation of the Rulesfrom the English language edition, is to be referredto the Bureau for resolution.

1/1.19.2 Surveyors

In case of disagreement between the Owners orbuilders and the Surveyors regarding the material,workmanship, extent of repairs, or application of theRules relating to any offshore installations classed orproposed to be classed by this Bureau, an appealmay be made in writing to the Committee who willorder a special survey to be held. Should the opinionof the Surveyor be confirmed, the expense of thisspecial survey is to be paid by the party appealing.

1/1.21 Limitation of Liability

The combined liability of the American Bureau ofShipping, its committees, officers, employees, agentsor subcontractors for any loss, claim, or damagearising from its negligent performance or nonper-formance of any of its services or from breach of anyimplied or expressed warranty of workmanlike per-formance in connection with those services, or fromany other reason, to any person, corporation, partner-ship, business entity, sovereign, country or nation,will be limited to the greater of a) $100,000 or b) anamount equal to ten (10) times the sum actually paidfor the services alleged to be deficient.

The limitation of liability may be increased upto an amount twenty-five times that sum paid forservices upon receipt of Client’s written request at orbefore the time of performance of services and uponpayment by Client of an additional fee of $10.00 forevery $1,000.00 increase in the limitation.



.

Surveys During Construction and Installation Section 2


Part 1Section 2 Surveys During Construction

and Installation

1/2.1 General

1/2.1.1 Scope

This section pertains to surveys during the construc-tion and installation of an offshore structure. The re-quirements of 1/2.1 are to apply to all structures cov-ered by these Rules regardless of structural type.Additional requirements specifically for steel struc-tures are contained in 1/2.3 and additional require-ments for concrete structures are contained in 1/2.5.

The phases of construction covered by this sec-tion include: material manufacture, fabrication, load-out, transportation, positioning, installation and finalfield erection.

1/2.1.2 Quality Control Program

A quality control program compatible with the type,size and intended function of the planned structure isto be developed and submitted to the Bureau for re-view. The Bureau will review, approve and, as neces-sary, request modification of this program. The Fabri-cator is to work with the attending Surveyor toestablish the required hold points on the quality con-trol program to form the basis for all future surveys atthe fabrication yard. As a minimum, the items enu-merated in the various applicable subsections beloware to be covered by the quality control program. Sur-veyors will be assigned to monitor the fabrication ofclassed structures and assure that all tests and inspec-tions specified in the quality control program are be-ing carried out by competent personnel. It is to benoted that the monitoring provided by the Bureau is asupplement to and not a replacement for inspections tobe carried out by the Fabricator or Operator.

1/2.1.3 Access and Notification

During construction, Surveyors are to have access tostructures at all reasonable times. The attending Sur-veyor is to be notified as to when and where parts ofthe structure may be examined. If, at any visit, Sur-veyors find occasion to recommend repairs or furtherinspection, notice is to be made to the Fabricator orhis representatives.

1/2.1.4 Identification of Materials

The fabricator is to maintain a system of materialtraceability to the satisfaction of the attending Sur-

veyor, for all special and primary applicationstructures. Data as to place of origin and results ofrelevant material tests for structural materials shallbe retained and made readily available during allstages of construction (see 1/2.3.12 and 1/2.5.9).Such data are to be available to the Surveyors uponrequest.

1/2.3 Steel Structures


The quality control program (see 1/2.1.2) for theconstruction of a steel structure is to include the fol-lowing items, as appropriate.

Material quality and traceabilitySteel FormingWelder qualification and recordsWelding procedure specifications and qualificationsWeld inspectionTolerances alignments and compartment testingCorrosion control systemsTightness and hydrostatic testing proceduresNondestructive testingInstallation of main structure

The items which are to be considered for each of thetopics, mentioned above are indicated in 1/2.3.2through 1/2.3.11.

1/2.3.2 Material Quality and Traceability

The properties of the material are to be in accor-dance with Section 1 of Part 2. Manufacturer’s cer-tificates are to be supplied with the material. Veri-fication of the material’s quality is to be done bythe Surveyor at the plant of manufacture, in accor-dance with Section 2/1 of ABS Requirements forMaterials and Welding. Alternatively materialmanufactured to recognized standards may be ac-cepted in lieu of the above Steel Requirements pro-vided the substitution of such materials is approvedby the Bureau. Materials used are to be in accor-dance with those specified in the approved designand all materials required for classification pur-poses are to be tested in the presence of an ABSSurveyor. The Constructor is to maintain a materialtraceability system for all the Primary and Specialapplication structures.



1/2.3.3 Steel Forming

When forming changes base plate properties beyondacceptable limits, appropriate heat treatments are tobe carried out to reestablish required properties. Un-less approved otherwise, the acceptable limits of thereestablished properties should meet the minimumsspecified for the original material before forming.The Bureau will survey formed members for theircompliance with the forming dimensional tolerancesrequired by the design.

1/2.3.4 Welder Qualification and Records

Welders who are to work on the structure are to bequalified in accordance with the welder qualificationtests specified in a recognized code or, as applicable,ABS Rule Requirements for Materials and Weldingto the satisfaction of the attending Surveyor. Certifi-cates of qualification are to be prepared to recordevidence of the qualification of each welder qualifiedby an approved standard/code, and such certificatesare to be available for the use of the Surveyors. Inthe event that welders have been previously tested inaccordance with the requirements of a recognizedcode and provided that the period of effectiveness ofthe previous testing has not lapsed, these welderqualification tests may be accepted.

1/2.3.5 Welding Procedure Specificationsand Qualifications

Welding procedures are to be approved in accor-dance with ABS Rule Requirements for Materialsand Welding. Welding procedures conforming to theprovisions of a recognized code may, at the Sur-veyor's discretion, be accepted. A written descriptionof all procedures previously qualified may be em-ployed in the structure’s construction provided it isincluded in the quality control program and madeavailable to the Surveyors. When it is necessary toqualify a welding procedure, this is to be accom-plished by employing the methods specified in therecognized code, and in the presence of the Sur-veyor.

1/2.3.6 Weld Inspection

As part of the overall quality control program, a de-tailed plan for the inspection and testing of welds isto be prepared and this plan is to include the applica-ble provisions of this Section of these Rules.

1/2.3.7 Tolerances and Alignments

The overall structural tolerances, forming tolerances,and local alignment tolerances are to commensuratewith those considered in developing the structuraldesign. Inspections are to be carried out to ensurethat the dimensional tolerance criteria are being met.

Particular attention is to be paid to the out-of-roundness of members for which buckling is an an-ticipated mode of failure. Structural alignment andfit-up prior to welding shall be monitored to ensureconsistent production of quality welds.

1/2.3.8 Corrosion Control Systems

The details of any corrosion control systems em-ployed for the structure are to be submitted for re-view. Installation and testing of the corrosion controlsystems are to be carried out to the satisfaction of theattending Surveyor in accordance with the approvedplans.

1/2.3.9 Tightness and Hydrostatic TestingProcedures

Compartments which are designed to be permanentlywatertight or to be maintained watertight during in-stallation are to be tested by a procedure approved bythe attending Surveyor. The testing is also to be wi t-nessed by the attending Surveyor.

1/2.3.10 Nondestructive Testing

A system of nondestructive testing is to be includedin the fabrication specification of the structures. Theminimum extent of nondestructive testing shall be inaccordance with these Rules or recognized designCode. All nondestructive testing records are to be re-viewed and approved by the attending Surveyor. Ad-ditional nondestructive testing may be requested bythe attending Surveyor if the quality of fabrication isnot in accordance with industry standards.

1/2.3.11 Installation of Main Structure

Upon completion of fabrication and when the struc-ture is to be loaded and transported to site for instal-lation, the load-out, tie-down and installation are tobe surveyed by an attending Surveyor from the Bu-reau. All load-out, transportation and installationprocedures are to be submitted to the Bureau for re-view and approval as described in Section 3/7.

The Surveyor is to verify the following activi-ties, as applicable to the planned structure, to ascer-tain whether they have been accomplished in a man-ner conforming to the approved procedures.

Load-out and Tie-downLaunching, flotation, lifting and up-endingPositioning at the site and levelingInstallation of Decks and ModulesPiling and GroutingWelding and Nondestructive TestingFinal field erection and levelingPre-Tensioning

Significant deviations from approved plans and pro-cedures or any incidents such as excessive titling of



the jacket or abnormal vibrations during pile drivingmay require resubmittal of supporting documentationto provide an assessment of the significance of devia-tion and any necessary remedial actions to be taken.

To ensure that overstressing of the structureduring transportation has not occurred, the Bureau isto have access to towing records to ascertain if con-ditions during the towing operations exceeded thoseemployed in the analyses required in Section 3/7.

1/2.3.12 Records

A data book of the records of construction activitiesis to be developed and maintained so as to compile arecord as complete as practicable. The pertinent rec-ords are to be adequately prepared and indexed to as-sure their usefulness, and they are to be stored so thatthey may be easily recovered.

For a steel structure, the construction record is toinclude, as applicable, the following: material trace-ability records including mill certificates, weldingprocedure specification and qualification records,shop welding practices, welding inspection records,construction specifications, structural dimensioncheck records, nondestructive testing records, rec-ords of completion of items identified in the qualitycontrol program and towing and pile driving records,position and orientation records, leveling and eleva-tion records, etc. The compilation of these records isa condition of classing the structure.

After fabrication and installation, these recordsare to be retained by the Operator or Fabricator forfuture references. The minimum time for record re-tention is not to be less than the greatest of the fol-lowing: the warranty period, the time specified inconstruction agreements, or the time required bystatute or governmental regulations.

1/2.5 Concrete Structures


The quality control program (see 1/2.1.2) for a con-crete structure is to cover the following items, as ap-propriate.

Inspections prior to concretingInspection of batching, mixing and placing concreteInspections of form removal and concrete curingInspection of prestressing and groutingInspection of jointsInspection of finished concreteInstallation of Main StructureTightness and Hydrostatic testing as applicable(See 1/2.3.9)

The items which are to be considered for each of thetopics mentioned above are indicated in 1/2.5.2through 1/2.5.8.

1/2.5.2 Inspections Prior to Concreting

Prior to their use in construction, the manufacturersof cement, reinforcing rods, prestressing tendons andappliances are to provide documentation of the perti-nent physical properties. These data are to be madeavailable to the attending Surveyor who will checkconformity with the properties specified in the ap-proved design.

As applicable, at the construction site, the Sur-veyor is to be satisfied that proper consideration isbeing given to the support of the structure duringconstruction, the storage of cement and prestressingtendons in weathertight areas, the storage of admix-tures and epoxies to manufacturer’s specifications,and the storage of aggregates to limit segregation,contamination by deleterious substances and mois-ture variations within the stock pile.

Forms and shores supporting the forms are to beinspected to insure that they are adequate in numberand type, and that they are located in accordancewith the approved plans. The dimensions and align-ment of the forms are to be verified by the attendingSurveyor, and the measurements are to be within theallowable finished dimensional tolerances specifiedin the approved design.

Reinforcing steel, prestressing tendons, post-tensioning ducts, anchorages and any included steelare to be checked, as appropriate to the plannedstructure, for size, bending, spacing, location, firm-ness of installation, surface condition, vent locations,proper duct coupling, and duct capping.

1/2.5.3 Inspection of Batching, Mixing andPlacing Concrete

The production and placing of the concrete are toemploy procedures which will provide a well mixedand well compacted concrete. Such procedures arealso to limit segregation, loss of material, contami-nation, and premature initial set during all opera-tions.

Mix components of each batch of concrete are tobe measured by a method specified in the qualitycontrol program. The designer is to specify the al-lowable variation of mix component proportions, andthe constructor is to record the actual proportions ofeach batch.

Testing during the production of concrete is tobe carried out following the procedures specified inthe quality control program. As a minimum, the fol-lowing concrete qualities are to be measured by theConstructor.

ConsistencyAir contentDensity or Specific GravityStrength



Field testing of aggregate gradation, cleanliness,moisture content, and unit weight is to be performedby the constructor following standards and schedulesspecified in the quality control program. The fre-quency of testing is to be determined taking into ac-count the uniformity of the supply source, volume ofconcreting, and variations of atmospheric conditions.Mix water is to be tested for purity following meth-ods and schedules specified in the quality controlprogram.

1/2.5.4 Inspections of Form Removal andConcrete Curing

The structure is to have sufficient strength to bear itsown weight, construction loads and the anticipatedenvironmental loads without undue deformations be-fore forms and form supports are removed. Theschedule of form removal is to be specified in thequality control program, giving due account to theloads and the anticipated strength.

Curing procedures for use on the structure are tobe specified in the quality control program. Whenconditions at the construction site cause a deviationfrom these procedures, justification for these devia-tions is to be fully documented and included in theconstruction records.

Where the construction procedures require thesubmergence of recently placed concrete, specialmethods for protecting the concrete from the effectsof salt water are to be specified in the quality controlprogram. Generally, concrete should not be sub-merged until 28 days after placing. (See also3/5.11.2e.)

1/2.5.5 Inspection of Prestressing and Grouting

A schedule indicating the sequence and anticipatedelongation and stress accompanying the tensioningof tendons are to be prepared. Any failures toachieve proper tensioning are to be immediately re-ported to the designer to obtain guidance as toneeded remedial actions.

Pre- or post-tensioning loads are to be deter-mined by measuring both tendon elongation and ten-don stress. These measurements are to be compared,and should the variation of measurements exceed thespecified amount, the cause of the variation is to bedetermined and any necessary corrective actions areto be accomplished.

The grout mix is to conform to that specified inthe design. The constructor is to keep records of themix proportions and ambient conditions during groutmixing. Tests for grout viscosity, expansion andbleeding, compressive strength, and setting time areto be made by the constructor using methods andschedules specified in the quality control program.Employed procedures are to ensure that ducts arecompletely filled.

Anchorages are to be inspected to ensure thatthey are located and sized as specified in the design.Anchorages are also to be inspected to assure thatthey will be provided with adequate cover to mitigatethe effects of corrosion.

1/2.5.6 Inspection of Joints

Where required, leak testing of construction joints isto be carried out using procedures specified in thequality control program. When deciding which jointsare to be inspected, consideration is to be given tothe hydrostatic head on the subject joint during nor-mal operation, the consequence of a leak at the sub-ject joint, and the ease of repair once the platform isin service.

1/2.5.7 Inspection of Finished Concrete

The surface of the hardened concrete is to be com-pletely inspected for cracks, honeycombing, pop-outs, spalling and other surface imperfections. Whensuch defects are found, their extent is to be reportedto the Surveyor and to the designer for guidance onany necessary repairs.

The structure is to be examined using a calibratedrebound hammer or a similar nondestructive testingdevice. Where the results of surface inspection, cylin-der strength tests or nondestructive testing do not meetthe design criteria, the designer is to be consulted re-garding remedial actions which are to be taken.

The completed sections of the structure are to bechecked for compliance to specified design toler-ances for thickness, alignment, etc., and to the extentpracticable, the location of reinforcing andprestressing steel and post-tensioning ducts. Varia-tions from the tolerance limits are to be reported tothe designer for evaluation and guidance as to anynecessary remedial actions.

1/2.5.8 Installation of Main Structure

Upon completion of fabrication and when the struc-ture is to be loaded and transported to site for instal-lation, the load-out, tie-down and installation proce-dures are to be surveyed by an attending Surveyorfrom the Bureau. All load-out, transportation and in-stallation procedures are to be submitted to the Bu-reau for review and approval.

The Surveyor is to witness the following opera-tions, as applicable to the planned structure, to verifythat they have been accomplished in a manner con-forming to plans or drawings covering these operations.

Load-out and tie-downTowing arrangementsPositioning at the siteInstallationFinal field erectionPre-Tensioning



Significant deviations from approved plans and pro-cedures may require resubmittal of supportingdocumentation to provide an assessment of the sig-nificance of the deviation and the remedial actions tobe taken.

To ensure that overstressing of the structureduring transportation has not occurred, the Bureau isto have access to towing records to ascertain if con-ditions during the towing operations exceeded thoseemployed in the analyses required in Section 3/7.Results are to be submitted to demonstrate compli-ance with the reviewed design analysis.

1/2.5.9 Records

Reference is to be made to 1/2.3.12 regarding theneed to compile construction records. For a concretestructure, the construction records are to include, asapplicable, all material certificates and test reports,tensioning and grouting records, concrete records in-cluding weight, moisture content and mix propor-tions, a listing of test methods and results, ambientconditions during the pours, calibration data for testequipment, towing records, data on initial structuralsettlements, and the inspector’s logs. These recordsare to be retained by the Operator.



.

Surveys After Construction Section 3


Part 1Section 3 Surveys After Construction(Platforms and Self-Elevating Units in Site Dependent Services)

1/3.1 Condition for Surveys afterConstruction

1/3.1.1 Damages, Failure and Repair

a Examination and RepairDamage, failure, deterioration or repair to the classedstructure, which affects or may affect classification,is to be submitted by the Owner or their representa-tives for examination by the Surveyor at first oppor-tunity. All repairs found necessary by the Surveyorare to be carried out to his satisfaction.

b Repairs on SiteWhere repairs to the structure, which affect or mayaffect classification, are intended to be carried out atsite, complete repair procedure including the extentof proposed repair and the need for Surveyor’s atten-dance on site is to be submitted to and agreed uponby the Surveyor reasonably in advance. The above isnot intended for routine maintenance.

c RepresentationNothing contained in this section or in a regulationof any government or other administration, or the is-suance of any report or certificate pursuant to thissection or such a rule or regulation, is to be deemedto enlarge upon the representations expressed inSection 1/1 of these rules.

1/3.1.2 Notification and Availability for Survey

The Surveyors are to have access to a classed struc-ture at all reasonable times. The Surveyors are to un-dertake all surveys on classed installation upon re-quest, with adequate notification, from the Ownersor their representatives and are to report thereon tothe committee. Should the Surveyors find occasionduring any survey to recommend repairs or furtherexamination, notification is to be given immediatelyto the Owners or their representatives so that appro-priate action may be taken. The Surveyors are toavail themselves of every convenient opportunity forcarrying out periodical surveys in conjunction withsurveys of damages and repairs in order to avoid du-plication of work. For the purpose of Surveyormonitoring, monitoring surveyors shall also have ac-cess to classed offshore installations at all reasonabletimes. Such access may include attendance at the

same time as the assigned Surveyor or during a sub-sequent visit without the assigned Surveyor.

1/3.1.3 Annual Surveys

Annual Class Surveys of the structures are to bemade within three months either way of each annualanniversary date of crediting of the previous specialsurvey or original construction date. Where Survey-ors are engaged in the survey of a grouping of struc-tures of similar design and location, and where re-quested by the operator, special consideration will begiven to the timing of annual surveys and specialsurveys such that all periodical survey due dates canbe harmonized.

1/3.1.4 Special Periodic Surveys

Special surveys are to carried out at least once everyfive years. If a special survey is not completed at onetime, it will be credited as of the completion date ofthe survey provided the due date of the special peri-odical survey is not overdue by more than six (6)months. Where Surveyors are engaged in the surveyof a grouping of structures of similar design and lo-cation, and where requested by the Operator, specialconsideration will be given to the timing of SpecialSurveys so that the due dates for all periodical sur-veys can be harmonized.

1/3.1.5 Continuous Surveys

At the request of the Operator, and upon approval ofthe proposed arrangement, a system of ContinuousSurveys may be undertaken whereby all the SpecialSurvey requirements are carried out in regular rota-tion and completed within the normal Special Surveyinterval. For Continuous Surveys, a suitable notationwill be entered in the Record and the date of com-pletion of the cycle published. If any defects arefound during this Survey, they are to be examinedand dealt with to the satisfaction of the Surveyor.

1/3.1.6 Reactivation Surveys

In the case of structures which have been out ofservice for an extended period, the requirements forsurveys on reactivation are to be specially consideredin each case, due regard being given to the status of



surveys at the time of the commencement of the de-activation period, the length of the period, and con-ditions under which the structure had been main-tained during that period.

1/3.1.7 Incomplete Surveys

When a survey is not completed, the Surveyors are toreport immediately upon the work done in order thatthe Operator and the Committee may be advised ofthe parts still to be surveyed.

1/3.1.8 Alterations

No major alterations which affect classification ofthe installation are to be made to a classed structureunless plans of the proposed alterations are submit-ted and approved by the Bureau before the altera-tions are undertaken. Such alterations are to be car-ried out to the satisfaction of the Surveyors. Nothingcontained in this section or in a rule or regulation ofany government or other administration, or the issu-ance of any report or certificate pursuant to this sec-tion or such a rule or regulation, is to be deemed toenlarge upon the representations expressed in 1/1.1.1through 1/1.1.4 and the issuance and use of any suchreports or certificates are to be, in all respects, gov-erned by 1/1.1.1 through 1/1.1.4

1/3.1.9 Self Elevating Units Deployed asOffshore Installation

Self-elevating Mobile Offshore Units which havebeen converted to site dependent platform structureswill be subjected to surveys as applicable in thesesections in addition to the applicable structural ex-aminations required by the ABS Rules for Buildingand Classing Mobile Offshore Drilling Units. Sur-veys are to include Annual and Special Surveys withan underwater exam in lieu of drydocking of theabove mudline sections of the legs, mats, spud cansand platform twice in each five year Special Surveyperiod in accordance with applicable sections of theABS Mobile Offshore Drilling Units Rules. Spudcans and mats which will be located below the mudline will be considered inaccessible and fatigue,structural and corrosion analyses shall be provided tojustify the integrity of these inaccessible areas for thedesign life of the installation.

1/3.1.10 Survey for Extension of Use

Existing installations to be used at the same locationfor an extended period of time beyond their originaldesign life are subject to additional surveys to deter-mine the actual condition of the platform. The extentof the survey will depend on the completeness of theexisting survey documents. ABS will review andverify maintenance manual, logs and records. Any

alterations, repairs or installation of equipment sinceinstallation should be included in the records.

Those survey requirements in 1/3.5 for the spe-cial survey have to be included in the survey for ex-tension of use. The surveys generally cover exami-nation of splash zone, inspection of above water andunderwater structural members and welds for dam-ages and deteriorations, examination and measure-ments of corrosion protection systems and marinegrowth, sea floor condition survey, examination ofsecondary structural attachments, risers and riserclamps. Special attention should be given to the fol-lowing critical areas.

— Areas of high stress— Areas of low fatigue life— Damage incurred during installation or while

in service— Repairs or modifications made while in service— Abnormalities found during previous surveys.— An inspection report of the findings is to be

submitted to ABS for review and evaluation ofthe condition of the platform.

The need for more frequent future periodical surveyswill be determined based on the calculated remainingfatigue life described in Section 4/1 and past inspec-tion results.

1/3.1.11 Relocation of Existing Installations

Existing installations that are classed at a specifiedlocation require special consideration when reloca-tion to a new site is proposed. The Owner is to ad-vise ABS of the proposal to change locations ad-dressing removal, transportation and re-installationaspects of the change. Survey requirements de-scribed in Section 1/2 and 1/3.1.10, wherever appli-cable, are to be complied with in addition to an engi-neering analyses required to justify the integrity ofthe installation for the design life at the new location.

1/3.3 Annual Surveys

Each annual survey is to include a thorough visualexamination of all above water structure. Special at-tention will be given to the splash zone for possibledamage or deterioration from corrosion. Addition-ally, where it appears that significant deterioration ordamage has occurred to an installation since the lastsurvey, a general examination, by diver, underwatercamera, submersible, or other suitable means, of theunderwater structure, the sea floor, and the corrosioncontrol system shall be carried out. Underwater ex-aminations are to be contracted by the Operator andmonitored by a Surveyor.

Any novel features incorporated in the designare to be given special attention according to proce-dures agreed to during review of the design.



Particular attention is to be given to significantmodifications or repairs made as a result of findingsat the previous survey.

The Annual Survey is also to include verifica-tion that the approved design life has not been ex-ceeded. The surveyor is to confirm the design lifelimits and the technical office should be consultedfor verification. If the end of the design life has beenreached then the provisions of Section 4/1 are to beapplied and specific requirements for maintaining theclass of the structure are to be obtained from thetechnical office.

1/3.5 Special Periodic Surveys

The requirements of the Annual Survey are to be metduring the Special Periodic Survey. Additionally,underwater inspection of selected areas of the in-stallation is to be carried out. Also, nondestructivetesting is to be carried out on representative joints ofthe structures and if found necessary, structural sup-ports of conductors and risers. The extent and meth-ods to be employed in such testing, cleaning, and in-spection of the structure are to be in accordance withan approved inspection plan. The inspection plan,which is to be submitted for approval, is to cover allspecial surveys for the design life of the structure. Itis to enumerate in detail the items to be surveyed, thetesting and inspection procedures to be employed,and where necessary, cleaning and nondestructivetesting procedures. The plan is to include sufficientlydetailed drawings which can be used by the Surveyorto reference and locate the items to be surveyed. Thetesting, cleaning, and inspection services are to beprovided by the Operator and monitored by a Sur-veyor. Divers carrying out structural inspections andnondestructive testing on the structures are to besuitably qualified.

The special survey is also to include monitoringof the effectiveness of the corrosion protection sys-tem. The effectiveness of the corrosion protectionsystem is to be monitored by taking measurements ofthe potential voltages generated by such systems.Scour in way of platform legs, tilt and subsidence arealso to be checked and witnessed by the attendingsurveyor.

The Special Periodic Survey is also to includeverification that the approved design life has notbeen exceeded. The surveyor is to confirm the designlife limits and the technical office should be con-sulted for verification. If the end of the design lifehas been reached then the provisions of Section 4/1are to be applied and specific requirements formaintaining the class of the structure are to be ob-tained from the technical office.

1/3.7 Gaugings

Thickness gaugings are required to be taken at eachSpecial Survey. Suspect areas including structures inway of the splash zone are to be tested for thickness andresults submitted to the attending Surveyor for review.Offshore installations consisting of converted self-elevating units will require thickness gauging where ac-cessible in accordance with the applicable gauging re-quirements contained in the ABS Rules for Buildingand Classing Mobile Offshore Drilling Units.

1/3.9 Structural Deterioration

Where thickness measurement and visual examinationshow evidence of significant structural deteriorationthe structural integrity of the structure for continuoususe shall be justified by engineering analyses. Deterio-rated structural members should be modeled such thatthey will add hydrodynamic or wind loads to the plat-form, but will not continue the strength of platform.Results of these analyses are to be submitted to ABSfor review and approval. If the results show that overall structural integrity of platform is adequate, the de-teriorated structural members may be left as it is. Oth-erwise the deteriorated structural members are to besuitably reinforced or removed and replaced with newmaterials having required dimensions and propertiesin accordance with approved procedure.

1/3.11 Maintenance of Marine Growth

During any Annual or Special Survey, assessment ofthe degree of marine growth shall be carried out.Should marine growth be found to be thicker thanthe original approved design, it is to be removed. Ifthe Operator decides to leave the marine growthgreater than what is allowed in the approved design,the Operator is to show justification that the higherhydrodynamic loading due to the additional marinegrowth will not affect the structural integrity of thestructure. The Operator is to at least submit an in-place analysis to justify that the installation is capa-ble of withstanding environmental wave loads re-sulting from the maximum marine growth that theOperator is prepared to maintain.

1/3.13 Statutory Certification

When the Bureau is authorized to perform certifica-tion on behalf of a governmental authority, or whenrequested by the Operator, requirements as specifiedby the governmental authority or Operator shall becertified accordingly and the reports and certificatesin accordance with those requirements shall be is-sued as appropriate.



.

Definitions and Design Documentation Section 4


Part 1Section 4 Definitions and Design

Documentation

1/4.1 Definitions

1/4.1.1 Recurrence Period

The recurrence period is a specified period of timewhich is used to establish design values of randomparameters such as wave height.

1/4.1.2 Owner

An owner is any person or organization who ownsthe platform.

1/4.1.3 Operator

An operator is any person or organization empow-ered to conduct operations on behalf of the Ownersof an installation.

1/4.1.4 Constructor

A constructor is any person or organization havingthe responsibility to perform any or all of the fol-lowing: fabrication, erection, inspection, testing,load-out, transportation, and installation.

1/4.1.5 Consultant

A consultant is any person who, through educationand experience, has established credentials of profes-sionalism and expertise in the stated field.

1/4.1.6 Surveyor

A Surveyor is a person employed by the Bureauwhose principal functions are the surveillance duringconstruction and the survey of marine structures andtheir components for compliance with Bureau-issuedRules or other standards deemed suitable by the Bu-reau.

1/4.1.7 Offshore Installations

A buoyant or nonbuoyant structure, supported by orattached to the sea floor, whose design is based onfoundation and long term environmental conditionsat a particular installation site where it is intended toremain. The sea floor attachment afforded to theplatform may be obtained by pilings, direct bearing,mooring lines, anchors, etc. The site-specific data foran offshore installation employed by the designer

and submitted for review by the Bureau will form apart of its classification.

Examples of structures covered by these Rulesare the types of fixed structures characterized as pilesupported or gravity platforms, various forms ofcompliant structures, and other moored buoyantstructures. Specifically excluded from the coverageof these Rules are mobile units and manned sub-mersibles, which are treated in Rules separately is-sued by the Bureau. Where doubt exists concerningthe applicability of these Rules clarification may beobtained from the Bureau.

An offshore installation consists of one or moreof the following.

I. Platform StructureII. Undersea Pipeline Systems and RisersIII. Offshore Facilities

a. Machinery, Electrical and Piping Systemsb. Production Equipment

1/4.1.8 Platform Structures

Various types of offshore structures to which theseRules may be applied are defined below.

Pile Supported Platform This type of structureis characterized by slender foundation elements, orpiles, driven into the sea floor.

Gravity Structure This type of structure restsdirectly on the sea floor. The geometry and weight ofthe structure are selected to mobilize the availablecohesive and frictional strength components of thesea floor soil to resist loadings.

Compliant Tower This type of structure con-sists of a slender tower supported at the sea floor byan installed foundation (or by a large spud can) andmay also be partially supported by buoyancy aids.Guy lines may or may not be used for lateral re-straint.

Various provisions of these Rules are to be ap-plied to partially or fully buoyant structures whichare permanently (see 1/1.5.1) connected to the seafloor by mooring lines or other non-rigid means.Structural types including the articulated buoyanttower and the tension leg platform, which are de-fined below, are included in this category. For thesebuoyant structures, classification will be based oncompliance with the applicable portions of theseRules, those of the Bureau’s Rules for Building and



Classing Mobile Offshore Drilling Units, and otherrequirements which the Bureau, in consultation withthe Owner, deems appropriate.

Articulated Buoyant Tower This type of struc-ture depends on buoyancy acting near the water sur-face to provide necessary righting stability. Becauseof its tendency towards relatively large horizontaldisplacements, the articulated buoyant structure canbe provided with a pivot near the sea floor.

Tension Leg Platform This type of structure isfully buoyant and is restrained below its natural flo-tation line by mooring elements which are attachedin tension to gravity anchors or piles at the sea floor.

Additionally, these Rules may be employed, asapplicable, in the classification of structural types notmentioned above, when they are to be used as per-manent offshore installations (see 1/1.5.3).

1/4.1.9 Extension of Use

An existing platform to be used at the same locationfor a specified period of time beyond its original de-sign life. See Section 4/1.

1/4.1.10 Reuse

An existing platform to be moved to a new locationto continue its operation for a specified period oftime. See Section 4/2.

1/4.3 Design Documentation

The design documentation to be submitted is to in-clude the reports, calculations, plans, and otherdocumentation necessary to verify the structural de-sign. The extensiveness of the submitted documen-tation should reflect the uniqueness of the structureor the lack of experience with conditions in the areawhere the structure is to be located. In general, sig-nificantly less detailed documentation is required fora pile supported platform in calm, shallow watersthan for an unusual structural configuration sited indeep waters. Existing documentation may be usedwhere applicable.

1/4.3.1 Reports

Reports by consultants and other specialists used as abasis for design are to be submitted for review. Thecontents of reports on environmental considerations,foundation data, and materials are, in general, tocomply with the recommended list of items givenbelow.

a Environmental Considerations Reports onenvironmental considerations are to describe all en-vironmental phenomena appropriate to the areas ofconstruction, transportation, and installation. Thetypes of environmental phenomena to be accountedfor, as appropriate to the type and location of the

structure, are: wind, waves, current, temperature,tide, marine growth, chemical components of air andwater, snow and ice, earthquake, and other pertinentphenomena.

The establishment of the environmental pa-rameters is to be based on appropriate original dataor, when permitted, data from analogous areas. De-monstrably valid statistical models to extrapolate tolong-term values are to be employed, and any calcu-lations required to establish the pertinent environ-mental parameters should be submitted.

Preferably, a report on the various environ-mental considerations is to present data and conclu-sions on the relevant environmental phenomena. Thereport is, however, required to separately present asummary showing the parameters necessary to definethe Design Environmental Condition and OperatingEnvironmental Conditions, as defined in Section 3/1;where applicable, the likely environmental condi-tions to be experienced during the transportation ofthe structure to its final site; and where necessary,the Strength and Ductility Level Earthquakes, as de-fined in 3/1.5.1.

The report on environmental considerations mayalso contain the calculations which quantify the ef-fects or loadings on the structure where these are notprovided in other documentation.

b Foundation Data A report on foundationdata is to present the results of investigations or,when applicable, data from analogous areas on geo-physical, geological and geotechnical considerationsexisting at and near the platform site. The manner inwhich such data is established and the specific itemsto be assessed are to be in compliance with 3/6.3.The report is to contain a listing of references tocover the investigation, sampling, testing, and inter-pretive techniques employed during and after the siteinvestigation.

The report is to include a listing of the predictedsoil-structure interaction, such as p-y data, to be usedin design. As appropriate to the planned structure,the items which may be covered are: axial and lateralpile capacities and response characteristics, the ef-fects of cyclic loading on soil strength, scour, settle-ments and lateral displacements, dynamic interactionbetween soil and structure, the capacity of pilegroups, slope stability, bearing and lateral stability,soil reactions on the structure, and penetration resis-tance.

Recommendations relative to any special antici-pated problem regarding installation are to be in-cluded in the report. Items such as the following areto be included, as appropriate: hammer sizes, soilerosion during installation, bottom preparation, andprocedures to be followed should pile installationprocedures significantly deviate from those antici-pated.



c Materials and Welding Reports on structuralmaterials and welding may be required for metallicstructures, concrete structures or welding procedureswhere materials and procedures are used which donot conform to those provided for in Sections 2/1and 2/2.

For metallic structures, when it is intended toemploy new alloys not defined by a recognizedspecification, reports are to be submitted indicatingthe adequacy of the material’s metallurgical proper-ties, fracture toughness, yield and tensile strengths,and corrosion resistance, with respect to their in-tended application and service temperatures.

For concrete structures, when it is not intended totest or define material properties in accordance withapplicable standards of the American Society forTesting and Materials (ASTM) as listed in Section2/1, a report is to be provided indicating the standardsactually to be employed and their relative adequacywith respect to the corresponding ASTM standards.

1/4.3.2 Calculations

Design and analysis calculations are to be submittedfor items relating to loadings, structural stresses anddeflections for in-place and marine operations. Inthis regard, calculations are to be in general compli-ance with the items listed below. Calculations whichmay be required in association with environmentalconsiderations, and foundation data have been dis-cussed in 1/4.3.1.

a Loadings Calculations for loadings are to besubmitted in accordance with Section 3/2.

b Structural Stresses and Deflections Thestress and deflection calculations to be submitted areto include, those required for nominal element ormember stresses and deflections. As applicable, andwhere required in subsequent sections of these Rules,calculations may also be required for the stresses inlocalized areas and structural joints, the dynamic re-sponse of the structure, and fatigue life of criticalmembers and joints. For pile supported structures,calculations for the stresses in piles and the load ca-pacity of the connection between the structure andthe pile are to be submitted. Similarly, for gravitystructures, calculations are to be submitted for the ef-fects of the soil’s reaction on the structure.

When accounting for the stress resultants de-scribed above, and those resulting from considerationof marine operations (see Section 3/7), calculations

are to demonstrate the adequacy of the structural ele-ments, members or local structure. Also, the calcula-tions are to demonstrate, as applicable, that the de-flections resulting from the applied loadings andoverall structural displacement and settlement do notimpair the structural performance of the platform.

c Marine Operations As applicable, calculationsare to be submitted in compliance with Section 3/7.

d Other Calculations As required, additionalcalculations which demonstrate the adequacy of theoverall design are to be submitted. Such calculationsshould include those performed in the design of thecorrosion protection system.

1/4.3.3 Plans and Other Data

Generally, structural plans and other data are to besubmitted in quadruplicate. These plans are to in-clude the following, where applicable.

Arrangement plans, elevations, and plan viewsclearly showing in sufficient detail the overallconfiguration, dimensions and layout of thestructure, its facilities and foundation

Layout plans indicating the locations of equipmentand locations of the equipment loads and otherdesign deck loads, fender loads, etc., which areimposed on the structure

Structural plans indicating the complete structural ar-rangement, dimensions, member sizes, platingand framing, material properties, and details ofconnections and attachments; for concretestructures, arrangements and descriptions of re-inforcement procedures for construction are tobe indicated

Pile plans indicating arrangements, nominal sizes,thicknesses and penetration

Welding details and procedures, and schedule ofnondestructive testing

Corrosion control systemsStructural plans indicating the complete arrange-

ments of structures, such as helidecks, cranepedestals, equipment foundations and manner ofreinforcement, fendering, various houses andother structures which are not normally consid-ered vital to the overall structural integrity of theoffshore structure

Various information in support of novel featuresutilized in the offshore structure design, such ashydrostatic and stability curves, elements of anymooring system, etc.



.

Materials Section 1


Part 2 Materials and Welding

Materials Section 1


.

Materials Section 1


Part 2Section 1 Materials

2/1.1 Structural Steels

2/1.1.1 General

a Scope This subsection covers specifications formaterials used for the construction of offshore steelstructures. It is not intended for metals used in rein-forced or prestressed concrete. (See 2/1.3.) All materi-als are to be suitable for intended service conditions,they are to be of good quality, defined by a recognizedspecification and free of injurious imperfections.

b Material Selection Materials used are requiredto exhibit satisfactory formability and weldability char-acteristics. As required, documentation is to be submit-ted to substantiate the applicability of a proposed steel.Reference can be made to Tables 2/1.2A and 2/1.2B forASTM and API steel grades and to Appendix A forguidance on the selection of ABS grades of steel.

When material other than steel is to be used as astructural material, documentation is to indicate thetensile, toughness, fatigue and corrosion characteris-tics of the proposed material.

c Corrosion Control Details of corrosion con-trol systems (such as coatings, sacrificial anodes orimpressed current systems) are to be submitted withadequate supporting data to show their suitability.Such information is to indicate the extent to whichthe possible existence of stress corrosion, corrosionfatigue, and galvanic corrosion due to dissimilarmetals to be considered. Where the intended sea en-vironment contains unusual contaminants, any spe-cial corrosive effects of such contaminants shouldalso be considered. Appropriate coatings may beused to achieve satisfactory corrosion protection formiscellaneous parts such as bolts and nuts.

d Toughness Materials are to exhibit fracturetoughness which is satisfactory for the intended ap-plication as supported by previous satisfactory serv-ice experience or appropriate toughness tests. Wherethe presence of ice is judged as a significant envi-ronmental factor, material selection may require spe-cial consideration.

e Through Thickness Stress In cases whereprincipal loads, from either service or weld residualstresses, are imposed perpendicular to the surface ofa structural member, the use of special steel with im-proved through thickness (Z-direction) propertiesmay be required.

2/1.1.2 Steel Properties

a General Material specifications are to besubmitted for review or approval. Due regard is to begiven to established practices in the country in whichmaterial is produced and the purpose for which thematerial is intended.

b Tensile Properties In Table 2/1.1, the desig-nation Group I, II or III is used to categorize tensileproperties.

c Toughness Appropriate supporting informa-tion or test data are to indicate that the toughness ofthe steels will be adequate for their intended applica-tion and minimum service temperature. Criteria in-dicative of adequate toughness are contained in2/1.1.3.

d Bolts and Nuts Bolts and nuts are to havemechanical and corrosion characteristics comparableto the structural elements being joined and are to bemanufactured and tested in accordance with recog-nized material standards.

2/1.1.3 Toughness Criteria for Steel Selection

a General When members are subjected tosignificant tensile stress, fracture toughness is to beconsidered in the selection of materials.

b Steel Classification Steels are to be classi-fied as Groups I, II or III according to their tensileproperties as listed in Table 2/1.1. It should be notedthat the yield strengths given in Table 2/1.1 are pro-vided only as a means of categorizing steels.

TABLE 2/1.1 Steel Tensile Properties

Yield Strength fy

Group ksi MPa

III fy < 40 fy < 275III 40 ≤ fy ≤ 60 275 ≤ fy ≤ 415III 60 ≤ fy ≤ 100 415 ≤ fy ≤ 690

Some of the typical ASTM and API steels belongingto the groups of Table 2/1.1 are shown in Tables2/1.2A and 2/1.2B. Steels other than those men-tioned therein may be used, provided that theirchemical composition, mechanical properties andweldability are similar to those listed.

Materials Section 1


TABLE 2/1.2A Structural Steel Plates and Shapes

Yield Strength Tensile StrengthGroup Specification & Grade ksi MPa ksi MPa

I ASTM A36-94 (to 2 in. thick)ASTM A131-94 Grade A (to ½ in. thick) (ABS Grade A)

3634

250235

58-8058-75

400-550400-515

ASTM A285-90 Grade C (to C\v in. thick) 30 205 55-75 380-515ASTM A131-94 Grades B, D (ABS Grades B, D) 34 235 58-75 400-515ASTM A516-90 Grade 65 35 240 65-85 450-585ASTM A573-93a Grade 65 35 240 65-77 450-530ASTM A709-93a Grade 36T2 36 250 58-80 400-550ASTM A131-94 Grade E (ABS Grade E) (ABS Grades CS, E) 34 235 58-75 400-515

II ASTM A572-94b Grade 42 (to 2 in. thick) 42 290 60 min. 415 min.ASTM A572-94b Grade 50 (to ½ in. thick*) 50 345 65 min. 450 min.ASTM A588-94 (to 2 in. thick) 50 345 70 min. 485 min.ASTM A709-93a Grades 50T2, 50T3 50 345 65 min. 450 min.ASTM A131-94 Grade AH32 (ABS Grade AH32) 46 315 68-85 470-585ASTM A131-94 Grade AH36 (ABS Grade AH36) 51 350 71-90 490-620API Spec 2H-Grade 42 42 290 62-80 425-550API Spec 2H-Grade 50 (to 2½ in. thick) 50 345 70-90 485-620

(over 2½ in. thick) 47 325 70-90 485-620API Spec 2W-Grade 42 (to 1 in. thick) 42-67 290-460 62 min. 425 min.

(over 1 in. thick) 42-62 290-430 62 min. 425 min.Grade 50 (to 1 in. thick) 50-75 345-515 65 min. 450 min.

(over 1 in. thick) 50-70 345-485 65 min. 450 min.Grade 50T (to 1 in. thick) 50-80 345-550 70 min. 485 min.

(over 1 in. thick) 50-75 345-515 70 min. 485 min.API Spec 2Y-Grade 42 (to 1 in. thick) 42-67 290-460 62 min. 425 min.

(over 1 in. thick) 42-62 290-430 62 min. 425 min.Grade 50 (to 1 in. thick) 50-75 345-515 65 min. 450 min.

(over 1 in. thick) 50-70 345-485 65 min. 450 min.Grade 50T (to 1 in. thick) 50-80 345-550 70 min. 485 min.

(over 1 in. thick) 50-75 345-515 70 min. 485 min.

ASTM A131-94 Grades DH32, EH32 (ABS Grades DH32, EH32 46 315 68-85 470-585Grades DH36, EH36 (ABS Grades DH36, EH36) 51 350 71-90 490-620

ASTM A537-91 Class 1 (to 2½ in. thick) 50 345 70-90 485-620ASTM A633-94a Grade A 42 290 63-83 435-570

Grades C, D 50 345 70-90 480-620ASTM A678-94a (80) Grade A 50 345 70-90 485-620

III ASTM A537-91 Class 2 60 415 80-100 550-690ASTM A633-94a Grade E 60 415 80-100 550-690ASTM A678-94a (80) Grade B 60 415 80-100 550-690API Spec 2W-Grade 60 (to 1 in. thick) 60-90 415-620 75 min. 515 min.

(over 1 in. thick) 60-85 415-585 75 min. 515 min.

API Spec 2Y-Grade 60 (to 1 in. thick) 60-90 415-620 75 min. 515 min.(over 1 in. thick) 60-85 415-585 75 min. 515 min.

ASTM A710-Grade A Class 3 (to 2 in. thick) 75 515 85 min. 585 min.65 450 75 min. 515 min.60 415 70 min 485 min.

*To 2 in. Thick for Type 1, Fully Killed, Fine Grain Practice.

Materials Section 1


TABLE 2/1.2B Structural Steel Pipes

Yield Strength Tensile StrengthGroup Specification & Grade ksi MPa ksi MPa

I API 5L-Grade B 35 240 60 min 415 min.ASTM A53-93a Grade B 35 240 60 min. 415 min.ASTM A135-93 Grade B 35 240 60 min. 415 min.ASTM A139-93a Grade B 35 240 60 min. 415 min.ASTM A381-93 Grade Y35 35 240 60 min. 415 min.ASTM A500-93 Grade A 33-39 230-270 45 min. 310 min.ASTM A501-93 36 250 58 min. 400 min.ASTM A106-94 Grade B 35 240 60 min. 415 min.ASTM A524-93 (strength varies with thickness) 30-35 205-240 55-85 380-585

II API 5L95 Grade X42 (2% max. cold expansion) 42 290 60 min. 415 min.API 5L95 Grade X52 (2% max. cold expansion) 52 360 66 min. 455 min.ASTM A500-93 Grade B 42-46 290-320 58 min. 400 min.ASTM A618-93 Grade Ia, Ib & II (to C\v in. thick) 50 345 70 min. 485 min.API 5L95 Grade X52 (with SR5, SR6, or SR8) 52 360 66 min. 455 min.

c Toughness Characteristics Satisfactorytoughness characteristics can be demonstrated by anyone of the following.Demonstration of past successful application, under

comparable conditions, of a steel, produced to arecognized standard (such as those of theASTM, API or other recognized standard)

Demonstration that a steel manufactured by a par-ticular producer using a specific manufacturingprocess has minimum toughness levels repre-sentative of those listed herein

Charpy impact testing in accordance with Table 2/1.3

2/1.1.4 Minimum Service Temperature

Minimum service temperature refers to the tempera-ture of the material and is generally to be establishedin accordance with a, b and c below. This tempera-ture is to be based on meteorological data taken overa period of not less than 10 years for the lowest aver-age daily temperature.

a Material below the Splash Zone For materialbelow the splash zone (see 3/3.5.5), the service tem-perature is defined as 0°C (32°F). A higher servicetemperature may be used if adequate supporting datacan be presented relative to the lowest average dailywater temperature applicable to the depths involved.

b Material within or above the Splash ZoneFor material within or above the splash zone, theservice temperature is the same as the lowest averagedaily atmospheric temperature. A higher servicetemperature may be used if the material above thewaterline is warmed by adjacent sea water tempera-ture or by auxiliary heating.

c Special Conditions In all cases where mate-rial temperature is reduced by localized cryogenicstorage or other cooling conditions, such factorsshould be taken into account in establishing mini-mum service temperature.

2/1.3 Materials for ConcreteConstruction

2/1.3.1 General

a Scope This subsection covers specificationsfor materials for concrete used in the construction ofoffshore platforms. It includes the metals used in rein-forced or prestressed concrete. All materials are to besuitable for intended service conditions and are to beof good quality, defined by recognized specificationsand free of injurious defects. Materials used in theconstruction of concrete structures are to be selectedwith due attention given to their strength and durabil-ity in the marine environment. Materials which do notconform to the requirements of this subsection may beconsidered for approval upon presentation of suffi-cient evidence of satisfactory performance.

b Zones Particular attention should be given ineach of the following zones (see 3/3.5.5) to the con-siderations indicated.

Submerged zone: chemical deterioration of the con-crete, corrosion of the reinforcement and hard-ware, and abrasion of the concrete

Splash zone: freeze-thaw durability, corrosion of thereinforcement and hardware, chemical deterio-ration of the concrete, and fire hazards

Ice zone: freeze-thaw durability, corrosion of the re-inforcement and hardware, chemical deteriora-tion of the concrete, fire hazards, and abrasionof the concrete

Atmospheric zone: freeze-thaw durability, corrosionof reinforcement and hardware, and fire hazards

2/1.3.2 Cement

a Type Cement is to be equivalent to Types Ior II Portland cement as specified by ASTM C150 orportland-pozzolan cement as specified by ASTM

Materials Section 1


C595. ASTM C150 Type III Portland cement may bespecially approved for particular applications.

b Tricalcium Aluminate The tricalcium alu-minate content of the cement is generally to be in the5% to 10% range.

c Oil Storage For environments which containdetrimental sulfur bearing materials (such as whereoil storage is planned and the oil is expected to con-tain sulphur compounds which are detrimental toconcrete durability), the maximum content of trical-cium aluminate is to be at the lower end of the 5% to10% range. Alternatively, pozzolans or pozzolansand fly ash may be added or a suitable coating em-ployed to protect the concrete.

2/1.3.3 Water

a Cleanliness Water used in mixing concreteis to be clean and free from injurious amounts ofoils, acids, alkalis, salts, organic materials or othersubstances that may be deleterious to concrete orsteel.

b Nonpotable Water If nonpotable water isproposed for use, the selection of proportions ofmaterials in the concrete is to be based on test con-crete mixes using water from the same source. Thestrength of mortar test cylinders made with nonpot-able water is not to be less than 90% of the strengthof similar cylinders made with potable water.Strength test comparisons should include 7-day and28-day strength data on mortars prepared and testedin accordance with recognized standards such asASTM C109.

2/1.3.4 Chloride or Sulphide Content

Water for structural concrete or grout should notcontain more than 0.07% chlorides as Cl by weightof cement, nor more than 0.09% sulfates as SO4when tested by ASTM D512. Chlorides in mix waterfor prestressed concrete or grout should be limited to0.04% by weight of cement.

Total chloride content, as Cl, of the concreteprior to exposure shall not exceed 0.10% by weightof the cement for normal reinforced concrete and0.06% by weight of cement for prestressed concrete.

2/1.3.5 Aggregates

a General Aggregates are to conform to therequirements of ASTM C33 or equivalent. Other ag-gregates may be used if there is supporting evidencethat they produce concrete of satisfactory quality.When specially approved, lightweight aggregatessimilar to ASTM C330 may be used for conditionsthat do not pose durability problems.

b Washing Marine aggregates are to bewashed with fresh water before use to remove chlo-rides and sulphates so that the total chloride and sul-

phate content of the concrete mix does not exceedthe limits defined in 2/1.3.4.

c Size The maximum size of the aggregate isto be such that the concrete can be placed withoutvoids. It is recommended that the maximum size ofthe aggregate should not be larger than the smallestof the following: one-fifth of the narrowest dimen-sion between sides of forms; one-third of the depthof slabs; three-fourths of the minimum clear spacingbetween individual reinforcing bars, bundles of bars,prestressing tendons or post-tensioning ducts.

2/1.3.6 Admixtures

a General The admixture is to be shown capa-ble of maintaining essentially the same compositionand performance throughout the work as the productused in establishing concrete proportions. Admix-tures containing chloride ions are not to be used iftheir use will produce a deleterious concentration ofchloride ions in the mixing water.

b Recognized Standards Admixtures are to bein accordance with applicable recognized standardssuch as ASTM C260, ASTM C494, ASTM C618 orequivalents.

c Pozzolan Content Pozzolan or pozzolan andfly ash content is not to exceed 15% by weight ofcement unless specially approved.

2/1.3.7 Steel Reinforcement

Steel reinforcement used in offshore concrete struc-tures is to be suitable for its intended service and inaccordance with recognized standards.

a Reinforcement for Non-Prestressed ConcreteNon-prestressed reinforcement is to be in accordancewith one of the following specifications or its equiva-lents.

Deformed reinforcing bars and plain bars: ASTM A615Bar and rod mats: ASTM A184Plain wire for spiral reinforcement: ASTM A82,

ASTM A704Welded plain wire fabric: ASTM A185Deformed wire: ASTM A496Welded deformed wire fabric: ASTM A497

b Welded Reinforcement Reinforcement whichis to be welded is to have the properties needed to pro-duce satisfactory welded connections. Welding is to bein accordance with recognized specifications such asthe American Welding Society (AWS) D1.l, or is to beproven to produce connections of satisfactory quality.

c Steel Reinforcement for Prestressed ConcreteSteel reinforcement for prestressed concrete is to bein accordance with one of the following specifica-tions or equivalent.

Seven-wire strand: ASTM A416Wire: ASTM A421

Materials Section 1


d Other Materials Other prestressing tendonsmay be approved upon presentation of evidence ofsatisfactory properties.

2/1.3.8 Concrete

The concrete is to be designed to assure sufficientstrength and durability. A satisfactory method forquality control of concrete is to be used which isequivalent to ACI 318. Mixing, placing and curing ofconcrete shall conform to recognized standards.

2/1.3.9 Water-Cement Ratios

Unless otherwise approved, water-cement ratios and28-day compressive strengths of concrete for thethree exposure zones are to be in accordance withTable 2/1.4.

2/1.3.10 Other Durability Requirements

a Cement Content Minimum cement contentshould insure an adequate amount of paste for rein-forcement protection and generally be not less than355 kg/m3 (600 lb/yd3).

b Freeze-Thaw Durability When freeze-thawdurability is required, the concrete is to contain en-trained air in accordance with a recognized standardsuch as ACI 211.1. Attention is to be paid to the ap-propriate pore distribution of the entrained air andthe spacing between pores in the hardened concrete.The calculated spacing factors are not to exceed0.25 mm (0.01 in.).

c Scouring When severe scouring action isexpected, the coarse aggregate should be as hard asthe material causing the abrasion, the sand content ofthe concrete mix should be kept as low as possible,and air entrainment is to be limited to the minimumappropriate to the application.

2/1.3.11 Grout for Bonded Tendons

a General Grout for bonded tendons is to con-form to ACI 318 or equivalent.

b Chlorides and Sulphates Grout is not tocontain chlorides or sulfates in amounts which aredetrimental to the structure. Limitations are includedin 2/1.3.4.

c Contents Grout is to consist of Portland ce-ment and potable water, or Portland cement, sand,and potable water. Admixtures may be used only af-ter sufficient testing to indicate that their use is bene-ficial and that they are free of harmful quantities ofchlorides, nitrates, sulfides, sulphates or any othermaterial which has been shown to be detrimental tothe steel or grout.

d Sand Sand, if used, is to conform to ASTMC144 or equivalent, except that gradation may bemodified as necessary to obtain increased workability.

e Preparation Proportions of grouting materi-als are to be based on results of tests on fresh andhardened grout prior to beginning work. The watercontent shall be the minimum necessary for properplacement but in no case more than 50% of the con-tent of cement by weight. Grout is to be properlymixed and screened.

f Temperature Temperature of members at thetime of grouting is to be above 10°C (50°F) and is tobe maintained at this temperature for at least 48hours.

TABLE 2/1.3 Charpy Toughness Specificationfor Steels

Energy Absorption(Longitudinal)

Group Section Size Joules ft-lb

I 6 mm < t < 19 mm 20 15(0.25 in. < t < 0.75 in.)

t > 19 mm (0.75 in.) 27 20II, III t > 6 mm (0.25 in.) 34 25

The following notes apply to Table 2/1.3.

1 Test Temperatures—The following applies forservice temperatures down to −30°C (−22°F); forlower service temperatures, test requirements areto be specially considered.a For structural members and joints whose per-

formance is vital to the overall integrity of thestructure and which experience an unusuallysevere combination of stress concentration,rapid loading, cold working, and restraint, theimpact test guidelines of Table 2/1.3 shall bemet at test temperatures as given below.

Group Test TemperatureMinimum Service

Temperature(As determined

by 2/1.1.4)

I, II 30°C (54°F) belowMinimum Service

TemperatureIII −40°C (−40°F) −10°C (32°F)

−50°C (−58°F) −10°C (14°F)−50°C (−58°F) −20°C (−4°F)−60°C (−76°F) −−30°C (−22°F)

b For structural members and joints which sus-tain significant tensile stress and whose frac-ture may pose a threat to the survival of thestructure, the impact test guidelines of Table2/1.3 should be met at test temperatures asgiven on the following page.

Materials Section 1


Group Test Temperature

Minimum ServiceTemperature

(As determinedby 2/1.1.4)

I, II 10°C (18°F) belowMinimum Service

TemperatureIII −30°C (−22°F) −10°C (32°F)

−40°C (−40°F) −10°C (14°F)−40°C (−40°F) −20°C (−4°F)−50°C (−58°F) −−30°C (−22°F)

c For primary structural members subjected tosignificant tensile stresses and whose usagewarrants impact toughness testing, the impacttest guidelines of Table 2/1.3 should be met atthe following test temperatures.

Group Test Temperature

Minimum ServiceTemperature

(As determinedby 2/1.1.4)

I, II At MinimumService Temperature

III Same as note 1b −

d For structural members which have sufficientstructural redundancy so that their fracturewould not pose a threat to the survivability ofthe structure, the toughness criteria specifiedfor c above may be relaxed provided the mate-rials used in such cases are appropriate for theloading conditions, loading rates, and tem-peratures encountered in service.

2 Impact tests are not necessary for section sizesbelow 6 mm (0.25 in.) in thickness.

3 Energy values in Table 2/1.3 are minimum aver-age values for full-size longitudinal specimens.Alternative toughness criteria which may be ap-plied are:

Under-sized longitudinal specimens: pro-portional reduction in Table 2/1.3 energyvalues in accordance with ASTM A20 orequivalent

Transverse specimens: 2/3 of the energyvalues shown in Table 2/1.3 but in nocase less than 20 Joules (15 ft-lb)

Longitudinal or transverse specimens: lat-eral expansion should not be less than0.5 mm (0.02 in.), or 0.38 mm (0.015in.), respectively

Nil-ductility temperature (NDT) as deter-mined by drop weight tests should be5°C (l0°F) below the test temperature in-dicated in item 1 above

Other fracture toughness tests as appropri-ate.

4 The minimum number of specimens to be testedper heat should be three; however, this numbershould be increased in accordance with usage ofthe material (see ASTM A673 or equivalent).

TABLE 2/1.4 W/C Ratios and CompressiveStrengths

ZoneMaximumw/c Ratio

Minimum28-day Cylinder

Compressive StrengthSubmerged 0.45 35 MPa (5000 psi)Splash andAtmospheric

0.40 to 0.45* 35 MPa (5000 psi)

*Depending upon severity of exposure

Welding and Fabrication Section 2


Part 2Section 2 Welding and Fabrication

2/2.1 Introduction

Welding for steel structures is to comply with thepertinent requirements of a recognized code, such asthe Structural Welding Code—Steel, D1.1, issued bythe American Welding Society, or Section 2/3 of theABS Rule Requirements for Materials and Welding.While the requirements of this section are to be ad-dressed using a recognized reference code, the refer-ence code may not provide coverage of all necessaryitems. Therefore, this section provides additional re-quirements which, as the need arises, extend thescope of the code to make it suitable for classifica-tion purposes. Also, because of the possible widevariation of requirements which may exist amongselected welding codes, 2/2.9 through 2/2.13 givesome specific requirements which are intended toensure a basic degree of uniformity in the weldingperformed for structures classed with the Bureau.

2/2.3 General

2/2.3.1 Plans and Specifications

Submitted plans or specifications are to be in accor-dance with Section 4 and they are to indicate clearlythe extent of welding for the main parts of the struc-ture. The plans or specifications should indicate theextent of nondestructive inspection of the weld. Thewelding process, filler metal and joint design are to beindicated on plans or in separate specifications sub-mitted for approval, which are to distinguish betweenmanual and automatic welding. The Surveyor is to beinformed of the planned sequences and procedures tobe followed in the erection and welding of the mainstructural members. In all instances, welding proce-dures and filler metals are to be applied which willproduce sound welds that have strength and toughnesscomparable to that of the base material.

2/2.3.2 Workmanship and Supervision

It is to be demonstrated that all welders and weldingoperators to be employed in the construction ofstructures to be classed are properly qualified and areexperienced in the type of work proposed and in theproper use of the welding processes and proceduresto be followed. A sufficient number of skilled super-visors is to be employed to ensure thorough supervi-sion and control of all welding operations. Inspectionof welds employing methods outlined in 2/2.7.9 is tobe carried out to the satisfaction of the Surveyor.

2/2.3.3 Welding Procedures

Procedures for the welding of all joints, includingtypes of electrodes, edge preparations, welding tech-niques and proposed positions, are to be establishedbefore construction begins. Details of proposedwelding procedures and sequences may be requiredto be submitted for review, depending on the in-tended application.

Special precautions, with regard to joint prepa-ration, preheat, welding sequence, heat input and in-terpass temperature, are to be taken for welding thicksections. Ultrasonic inspection to insure the absenceof injurious laminations may be required for materialused where through thickness (Z-direction) proper-ties are important.

2/2.5 Preparation for Welding

2/2.5.1 Edge Preparation and Fitting

Edge preparations are to be accurate and uniform andthe parts to be welded are to be fitted in accordancewith the approved joint detail. All means adopted forcorrecting improper fitting are to be to the satisfac-tion of the Surveyor. Where excessive root openingsare encountered for butt weld connections, weldbuild-up of the edges may be approved by the Sur-veyor, depending upon the location of the joint andthe welding procedures employed. Unless speciallyapproved, such build-up of each edge, where per-mitted, is not to exceed t/2 or 12.5 mm (Z\x in.),whichever is less, where t is the thickness of thethinner member being welded. Where sections to bebutt welded differ in thickness and have an offset onany side of more than 3 mm (Z\, in.), a suitabletransition taper is to be provided. In general, thetransition taper length is to be not less than threetimes the offset. The transition may be formed by ta-pering the thicker member or by specifying a weldjoint design which will provide the required transi-tion.

2/2.5.2 Alignment

Means are to be provided for maintaining the mem-bers to be welded in correct position and alignmentduring the welding operation. In general, strongbacksor other appliances used for this purpose are to be soarranged as to allow for expansion and contractionduring production welding. The removal of suchitems is to be carried out to the satisfaction of theSurveyor.



2/2.5.3 Cleanliness

All surfaces to be welded are to be free from mois-ture, grease, loose mill scale, excessive rust andpaint. Primer coatings of ordinary thicknesses, thincoatings of linseed oil or equivalent coatings may beused, provided it is demonstrated that their use hasno adverse effect on the production of satisfactorywelds. Slag and scale are to be removed not onlyfrom the edges to be welded but also from each passor layer before the deposition of subsequent passesor layers. Weld joints prepared by arc-air gougingmay require additional preparation by grinding orchipping and wire brushing prior to welding, tominimize the possibility of excessive carbon on thesurfaces. Compliance with these cleanliness re-quirements is of prime importance in the welding ofhigher-strength steels (see 2/1.1.3), especially thosewhich are quenched and tempered.

2/2.5.4 Tack Welds

Tack welds of consistent good quality, made with thesame grade of filler metal as intended for productionwelding and deposited in such a manner as not tointerfere with the completion of the final weld, neednot be removed, provided they are found upon ex-amination to be thoroughly clean and free fromcracks or other defects. Preheat may be necessaryprior to tack welding when the materials to be joinedare highly restrained. Special consideration is to begiven to using the same preheat as specified in thewelding procedure when tack welding higher-strength steels, particularly those materials which arequenched and tempered. These same precautions areto be followed when making any permanent weldedmarkings.

2/2.5.5 Run-on and Run-off Tabs

When used, run-on and run-off tabs are to be de-signed to minimize the possibility of high stress con-centrations and base-metal and weld-metal cracking.

2/2.7 Production Welding

2/2.7.1 Environment

Proper precautions are to be taken to insure that allwelding is done under conditions where the weldingsite is protected against the deleterious effects ofmoisture, wind, and severe cold.

2/2.7.2 Sequence

Welding is to be planned to progress symmetricallyso that shrinkage on both sides of the structure willbe equalized. The ends of frames and stiffenersshould be left unattached to the plating at the subas-sembly stage until connecting welds are made in the

intersecting systems of plating, framing and stiffen-ers at the erection stage. Welds are not to be carriedacross an unwelded joint or beyond an unweldedjoint which terminates at the joint being welded, un-less specially approved.

2/2.7.3 Preheat and Postweld Heat Treatment

The use of preheat is to be considered when weldinghigher-strength steels, materials of thick cross sec-tion, materials subject to high restraint, and whenwelding is performed under high humidity condi-tions or when the temperature of the steel is below0°C (32°F). The control of interpass temperature isto be specially considered when welding quenchedand tempered higher-strength steels. When preheat isused, the temperature is to be in accordance with theaccepted welding procedure. Postweld heat treat-ment, when specified, is to be carried out using anapproved method.

2/2.7.4 Low-hydrogen Electrodes or WeldingProcesses

Unless otherwise approved, the use of low-hydrogenelectrodes or welding processes is required forwelding all higher-strength steels, and may also beconsidered for ordinary-strength steel weldmentssubject to high restraint. When using low-hydrogenelectrodes or processes, proper precautions are to betaken to ensure that the electrodes, fluxes and gasesused for welding are clean and dry.

2/2.7.5 Back Gouging

Chipping, grinding, arc-air gouging or other suit-able methods are to be employed at the root or un-derside of the weld to obtain sound metal beforeapplying subsequent beads for all full-penetrationwelds. When arc-air gouging is employed, the se-lected technique is to minimize carbon buildup andburning of the weld or base metal. Quenched andtempered steels are not to be flame gouged usingoxy-fuel gas.

2/2.7.6 Peening

The use of peening is not recommended for single-pass welds and the root or cover passes on multi-pass welds. Peening, when used to correct distor-tion or to reduce residual stresses, is to be effectedimmediately after depositing and cleaning eachweld pass.

2/2.7.7 Fairing and Flame Shrinking

Fairing by heating or flame shrinking, and othermethods of correcting distortion or defective work-manship in fabrication of main strength membersand other members which may be subject to high



stresses, are to be carried out only with the expressedapproval of the Surveyor. These corrective measuresare to be kept to an absolute minimum when higher-strength quenched and tempered steels are involved,due to high local stresses and the possible degrada-tion of the mechanical properties of the base mate-rial.

2/2.7.8 Weld Soundness and SurfaceAppearance

Production welds are to be sound, crack-free andreasonably free from lack of fusion or penetration,slag inclusions and porosity. The surfaces of weldsare to be visually inspected and are to be regular anduniform with a minimum amount of reinforcementand reasonably free from undercut and overlap andfree from injurious arc strikes. Contour grindingwhen required by an approved plan or specificationor where deemed necessary by the Surveyor is to becarried out to the satisfaction of the Surveyor.

2/2.7.9 Inspection of Welds

Inspection of welded joints in important locations isto be carried out preferably by established nonde-structive test methods such as radiographic, ultra-sonic, magnetic-particle or dye-penetrant inspection.An approved acceptance criterion or the Bureau’sRules for Nondestructive Inspection of Hull Weldsare to be used in evaluating radiographs and ultra-sonic indications (see also 2/2.7.11). Radiographic orultrasonic inspection, or both, are to be used whenthe overall soundness of the weld cross section is tobe evaluated. Magnetic-particle or dye-penetrant in-spection may be used when investigating the outersurface of welds, as a check of intermediate weldpasses such as root passes, and to check backchipped, ground or gouged joints prior to depositingsubsequent passes. Surface inspection of importanttee or corner joints in critical locations, using an ap-proved magnetic-particle or dye-penetrant method, isto be conducted to the satisfaction of the Surveyor.Some steels, especially higher-strength steels, maybe susceptible to delayed cracking. When weldingthese materials, the final nondestructive testing is tobe delayed for a suitable period to permit detectionof such defects. Weld run-on or run-off tabs may beused where practicable and these may be sectionedfor examination. The practice of taking weld plugs orsamples by machining or cutting from the weldedstructure is not recommended and is to be used onlyin the absence of other suitable inspection methods.When such weld plugs or samples are removed fromthe welded structure, the holes or cavities thusformed are to be properly prepared and welded, us-ing a suitable welding procedure as established forthe original joint.

2/2.7.10 Extent of Inspection of Welds

a General The minimum extent of nonde-structive testing to be conducted is indicated in d or ebelow. The distribution of inspected welds is to bebased on the classification of application of thewelds, as mentioned in c, and the variety of weldsizes used in the structure. Nondestructive testing isgenerally to be carried out after all forming andpostweld heat treatment, and procedures should beadequate to detect delayed cracking. Welds whichare inaccessible or difficult to inspect in service areto be subjected to increased levels of nondestructiveinspection. Nondestructive examination of full pene-tration butt welds is generally to be carried out byradiographic or ultrasonic methods. Where a method(such as radiography or ultrasonics) is selected as theprimary nondestructive method of inspection, the ac-ceptance standards of such a method govern. Whereinspection by any method indicates the presence ofdefects that could jeopardize the integrity of thestructure, removal and repair of such defects are tobe carried out to the satisfaction of the attendingSurveyor. Should the ultrasonic method be used asthe primary inspection method, such testing shouldbe supplemented by a reasonable amount of radio-graphic inspection to determine that adequate qualitycontrol is being achieved. To assess the extent of sur-face imperfections in welds made in Group III steelsused in critical structural locations, representative in-spection by the magnetic-particle or dye-penetrantmethod should also be accomplished.

b Plans A plan for nondestructive testing ofthe structure is to be submitted. This plan should in-clude, but not be restricted to, visual inspection of allwelds, representative magnetic-particle or dye-penetrant inspection of tee and fillet welds not sub-jected to ultrasonic inspection, and the inspection ofall field welds by appropriate means. The extent andmethod of inspection are to be indicated on the plan,and the extent of inspection is to be based on thefunction of the structure and the accessibility of thewelds after the structure is in service. For automatedwelds for which quality assurance techniques indi-cate consistent satisfactory performance a lesser de-gree of inspection may be permitted.

c Classification of Application Welds are tobe designated as being either special, primary or sec-ondary depending on the function and severity ofservice of the structure in which the welds are lo-cated. Special welds are those occurring in structurallocations of critical importance to the integrity of thestructure or its safe operation. Secondary welds arethose occurring in locations of least importance tothe overall integrity of the structure. Primary weldsare those occurring in locations whose importance isintermediate between the special and secondary clas-sifications. Reference can be made to Table A.1



(Appendix A) for examples of applications followingthis classification system.

d Extent of Nondestructive Inspection—SteelJacket Type Structures In general, the number ofpenetration type welds (i. e., butt, T, K and Yjoints) to be inspected in each classification is to bebased on the percentages stated below. Alterna-tively, the extent of radiographic and ultrasonic in-spection may be based on other methods, providedthe alternative will not result in a lesser degree ofinspection. Where the extent of welds to be in-spected is stated as a percentage, such as 20% ofprimary welds, this means that complete inspectionof 20% of the total number of welds considered tobe primary is required.

All welds considered special are to be inspected100% by the ultrasonic or radiographic method.Twenty percent of all welds considered primary areto be inspected by the ultrasonic or radiographicmethod. Welds considered to be secondary are to beinspected on a random basis using an appropriatemethod. In locations where ultrasonic test results arenot considered reliable, the use of magnetic-particleor dye-penetrant inspection as a supplement to ultra-sonic inspection is to be conducted. For T, K, or Yjoints, approval may be given to substituting mag-netic-particle or dye-penetrant inspection for ultra-sonic inspection when this will achieve a sufficientinspection quality.

Magnetic-particle or dye-penetrant inspection offillet welds is to be accomplished for all permanentfillet welds used in jacket construction, all jacket-to-pile shim connections, and all fillet welds in specialapplication areas of the deck structure. The randominspection of other deck fillet welds is to be carriedout at the discretion of the Surveyor.

e Extent of Nondestructive Inspection SteelPlate or Shell Type Structures The minimum extentof the ultrasonic or radiographic inspection of plateor shell type structures is to be equivalent to, or ingeneral agreement with, the number of check pointsobtained by the following equations. As practicable,the length of each check point is to be at least750 mm (30 in.).

For structures which are rectangular in shape

Metric Units English Unitsn = L (B + D)/46.5 n = L (B + D)/500

n = number of check points (2 is minimum number)L = length of greatest dimension of structure, in

m (ft)B = greatest breadth, in m (ft)D = greatest depth at the center, in m (ft)

For structures which are other than rectangular aproportional number of check points should be sup-plied.

For structures which are circular in shape

Metric Units English Unitsn = Ld/46.5 n = Ld/500

n = number of check points (2 is minimum number)L = length of the circular structure, in m (ft)d = diameter of the circular structure, in m (ft)

f Additional Inspection—Special ConditionsAdditional inspection may be required depending onthe type and use of the structure, the material andwelding procedures involved, and the quality controlprocedures employed.

g Additional Inspection—Production Experi-ence If the proportion of unacceptable welds be-comes abnormally high, the frequency of inspectionis to be increased.

h High Through Thickness (Z-Direction)Stresses At important intersections welds which im-pose high stresses perpendicular to the memberthicknesses (Z-direction loading) are to be ultrasoni-cally inspected to assure freedom from lamellartearing after welding.

2/2.7.11 Acceptance Criteria

As stated in 2/2.7.9, recognized acceptance criteriasuch as those issued by the AWS are to be employed.

When employing the Bureau’s Rules for Nonde-structive Inspection of Hull Welds, Class A and ClassB criteria are to be applied as follows.

Class A acceptance criteria are to be used for specialapplication structure and critical locations withinprimary application structure such as circumfer-ential welds of cylindrical and built up columnsor legs, weld intersections of external plating inplatforms, etc.

Class B acceptance criteria are to be used for pri-mary application structure where Class A ac-ceptance does not apply

Twice Class B acceptance criteria are to be used forsecondary application structure

When radiographic or ultrasonic inspection is speci-fied for other types of connections, such as partialpenetration and groove type tee or corner welds,modified procedures and acceptance criteria are to bespecified which adequately reflect the application.

2/2.12 Repair Welding

Defective welds and other injurious defects, as de-termined by visual inspection, nondestructive testmethods, or leakage under hydro static tests, are tobe excavated in way of the defects to sound metaland corrected by rewelding, using a suitable repairwelding procedure to be consistent with the materialbeing welded. Removal by grinding of minor surfaceimperfections such as scars, tack welds and arc



strikes may be permitted. Special precautions, suchas the use of both preheat and low-hydrogen elec-trodes, are to be considered when repairing welds inhigher-strength steel, materials of thick cross sectionor materials subject to high restraint.

2/2.9 Butt Welds

2/2.9.1 Manual Welding Using Stick Electrodes

Manual welding using stick electrodes may be em-ployed for butt welds in members not exceeding6.5 mm (Z\v in.) in thickness without beveling theabutting edges. Members exceeding 6.5 mm (Z\vin.) are to be prepared for welding using an appropri-ate edge preparation, root opening and root face(land) to provide for welding from one or both sides.For welds made from both sides, the root of the firstside welded is to be removed to sound metal by anapproved method before applying subsequent weldpasses on the reverse side. When welding is to bedeposited from one side only, using ordinary weld-ing techniques, appropriate backing (either perma-nent or temporary) is to be provided. The backing isto be fitted so that spacing between the backing andthe members to be joined is in accordance with es-tablished procedures. Unless specially approved oth-erwise, splices in permanent backing strips are to bewelded with full penetration welds prior to makingthe primary weld.

2/2.9.2 Submerged-arc Welding

Submerged-arc welding, using wire-flux combina-tions for butt welds in members not exceeding16 mm (B\, in.) in thickness, may be employedwithout beveling the abutting edges. Members ex-ceeding 16 mm (B\, in.) are normally to be preparedfor welding using an appropriate edge preparation,root opening and root face (land) to provide forwelding from one or both sides. When it is deter-mined that sound welds can be made without goug-ing, the provisions of 2/2.7.5 are not applicable.Where the metal is to be deposited from one sideonly, using ordinary welding techniques, backing(either permanent or temporary) is to be providedand the members are to be beveled and fitted in ac-cordance with established procedures.

2/2.9.3 Gas Metal-arc and Flux Cored-arcWelding

Manual semi-automatic or machine automatic gasmetal-arc welding, and flux cored-arc welding usingwire-gas combinations and associated processes,may be ordinarily employed utilizing the conditionsspecified in 2/2.9.1 except that specific joint designsmay differ between processes.

2/2.9.4 Electroslag and Electrogas Welding

The use of electroslag and electrogas welding proc-esses will be subject to special consideration, de-pending upon the specific application and the me-chanical properties of the resulting welds and heat-affected zones.

2/2.9.5 Special Welding Techniques

Special welding techniques employing any of the ba-sic welding processes mentioned in 2/2.9.1 through2/2.9.4 will be specially considered, depending uponthe extent of the variation from the generally ac-cepted technique. Such special techniques includeoneside welding, narrow-gap welding, tandem-arcwelding, open-arc welding and consumable-nozzleelectroslag welding. The use of gas tungsten-arcwelding will also be subject to special consideration,depending upon the application and whether the pro-cess is used manually or automatically.

2/2.11 Fillet Welds

2/2.11.1 General

The sizes of fillet welds are to be indicated on detailplans or on a separate welding schedule and aresubject to approval. The weld throat size is not to beless than 0.7 times the weld leg size. Fillet weldsmay be made by an approved manual or automatictechnique. Where the gap between the faying sur-faces of members exceeds 2 mm (Z\zn in.) and isnot greater than 5 mm (C\zn in.), the weld leg sizeis to be increased by the amount of the opening.Where the gap between members is greater than 5mm (C\zn in.), fillet weld sizes and weld proce-dures are to be specially approved by the Surveyor.Completed welds are to be to his satisfaction. Specialprecautions such as the use of preheat or low-hydrogen electrodes or low hydrogen welding proc-esses may be required where small fillets are used toattach heavy members or sections. When heavy sec-tions are attached to relatively light members, theweld size may be required to be modified.

2/2.11.2 Tee Connections

Except where otherwise indicated under 2/2.11.1, thefillet weld requirement for tee connections is to bedetermined by the lesser thickness member beingjoined. Where only the webs of girders, beams orstiffeners are to be attached, it is recommended thatthe unattached face plates or flanges be cut back. Ex-cept for girders of thickness greater than 25 mm(1 in.), reduction in fillet weld sizes may be speciallyapproved in accordance with either a or b specifiedbelow. However, in no case is the reduced leg size tobe less than 5 mm (C\zn in.).



a Where quality control facilitates working to agap between members being attached of 1 mm(0.04 in.) or less, a reduction in fillet weld legsize of 0.5 mm (0.02 in.) may be permittedprovided that the reduced leg size is not lessthan 8 mm (C\zn in.).

b Where automatic double continuous filletwelding is used and quality control facilitatesworking to a gap between members being at-tached of 1 mm (0.04 in.) or less, a reductionin fillet weld leg size of 1.5 mm (Z\zn in.)may be permitted provided that the penetrationat the root is at least 1.5 mm (Z\zn in.) intothe members being attached and the reducedleg size is not less than 5 mm (C\zn in.).

2/2.11.3 Lapped Joints

Lapped joints are generally not to have overlaps ofless width than twice the thinner plate thickness plus25 mm (1 in.). Both edges of an overlapped joint areto have continuous fillet welds in accordance with2/2.11. or 2/2.11.4.

2/2.11.4 Overlapped End Connections

Overlapped end connections of structural memberswhich are considered to be effective in the overallstrength of the unit are to have continuous filletwelds on both edges equal in leg size to the thicknessof the thinner of the two members joined. All otheroverlapped end connections are to have continuousfillet welds on each edge of leg sizes such that the

sum of the two is not less than 1.5 times the thick-ness of the thinner member.

2/2.11.5 Overlapped Seams

Unless specially approved, overlapped seams are tohave continuous welds on both edges of the sizes re-quired by the approved plans and are to be in accor-dance with the applicable provisions of 2/2.11.1.

2/2.11.6 Plug Welds or Slot Welds

Plug welds or slot welds may be specially approvedfor particular applications. Where used in the bodyof doublers and similar locations, such welds may begenerally spaced about 300 mm (12 in.) betweencenters in both directions. Slot welds generallyshould not be filled with weld metal. For plate thick-nesses up to 13 mm (Z\x in.), fillet sizes are to beequal to plate thickness but not greater than 9.5 mm(C\, in.); for thicknesses over 13 mm (Z\x in.) to25 mm (1 in.) fillet sizes are to be 16 mm (B\, in.)maximum.

2/2.13 Full Penetration Corner or Tee Joints

Measures taken to achieve full penetration corner ortee joints, where specified, are to be to the satisfac-tion of the attending Surveyor. Ultrasonic inspectionof the member in way of the connection may be re-quired to assure the absence of injurious laminationsprior to fabrication which could interfere with theattainment of a satisfactory welded joint.

Environmental Conditions Section 1


Part 3 Design



.



Part 3Section 1 Environmental Conditions

3/1.1 General

The environmental conditions to which an offshoreinstallation may be exposed during its life are to bedescribed using adequate data for the areas in whichthe structure is to be transported and installed. Forstructures requiring substantial near-shore construc-tion (e.g., concrete gravity installations), environ-mental studies are to be commensurate with the du-ration of construction operations and the relativeseverity of expected conditions.

The environmental phenomena which may in-fluence the transport, installation, and operation ofthe structure are to be described in terms of the char-acteristic parameters relevant to the evaluation of thestructure. Statistical data and realistic statistical andmathematical models which describe the range ofpertinent expected variations of environmental con-ditions are to be employed. All data used are to befully documented with the sources and estimated re-liability of data noted.

Methods employed in developing available datainto design criteria are to be described and submitted inaccordance with Section 1/4. Probabilistic methods forshort-term, long-term and extreme-value prediction areto employ statistical distributions appropriate to the en-vironmental phenomena being considered, as evidencedby relevant statistical tests, confidence limits and othermeasures of statistical significance. Hindcasting meth-ods and models are to be fully documented.

Generally, suitable data and analyses suppliedby consultants will be accepted as the basis for de-sign. For installations in areas where published de-sign standards and data exist, such standards anddata may be cited as documentation.

3/1.3 Environmental Factors to beConsidered

In general, the design of an offshore installation willrequire investigation of the following environmentalfactors.

WavesWindCurrentsTides and storm surgesAir and sea temperaturesIce and snowMarine growthSeismicitySea ice

Other phenomena, such as tsunamis, submarineslides, seiche, abnormal composition of air and wa-ter, air humidity, salinity, ice drift, icebergs, icescouring, etc. may require investigation dependingupon the specific installation site.

The required investigation of seabed and soilconditions is described in Section 3/6.

3/1.5 Environmental Design Criteria

The combination and severity of environmental con-ditions for use in design are to be appropriate to theinstallation being considered and consistent with theprobability of simultaneous occurrence of the envi-ronmental phenomena. It is to be assumed that envi-ronmental phenomena may approach the installationfrom any direction unless reliable site-specific dataindicate otherwise. The direction, or combination ofdirections, which produces the most unfavorable ef-fects on the installation is to be accounted for in thedesign.

3/1.5.1 Design Environmental Condition

In these Rules, the combination of environmentalfactors producing the most unfavorable effects on thestructure, as a whole and as defined by the parame-ters given below, is referred to as the Design Envi-ronmental Condition. This condition is to be de-scribed by a set of parameters representing anenvironmental condition which has a high probabil-ity of not being exceeded during the life of thestructure and will normally be composed of

a The maximum wave height corresponding tothe selected recurrence period together withthe associated wind, current and limits of waterdepth, and appropriate ice and snow effects

b The extreme air and sea temperaturesc The maximum and minimum water level due

to tide and storm surgeHowever, depending upon site-specific condi-

tions, consideration should be given to the combina-tions of events contained in item a above. The recur-rence period chosen for events a, b, and c above isnormally not to be less than one hundred years, un-less justification for a reduction can be provided. Forplatforms that are unmanned, or can be easily evacu-ated during the design event, or platforms withshorter design life than typical 20 years may use arecurrence interval less than 100 years for events a, band c above. However, the recurrence interval is notto be less than 50 years.



For installation sites located in seismically ac-tive areas (see 3/1.7.8), an earthquake of magnitudewhich has a reasonable likelihood of not being ex-ceeded during the platform life to determine the riskof damage, and a rare intense earthquake to evaluatethe risk of structural collapse are to be considered inthe design. The earthquakes so described are hereinreferred to as the Strength Level and Ductility LevelEarthquakes respectively. The magnitudes of the pa-rameters characterizing these earthquakes having re-currence periods appropriate to the design life of thestructure are to be determined. The effects of theearthquakes are to be accounted for in design but,generally, need not be taken in combination withother environmental factors.

For installations located in areas susceptible totsunami waves, submarine slides, seiche or otherphenomena, the effects of such phenomena are tobe based on the most reliable estimates availableand, as practicable, the expected effects are to beaccounted for in design. Generally, for such phe-nomena, suitable data and recommendations sub-mitted by consultants will be accepted as a basis fordesign.

3/1.5.2 Operating Environmental Conditions

For each intended major function or operation of theinstallation, a set of characteristic parameters for theenvironmental factors which act as a limit on the safeperformance of an operation or function is to be de-termined. Such operations may include, as appropri-ate, transportation, offloading and installation of thestructure, drilling or producing operations, evacua-tion of the platform, etc. These sets of conditions areherein referred to as Operating Environmental Con-ditions.

3/1.7 Specific EnvironmentalConditions

3/1.7.1 Waves

a General Statistical wave data from whichdesign parameters are determined are normally to in-clude the frequency of occurrence of various waveheight groups, associated wave periods and direc-tions. Published data and previously established de-sign criteria for particular areas may be used wheresuch exist. Hindcasting techniques which adequatelyaccount for shoaling and fetch limited effects onwave conditions at the site may be used to augmentavailable data. Analytical wave spectra employed toaugment available data are to reflect the shape andwidth of the data, and they are to be appropriate tothe general site conditions.

b Long-Term Predictions All long-term andextreme-value predictions employed for the determi-nation of design wave conditions are to be fully de-scribed and based on recognized techniques. Designwave conditions may be formulated for use in eitherdeterministic or probabilistic methods of analysis,but the method of analysis is to be appropriate to thespecific topic being considered.

c Data The development of wave data to beused in required analyses is to reflect conditions atthe installation site and the type of structure. As re-quired, wave data may have to be developed to de-termine the following.

Provision for air gapMaximum mud line shear force and overturning

momentDynamic response of the structureMaximum stressFatigueImpact of local structure

Breaking wave criteria are to be appropriate tothe installation site and based on recognized tech-niques. Waves which cause the most unfavorable ef-fects on the overall structure may differ from waveshaving the most severe effects on individual struc-tural components. In general, more frequent wavesof lesser heights, in addition to the most severe waveconditions, are to be investigated when fatigue anddynamic analyses are required.

3/1.7.2 Wind

a General Statistical wind data is normally toinclude information on the frequency of occurrence,duration and direction of various wind speeds. Pub-lished data and data from nearby land and sea sta-tions may be used if available. If on-site measure-ments are taken, the duration of individualmeasurements and the height above sea-level ofmeasuring devices is to be stated. Sustained windsare to be considered those having durations equal toor greater than one minute, while gust winds arewinds of less than one minute duration.

b Long-Term and Extreme-Value PredictionsLong-term and extreme-value predictions for sus-tained and gust winds are to be based on recognizedtechniques and clearly described. Preferably, the sta-tistical data used for the long-term distributions ofwind speed should be based on the same averagingperiods of wind speeds as are used for the determi-nation of loads. Vertical profiles of horizontal windare to be determined on the basis of recognized sta-tistical or mathematical models.



TABLE 3/1.1 Wind Speed for Time-Averaging Period tRelative to the 1-Hour Wind Speed at 10 m (33 ft) above SWL (see 3/1.7.4)t 1 hr. 10 min. 1 min. 15 sec. 5 sec. 3 sec.Factor 1.00 1.04 1.16 1.26 1.32 1.35

c Vertical Profiles of Horizontal Wind Verti-cal profiles of horizontal wind for use in design canbe determined using the following equation.

Vy = VH (y/h)l/n

Vy = wind speed at height y above a reference wa-ter depth, in m/s (ft/s)

VH = wind speed at reference height H, usually 10m (33 ft) above a reference water depth, inm/s (ft/s)

1/n = exponent dependent upon the time-averagingperiod of the measured wind speed VH

The value of n typically ranges from 7 for sus-tained winds to 13 for gust winds of brief duration.For sustained winds of 1-minute duration, n equal to7 may be used; for gust winds of 3-second duration,n equal to 12 may be used.

d Lack of Data In the event that wind speeddata is not available for the time-averaging periodsdesired for use in design, conversions to the desiredtime-averaging periods may be made on the basis ofTable 3/1.1.

Linear interpolation may be used with Table3/1.1 to determine the factor to be applied to thetime-averaging period wind speed relative to the 1-hour wind speed.

For wind speeds given in terms of the “fastestmile of wind”, Vf, the corresponding time-averagingperiod t in seconds is given by

t = 3600/Vf

where Vf is the fastest mile of wind at a referenceheight of 10 m (33 ft), in miles per hour.

3/1.7.3 Currents

a General Data for currents are generally toinclude information on current speed, directions andvariation with depth. The extent of informationneeded is to be commensurate with the expected se-verity of current conditions at the site in relation toother load causing phenomena, past experience inadjacent or analogous areas and the type of structureand foundation to be installed. On-site data collec-tion may be indicated for previously unstudied areasand/or areas expected to have unusual or severe con-ditions. Consideration is to be given to the followingtypes of current, as appropriate to the installationsite: tidal, wind-generated, density, circulation andriver-outflow.

b Velocity Profiles Current velocity profilesare to be based on site-specific data or recognizedempirical relationships. Unusual profiles due to bot-tom currents and stratified effects due to river out-flow currents are to be accounted for.

3/1.7.4 Tides

a General Tides, when relevant, are to be con-sidered in the design of an offshore fixed structure.Tides may be classified as lunar or astronomicaltides, wind tides, and pressure differential tides. Thecombination of the latter two is commonly called thestorm surge. The water depth at any location consistsof the mean depth, defined as the vertical distancebetween the sea bed and an appropriate near-surfacedatum, and a fluctuating component due to astro-nomical tides and storm surges. Astronomical tidevariations are bounded by highest astronomical tide,HAT, and lowest astronomical tide, LAT, still waterlevel (SWL) should be taken as the sum of the high-est astronomical level plus the storm surge.

Storm surge is to be estimated from availablestatistics or by mathematical storm surge modeling.

b Design Environmental Wave Crest For de-sign purposes, the design environmental wave crestelevation is to be superimposed on the SWL. Varia-tions in the elevation of the daily tide may be used indetermining the elevations of boat landings, bargefenders and the corrosion prevention treatment ofstructure in the splash zone. Water depths assumedfor various topics of analysis are to be clearly stated.

3/1.7.5 Temperature

Extreme values of air, sea and seabed temperatures areto be expressed in terms of recurrence Periods and as-sociated highest and lowest values. Temperature datais to be used to evaluate selection of structural materi-als, ambient ranges and conditions for machinery andequipment design, and for determination of thermalstresses, as relevant to the installation.

3/1.7.6 Ice and Snow

For structures intended to be installed in areas whereice and snow may accumulate or where sea ice haz-ards may develop, estimates are to be made of theextent to which ice and snow may accumulate on thestructure. Data may be derived from actual fieldmeasurements, laboratory data or data from analo-gous areas.



3/1.7.7 Marine Growth

Marine growth is to be considered in the design of anoffshore installation. Estimates of the rate and extentof marine growth may be based on past experienceand available field data. Particular attention is to bepaid to increases in hydrodynamic loading due to in-creased diameters and surface roughness of memberscaused by marine fouling as well as to the addedweight and increased inertial mass of submergedstructural members. Consideration should be givento the types of fouling likely to occur and their pos-sible effects on corrosion protection coatings.

3/1.7.8 Seismicity and Earthquake RelatedPhenomena

a Effects on Structures The effects of earth-quakes on structures located in areas known to be seis-mically active are to be taken into account. The antici-pated seismicity of an area is, to the extent practicable,to be established on the basis of regional and site spe-cific data including, as appropriate, the following.

Magnitudes and recurrence intervals of seismicevents

Proximity to active faultsType of faultingAttenuation of ground motion between the faults and

the siteSubsurface soil conditionsRecords from past seismic events at the site where

available, or from analogous sites

b Ground Motion The seismic data are to beused to establish a quantitative strength level andductility level earthquake criteria describing the

earthquake induced ground motion expected duringthe life of the structure. In addition to ground mo-tion, and as applicable to the site in question, thefollowing earthquake related phenomena should betaken into account.

Liquefaction of subsurface soilsSubmarine slidesTsunamisAcoustic overpressure shock waves

3/1.7.9 Sea Ice

The effects of sea ice on structures must consider thefrozen-in condition (winter), break-out in the spring,and summer pack ice invasion as applicable. Impact,both centric and eccentric, must be considered wheremoving ice may impact a structure.

Impact should consider both that of large masses(multi-year floes and icebergs) moving under the ac-tion of current, wind, and Coriolis effect, and that ofsmaller ice masses which are accelerated by stormwaves.

The interaction between ice and the structureproduces responses both in the ice and the structure-soil system, and this compliance should be taken intoaccount as applicable.

The mode of ice failure (tension, compression,shear, etc.) depends on the shape and roughness ofthe surface and the presence of adfrozen ice, as wellas the ice character, crystallization, temperature, sa-linity, strain rate and contact area. The force exertedby the broken or crushed ice in moving past thestructure must be considered. Limiting force con-cepts may be employed if thoroughly justified bycalculations.

Loads Section 2


Part 3Section 2 Loads

3/2.1 General

This section pertains to the identification, definitionand determination of the loads to which an offshorestructure may be subjected during and after its trans-portation to site and its installation. As appropriate tothe planned structure, the types of loads described in3/2.3 are to be accounted for in design.

3/2.3 Types of Loads

Loads applied to an offshore structure are, for pur-poses of these Rules, categorized as follows.

3/2.3.1 Dead Loads

Dead loads are loads which do not change during themode of operation under consideration. Dead loadsinclude the following.

Weight in air of the structure including, as appropri-ate, the weight of the principal structure (e.g.,jacket, tower, caissons, gravity foundation, pil-ing), grout, module support frame, decks, mod-ules, stiffeners, piping, helideck, skirt, columnsand any other fixed structural parts

Weight of permanent ballast and the weight of per-manent machinery

External hydrostatic pressure and buoyancy calcu-lated on the basis of the still water level

Static earth pressure

3/2.3.2 Live Loads

Live loads associated with the normal operation anduse of the structure are loads which may changeduring the mode of operation considered. (Thoughenvironmental loads are live loads, they are catego-rized separately; see 3/2.3.4) Live loads acting afterconstruction and installation include the following.

a The weight of drilling or production equipmentwhich can be removed, such as derrick, drawworks, mud pumps, mud tanks, rotating equip-ment, etc.

b The weight of crew and consumable supplies,such as mud, chemicals, water, fuel, pipe, cable,stores, drill stem, casing, etc.

c Liquid in the vessels and pipes during operationd Liquid in the vessels and pipes during testinge The weight of liquids in storage and ballast tanksf The forces exerted on the structure due to opera-

tions, e.g., maximum derrick reaction

g The forces exerted on the structure during the op-eration of cranes and vehicles

h The forces exerted on the structure by vesselsmoored to the structure or accidental impact con-sideration for a typical supply vessel that wouldnormally service the installation

i The forces exerted on the structure by helicoptersduring take-off and landing, or while parked onthe structure

Where applicable, the dynamic effects on the struc-ture of items d through g are to be taken into ac-count. Where appropriate, some of the items of liveload listed above may be adequately accounted forby designing decks, etc. to a maximum, uniform areaload as specified by the Operator, or past practice forsimilar conditions.

Live loads occurring during transportation andinstallation are to be determined for each specificoperation involved and the dynamic effects of suchloads are to be accounted for as necessary (see Sec-tion 3/7).

3/2.3.3 Deformation Loads

Deformation loads are loads due to deformations im-posed on the structure. The deformation loads in-clude those due to temperature variations (e.g., hotoil storage) leading to thermal stress in the structureand, where necessary, loads due to soil displace-ments (e.g., differential settlement or lateral dis-placement) or due to deformations of adjacentstructures. For concrete structures, deformation loadsdue to prestress, creep, shrinkage and expansion areto be taken into account.

3/2.3.4 Environmental Loads

Environmental loads are loads due to wind, waves,current, ice, snow, earthquake, and other environ-mental phenomena. The characteristic parameters de-fining an environmental load are to be appropriate tothe installation site and in accordance with the re-quirements of Section 3/1. Operating EnvironmentalLoads are those loads derived from the parameterscharacterizing Operating Environmental Conditions(see 3/1.5.2). Design Environmental Loads are thoseloads derived from the parameters characterizing theDesign Environmental Condition (see 3/1.5.1).

The combination and severity of Design Envi-ronmental Loads are to be in accordance with3/1.5.1.

Loads Section 2


Environmental loads are to be applied to thestructure from directions producing the most unfa-vorable effects on the structure, unless site-specificstudies provide evidence in support of a less strin-gent requirement. Directionality may be taken intoaccount in applying the environmental criteria.

Earthquake loads and loads due to accidents orrare occurrence environmental phenomena need notbe combined with other environmental loads, unlesssite-specific conditions indicate that such combina-tions are appropriate.

3/2.5 Determination of EnvironmentalLoads

3/2.5.1 General

Model or field test data may be employed to estab-lish environmental loads. Alternatively, environ-mental loads may be determined using analyticalmethods compatible with the data established incompliance with Section 3/1. Any recognized loadcalculation method may be employed provided it hasproven sufficiently accurate in practice, and it isshown to be appropriate to the structure’s character-istics and site conditions. The calculation methodspresented herein are offered as guidance representa-tive of current acceptable methods.

3/2.5.2 Wave Loads

a Range of Wave Parameters A sufficientrange of realistic wave periods and wave crest posi-tions relative to the structure are to be investigated toensure an accurate determination of the maximumwave loads on the structure. Consideration should begiven to other wave induced effects such as waveimpact loads, dynamic amplification and fatigue ofstructural members. The need for analysis of theseeffects is to be assessed on the basis of the configu-ration and behavioral characteristics of the structure,the wave climate and past experience.

b Determination of Wave Loads For struc-tures composed of members having diameters whichare less than 20% of the wave lengths being consid-ered, semi-empirical formulations such as Morison’sequation are considered to be an acceptable basis fordetermining wave loads. For structures composed ofmembers whose diameters are greater than 20% ofthe wave lengths being considered, or for structuralconfigurations which substantially alter the incidentflow field, diffraction forces and the hydrodynamicinteraction of structural members are to be accountedfor in design.

c Morison’s Equation The hydrodynamicforce acting on a cylindrical member, as given byMorison’s equation, is expressed as the sum of theforce vectors indicated in the following equation.

F = FD + Fl

F = hydrodynamic force vector per unit lengthalong the member, acting normal to the axisof the member

FD = drag force vector per unit lengthFl = inertia force vector per unit length

The drag force vector for a stationary, rigid memberis given by

FD = (c/2g) D CD un |un|

c = weight density of water, in N/m3 (lb/ft3)g = gravitational acceleration, in m/s2 (ft/s2)D = projected width of the member in the direc-

tion of the crossflow component of velocity(in the case of a circular cylinder, D denotesthe diameter), in m (ft)

CD = drag coefficient (dimensionless)un = component of the fluid velocity vector nor-

mal to the axis of the member, in m/s (ft/s)un = absolute value of un, in m/s (ft/s)

The inertia force vector for a stationary, rigid mem-ber is given by

Fl = (c/g) (p D2/4) CM an

CM = Inertia coefficient based on the displacedmass of fluid per unit length (dimensionless)

an = component of the fluid acceleration vectornormal to the axis of the member, in m/s2

(ft/s2)

For compliant structures which exhibit substantialrigid body oscillations due to the wave action, themodified form of Morison’s equation given belowmay be used to determine the hydrodynamic force.

F = FD + Fl (c/2g) DCD (un-ún) |un-ún|

+ (c/g) (p D2/4) an + (c/g) (p D2/4) Cm (an-án)

ún = component of the velocity vector of thestructural member normal to its axis, in m/s(ft/s)

Cm = added mass coefficient, i.e., Cm = CM − 1án = component of the acceleration vector of the

structural member normal to its axis, in m/s2

(ft/s2)

For structural shapes other than circular cylinders,the term p D2/4 in the above equations is to be re-placed by the actual cross-sectional area of theshape.

Values of un and an for use in Morison’s equa-tion are to be determined using a recognized wavetheory appropriate to the wave heights, wave peri-ods, and water depth at the installation site. Valuesfor the coefficients of drag and inertia to be used inMorison’s equation are to be determined on the basisof model tests, full scale measurements, or previousstudies which are appropriate to the structural con-

Loads Section 2


figuration, surface roughness, and pertinent flow pa-rameters (e.g., Reynolds number).

Generally, for pile-supported template typestructures, values of CD range between 0.6 and 1.2;values of CM range between 1.5 and 2.0.

d Diffraction Theory For structural configura-tions which substantially alter the incident wavefield, diffraction theories of wave loading are to beemployed which account for both the incident waveforce (i.e., Froude-Kylov force) and the force result-ing from the diffraction of the incident wave due tothe presence of the structure.

The hydrodynamic interaction of structuralmembers is to be taken into account. For structurescomposed of surface piercing caissons or for instal-lation sites where the ratio of water depth to wavelength is less than 0.25, nonlinear effects of waveaction are to be taken into account. This may be doneby modifying linear diffraction theory to account fornonlinear e ffects or by performance of model tests.

3/2.5.3 Wind Loads

Wind loads and local wind pressures are to be de-termined on the basis of analytical methods or windtunnel tests on a representative model of the struc-ture. In general, the wind load on the overall struc-ture to be combined with other design environmentalloads is to be determined using a one-minute sus-tained wind speed. For installations with negligibledynamic response to wind, a one-hour sustainedwind speed may be used to calculate the wind loadson the overall structure. Wind loads on broad, essen-tially flat structures such as living quarters, walls,enclosures, etc. are to be determined using a fifteen-second gust wind speed. Wind pressures on individ-ual structural members, equipment on open decks,etc. are to be determined using a three second gustwind speed.

For wind loads normal to flat surfaces or normalto the axis of members not having flat surfaces, thefollowing relation may be used.

Fw = (ca/2g) Cs Vy2 A

Fw = wind load, in N (lb)g = gravitational acceleration, in m/s2 (ft/s2)ca = weight density of air, in N/m3 (lb/ft3)Cs = shape coefficient (dimensionless)Vy = wind speed at altitude y, in m/s (ft/s)A = projected area of member on a plane normal

to the direction of the considered force, in m2

(ft2)

For any direction of wind approach to thestructure, the wind force on flat surfaces should beconsidered to act normal to the surface. The windforce on cylindrical objects should be assumed to actin the direction of the wind.

In the absence of experimental data, values for theshape coefficient (Cs) may be assumed as follows.

TABLE 3/2.1 Values of Cs

Shape Cs

Cylindrical shape 0.50Major flat surfaces and overall projected

area of platform 1.00Isolated structural shapes (cranes, angles,

beams, channels, etc.) 1.50Under-deck areas (exposed beams and

girders) 1.30Derricks or truss cranes (each face) 1.25Sides of buildings 1.50

The area of open trussworks commonly used for der-ricks and crane booms may be approximated by tak-ing 30% of the projected area of both the windwardand leeward sides with the shape coefficient taken inaccordance with Table 3/2.1.

Where one structural member shields anotherfrom direct exposure to the wind, shielding may betaken into account. Generally, the two structuralcomponents are to be separated by not more thanseven times the width of the windward componentfor a reduction to be taken in the wind load on theleeward member.

Where appropriate, dynamic effects due to thecyclic nature of gust wind and cyclic loads due tovortex induced vibration are to be investigated. Bothdrag and lift components of load due to vortex in-duced vibration are to be taken into account. The ef-fects of wind loading on structural members or com-ponents that would not normally be exposed to windloads after platform installation are to be considered.This would especially apply to fabrication or trans-portation phases.

3/2.5.4 Current Loads

Current induced loads on immersed structural mem-bers are to be determined on the basis of analyticalmethods, model test data or full-scale measurements.When currents and waves are superimposed, the cur-rent velocity is to be added vectorially to the waveinduced particle velocity prior to computation of thetotal force. Current profiles used in design are to berepresentative of the expected conditions at the in-stallation site. Where appropriate, flutter and dy-namic amplification due to vortex shedding are to betaken into account.

For calculation of current loads in the absence ofwaves, the lift force normal to flow direction, and thedrag force may be determined as follows.

FL = CL (c/2g) V2 Al

FD = CD (c/2g) V2 Al

Loads Section 2


FL = total lift force per unit length, in N/m (lb/ft)CL = lift coefficient (dimensionless)

c = weight density of water, in N/m3 (lb/ft3)V = local current velocity, in m/s (ft/s) (see

3/1.7.3)Al = projected area per unit length in a plane nor-

mal to the direction of the force, in m2/m(ft2/ft)

FD = total drag force per unit length, in N/m (lb/ft)CD = drag coefficient (see 3/2.5.2c)

In general, lift force may become significant for longcylindrical members with large length-diameter ra-tios and should be checked in design under theseconditions. The source of CL values employed is tobe documented.

3/2.5.5 Ice and Snow Loads

At locations where structures are subject to ice andsnow accumulation the following effects are to beaccounted for, as appropriate to the local condi-tions.

Weight and change in effective area of structuralmembers due to accumulated ice and snow

Incident pressures due to pack ice, pressure ridgesand ice island fragments impinging on thestructure

For the design of structures that are to be installed inservice in extreme cold weather such as arctic re-gions, reference is to be made to API Bulletin 2N:“Planning, Designing, and Constructing Fixed Off-shore Platforms in Ice Environments.”

3/2.5.6 Earthquake Loads

For structures located in seismically active areasstrength level and ductility level earthquake inducedground motions (see 3/1.5.1) are to be determined onthe basis of seismic data applicable to the installationsite. Earthquake ground motions are to be describedby either applicable ground motion records or re-sponse spectra consistent with the recurrence periodappropriate to the design life of the structure. Avail-able standardized spectra applicable to the region ofthe installation site are acceptable provided suchspectra reflect site-specific conditions affecting fre-quency content, energy distribution, and duration.These conditions include: the type of active faults inthe region, the proximity of the site to the potentialsource faults, the attenuation or amplification ofground motion between the faults and the site, andthe soil conditions at the site.

The ground motion description used in design isto consist of three components corresponding to twoorthogonal horizontal directions and the vertical di-rection. All three components are to be applied to thestructure simultaneously.

When a standardized response spectrum, such asgiven in the American Petroleum Institute (API) RP2A, is used for structural analysis, input values ofground motion (spectral acceleration representation)to be used are not to be less severe than the follow-ing.

100% in both orthogonal horizontal directions50% in the vertical direction

When three-dimensional, site-specific groundmotion spectra are developed, the actual directionalaccelerations are to be used. If single site-specificspectra are developed, accelerations for the remain-ing two orthogonal directions should be applied inaccordance with the factors given above.

If time history method is used for structuralanalysis, at least three sets of ground motion timehistories are to be employed. The manner in whichthe time histories are used is to account for the po-tential sensitivity of the structure’s response tovariations in the phasing of the ground motion rec-ords.

Structural appurtenances, equipment, modules,and piping are to be designed to resist earthquake in-duced accelerations at their foundations.

As appropriate, effects of soil liquefaction,shear failure of soft muds and loads due to accel-eration of the hydrodynamic added mass by theearthquake, submarine slide, tsunamis and earth-quake generated acoustic shock waves are to betaken into account.

3/2.5.7 Marine Growth

The following effects of anticipated marine growthare to be accounted for in design.

Increase in hydrodynamic diameterIncrease in surface roughness in connection with the

determination of hydrodynamic coefficients(e.g., lift, drag and inertia coefficients)

Increase in dead load and inertial mass

The amount of accumulation assumed for design isto reflect the extent of and interval between cleaningof submerged structural parts.

3/2.5.8 Sea Ice

The global forces exerted by sea ice on the structureas a whole and local concentrated loads on structuralelements are to be considered. The effects of rubblepiles on the development of larger areas, and theirforces on the structure, need to be considered.

The impact effect of a sea ice feature must con-sider mass and hydrodynamic added mass of the ice,its velocity, direction and shape relative to the struc-ture, the mass and size of the structure, the addedmass of water and soil accelerating with the struc-

Loads Section 2


ture, the compliance of the structure-soil interactionand the failure mode of the ice-structure interaction.The dynamic response of structure to ice may be im-portant in flexible structures. As appropriate, lique-faction of the underlying soils due to repetitive com-pressive failures of the ice against the structure is tobe taken into account.

3/2.5.9 Subsidence

The effects of subsidence should be considered in theoverall foundation and structural design. This wouldbe especially applicable to facilities where uniquegeotechnical conditions exist such that significantsea floor subsidence could be expected to occur as aresult of depiction of the subsurface reservoir.

Loads Section 2


.

General Design Requirements Section 3


Part 3Section 3 General Design Requirements

3/3.1 General

This section of the Rules outlines general conceptsand considerations which may be incorporated in de-sign. In addition, considerations for particular typesof offshore structures are enumerated. Subsequentsections of these Rules dealing specifically withsteel, concrete and foundation design are to beviewed in light of the requirements given in this sec-tion. Wherever references are made in the Rules toAPI-RP2A, if applicable, other industry standardssuch as ISO documents may also be used.

The design assessment of service life extensionand reuse of existing platforms is to be based on thecondition and usage of the platform is to be in accor-dance with those described in Part 4 of these Rules.

3/3.3 Analytical Approaches

3/3.3.1 Format of Design Specifications

The design requirements of these Rules are generallyspecified in terms of a working stress format for steelstructures and an ultimate strength format for con-crete structures. In addition, the Rules require thatconsideration be given to the serviceability of struc-ture relative to excessive deflection, vibration, and,in the case of concrete, cracking.

The Bureau will give special consideration tothe use of alternative specification formats, such asthose based on probabilistic or semiprobabilisticlimit state design concepts.

3/3.3.2 Loading Formats

With reference to Sections 3/1 and 3/2, either a de-terministic or spectral format may be employed todescribe various load components. When a static ap-proach is used, it is to be demonstrated, where rele-vant, that consideration has been given to the generaleffects of dynamic amplification. The influence ofwaves other than the highest waves is to be investi-gated for their potential to produce maximum peakstresses due to resonance with the structure.

When considering an earthquake in seismicallyactive areas (see Section 3/2), a dynamic analysis isto be performed. A dynamic analysis is also to beconsidered to assess the effects of environmental orother types of loads where dynamic amplification isexpected. When a fatigue analysis is performed, along-term distribution of the stress range, withproper consideration of dynamic effects, is to be

obtained for relevant loadings anticipated duringthe design life of the structure (see 3/4.13 and3/5.5.4).

If the modal method is employed in dynamicanalysis, it should be recognized that the number ofmodes to be considered is dependent on the charac-teristics of the structure and the conditions beingconsidered.

For earthquake analysis, a minimum number ofmodes is to be considered to provide approximately90% of the total energy of all modes. Normally, atleast six modes with the highest energy content areto be considered. The correlation between the indi-vidual modal responses in determining the total re-sponse is to be investigated. The complete quadraticcombination (CQC) method may be used for com-bining modal responses. If the correlation betweenthe individual modal responses is small, the total re-sponse may be calculated as the square root of thesum of the squares of the individual modal re-sponses.

For extreme wave and fatigue analyses, dynamicresponse is to be considered for structural modeshaving periods greater than 3.0 seconds. For signifi-cant modes with periods of 3.0 seconds or less, thedynamic effect need not be considered provided thefull static effect (including flexure of individualmembers due to localized wave forces) is considered.

3/3.3.3 Combination of Loading Components

Loads imposed during and after installation are to betaken into account. In consideration of the variousloads described in Section 3/2, loads to be consid-ered for design are to be combined consistent withtheir probability of simultaneous occurrence. How-ever, earthquake loadings may be applied withoutconsideration of other environmental effects unlessconditions at the site necessitate their inclusion. Ifsite-specific directional data is not obtained, the di-rection of applied environmental loads is to be suchas to produce the highest possible influence on thestructure.

Loading combinations corresponding to condi-tions after installation are to reflect both operatingand design environmental loadings (see 3/1.5). Ref-erence is to be made to Sections 3/4, 3/5 and 3/6 re-garding the minimum load combinations to be con-sidered. The Operator is to specify the operatingenvironmental conditions and the maximum toler-able environmental loads during installation.



3/3.5 Overall Design Considerations

3/3.5.1 Design Life

The design life of the structure is to be specified bythe Operator. Continuance of classification beyondthe Design Life will be subject to a special surveyand engineering analysis as indicated in Part 4 ofthese Rules.

3/3.5.2 Air Gap

An air gap of at least 1.5 m (5 ft) is to be providedbetween the maximum wave crest elevation and thelowest protuberance of the superstructure for whichwave forces have not been included in the design.After accounting for the initial and expected long-term settlements of the structure, due to consolida-tion and subsidence in a hydrocarbon or other res-ervoir area, the design wave crest elevation is to besuperimposed on the still water level (see 3/1.7.4)and consideration is to be given to wave run-up,tilting of the structure and, where appropriate, tsu-namis.

3/3.5.3 Long-Term and Secondary Effects

Consideration is to be given to the following effects,as appropriate to the planned structure.

Local vibration due to machinery, equipment andvortex shedding

Stress concentrations at critical jointsSecondary stresses induced by large deflection (P-δ

effects)Cumulative fatigueCorrosionAbrasion due to iceFreeze-thaw action on concrete and coatings

3/3.5.4 Reference Marking

For large or complex structures, consideration shouldbe given to installing permanent reference markingsduring construction to facilitate future surveys.Where employed, such markings may consist ofweld beads, metal or plastic tags, or other permanentmarkings. In the case of a concrete structure, mark-ings may be provided using suitable coatings or per-manent lines molded into the concrete.

3/3.5.5 Zones of Exposure

Measures taken to mitigate the effects of corrosion asrequired by 3/4.1.2 and 3/5.1.2 are to be specifiedand described in terms of the following definitionsfor corrosion protection zones.

a Submerged Zone That part of the installa-tion below the splash zone.

b Splash Zone The part of the installationcontaining the areas above and below the still waterlevel (see 3/1.7.4) which are regularly subjected towetting due to wave action. Characteristically, thesplash zone is not easily accessible for field painting,nor protected by cathodic protection.

c Atmospheric Zone That part of the installa-tion above the splash zone.

Additionally, for structures located in areassubject to floating or submerged ice, that portion ofthe structure which may reasonably be expected tocome into contact with floating or submerged ice isto be designed with consideration for such contact.

3/3.7 Considerations for ParticularTypes of Structures

3/3.7.1 General

In this subsection are listed specific design consid-erations which are to be taken into account for par-ticular types of structures. They constitute additionalpertinent factors which affect the safety and per-formance of the structure and are not intended tosupplant or modify other criteria contained in theseRules.

Where required, the interactive effects betweenthe platform and conductor or riser pipes due to plat-form motions are to be investigated. For compliantstructures which exhibit significant wave inducedmotions, determination of such interactive effectsmay be of critical importance.

3/3.7.2 Pile-Supported Steel Platforms

a Factors to be Considered Factors to be con-sidered in the structural analysis are to include thesoil-pile interaction and the loads imposed on thetower or jacket during towing and launching.

b Installation Procedures Carefully controlledinstallation procedures are to be developed so thatthe bearing loads of the tower or jacket on the soilare kept within acceptable limits until the piles aredriven.

c Special Procedures Special procedures mayhave to be used to handle long, heavy piles until theyare self-supporting in the soil. Pile driving delays areto be minimized to avoid set-up of the pile sections.

d Dynamic Analysis For structures likely to besensitive to dynamic response, the natural period ofthe structure should be checked to insure that it is notin resonance with waves having significant energycontent.

e Instability Instability of structural membersdue to submersion is to be considered, with due ac-count for second-order effects produced by factorssuch as geometrical imperfections.



3/3.7.3 Concrete or Steel Gravity Platforms

a Positioning The procedure for transportingand positioning the structure and the accuracy ofmeasuring devices used during these procedures areto be documented.

b Repeated Loadings Effects of repeatedloadings on soil properties, such as pore pressure,water content, shear strength and stress strain be-havior, are to be investigated.

c Soil Reactions Soil reactions against thebase of the structure during installation are to be in-vestigated. Consideration should be given to the oc-currence of point loading caused by sea bottom ir-regularities. Suitable grouting between base slab andsea floor can be employed to reduce concentration ofloads.

d Maintenance The strength and durability ofconstruction materials are to be maintained. Wheresulphate attack is anticipated, as from stored oil, ap-propriate cements are to be chosen, pozzolans incor-porated in the mix, or the surfaces given suitablecoatings.

e Reinforcement Corrosion Means are to beprovided to minimize reinforcing steel corrosion.

f Instability Instability of structural membersdue to submersion is to be considered, with due ac-count for second-order effects produced by factorssuch as geometrical imperfections.

g Horizontal Sliding Where necessary, pro-tection against horizontal sliding along the sea flooris to be provided by means of skirts, shear keys orequivalent means.

h Dynamic Analysis A dynamic analysis, in-cluding simulation of wave-structure response andsoil-structure interaction, should be considered forstructures with natural periods greater than approxi-mately 3 seconds.

i Long Term Resistance The long term resis-tance to abrasion, cavitation, freeze-thaw durabilityand strength retention of the concrete are to be con-sidered.

j Negative Buoyancy Provision is to be madeto maintain adequate negative buoyancy at all timesto resist the uplift forces from waves, currents, andoverturning moments. Where this is achieved byballasting oil storage tanks with sea water, continu-ously operating control devices should be used tomaintain the necessary level of the oil-water inter-face in the tanks.

3/3.7.4 Concrete-Steel Hybrid Structures

a Horizontal Loading Where necessary, theunderside of the concrete base is to be provided withskirts or shear keys to resist horizontal loading; steelor concrete keys or equivalent means may be used.

b Steel and Concrete Interfaces Special at-tention is to be paid to the design of the connectionsbetween steel and concrete components.

c Other Factors Pertinent design factors forthe concrete base listed in 3/3.7.3 are also to be takeninto account.

3/3.7.5 Guyed Compliant Towers

a Clump Weights Where necessary, clumpweights (with or without buoyancy units) betweenthe tower and anchors on the sea floor are to be pro-vided to minimize the uplift forces on the anchors, tohold the guylines taut, and to restrict the lateralmovement of the tower.

b Lifting Clump Weights As required by thedesign, clump weights should have provisions forbeing lifted off the sea floor during a storm. Liftingof the clump weights will decrease the stiffness ofthe mooring system, and allow the tower to displacemore with the large waves.

c Swiveling Fairleads Consideration should begiven to locating the swiveling fairleads on the toweras close as possible to the center of pressure of the de-sign wind, wave and current loads in order to mini-mize horizontal forces at the bottom of the tower.

d Foundations Foundations supporting thebase are to be embedded in the sea floor to a depthsufficient to attain the desired load-carrying capacity.

3/3.7.6 Tension Leg Platform (TLP)

a Factors to be Considered In determining thedesign environmental criteria, the design events thatwill produce the worst response to each componentof the structure are to be considered. The largest re-sponses of different components of the structure arenot necessarily produced by the highest wave condi-tion.

b Tendons The pretension is to be selected to re-sult in positive tension at the foundation tendon con-nection for all design and operating load conditions.

c Analysis Frequency domain or time domainanalysis may be performed to determine the re-sponses of the structure such as extreme offset andyaw, minimum and maximum tendon tension, anddeck clearance.

d Damaged Condition Analysis In addition tointact operating and design environmental condi-tions, damaged conditions such as accidental flood-ing of a buoyant compartment and/or a missing orflooded tendon are to be considered in the design.

e Foundation Foundations used to anchor thetendon leg system to the sea floor are to be sufficientto attain the required load-carrying capacity.

f Transportation and Installation ProceduresTransportation and installation procedures are to bedeveloped which minimize the stresses of the struc-



tural components. For the design of TLP, reference isto be made to API RP2T: “Recommended Practicefor Planning, Designing, and Constructing TensionLeg Platforms”.

3/3.7.7 Minimum Structures

a Design Considerations These Rules are tobe applied in design of minimum structures whereverapplicable. Minimum structures generally have lessstructural redundancy and more prominent dynamicresponses due to the flexible nature of the structuralconfiguration. The dynamic effects on the structureare to be considered in the structural analysis whenthe structure has a natural period greater than 3 sec-onds. The pertinent design factors listed in 3/3.7.2are to be taken into account.

b Mechanical Connections Connections otherthan welded joints are commonly used in minimumstructures. For these mechanical connections such asclamps, connectors and bolts, joining diagonal bracesto the column or piles to the minimum structure, thestrength and fatigue resistance are to be assessed byanalytical methods or testing.

3/3.7.8 Site Specific Self-Elevating MobileOffshore Units

a Design Considerations Self-Elevating Mo-bile Offshore Units converted to site dependent plat-

form structures are to be designed in accordance withthese Rules along with the ABS “Rules for Buildingand Classing Mobile Offshore Drilling Units”, wher-ever applicable.

b Foundation When selecting a unit for a par-ticular site, due consideration should be given to soilconditions at the installation site. The bearing capac-ity and sliding resistance of the foundation are to beinvestigated. The foundation design is to be in ac-cordance with 3/6.9. As applicable, the foot printsleft by Jack-up rigs and scour are to be considered inthe foundation design.

c Structural Analysis In the structural analy-sis, the leg to hull connections and soil/structure in-teraction are to be properly considered. The upperand lower guide flexibility, stiffness of the elevat-ing/holding system, and any special details regardingits interaction with the leg should be taken into con-sideration. For units with spud cans, the legs may beassumed pinned at the reaction point. For mat sup-ported units, the soil structure interaction may bemodelled using springs.

d Holding Capacity While used as a site de-pendent platform structure, the calculated loads areto demonstrate that the maximum holding capacityof the jacking system will not be exceeded.

e Preload Units with spud cans are to be pre-loaded on installation in order to minimize the possi-bility of significant settlement under severe stormconditions.

Steel Structures Section 4


Part 3Section 4 Steel Structures

3/4.1 General

The requirements of this section are to be applied inthe design and analysis of the principal componentsof steel structures intended for offshore applications.Items to be considered in the design of welded con-nections are specified in Section 2/2.

3/4.1.1 Materials

The requirements of this section are intended forstructures constructed of steel manufactured andhaving properties as specified in Section 2/1. Whereit is intended to use steel or other materials havingproperties differing from those specified in Section2/1, their applicability will be considered subject to areview of the specifications for the alternative mate-rials and the proposed methods of fabrication.

3/4.1.2 Corrosion Protection

Materials are to be protected from the effects of cor-rosion by the use of a corrosion protection systemincluding the use of coatings. The system is to be ef-fective from the time the structure is initially placedon site. Where the sea environment contains unusualcontaminants, any special corrosive effects of suchcontaminants are also to be considered. For the de-sign of protection systems, reference is to be made tothe National Association of Corrosion Engineers(NACE) publication RP 0176-94, or other appropri-ate references

3/4.1.3 Access for Inspection

In the design of the structure, consideration shouldbe given to providing access for inspection duringconstruction and, to the extent practicable, for surveyafter construction.

3/4.1.4 Steel-Concrete Hybrid Structures

The steel portions of a steel-concrete hybrid structureare to be designed in accordance with the require-ments of this section, and the concrete portions are tobe designed as specified in Section 3/5. Any effectsof the hybrid structure interacting on itself in areassuch as corrosion protection should be considered.

3/4.3 General Design Criteria

Steel structures are to be designed and analyzed forthe loads to which they are likely to be exposed dur-

ing construction, installation and in-service opera-tions. To this end, the effects on the structure of aminimum set of loading conditions, as indicated in3/4.5, are to be determined, and the resulting struc-tural responses are not to exceed the safety andserviceability criteria given below.

The use of design methods and associated safetyand serviceability criteria, other than those specifi-cally covered in this section, is permitted where itcan be demonstrated that the use of such alternativemethods will result in a structure possessing a levelof safety equivalent to that provided by the directapplication of these requirements.

The contents of Sections 3/2 and 3/3 are to beconsulted regarding definitions and requirementspertinent to the determination of loads and generaldesign requirements.

3/4.5 Loading Conditions

Loadings which produce the most unfavorable effectson the structure during and after construction and in-stallation are to be considered. Loadings to be investi-gated for conditions after installation are to include atleast those relating to both the realistic operating anddesign environmental conditions combined with otherpertinent loads in the following manner.

Operating environmental loading combined withdead and maximum live loads appropriate to thefunction and operations of the structure

Design environmental loading combined with deadand live loads appropriate to the function andoperations of the structure during the design en-vironmental condition

For structures located in seismically active ar-eas, earthquake loads (see 3/2.5.6 and 3/3.3.3) are tobe combined with dead and live loads appropriate tothe operation and function of the structure whichmay be occurring at the onset of an earthquake.

3/4.7 Structural Analysis

a The nature of loads and loading combinations aswell as the local environmental conditions are tobe taken into consideration in the selection of de-sign methods. Methods of analysis and their asso-ciated assumptions are to be compatible with theoverall design principles. Linear, elastic methods(working stress methods) can be employed in de-sign and analysis provided proper measures are



taken to prevent general and local buckling fail-ure, and the interaction between soil and structureis adequately treated. When assessing structuralinstability as a possible mode of failure, the ef-fects of initial stresses and geometric imperfec-tions are to be taken into account. Constructiontolerances are to be consistent with those used inthe structural stability assessment.

b Dynamic effects are to be accounted for if thewave energy in the frequency range of the struc-tural natural frequencies is of sufficient magni-tude to produce significant dynamic response inthe structure. In assessing the need for dynamicanalyses of deep water or unique structures, in-formation regarding the natural frequencies of thestructure in its intended position is to be obtained.The determination of dynamic effects is to be ac-complished either by computing the dynamic am-plification effects in conjunction with a determi-nistic analysis or by a random dynamic analysisbased on a probabilistic formulation. In the lattercase, the analysis is to be accompanied by a sta-tistical description and evaluation of the relevantinput parameters.

c For static loads, plastic methods of design andanalysis can be employed only when the proper-ties of the steel and the connections are such thatthey exclude the possibility of brittle fracture, al-low for formation of plastic hinges with sufficientplastic rotational capability, and provide adequatefatigue resistance.

d In a plastic analysis, it is to be demonstrated thatthe collapse mode (mechanism) which corre-sponds to the smallest loading intensities has beenused for the determination of the ultimate strengthof the structure. Buckling and other destabilizingnonlinear effects are to be taken into account inthe plastic analysis. Whenever non-monotonic orrepeating loads are present, it is to be demon-strated that the structure will not fail by incre-mental collapse or fatigue.

e Under dynamic loads, when plastic strains mayoccur, the considerations specified in c are to besatisfied and any buckling and destabilizing non-linear effects are to be taken into account.

3/4.9 Allowable Stresses and LoadFactors

3/4.9.1 Working Stress Approach

When a design is based on a working stress method(see 3/4.7a and 3/3.3), the safety criteria are to beexpressed in terms of appropriate basic allowablestresses in accordance with requirements specifiedbelow.a For structural members and loadings covered by

Part 5 of the American Institute of Steel Con-

struction (AISC) Manual of Steel Construction,ASD, with the exception of earthquake loadings(see d below) and tubular structural members un-der the combined loading of axial compressionand bending, the basic allowable stresses of themembers are to be obtained using the AISCSpecification. For tubular members subjected tothe aforementioned interaction, stress limits are tobe in accordance with the API RP 2A.

b Where stresses in members described in a areshown to be due to forces imposed by the designenvironmental condition acting alone or in com-bination with dead and live loads, the basic al-lowable stresses cited in a may be increased byone-third provided the resulting structural mem-ber sizes are not less than those required for theoperating environment loading combined withdead and live loads without the one-third increasein allowable stresses.

c When considering loading combinations whichinclude earthquake loads (see 3/4.5) on individualmembers or on the overall structure, the allowablestress may be set equal to 1.7 times the basic al-lowable stress of the member.

d The allowable stresses specified in b are to be re-garded as the limits for stresses in all structuralparts for the marine operations covered in Section3/7, except for lifting, where the one third in-crease in the basic allowable stress is not permit-ted. The lifting analysis should adequately ac-count for equipment and fabrication weightincrease.

e For any two- or three-dimensional stress fieldwithin the scope of the working stress formula-tion, the equivalent stress (e.g., the von Misesstress intensity) is to be limited by an appropriateallowable stress less than yield stress, with theexception of those stresses of a highly localizednature. In the latter case, local yielding of thestructure may be accepted provided it can bedemonstrated that such yielding does not lead toprogressive collapse of the overall structure andthat the general structural stability is maintained.

f Whenever elastic instability, overall or local, mayoccur before the stresses reach their basic allow-able levels, appropriate allowable bucklingstresses govern.

3/4.9.2 Plastic Design Approach

Whenever the ultimate strength of the structure isused as the basis for the design of its members, thesafety factors or the factored loads are to be for-mulated in accordance with the requirements inChapter N (Plastic Design) of Part 5 of the AISCManual of Steel Construction, ASD or an equivalentcode. The capability of the principal structuralmembers to develop their predicted ultimate load



capacity is to be demonstrated. For safety againstbrittle fracture, special attention is to be given todetails of high stress concentration and to improvedmaterial quality.

3/4.11 Structural Response toEarthquake Loads

Structures located in seismically active areas are to bedesigned to possess adequate strength and stiffness towithstand the effects of strength level earthquake, aswell as sufficient ductility to remain stable during raremotions of greater severity associated with ductilitylevel earthquake. The sufficiency of the structuralstrength and ductility is to be demonstrated bystrength and, as required, ductility analyses.

For strength level earthquake, the strengthanalysis is to demonstrate that the structure is ade-quately sized for strength and stiffness to maintainall nominal stresses within their yield or bucklinglimits.

In the ductility analysis, it is to be demonstratedthat the structure has the capability of absorbing theenergy associated with the ductility level earthquakewithout reaching a state of incremental collapse.

In United States offshore regions, reference maybe made to the API RP 2A for design criteria forearthquake. In other seismically active locationsaround the world a seismic report should be submit-ted.

3/4.13 Fatigue Assessment

For structural members and joints where fatigue is aprobable mode of failure, or for which past experi-ence is insufficient to assure safety from possiblecumulative fatigue damage, an assessment of fa-tigue life is to be carried out. Emphasis is to begiven to joints and members in the splash zone,those that are difficult to inspect and repair once thestructure is in service, and those susceptible tocorrosion-accelerated fatigue.

For structural members and joints which requirea detailed assessment of cumulative fatigue damage,the results of the assessment are to indicate a mini-mum expected fatigue life of twice the design life ofthe structure where sufficient structural redundancyexists to prevent catastrophic failure of the structureof the member or joint under consideration. Wheresuch redundancy does not exist or where the desir-able degree of redundancy is significantly reduced asa result of fatigue damage, the result of a fatigue as-sessment is to indicate a minimum expected fatiguelife of three or more times the design life of thestructure.

A spectral fatigue analysis technique is recom-mended to calculate the fatigue life of the structure.

Other rational analysis methods are also acceptable ifthe forces and member stresses can be properly rep-resented. The dynamic effects should be taken intoconsideration if they are significant to the structuralresponse.

3/4.15 Stresses in Connections

Connections of structural members are to be devel-oped to insure effective load transmission betweenjoined members, to minimize stress concentrationand to prevent excessive punching shear. Connectiondetails are also to be designed to minimize undueconstraints against overall ductile behavior and tominimize the effects of postweld shrinkage. Undueconcentration of welding is to be avoided.

The design of tubular joints may be in accor-dance with the API RP 2A.

3/4.17 Structure-Pile Connections

The attachment of the structure to its foundation is tobe accomplished by positive, controlled means suchas welding or grouting, with or without the use ofmechanical shear keys or other mechanical connec-tors. Details of mechanical connectors are to besubmitted for review. Such attachments are to be ca-pable of withstanding the static and long-term cyclicloadings to which they will be subjected. Generalreferences may be made to the API RP 2A where theratio of the diameter to thickness of either the pile orthe sleeve is less than or equal to 80. Where a ratioexceeds 80, special consideration is to be given tothe effects of reduced confinement on allowablebond stress. Particulars of grouting mixtures are tobe submitted for review.

The allowable stresses or load factors to be em-ployed in the design of foundation structure for steelgravity bases or piles are to be in accordance with3/4.9.1 or 3/4.9.2, and with regard to laterally loadedpiles in accordance with 3/6.7.5.

3/4.19 Structural Response toHydrostatic Loads

Analyses of the structural stability are to be per-formed to demonstrate the ability of structural partsto withstand hydrostatic collapse at the water depthsat which they will be located.

3/4.21 Deflections

The platform deflections which may affect the de-sign of piles, conductors, risers and other structuresin way of the platform are to be considered. Whereappropriate, the associated geometric nonlinearity isto be accounted for in analysis.



3/4.23 Local Structure

Structures which do not directly contribute to theoverall strength of the fixed offshore structure, i.e.,their loss or damage would not impair the structuralintegrity of the offshore structure, are considered tobe local structure.

Local structures are to be adequate for the natureand magnitude of applied loads. Allowable stressesspecified in 3/4.9 are to be used as stress limits ex-cept for those structural parts whose primary func-tion is to absorb energy, in which case sufficientductility is to be demonstrated.

Concrete Structures Section 5


Part 3Section 5 Concrete Structures

3/5.1 General

The requirements of this section are to be applied tooffshore installations of reinforced and prestressedconcrete construction.

3/5.1.1 Materials

Unless otherwise specified, the requirements of thissection are intended for structures constructed of ma-terials manufactured and having properties as speci-fied in Section 2/1. Where it is intended to use materi-als having properties differing from those specified inSection 2/1, the use of such materials will be speciallyconsidered. Specifications for alternative materials,details of the proposed methods of manufacture and,where available, evidence of satisfactory previous per-formance, are to be submitted for approval.

3/5.1.2 Durability

Materials, concrete mix proportions, constructionprocedures and quality control are to be chosen toproduce satisfactory durability for structures locatedin a marine environment. Problems to be specificallyaddressed include chemical deterioration of concrete,corrosion of the reinforcement and hardware, abra-sion of the concrete, freeze-thaw durability, and firehazards as they pertain to the zones of exposure de-fined in 3/3.5.5.

Test mixes should be prepared and tested early inthe design phase to ensure that proper values ofstrength, creep, alkali resistance, etc. will be achieved.

3/5.1.3 Access for Inspection

The components of the structure are to be designed toenable their inspection during construction and, to theextent practicable, periodic survey after installation.

3/5.1.4 Steel-Concrete Hybrid Structures

The concrete portions of a hybrid structure are to bedesigned in accordance with the requirements of thissection, and the steel portions in accordance with therequirements of Section 3/4.

3/5.3 General Design Criteria

3/5.3.1 Design Method

a General The requirements of this sectionrelate to the ultimate strength method of design.

b Load Magnitude The magnitude of a designload for a given type of loading k is obtained bymultiplying the load, Fk, by the appropriate loadfactor, ck, i.e., design load = ckFk.

c Design Strength In the analysis of sections,the design, strength of a given material is obtainedby multiplying the material strength, fk, by the ap-propriate strength reduction factor, f, i.e., designstrength = f fk. The material strength, fk, for concreteis the specified compression strength of concrete (fc')after 28 days and for steel is the minimum specifiedyield strength (fy).

3/5.3.2 Load Definition

a Load Categories The load categories re-ferred to in this section, i.e., dead loads, live loads,deformation loads, and environmental loads, are de-fined in 3/2.3.

b Combination Loads Loads taken in combi-nation for the Operating Environmental Conditionsand the Design Environmental Condition are indi-cated in 3/5.5.2.

c Earthquake and Other Loads Earthquakeloads and loads due to environmental phenomena ofrare occurrence need not be combined with other en-vironmental loads unless site-specific conditions in-dicate that such combination is appropriate.

3/5.3.3 Design Reference

Design considerations for concrete structures not di-rectly addressed in these Rules are to follow the re-quirements of the American Concrete Institute (ACI)318 and ACI 357, or equivalent.

3/5.5 Design Requirements

3/5.5.1 General

The strength of the structure is to be such that ade-quate safety exists against failure of the structure orits components. Among the modes of possible failureto be considered are the following.

Loss of overall equilibriumFailure of critical sectionsInstability resulting from large deformationsExcessive plastic or creep deformation

The serviceability of the structure is to be assessed.The following items are to be considered in relation



to their potential influences on the serviceability ofthe structure.

Cracking and spallingDeformationsCorrosion of reinforcement or deterioration of concreteVibrationsLeakage

3/5.5.2 Required Strength (Load Combinations)

The required strength (U) of the structure and eachmember is to be equal to, or greater than, the maxi-mum of the following.

U = 1.2(D + T) + 1.6 Lmax + 1.3 Eo

U = 1.2(D + T) + 1.2 Lmax + cE Emax

U = 0.9(D + T) + 0.9 Lmin + cE Emax

in which cE assumes the following values:

cE = 1.3 for wave, current, wind, or ice loadcE = 1.4 for earthquake loads

In the preceding relations, the symbols D, T, and Lrepresent dead load, deformation load, and live load,respectively (see 3/5.3.2). The symbol EO representsoperating environmental loads, while Emax representsdesign environmental loads. The symbol Lmin repre-sents minimum expected live loads, while Lmax repre-sents maximum expected live loads.

For loads of type D, the load factor 1.2 is to bereplaced by 1.0 if it leads to a more unfavorable loadcombination. For loads of type EO the load factor 1.3may be reduced if a more unfavorable load combi-nation results. For strength evaluation the effects ofdeformation load may be ignored provided adequateductility is demonstrated.

While the critical design loadings will be identi-fied from the load combinations given above, theother simultaneously occurring load combinationsduring construction and installation phases are to beconsidered if they can cause critical load effects.

3/5.5.3 Strength Reduction Factors

The strength of a member or a cross section is to becalculated in accordance with the provisions of 3/5.7and it is to be multiplied by the following strengthreduction factor, f.

a For bending with or without axial tension, f = 0.90b For axial compression or axial compression com-

bined with bending.Reinforced members with spiral reinforcement,f = 0.75Other reinforced members (excluding slabs andshells), f = 0.70The values given above may be increased linearlyto 0.9 as Pu decreases from 0.1 fc'Ag or Pb, which-ever is smaller, to zero.

fc' = specified compression strength of concreteAg = gross area of sectionPu = axial design load in compression memberPb = axial load capacity assuming simultaneous

occurrence of the ultimate strain of concreteand yielding of tension steel

Slabs and shells, f = 0.70c For shear and torsion, f = 0.85d For bearing on concrete, f = 0.70

Alternatively, the expected strength of concretemembers can be determined by using idealizedstress-strain curves and material factors (cM) given inACI 357R. The material factors applied to the stress-strain curves limit the maximum stress to achieve thedesired reliability similar to using the strength re-duction factors given above. The strength reductionfactors (f) and the material factors (cM) are not to beused simultaneously.

3/5.5.4 Fatigue

The fatigue strength of the structure will be consid-ered satisfactory if under the unfactored operatingloads the following conditions are satisfied.

The stress range in reinforcing or prestressing steeldoes not exceed 138 MPa (20,000 psi), or wherereinforcement is bent, welded or spliced, 69MPa (10,000 psi)

There is no membrane tensile stress in concrete andnot more than 1.4 MPa (200 psi) flexural tensilestress in concrete

The stress range in compression in concrete does notexceed 0.50fc' where fc' is the specified compres-sive strength of concrete

Where maximum shear exceeds the allowable shearof the concrete alone, and where the cyclic rangeis more than half the maximum allowable shearin the concrete alone, all shear is taken by rein-forcement. In determining the allowable shear ofthe concrete alone, the influence of permanentcompressive stress may be taken into account

In situations where fatigue stress ranges allowgreater latitude than those under the serviceabil-ity requirements given in Table 3/5.1, the lattercondition shall assume precedence

Bond stress does not exceed 50% of that permittedfor static loads.

Where the above nominal values are exceeded, anin-depth fatigue analysis is to be performed. In suchan analysis the possible reduction of materialstrength is to be taken into account on the basis ofappropriate data (S-N curves) corresponding to the95th percentile of specimen survival. In this regard,consideration is to be given not only to the effectsof fatigue induced by normal stress, but also to fa-tigue effects due to shear and bond stress. Particular



attention is to be given to submerged areas sub-jected to the low-cycle, high-stress components ofthe loading history. Where an analysis of the fa-tigue life is performed, the expected fatigue life ofthe structure is to be at least twice the design life.In order to estimate the cumulative fatigue damageunder variable amplitude stresses, a recognized cu-mulative rule is to be used. Miner’s rule is an ac-ceptable method for the cumulative fatigue damageanalysis.

3/5.5.5 Serviceability Requirements

a Serviceability The serviceability of thestructure is to be checked by the use of stress-straindiagrams (Figures 3/5.1 and 3/5.2) with strength re-duction factor, f = 1.0, and the unfactored load com-bination

U = D + T + L + EO

where L is the most unfavorable live load and allother terms are as previously defined.

Using this method the reinforcing stresses are tobe limited in compliance with Table 3/5.1. Addition-ally for hollow structural cross sections, the maxi-

mum permissible membrane strain across the wallsshould not cause cracking under any combination ofD, L, T and Emax using load factors taken as 1.0. Forstructures prestressed in one direction only, tensilestresses in reinforcement transverse to the prestress-ing steel shall be limited so that the strains at theplane of the prestressing steel do not exceed Dps/ES.Where Dps is as defined in Table 3/5.1 and ES is themodulus of elasticity of reinforcement (see section3/5.7.2 g).

Alternative criteria such as those which directlylimit crack width will also be considered.

b Liquid-Containing Structures The follow-ing criteria are to be satisfied for liquid-containingstructures to ensure adequate design against leakage.

The reinforcing steel stresses are to be in accordancewish section 3/5.5.5a

The compression zone is to extend over 25% of thewall thickness or 205 mm (8 in.), whichever isless

There is to be no membrane tensile stress unlessother construction arrangements are made, suchas the use of special barriers to prevent leakage

TABLE 3/5.1 Allowable Tensile Stresses for Prestress and Reinforcing Steel to Control Cracking

Allowable Stress, MPa (ksi)

Stage LoadingReinforcing

Steel, fs

PrestressingTendons Dps

Construction: where crackingduring construction would bedetrimental to the completedstructure

All loads on the structureduring construction

160 (23.0) 130 (18.5)

Construction: where crackingduring construction is notdetrimental to the completedstructure

All loads on the structureduring construction

210 (30.0) or0.6 fy, whichever

is less

130 (18.5)

Transportation and installation All loads on the structureduring transportation andinstallation

160 (23.0) 130 (18.5)

At offshore site Dead and live plus operatingenvironmental loads

120 (17.0) 75 (11.0)

At offshore site Dead and live loads plus designenvironmental loads

0.8 fy

fy = yield stress of the reinforcing steelfs = allowable stress in the reinforcing steelDps = increase in tensile stress in prestressing steel with reference to the stress at zero strain in the concrete.



cf

cf

3/5.7 Analysis and Design

3/5.7.1 General

Generally, the analysis of structures may be per-formed under the assumptions of linearly elasticmaterials and linearly elastic structural behavior,following the requirements of ACI 318 and the addi-tional requirements of this subsection. The materialproperties to be used in analysis are to conform to3/5.7.2. However, the inelastic behavior of concretebased on the true variation of the modulus of elastic-ity with stress and the geometric nonlinearities, in-cluding the effects of initial deviation of the structurefrom the design geometry, are to be taken into ac-count whenever their effects reduce the strength ofthe structure. The beneficial effects of the concrete’snonlinear behavior may be accounted for in theanalysis and design of the structure to resist dynamicloadings.

When required, the dynamic behavior of con-crete structures may be investigated using a linearstructural model, but soil-structural impedances areto be taken into account. The analysis of the structureunder earthquake conditions may be performed un-der the assumption of elasto-plastic behavior due toyielding, provided that the requirements of 3/5.7.7are satisfied.

3/5.7.2 Material Properties for StructuralAnalysis

a Specified Compressive Strength The speci-fied compressive strength of concrete, fc', is to bebased on 28-day tests performed in accordance withspecifications ASTM C172, ASTM C31 and ASTMC39.

b Early Loadings For structures which aresubjected to loadings before the end of the 28-dayhardening period of concrete, the compressivestrength of concrete is to be taken at the actual age ofconcrete at the time of loading.

c Early-Strength Concrete For early-strengthconcrete, the age for the tests for fc' may be deter-mined on the basis of the cement manufacturer’scertificate.

d Modulus of Elasticity-Concrete For the pur-poses of structural analyses and deflection checks,the modulus of elasticity of normal weight concretemay be assumed as equal to 4733 MPa (57,000 psi) or determined from stress-strain curves de-veloped by tests (see Figure 3/5.1) the latter methodis used, the modulus of elasticity is to be determinedusing the secant modulus for the stress equal to0.5 fc'.

e Uniaxial Compression-Concrete In lieu oftests, the stress-strain relation shown in Figure 3/5.1may be used for uniaxial compression of concrete.

f Poisson Ratio The Poisson ratio of concretemay be taken equal to 0.17.

g Modulus of Elasticity-Reinforcement Themodulus of elasticity, ES of non-prestressed steelreinforcement is to be taken as 200 × 103 MPa(29 × 106 psi). The modulus of elasticity ofprestressing tendons is to be determined by tests.

h Uniaxial Tension-Reinforcement Thestress-strain relation of non-prestressed steel rein-forcement in uniaxial tension is to be assumed asshown in Figure 3/5.2. The stress-strain relation ofprestressing tendons is to be determined by tests, ortaken from the manufacturers certificate.

i Yield Strength-Reinforcement If the speci-fied yield strength, fy, of non-prestressed reinforce-ment exceeds 420 MPa (60,000 psi), the value of fy

used in the analysis is to be taken as the stress corre-sponding to a strain of 0.35%.

3/5.7.3 Analysis of Plates, Shells, and Folded

Plates

In all analyses of shell structures, the theory em-ployed in analysis is not to be based solely on mem-brane or direct stress approaches. The bucklingstrength of plate and shell structures is to be checkedby an analysis which takes into account the geomet-rical imperfections of the structure, the inelastic be-havior of concrete and the creep deformations ofconcrete under sustained loading. Special attention isto be devoted to structures subjected to external pres-sure and the possibility of their collapse (implosion)by failure of concrete in compression.

3/5.7.4 Deflection Analysis

Immediate deflections may be determined by the meth-ods of linear structural analysis. For the purposes of de-flection analysis, the member stiffnesses are to be com-puted using the material properties specified in thedesign and are to take into account the effect of cracksin tension zones of concrete. The effect of creep strainin concrete is to be taken into account in the computa-tions of deflections under sustained loadings.

3/5.7.5 Analysis and Design for Shear andTorsion

The applicable requirements of ACI 318 or theirequivalent are to be complied with in the analysisand design of members subject to shear or torsion orto combined shear and torsion.

3/5.7.6 Analysis and Design for Bending andAxial Loads

a Assumed Conditions The analysis and de-sign of members subjected to bending and axialloads are to be based on the following assumptions.



The strains in steel and concrete are proportional tothe distance from the neutral axis

Tensile strength of the concrete is to be neglected,except in prestressed concrete members underunfactored loads, where the requirements in3/5.5.5 apply

The stress in steel is to be taken as equal to ES timesthe steel strain, but not larger than fy

The stresses in the compression zone of concrete areto be assumed to vary with strain according tothe curve given in Figure 3/5.1 or any other con-servative rule. Rectangular distribution of com-pressive stresses in concrete specified by ACI318 may be used

The maximum strain in concrete at the ultimate stateis not to be larger than 0.30%

b Failure The members in bending are to bedesigned in such a way that any section yielding ofsteel occurs prior to compressive failure of concrete.

3/5.7.7 Seismic Analysis

a Dynamic Analysis For structures to be lo-cated at sites known to be seismically active (see3/5.7.8), dynamic analysis is to be performed to de-termine the response of the structure to design earth-quake loading. The structure is to be designed towithstand this loading without damage. In addition, aductility check is also to be performed to ensure thatthe structure has sufficient ductility to experience de-flections more severe than those resulting from thedesign earthquake loading without the collapse of theplatform structure, its foundation or any majorstructural component.

b Design Conditions The dynamic analysis forearthquake loadings is to be performed taking intoaccount

The interaction of all components of the structureThe compliance of the soil and the dynamic soil-

structure interactionThe dynamic effects of the ambient and contained

fluids.

c Method of Analysis The dynamic analysisfor earthquake loadings may be performed by anyrecognized method, such as determination of timehistories of the response by direct integration of theequations of motion, or the response spectra method.

d Ductility Check In the ductility check, dis-tortions at least twice as severe as those resultingfrom the design earthquake are to be assumed. If theductility check is performed with the assumption ofelasto-plastic behavior of the structure, the selectedmethod of analysis is to be capable of taking into ac-count the nonlinearities of the structural model. Thepossibility of dynamic instability (dynamic buckling)

of individual members and of the whole structureshould be considered.

3/5.7.8 Seismic Design

a Compressive Strain The compressive strainin concrete at it critical sections (including plastichinge locations) is to be limited to 0.3%, exceptwhen greater strain may be accommodated by con-fining steel.

b Flexural Bending or Load Reversals Forstructural members or sections subjected to flexuralbending or to load reversals, where the percentage oftensile reinforcement exceeds 70% of the reinforce-ment at which yield stress in the steel is reached si-multaneously with compression failure in the con-crete, special confining reinforcement and/orcompressive reinforcement are to be provided toprevent brittle failure in the compressive zone ofconcrete.

c Web Reinforcement Web reinforcement(stirrups) of flexural members is to be designed forshear forces which develop at full plastic bendingcapacity of end sections. In addition,

The diameter of rods used as stirrups is not to be lessthan 10 mm (#3 bar)

Only closed stirrups (stirrup ties) are to be usedThe spacing of stirrups is not to exceed the lesser of

d/2 or 16 bar diameters of compressive rein-forcement, where d is the distance from the ex-treme compression fiber to the centroid of tensilereinforcement. Tails of stirrups are to be anchoredwithin a confined zone, i.e., turned inward.

d Splices No splices are allowed within a dis-tance d from a plastic hinge. Lap splices are to be atleast 30 bar diameters long but not less than 460 mm(18 in.).

3/5.9 Design Details

3/5.9.1 Concrete Cover

a General The following minimum concretecover for reinforcing bars is required.

Atmospheric zone not subjected to salt spray: 50 mm(2 in.)

Splash and atmospheric zones subjected to salt sprayand exposed to soil: 65 mm (2.5 in.)

Submerged zone: 50 mm (2 in.)Areas not exposed to weather or soil: 40 mm (1.5 in.)Cover of stirrups may be 13 mm (0.5 in.) less than

covers listed above.

b Tendons and Ducts The concrete cover ofprestressing tendons and post-tensioning ducts is to beincreased 25 mm (1 in.) above the values listed in a.



c Sections Less Than 500 mm (20 in.) ThickIn sections less than 500 mm (20 in.) thick, the con-crete cover of reinforcing bars and stirrups may bereduced below the values listed in a; however, thecover is not to be less than the following.

1.5 times the nominal aggregate size1.5 times the maximum diameter of reinforcement,

or 19 mm (0.75 in.)Tendons and post-tensioning duct covers are to have

12.5 mm (0.5 in.) added to the above.

3/5.9.2 Minimum Reinforcement

For loadings during all phases of construction, trans-portation, and operation (including design environ-mental loading) where tensile stresses occur on aface of the structure, the following minimum rein-forcement on the face is required.

AS = (ft /fy) bde

AS = total cross-section area of reinforcementft = mean tensile strength of concretefy = yield stress of the reinforcing steelb = width of structural element

de = effective tension zone, to be taken as 1.5c +10db

c = cover of reinforcementdb = diameter of reinforcement bar

de should be at least 0.2 times the depth of the sec-tion but not greater than 0.5 (h-x), where x is thedepth of the compression zone prior to cracking andh is the section thickness.

At intersections between structural elements,where transfer of shear forces is essential to the in-tegrity of the structure, adequate transverse rein-forcement is to be provided.

3/5.9.3 Reinforcement Details

Generally, lapped joints should be avoided in struc-tural members subjected to significant fatigue load-ing. If lapped splices are used in members subject tofatigue, the development length of reinforcing bars isto be twice that required by ACI 318, and lappedbars are to be tied with tie wire. Reinforcing steel isto comply with the chemical composition specifica-tions of ACI 359 if welded splices are used.

For anchorage of shear reinforcement as well asfor anchorage of main reinforcement, mechanically-headed bars (T-headed bars) may be used if their ef-fectiveness has been verified by static and dynamictesting.

3/5.9.4 Post Tensioning Ducts

Ducting for post-tensioning ducts may be rigid steelor plastic, (polyethylene or polystyrene). Steel tubingshall have a minimum wall thickness of 1 mm. Plas-

tic tubing shall have a minimum wall thickness of 2mm. Ducts may also be semi-rigid steel, spirallywrapped, of minimum thickness of 0.75 mm, andshall be grout-tight. All splices in steel tubes andsemi-rigid duct shall be sleeved and the joints sealedwith heat-shrink tape. Joints in plastic duct shall befused or sleeved and sealed.

The inside diameter of ducts shall be at least 6mm (0.25 in.) larger than the diameter of the post-tensioning tendon in order to facilitate grout injection.

3/5.9.5 Post-Tensioning Anchorages and

Couplers

Anchorages for unbonded tendons and couplers areto develop the specified ultimate capacity of the ten-dons without exceeding anticipated set. Anchoragesfor bonded tendons are to develop at least 90% of thespecified ultimate capacity of the tendons, whentested in an unbonded condition without exceedinganticipated set. However, 100% of the specified ul-timate capacity of the tendons is to be developed af-ter the tendons are bonded in the member.

Anchorage and end fittings are to be perma-nently protected against corrosion. Post-tensioninganchorages shall preferably be recessed in a pocketwhich is then filled with concrete. The fill should bemechanically-tied to the structure by reinforcementsas well as bonded by epoxy or polymer.

Anchor fittings for unbonded tendons are to becapable of transferring to the concrete a load equal tothe capacity of the tendon under both static and cy-clic loading conditions.

3/5.11 Construction

3/5.11.1 General

Construction methods and workmanship are to fol-low accepted practices as described in ACI 318, ACI357, API RP 2A, and the specifications referred toby these codes. Additional requirements relevant toconcrete offshore structures are included below.

3/5.11.2 Mixing, Placing, and Curing ofConcrete

a Mixing Mixing of concrete is to conformwith the requirements of ACI 318 and ASTM C94.

b Cold Weather In cold weather, concreting inair temperatures below 2°C (35°F) should be carriedout only if special precautions are taken to protectthe fresh concrete from damage by frost. The tem-perature of the concrete at the time of placing is to beat least 4°C (40°F) and the concrete is to be main-tained at this or a higher temperature until it hasreached a strength of at least 5 MPa (700 psi).

Protection and insulation are to be provided tothe concrete where necessary. The aggregates and



water used in the mix are to be free from snow, iceand frost. The temperature of the fresh concrete maybe raised by heating the mixing water or the aggre-gates or both. Cement should never be heated norshould it be allowed to come into contact with waterat a temperature greater than 60°C (140°F).

c Hot Weather During hot weather, proper at-tention is to be given to ingredients, productionmethods, handling, placing, protection and curing toprevent excessive concrete temperatures or waterevaporation which will impair the required strengthor serviceability of the member or structure. Thetemperature of concrete as placed is not to exceed30°C (90°F) and the maximum temperature due toheat of hydration is not to exceed 65°C (145°F).

d Curing Special attention is to be paid to thecuring of concrete in order to ensure maximum dura-bility and to minimize cracking. Concrete should becured with fresh water, whenever possible, to ensurethat the concrete surface is kept wet during harden-ing. Care should be taken to avoid the rapid loweringof concrete temperatures (thermal shock) caused byapplying cold water to hot concrete surfaces.

e Sea Water Sea water is not to be used forcuring reinforced or prestressed concrete, although,if demanded by the construction program, “young”concrete may be submerged in sea water provided ithas gained sufficient strength to withstand physicaldamage. When there is doubt about the ability tokeep concrete surfaces permanently wet for thewhole of the curing period, a heavy duty membranecuring compound should be used.

f Temperature Rise The rise of temperature inthe concrete, caused by the heat of hydration of thecement, is to be controlled to prevent steep tem-perature stress gradients which could cause crackingof the concrete. Since the heat of hydration maycause significant expansion, members must be free tocontract, so as not to induce excessive cracking. Ingeneral, when sections thicker than 610 mm (2 ft) areconcreted, the temperature gradients between inter-nal concrete and external ambient conditions are tobe kept below 20°C (68°F).

g Joints Construction joints are to be madeand located in such a way as not to impair thestrength and crack resistance of the structure. Wherea joint is to be made, the surface of the concrete is tobe thoroughly cleaned and all laitance and standingwater removed. Vertical joints are to be thoroughlywetted and coated with neat cement grout or equiva-lent enriched cement paste or epoxy coating immedi-ately before placing of new concrete.

h Watertight Joints Whenever watertight con-struction joints are required, in addition to the aboveprovisions, the heavy aggregate of the existing con-crete is to be exposed and an epoxide-resin bondingcompound is to be sprayed on just before concreting.In this case, the neat cement grout can be omitted.

3/5.11.3 Reinforcement

The reinforcement is to be free from loose rust,grease, oil, deposits of salt or any other materiallikely to affect the durability or bond of the rein-forcement. The specified cover to the reinforcementis to be maintained accurately. Special care is to betaken to correctly position and rigidly hold the rein-forcement so as to prevent displacement during con-creting.

3/5.11.4 Prestressing Tendons, Ductsand Grouting

a General Further guidance on prestressingsteels, sheathing, grouts and procedures to be usedwhen storing, making up, positioning, tensioning andgrouting tendons will be found in the relevant sec-tions of ACI 318, Prestressed Concrete Institute(PCI) publications, Federation Internationale de laPrecontrainte (FIP) Recommended Practices, and thespecialist literature.

b Cleanliness All steel for prestressing ten-dons is to be clean and free from grease, insolubleoil, deposits of salt or any other material likely to af-fect the durability or bond of the tendons.

c Storage During storage, prestressing tendonsare to be kept clear of the ground and protected fromweather, moisture from the ground, sea spray andmist. No welding, flame cutting or similar operationsare to be carried out on or adjacent to prestressingtendons under any circumstances where the tem-perature of the tendons could be raised or weldsplash could reach them.

d Protective Coatings Where protective wrap-pings or coatings are used on prestressing tendons,they are to be chemically neutral so as not to producechemical or electrochemical corrosive attack on thetendons.

e Entry of Water All ducts are to be watertightand all splices carefully taped to prevent the ingressof water, grout or concrete. During construction, theends of ducts are to be capped and sealed to preventthe entry of sea water. Ducts may be protected fromexcessive rust by the use of chemically neutral pro-tective agents such as vapor phase inhibitor powder.

f Grouting Where ducts are to be grouted, alloil or similar material used for internal protection ofthe sheathing is to be removed before grouting.However, water-soluble oil used internally in theducts or on the tendons may be left on, to be re-moved by the initial portion of the grout.

g Air Vents Air vents are to be provided at allcrests in the duct profile. Threaded grout entries,which permit the use of a screwed connector fromthe grout pump, may be used with advantage.

h Procedures For long vertical tendons, thegrout mixes, admixtures and grouting procedures are



to be checked to ensure that no water is trapped atthe upper end of the tendon due to excessive bleed-ing or other causes. Suitable admixtures known tohave no injurious effects on the metal or concretemay be used for grouting to increase workability andto reduce bleeding and shrinkage. Temperature of

members must be maintained above 10°C (50°F) forat least 48 hours after grouting. General guidance ongrouting will be found in the specialist literature.Holes left by unused ducts or by climbing rods ofslipforms are to be grouted in the same manner asdescribed above.

FIGURE 3/5.1 Stress-Strain Relation for Concrete in Uniaxial Compression



FIGURE 3/5.2 Stress-Strain Relation for Non-Prestressed Steel in Uniaxial Tension



.

Foundations Section 6


Part 3Section 6 Foundations

3/6.1 General

Soil investigations, design considerations for thesupporting soil and the influence of the soil on thefoundation structure are covered in this section. Thedegree of design conservatism should reflect priorexperience under similar conditions, the mannerand extent of data collection, the scatter of designdata, and the consequences of failure. For caseswhere the limits of applicability of any method ofcalculation employed are not well defined, or wherethe soil characteristics are quite variable, more thanone method of calculation or a parametric study ofthe sensitivity of the relevant design data is to beused.

3/6.3 Site Investigation

3/6.3.1 General

The actual extent, depth and degree of precision ap-plied to the site investigation program are to reflectthe type, size and intended use of the structure, fa-miliarity with the area based on previous site studiesor platform installations, and the consequenceswhich may arise from a failure of the foundation. Formajor structures, the site investigation program is toconsist of the following three phases.

Sea Floor Survey (see 3/6.3.2) to obtain relevantgeophysical data

Geological Survey (see 3/6.3.3) to obtain data of aregional nature concerning the site

Subsurface Investigation and Testing (see 3/6.3.4) toobtain the necessary geotechnical data

The results of these investigations are to be the basesfor the additional site related studies which are listedin 3/6.3.5.

A complete site investigation program is to beaccomplished for each offshore structure. However,use of the complete or partial results of a previouslycompleted site investigation as the design basis foranother similarly designed and adjacent offshorestructure is permitted when the adequacy of the pre-vious site’s investigation for the new location is sat-isfactorily demonstrated.

When deciding the area to be investigated, dueallowance is to be given to the accuracy of position-ing devices used on the vessel employed in the siteinvestigation to ensure that the data obtained arepertinent to the actual location of the structure.

3/6.3.2 Sea Floor Survey

Geophysical data for the conditions existing at andnear the surface of the sea floor are to be obtained.The following information is to be obtained whereapplicable to the planned structure.

Soundings or contours of the sea bedPosition of bottom shapes which might affect scourThe presence of boulders, obstructions, and small

cratersGas seepsShallow faultsSlump blocksIce scour of sea floor sedimentsSubsea permafrost or ice bonded soils

3/6.3.3 Geological Survey

Data of the regional geological characteristics whichcan affect the design and siting of the structure. Suchdata are to be considered in planning the subsurfaceinvestigation, and they are also to be used to assurethat the findings of the subsurface investigation areconsistent with known geological conditions.

Where necessary, an assessment of the seismicactivity at the site is to be made. Particular emphasisis to be placed on the identification of fault zones,the extent and geometry of faulting and attenuationeffects due to conditions in the region of the site.

For structures located in a producing area, thepossibility of sea floor subsidence due to a drop inreservoir pressure is to be considered.

3/6.3.4 Subsurface Investigation and Testing

The subsurface investigation and testing program isto obtain reliable geotechnical data concerning thestratigraphy and engineering properties of the soil.These data are to be used to assess whether the de-sired level of structural safety and performance canbe obtained and to assess the feasibility of the pro-posed method of installation.

Consistent with the stated objective, the soiltesting program is to consist of an adequate numberof in-situ tests, borings and samplings to examine allimportant soil and rock strata. The testing program isto reveal the necessary strength, classification anddeformation properties of the soil. Further tests are tobe performed as needed, to describe the dynamiccharacteristics of the soil and the static and cyclicsoil-structure interaction.



For pile-supported structures, the minimumdepth of at least one bore hole, for either individualor clustered piles, is to be the anticipated length ofpiles plus a zone of influence. The zone of influenceis to be at least 15.2 m (50 ft) or 1.5 times the di-ameter of the cluster, whichever is greater, unless itcan be shown by analytical methods that a lesserdepth is justified. Additional bore holes of lesserdepth are required if discontinuities in the soil arelikely to exist within the area of the structure.

For a gravity-type foundation, the required depthof at least one boring is to be at least equal to thelargest horizontal dimension of the base. In-situ testsare to be carried out, where possible, to a depth thatwill include the anticipated shearing failure zone.

A reasonably continuous profile is to be ob-tained during recovery of the boring samples. Thedesired extent of sample recovery and field testing isto be as follows.

The recovery of the materials to a depth of 12 m(40 ft) below the mudline is to be as complete aspossible. Thereafter, samples at significantchanges in strata are to be obtained, at approxi-mately 3 m (10 ft) intervals to 61 m (200 ft) andapproximately 8 m (25 ft) intervals below 61 m(200 ft).

At least one undrained strength test (vane, drop cone,unconfined compression, etc.) on selected re-covered cohesive samples is to be performed inthe field.

Where practicable, a standard penetration test orequivalent on each significant sand stratum is tobe performed, recovering samples where possi-ble.

Field samples for laboratory work are to be retainedand carefully packaged to minimize changes inmoisture content and disturbance.

Samples from the field are to be sent to a recog-nized laboratory for further testing. They are to beaccurately labeled and the results of visual inspectionrecorded. The testing in the laboratory is to includeat least the following.

Perform unconfined compression tests on clay stratawhere needed to supplement field data

Determine water content and Atterberg limits on se-lected cohesive samples

Determine density of selected samplesAs necessary, develop appropriate constitutive pa-

rameters or stress-strain relationships from eitherunconfined compression tests, unconsolidatedundrained triaxial compression tests, or consoli-dated undrained triaxial compression tests

Perform grain size sieve analysis, complete with per-centage passing 200 sieve, on each significantsand or silt stratum

For pile-supported structures, consideration is tobe given to the need for additional tests to adequatelydescribe the dynamic characteristics of the soil andthe static and cyclic lateral soil-pile

For gravity structures, laboratory tests are alsoto include, where necessary, the following.

Shear strength tests with pore pressure measure-ments. The shear strength parameters and pore-water pressures are to be measured for the rele-vant stress conditions

Cyclic loading tests with deformation and pore pres-sure measurements to determine the soil behav-ior during alternating stress

Permeability and consolidation tests performed asrequired

3/6.3.5 Documentation

The foundation design documentation mentioned inSection 4 of Part 1 is to be submitted for review. Asapplicable, the results of studies to assess the fol-lowing effects are also to be submitted.

Scouring potential of the sea floorHydraulic instability and the occurrence of sand

wavesInstability of slopes in the area where the structure is

to be placedLiquefaction and other soil instabilitiesFor Arctic areas, possible degradation of subsea

permafrost layers as a result of the production ofhot oil

Soils conditions in the vicinity of footprints left bytemporarily situated drilling units or other serv-ice units

Effects of volcanic sands, organic matter, carbonatesoil, calcareous sands and other substanceswhich degrade the strength of the soil founda-tion

In these studies, the structure is to be considered pre-sent.

3/6.5 Foundation Design Requirements

3/6.5.1 General

The loadings used in the analysis of the safety of thefoundation are to include those defined in 3/6.5.7 andthose experienced by the foundation during installa-tion. Foundation displacements are to be evaluated tothe extent necessary to assure that they are withinlimits which do not impair the intended function andsafety of the structure.

The soil and the structure are to be considered asan interactive system, and the results of analyses, asrequired in subsequent paragraphs, are to be evalu-ated from this point of view.



3/6.5.2 Cyclic Loading Effects

The influence of cyclic loading on soil properties isto be considered. For gravity structures in particular,possible reduction of soil strength is to be investi-gated and employed in design. In particular the fol-lowing conditions are to be considered.

Design storm during the initial consolidation phaseShort-term effects of the design stormLong-term cumulative effects of several storms, in-

cluding the design storm

Reduced soil strength characteristics resultingfrom these conditions are to be employed in design.

In seismically active zones, similar deterioratingeffects due to repeated loadings are to be considered.

Other possible cyclic load effects, such aschanges in load-deflection characteristics, liquefac-tion potential and slope stability are also to be con-sidered, and these effects should be accounted forwhen they will affect the design.

3/6.5.3 Scour

Where scour is expected to occur, either effectiveprotection is to be furnished soon after the installa-tion of the structure, or the depth and lateral extent ofscouring, as evaluated in the site investigation pro-gram, is to be accounted for in design.

3/6.5.4 Deflections and Rotations

Tolerable limits of deflections and rotations are to beestablished based on the type and function of theplatform, and the effects of those movements on ris-ers, piles and other structures which interact with theplatform. Maximum allowable values of platformmovements, as limited by these structural considera-tions or overall platform stability, are to be consid-ered in the design.

3/6.5.5 Soil Strength

The ultimate strength or stability of soil is to be de-termined using test results which are compatible withthe method selected. In a total stress approach thetotal shear strength of the soil obtained from simpletests is used. A total stress approach largely ignoreschanges in the soil’s pore water pressure undervarying loads and the drainage conditions at the site.When an effective stress approach is used effectivesoil strength parameters and pore water pressures aredetermined from tests which attempt to predict in-situ total stresses and pore pressures.

3/6.5.6 Dynamic and Impact Considerations

For dynamic and impact loading conditions, a real-istic and compatible treatment is to be given to the

interactive effects between the soil and structure.When analysis is required it may be accomplishedby lumped parameter, foundation impedance func-tions, or by continuum approaches including theuse of finite element methods. Such models are toinclude consideration of the internal and radiationaldamping provided by the soil and the effects of soillayering.

Studies of the dynamic response of the structureare to include, where applicable, consideration of thenonlinear and inelastic characteristics of the soil, thepossibilities of deteriorating strength and increasedor decreased damping due to cyclic soil loading, andthe added mass of soil subject to acceleration. Whereapplicable, the influence of nearby structures is to beincluded in the analysis.

3/6.5.7 Loading Conditions

Those loadings which produce the worst effects onthe foundation during and after installation are to betaken into account. Post installation loadings to bechecked are to include at least those relating to boththe operating and design environmental conditions,combined in the following manner.

a Operating environmental loading combined withdead and maximum live loads appropriate to thefunction and operations of the structure

b Design environmental loading combined withdead and live loads appropriate to the functionand operations of the structure during the designenvironmental condition

c Design environmental loading combined withdead load and minimum live loads appropriate tothe function and operations of the structure duringthe design environmental conditions

For areas with potential seismic activity, thefoundation is to be designed for sufficient strength tosustain seismic loads.

3/6.5.8 Anchoring System

Where the anchoring utilizes piles, the requirementsin these Rules applicable to piles are to be used. Theloads at the mooring line attachments are to be cal-culated and the pile’s local strength is to be checked.Where the anchoring utilizes gravity anchors, the re-quirements in these Rules applicable to gravity basedstructures are to be used.

Where platforms such as guyed towers andcompliant towers are permanently and partially sup-ported by a mooring system, the analysis of the plat-form’s foundation is to include the interactive effectsof the mooring system.

Other types of anchoring will be specially con-sidered.



3/6.5.9 Loads and Soil Conditions Due to

Temporarily Situated Structures

Changes in soil conditions due to temporarily situ-ated platforms such as self-elevating drilling units,workover rigs or tender rigs placed near the structureare to be assessed and investigated. These changesand their influence on the structure are to be incorpo-rated in the foundation design to ensure that struc-ture’s function and safety are not impaired.

3/6.7 Pile Foundations

3/6.7.1 General

The effects of axial, bending and lateral loads are tobe accounted for in the design of individual andgroup piles. The design of a pile is to reflect the in-teractive behavior between the soil and the pile andbetween the pile and the structure.

Methods of pile installation are to be consistentwith the type of soil at the site, and with the installa-tion equipment available. Pile driving is to be carriedout and supervised by qualified and experienced per-sonnel, and driving records are to be obtained andsubmitted for review.

Should unexpectedly high or low driving resis-tance or other conditions be encountered which leadto a failure of the pile to reach its desired penetra-tion, a reevaluation of the pile’s capacity is to be car-ried out considering the parameters resulting fromthe actual installation.

Where necessary, the effects of bottom instabil-ity in the vicinity of the structure are to be assessed.

3/6.7.2 Axial Piles

For piles in compression, the axial capacity is to beconsidered to consist of the skin friction, Qf devel-oped along the length of the pile, and the end bear-ing, Qp at the tip of the pile. The axial capacity of apile subjected to tension is to be equal to or less thanthe skin friction alone. Predictions of the various pa-rameters needed to evaluate Qf and Qp are to be ac-complished using a recognized analytical method,such as that found in the API RP 2A, or anothermethod shown to be more appropriate to the condi-tions at the site. When required, the acceptability ofany method used to predict the components of pileresistance is to be demonstrated by showing satis-factory performance of the method under conditionssimilar to those existing at the actual site. The resultsof dynamic pile driving analysis alone are not to beused to predict the axial load capacity of a pile.

3/6.7.3 Factors of Safety for Axial Piles

When the pile is subjected to the three loading casesdescribed in 3/6.5.7 and the ultimate capacities are

evaluated using the above cited API method, the al-lowable values of axial pile bearing and pullout loadsare to able values of axial pile bearing and pulloutloads are to be determined by dividing the ultimatecapacities obtained above by a factor of safety tabu-lated below.

Loading Condition Factor of Safety3/6.5.7a 2.0

3/6.5.7b, 3/6.5.7c 1.5

For the Design Earthquake, the factor of safety willbe specially considered.

3/6.7.4 Laterally Loaded Piles

In the evaluation of the pile’s behavior under lateralloadings, the combined-load-deflection characteris-tics of the soil and pile, and the pile and the structureare to be taken into account. The representation ofthe soil’s lateral deflection when it is subjected tolateral loads is to adequately reflect the deteriorationof the lateral bearing capacity when the soil is sub-jected to cyclic loading.

The description of the lateral load versus deflec-tion characteristics for the various soil strata is to bebased on constitutive data obtained from suitable soiltests. Reference is to be made to the API RP 2A for aprocedure to evaluate the load-deflection character-istics of laterally loaded piles. However, the use ofalternative methods is permitted when they are moreappropriate for conditions at the site.

Where applicable, the rapidly deteriorating cy-clic bearing capacity of stiff clays, especially thoseexhibiting the presence of a secondary structure, is tobe accounted for in the design.

3/6.7.5 Anchor Piles

When lateral loads are directly applied to a pile suchas in the case when it is used to anchor a mooringline suitable load factors greater than 1.0 are to beused to increase the magnitudes of the lateral load ef-fects resulting from the load conditions of 3/6.5.7.Calculation of the soil capacity and the pile stressesis to be based on consideration of the modified loads.

3/6.7.6 Pile Groups

Where applicable, the effects of close spacing on theload and deflection characteristics of pile groups areto be determined. The allowable load for a group,both axial and lateral, is not to exceed the sum of theapparent individual pile allowable loads reduced by asuitable factor.

3/6.7.7 Connections Between Piles and Structure

The loads acting on the platform may be transferredto the piles by connecting the jacket legs or pile



sleeves to the piles by welding, grouting the annulusbetween the jacket leg or pile sleeve and the pile, orby use of mechanical devices such as pile grippers.

The design of the grouted pile to structure con-nection should consider the use of mechanical shearconnectors as their presence increase the strength ofthe connection, and eliminates any effect of long termgrouting shrinkage. Adequate clearance between thepile and the jacket leg should be provided for properplacement of the grout. Reliable means for the intro-duction of the grout to the annulus should be providedto ensure complete filling of the annulus and to mini-mize the possibility of dilution of the grout and theformation of voids in the grout. Wipers or similar de-vices should be used to minimize intrusion of mudinto the annulus during installation. For the design ofthe grouted connections, reference is to be made toAPI RP2A or other appropriate references.

If mechanical devices are used their strength andfatigue characteristics are to be adequately demon-strated by analysis, testing or experience.

3/6.9 Gravity Structures

3/6.9.1 General

The stability of the foundation with regard to bearingand sliding failure modes is to be investigated em-ploying the soil shear strengths determined in accor-dance with 3/6.3.4 and 3/6.5.2. The effects of adja-cent structures and the variation of soil properties inthe horizontal direction are to be considered whererelevant.

Where leveling of the site is not carried out, thepredicted tilt of the overall structure is to be based onthe average bottom slope of the sea floor and the tol-erance of the elevation measuring device used in thesite investigation program. Differential settlement isalso to be calculated and the tilting of the structurecaused by this settlement is to be combined with thepredicted structural tilt. Any increased loading ef-fects caused by the tilting of the structure are to beconsidered in the foundation stability requirementsof 3/6.9.2.

When an underpressure or overpressure is experi-enced by the sea floor under the structure, provision isto be made to prevent piping which could impair theintegrity of the foundation. The influence of hydraulicand slope instability, if any, is to be determined for thestructural loading cases b and c of 3/6.5.7.

Initial consolidation and secondary settlements,as well as permanent horizontal displacements, are tobe calculated.

3/6.9.2 Stability

The bearing capacity and lateral resistance are to becalculated under the most unfavorable combination

of loads. Possible long-term redistribution of bearingpressures under the base slab are to be considered inorder to ensure that the maximum edge pressures areused in the design of the perimeter of the base.

The lateral resistance of the platform is to be in-vestigated with respect to various potential shearingplanes. Special consideration is to be given to anylayers of soft soil.

Calculations for overturning moment and verti-cal forces induced by the passage of a wave are toinclude the vertical pressure distribution across thetop of the foundation and along the sea floor.

The capacity of the foundation to resist a deep-seated bearing failure is to be analyzed. In lieu of amore rigorous analysis, where uniform soil condi-tions are present or where conservatively chosen soilproperties are used to approximate a non-uniformsoil condition, and where a trapezoidal distributionof soil pressure is a reasonable expectation, the ca-pacity of the foundation to resist a deep-seated bear-ing failure can be calculated by standard bearing ca-pacity formulas applicable to eccentrically loadedshallow foundations. Alternatively, slip-surfacemethods, covering a range of kinematically possibledeep rupture surfaces can be employed in the bearingcapacity calculations.

The maximum allowable shear strength of thesoil is to be determined by dividing the ultimateshear strength of the soil by the minimum safetyfactors given below.

When the ultimate soil strength is determined byan effective stress method, the safety factor is to beapplied to both the cohesive and frictional terms. If atotal stress method is used, the safety factor is to beapplied to the undrained shear strength. The mini-mum safety factors to be obtained, when employinga standard bearing capacity formulation and varioustrial sliding failure planes with the loading condi-tions of 3/6.5.7, are 2.0 for loading case a, and 1.5for loading cases b and c. The safety factors to beobtained when considering the Design Earthquakewill be specially considered.

Where present, the additional effects of pene-trating walls or skirts which transfer vertical and lat-eral loads to the soil are to be investigated as to theircontribution to bearing capacity and lateral resis-tance.

3/6.9.3 Soil Reaction on the Structure

For conditions during and after installation, the reac-tion of the soil against all structural members seatedon or penetrating into the sea floor is to be deter-mined and accounted for in the design of thesemembers. The distribution of soil reactions is to bebased on the results obtained in 3/6.3.4. Calculationsof soil reactions are to account for any deviationfrom a plane surface, the load-deflection characteris-



tics of the soil and the geometry of the base of thestructure.

Where applicable, effects of local soil stiffening,nonhomogeneous soil properties, as well as the pres-ence of boulders and other obstructions, are to be ac-counted for in design. During installation, considera-tion is to be given to the possibility of local contactpressures due to irregular contact between the baseand the sea floor; these pressures are additive to thehydrostatic pressure.

An analysis of the penetration resistance ofstructural elements projecting into the sea floor be-low the foundation structure is to be performed. Thedesign of the ballasting system is to reflect uncer-tainties associated with achieving the required pene-tration of the structure. Since the achievement of therequired penetration of the platform and its skirts isof critical importance, the highest expected values ofsoil strength are to be used in the calculation ofpenetration.

Marine Operations Section 7


Part 3Section 7 Marine Operations

3/7.1 General

The effects which may be induced in the structureduring the marine operations required for the trans-portation and installation of the structure and equip-ment are to be accounted for. The emphasis of thissection is on the influence which these operations mayhave on the safety and integrity of the structure. How-ever, the adequacy of the tie-down and barge strengthshould also be evaluated. In these Rules, marine op-erations will generally include the following activitiesas appropriate to the planned installation.

Lifting and mooring operationsLoad-outConstruction afloatTowingLaunching and uprightingSubmergenceMatingPile installationFinal field erectionRemoval operations

For all marine operations except towing, the Sur-veyor is to be satisfied that skilled supervision isbeing provided and that the operations are being exe-cuted satisfactorily. During a tow, the towage masteris to assure that proper procedures are followed. TheOperator may also optionally request that a Surveyorbe present during a tow.

3/7.3 Documentation

The extent of documentation and analysis (3/7.5) ofmarine operations is to be commensurate with thesize and type of structure involved, the particular op-eration being considered, the extent of past experi-ence with similar operations, and the severity of theexpected environmental conditions.

A report on the marine operations planned totransport and install the structure is to be developedand submitted for use in association with the reviewof the analyses required in 3/7.5. For structures re-quiring a significant amount of construction whileafloat (e.g., large concrete gravity structures), docu-mentation of the operations involved is to be in-cluded in this report. The purpose of this report is todemonstrate that the strength and integrity of thestructure are not reduced or otherwise jeopardized bythe marine operations.

Generally, this report is to contain the followinginformation.

Description of the marine operations to be performedand the procedures to be employed

For operations which do not govern design of thestructure, a description of the engineering logic,experience or preliminary calculations support-ing this conclusion

For operations which govern design of the structure,the assumptions, calculations and results of theanalyses required in 3/7.5

For structures to be uprighted or submerged by se-lective ballasting, a detailed description of themechanical, electrical and control systems to beemployed, the ballasting schedule and support-ing calculations

3/7.5 Analysis

3/7.5.1 Loads

Analyses are to be performed to determine the typeand magnitude of the loads and load combinations towhich the structure will be exposed during the per-formance of marine operations. Particular attention isto be given to inertial, impact, and local loads whichare likely to occur during marine operations. Wheresignificant fatigue damage occurs during marine op-erations, it shall be included in calculating the totalfatigue lives.

3/7.5.2 Stress

Where temporary attachments or appurtenances (tie-downs, skid beams, etc.) are utilized, analyses are tobe performed to ensure that these items and theirsupporting structure have sufficient strength to with-stand the type and magnitude of loads with the ap-propriate factor of safety. The strength criteria ofSections 4 and 5 of Part 3 for steel structures andconcrete structures, respectively, are to be employedin this determination.

3/7.5.3 Stability

Analyses are to be performed to ensure that thestructure, or its means of support where such exist,has sufficient hydrostatic stability and reserve buoy-ancy to allow for successful execution of all phasesof marine operations.

Marine Operations Section 7


For large or unusual structures, an experimentaldetermination of the center of gravity of the structureand its means of support, where such exist, is to beperformed.

3/7.7 Fitness to Tow Certificate

Upon request by the Operator and where authorizedto do so, the Bureau will undertake the services re-quired for the issuance of a fitness to tow certificate.

The adequacy of the towlines, attachments and tow-ing vessels will not be reviewed by the Bureau. Re-view by the Bureau solely for the purposes of classi-fication is not to be considered a replacement for thereview commonly required for the issuance of a tow-age certificate.

Extension of Use Section 1


Part 4 Extension of Use and Reuse



.



Part 4Section 1 Extension of Use

4/1.1 General

This section pertains to the classification or continu-ance of classification of an existing platform for ex-tension of service beyond the design life. The classi-fication requires special considerations with respectto the review, surveys and structural analyses in or-der to verify the adequacy of the platform for its in-tended services. In general, the reviews and surveysdescribed in 4/1.2 should be followed, However, ifthe platform is currently classed with ABS, some ofthese reviews and surveys may not be necessary de-pending on the condition of the platform.

4/1.2 Extension of Use

The approach for the classification of an existingplatform for extended service is as follows:

a Review original design documentation, plans,structural modification records and survey re-ports.

b Survey structure to establish condition of plat-form.

c Review the results of the structural analysis util-izing results of survey, original plans, specialistgeotechnical and oceanographic reports and pro-posed modifications which affect the dead, live,environmental and earthquake loads, if applica-ble, on the structures.

d Resurvey platform utilizing results from structuralanalysis. Make any alterations necessary for ex-tending the service of the structure.

e Review a program of continuing surveys to assurethe continued adequacy of the platform.

Items a and b are to assess the platform to determinethe possibility of continued use. If the conclusion isfavorable from this assessment, structural analysesshould be carried out.

The in-place analysis is to follow 3/4.7 and is tobe in accordance with 4/1.2.3. The results of the in-place analysis can be utilized to identify the areasmost critical for inspection at the resurvey.

The fatigue life can be calculated by means ofan analysis as described in 3/4.13. This analysis issensitive to the waves encountered during the pastservice and future prediction, therefore the long termenvironmental data are to be properly represented.The remaining fatigue lives of all the structuralmembers and joints are not to be less than twice theextended service life. Should any area found to be

deficient, these areas shall be required to bestrengthened to achieve the required fatigue life ormonitoring programs developed to monitor these ar-eas with nondestructive testing on a periodical basisor grinding the weld toe to improve fatigue life forjoints with full penetration welds.

The fatigue analysis may not be needed pro-vided all of the following conditions are satisfied.

a The original fatigue analysis indicates that the fa-tigue lives of all joints are sufficient to cover theextension of use.

b The fatigue environmental data used in the origi-nal fatigue analysis remain valid or deemed to bemore conservative.

c Cracks are not found during the condition surveyor damaged joints and members are being re-paired.

d Marine growth and corrosion is found to bewithin the allowable design limits.

Surveys on a periodic basis based on 1/3 should beundertaken to ascertain the satisfactory condition ofthe platform. Additional surveys may be required forplatforms which have unique features.

4/1.2.1 Review of Platform Design Documents

Platform design information is to be collected to al-low an engineering assessment of a platform’s over-all structural integrity. It is essential to have theoriginal design reports, documents, original and as-isplans, specifications, survey records during fabrica-tion, installation and past service. The operatorshould ensure that any assumptions made are reason-able and information gathered is both accurate andrepresentative of actual conditions at the time of theassessment. If the information can not be provided,an assumption of lower design criteria, actual meas-urements or testings should be carried out to estab-lish a reasonable and conservative assumption.

4/1.2.2 Survey of Platforms

Surveying an existing platform witnessed andmonitored by a Bureau Surveyor is necessary to de-termine a base condition upon which justification ofcontinued service can be made. Reports of previoussurveys and maintenance will be reviewed, an in-spection procedure developed, and a complete un-derwater inspection required to assure that an accu-rate assessment of the platform’s condition is



obtained. Survey requirements are outlined in1/3.1.10. The surveyors will witness and monitor allsurvey activities for the platform.

The corrosion protection system is to be re-evaluated to ensure that existing anodes are capableof serving the extended design life of the platform. Iffound necessary by the re-evaluation, replacement ofthe existing anodes or additional new anodes mayhave to be carried out. If the increase in hydrody-namic loads due to the addition of new anodes is sig-nificant, this additional load should be taken into ac-count in the structural analysis. Condition ofprotective coatings in the splash zone shall be recti-fied and placed in satisfactory condition.

4/1.2.3 Structural Analyses

The structural analyses of an existing structure mustincorporate the results of the platform survey. Spe-cifically, deck loads, wastage, marine growth, scour,and any platform modifications and damages mustbe incorporated into the analysis model. The originalfabrication materials and fit-up details must be es-tablished such that proper material characteristics areused in the analysis and any stress concentrations areaccounted for. The pile driving records should bemade available so that the foundation can be accu-rately modelled. For areas where the design is con-trolled by earthquake or ice conditions, the analysesfor such conditions should also be carried out.

The results of the analyses are considered to bean indicator of areas needing careful inspection. Pos-

sible alterations of platforms to allow continued useare developed by altering the analysis model toevaluate the effect of the alterations. Members andjoints indicated overstressed or low in fatigue lifemay be improved by reducing deck load and remov-ing unused structures such as conductors, conductorguides framing, and boat landing. The results ofthese load reduction on the structure should beevaluated to determine whether the repairs/alterations is needed.

An analysis based on an ultimate strengthmethod is also acceptable if the method and safetyfactors used are proven to be appropriate.

4/1.2.4 Implementing Repairs/Reinspection

The initial condition survey in conjunction withstructural analysis will form the basis for determin-ing the extent of repairs/alterations which will benecessary to class the platform for continued opera-tion.

A second survey may be necessary to inspect ar-eas where the analysis results indicate as being themore highly stressed regions of the structure. Mem-bers and Joints found overstressed should bestrengthened. Joints with low fatigue lives may beimproved either by strengthening or grinding thewelds. If grinding is used, the details of the grindingare to be submitted to ABS for review and approval.Interval of future periodic surveys should be deter-mined based on the remaining fatigue lives of thesejoints.

Reuse Section 2


Part 4Section 2 Reuse

4/2.1 General

The classification of a platform to be reused requiresspecial considerations. with respect to the review,surveys, structural analyses, and the removal andreinstallation operation. In general, the design re-quirements stated in these Rules should be followedparticularly, the requirements described in Section4/1, whenever applicable and survey requirementsgiven in 1/3.1.11 for platform reuse.

Since the platform is to be reused at a new site,the environmental and geotechnical data used in theanalysis should be in accordance with those for thenew site. Any alteration to the structure to enable theremoval operation to be successfully performed andto suit the new site shall be incorporated in theanalysis.

The platform reuse involves the platform re-moval and reinstallation process which requires spe-cial plans in order to achieve its intended services.

4/2.2 Removal and ReinstallationOperation

Removal of the sub-structure and superstructure mayentail the reversing of the initial installation se-quence. Cutting piles, re-floatation and lifting as en-visaged in the platform removal procedure shall bewell planned and analyzed to verify that the integrityof the structure has not been compromised. Platformremoval plans, procedures, seafastening drawings,transportation, together with the analysis calculationsshould be submitted to the Bureau for review. Ingeneral, Section 3/7 should be followed for the rein-stallation of used platforms.

Reuse Section 2


.

Material Selection Appendix A


Appendices



.



Appendix A Material Selection

A.1 General

For the classification of offshore fixed structures it is necessary to take into account minimum expected servicetemperature, the structural element category and material thickness when selecting structural materials. Thevarious parts of the structure are to be grouped according to their material application categories. The structuralelements falling into these categories are described, in general, in A.3.

A.3 Classification of Applications

The application of structural members in an offshore fixed structure is to be in accordance with the categorieslisted in this paragraph.

Special application refers to highly stressed materials, located at intersections of main structural elementsand other areas of high stress concentration where the occurrence of a fracture could induce a major structuralfailure.

Primary application refers to primary load carrying members of a structure where the occurrence of a frac-ture could induce a major structural failure.

Secondary application refers to less critical members due to a combination of lower stress and favorable ge-ometry or where an incidence of fracture is not likely to induce a major structural failure.

A.3.1 Examples of Applications

The following are typical examples of application categories.

a Secondary Application Structure (Least Critical)Internal structure including bulkheads and girders in legs or columnsDeck plating not essential for overall structural integrityLow-stressed deck beams in parallel and bracing, except where structure is considered primary or special

applicationPlating of certain columns with low slenderness ratios, except at intersections

b Primary Application Structure (Intermediate)Plating of lattice legsExternal shell plating of caissonsDeck plating and structure which is not considered special or secondaryMain braces, jacket legs and deck legs, except where considered specialHeavy flanges and deep webs of major load supporting members, which form box or I type supporting

structure, and which do not receive major concentrated loadsMembers which provide local reinforcement or continuity of structure in way of intersections, including

main deck load plating where the structure is considered special applicationc Special Application Structure (Most Critical)

External shell or deck structure in way of intersections of vertical columnsPortions of deck plating, heavy flanges, and deep webs at major load supporting members within the deck,

which form box or 1 type supporting structure, and which receive major concentrated loadsIntersection of major bracing members and critical joint nodesMembers which receive immediate concentrated loads at intersections of major structural members



A.5 Steel Selection Criteria

Table A.1 shows selection criteria for each structural element category for ABS grades of ordinary and higherstrength structural steel.

Selection of steel grades and thicknesses other than those shown in Table A.1 should be based on the funda-mental principles specified in 2/1.1.3.

TABLE A.1 Material Selection Guidelines for ABS Grades of Structural Steel*

Numbers in table are maximum thickness in mm (in.). Blank areas indicate no application.Application Areas

Secondary Applications Primary Applications Special ApplicationsServiceTemp.

C 0 −10 −20 −30 −40 −50 0 −10 −20 −30 −40 −50 0 −10 −20 −30 −40 −50(F) (32) (14) (−4) (−22) (−40) (−58) (32) (14) (−4) (−22) (−40) (−58) (32) (14) (−4) (−22) (−40) (−58)

GradeA 30 20 10 20 10

(1.18) (0.79) (0.39) (0.79) (0.39)B 40 30 20 10 25 20 10 15

(1.57) (1.18) (0.79) (0.39) (1.00) (0.79) (0.39) (0.59)

D 50 50 45 35 25 15 45 40 30 20 10 30 20 10(2.00) (2.00) (1.77) (1.38) (1.00) (0.59) (1.77) (1.57) (1.18) (0.79) (0.39) (1.18) (0.79) (0.39)

E 50 50 50 50 45 35 50 50 50 40 30 20 50 45 35 25 15(2.00) (2.00) (2.00) (2.00) (1.77) (1.38) (2.00) (2.00) (2.00) (1.57) (1.18) (0.79) (2.00) (1.77) (1.38) (1.00) (0.59)

AH 40 30 20 10 25 20 10 15(1.57) (1.18) (0.79) (0.39) (1.00) (0.79) (0.39) (0.59)

DH 50 50 45 35 25 15 45 40 30 20 10 30 20 10(2.00) (2.00) (1.77) (1.38) (1.00) (0.59) (1.77) (1.57) (1.18) (0.79) (0.39) (1.18) (0.79) (0.39)

EH 50 50 50 50 45 35 50 50 50 40 30 20 50 45 35 25 15(2.00) (2.00) (2.00) (2.00) (1.77) (1.38) (2.00) (2.00) (2.00) (1.57) (1.18) (0.79) (2.00) (1.77) (1.38) (1.00) (0.59)

FH 50 50 50 50 50 50 50 50 50 50 50 40 50 50 50 50 40 30(2.00) (2.00) (2.00) (2.00) (2.00) (2.00) (2.00) (2.00) (2.00) (2.00) (2.00) (1.57) (2.00) (2.00) (2.00) (2.00) (1.57) (1.18)

AQ 40 25 10 20(1.57) (1.00) (.039) (0.79)

DQ 50 45 35 25 15 45 35 25 15 25 15(2.00) (1.77) (1.38) (1.00) (0.59) (1.77) (1.38) (1.00) (0.59) (1.00) (0.59)

EQ 50 50 50 45 35 25 50 50 45 35 25 15 50 40 30 20 10(2.00) (2.00) (2.00) (1.77) (1.38) (1.00) (2.00) (2.00) (1.77) (1.38) (1.00) (0.59) (2.00) (1.57) (1.18) (0.79) (0.39)

FQ 50 50 50 50 50 45 50 50 50 50 45 35 50 50 50 40 30 20(2.00) (2.00) (2.00) (2.00) (2.00) (1.77) (2.00) (2.00) (2.00) (2.00) (1.77) (1.38) (2.00) (2.00) (2.00) (1.57) (1.18) (0.79)

*See “ABS Rule Requirements for Material and Welding” for description of ABS Grades of Structural Steel

References by Organization Appendix B


Appendix B References by Organization

ABS-American Bureau of Shipping Two World Trade Center: 106th FloorNew York, NY 10048, USA

Code No. Year/Edition Title— 1997 Rules for Building and Classing Mobile Offshore Drilling Units— 1997 Rules for Building and Classing Steel Vessels— 1986 Rules for Nondestructive Inspection of Hull Welds

ACI—American Concrete Institute Box 19150Redford StationDetroit, MI 48219-0150

Code No. Year/Edition Title318.2R 1989 (1992) Building Code Requirements for Reinforced Concrete357R.2 1984 (1992) Guide for the Design and Construction of Fixed Offshore Concrete

Structures357.2R 1988 (1992) State-of-the-Art Report on Barge-Like Concrete Structures359.2R 1989 (1992) Code for Concrete Reactor Vessels and Containments211.1R 1991 (1992) Standard Practice for Selecting Proportions for Normal, Heavyweight

and Mass Concrete

AISC—American Institute of SteelConstruction

1 East Wacker Drive, Suite 3100Chicago, IL 60601

Code No. Year/Edition Title1989/9th Manual of Steel Construction, ASD

API—American Petroleum Institute 1220 L Street, NWWashington, DC 20005(Production Dept.—1201 Main Street,Suite 2535, Dallas, TX 75202-3994)

Code No. Year/Edition TitleRP 2A 1993/1st Recommended Practice for Planning, Designing and Constructing

Fixed Offshore Platforms—Load and Resistance Factor DesignRP 2N 1988/1st Recommended Practice for Planning, Designing and Constructing

Fixed Offshore Platforms in Ice EnvironmentsRP 2T 1987/1st Recommended Practice for Planning, Designing and Constructing

Tension Leg Platforms

ASTM—American Society forTesting and Materials

1916 Race St.,Philadelphia, PA 19103

Code No. Year/Edition TitleA20 1994 Standard Specification for General Requirements for Steel Plates for

Pressure VesselsA82 1994 Standard Specification for Cold Drawn Steel Wire for Concrete

ReinforcementA184 1990 Standard Specification for Fabricated Deformed Steel Bar Mats for

Concrete ReinforcementA185 1994 Standard Specification for Welded Steel Wire Fabric for Concrete

ReinforcementA416 1994 Standard Specification for Uncoated Seven-Wire Stress-Relieved

Strand for Prestressed ConcreteA421 1991 Standard Specification for Uncoated Stress-Relieved Wire for

Prestressed Concrete

References by Organization Appendix B


A496 1994 Standard Specification for Deformed Steel Wire for ConcreteReinforcement

A497 1994a Standard Specification for Welded Deformed Steel Wire Fabric forConcrete Reinforcement

Code No. Year/Edition TitleA615 1994 Standard Specification for Deformed and Plain Billet-Steel Bars for

Concrete ReinforcementA673 1990b Standard Specification for Sampling Procedure for Impact Testing of

Structural SteelA704 1990 Standard Specification for Welded Steel Plain Bars or Rod Mats for

Concrete ReinforcementC31 1991 Standard Method of Making and Curing Concrete Test Specimens in

the FieldC33 1992a Standard Specification for Concrete AggregatesC39 1986 Standard Test Method for Compressive Strength of Cylindrical

Concrete SpecimensC94 1992a Standard Specification for Ready-Mixed ConcreteC109 1992 Standard Test Method for Compressive Strength of Hydraulic Cement

Mortars (using 2-in. or 50-mm Cube Specimens)C144 1993 Standard Specification for Aggregate for Masonry MortarC150 1992 Standard Specification for Portland CementC172 1990 Standard Method of Sampling Fresh ConcreteC260 1986 Standard Specification for Air-Entraining Admixtures for ConcreteC330 1989 Standard Specification for Lightweight Aggregates for Structural

ConcreteC494 1992 Standard Specification for Chemical Admixtures for ConcreteC595 1993 Standard Specification for Blended Hydraulic CementsC618 1993 Standard Specification for Fly Ash and Raw or Calcined Natural

Pozzolan for Use as a Mineral Admixture in Portland Cement ConcreteD512 1989 Standard Test Method for Chloride Ion in Water and Waste Water

AWS—American Welding Society 550 N. W. LeJeune RoadMiami, FL 33126

Code No. Year/Edition TitleD1.1 1996 Structural Welding Code—Steel

ISO 13819: 1995 (Offshore Structures—Part 1: General)

ISO 13819: 1995 (Offshore Structures—Part 2: Fixed Steel Structures)

NACE—National Association ofCorrosion Engineers

P.O. Box 218340Houston, TX 77218-8340

Code No. Year/Edition TitleRP-01-76 1994 Recommended Practice—Control of Corrosion on Steel, Fixed

Offshore Platforms Associated with Petroleum Production

General reference is made in these Rules to publications of the following organizationsFIP—Federation Internationale de la Precontrainte

(for publications write to Cement and Concrete Association, Wexham Springs, Slough SL3 6PL, (UnitedKingdom)

PCI—Prestressed Concrete Institute175 West Jackson Blvd.Chicago, IL 60604

Rules for Building and Classing Offshore Installations

Documents

Transcript of Rules for Building and Classing Offshore Installations