DNV-OS-C107: Structural Design of Ship-shaped Drilling … · STRUCTURAL DESIGN OF SHIP-SHAPED...

OFFSHORE STANDARD

DET NORSKE VERITAS

DNV-OS-C107

STRUCTURAL DESIGN OF SHIP-SHAPED DRILLING AND

WELL SERVICE UNITS

APRIL 2008

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Offshore Standard DNV-OS-C107, April 2008 Contents –

CONTENTS

Sec. 1 Introduction........................................................... 5A. General....................................................................................5A 100 Objectives ......................................................................... 5A 200 Classification .................................................................... 5B. Assumptions and Applications ...............................................5B 100 General.............................................................................. 5C. Definitions .............................................................................5C 100 Verbal forms ..................................................................... 5C 200 Terms ................................................................................ 5C 300 Symbols ............................................................................ 5C 400 Abbreviations.................................................................... 5D. References ..............................................................................5D 100 DNV Offshore Standards, Rules and

Classification Notes .......................................................... 5

Sec. 2 Structural Categorisation, Material Selection and Inspection Principles ............................................ 7

A. Selection of Material ..............................................................7A 100 General.............................................................................. 7A 200 Design temperature for elements not specified by the

DAT(-X°C) notation......................................................... 7A 300 Structural categorisation ................................................... 7A 400 Material Class for structural member not covered by the

DNV Rules for Classification of Ships Pt.3 Ch.1............. 7B. Inspection Principles...............................................................8B 100 General.............................................................................. 8B 200 Hull structure .................................................................... 8B 300 Topside structure............................................................... 8

Sec. 3 Design Principles ................................................ 10A. Introduction ..........................................................................10A 100 Overall design principles ................................................ 10A 200 Operational modes .......................................................... 10A 300 Local design loads .......................................................... 10A 400 Still water loading conditions ......................................... 10B. Hull Strength ........................................................................ 10B 100 Hull girder and hull girder structural members .............. 10C. Topside facilities and supporting structure...........................10C 100 General design principles................................................ 10C 200 Load combinations.......................................................... 10C 300 Working Stress Design method (WSD).......................... 11C 400 Basic usage factors.......................................................... 11C 500 Yield check ..................................................................... 11C 600 Design accelerations, bending moments and

shear forces ..................................................................... 11C 700 Combination of hull responses ...................................... 11C 800 Capacity models for strength .......................................... 11C 900 Capacity models for fatigue............................................ 12

Sec. 4 Design Loads ....................................................... 13A. Introduction ..........................................................................13A 100 General............................................................................ 13A 200 Definitions ...................................................................... 13B. Local static loads in topside structure...................................13B 100 Local loads on decks and bulkheads............................... 13B 200 Liquid in tanks ................................................................ 13C. Global static loads in topside structure.................................13C 100 General ........................................................................... 13D. Global static and dynamic loads in topside structure ...........14D 100 General............................................................................ 14E. Combination of accelerations, bending moments and

shear forces ...........................................................................14E 100 Basic responses ............................................................... 14E 200 Transit conditions ........................................................... 14E 300 Operating conditions....................................................... 14F. Hull deformation...................................................................15F 100 General............................................................................ 15

Sec. 5 Strength of Topside Structures.......................... 16A. Introduction ..........................................................................16A 100 General............................................................................ 16

B. Permissible stresses ..............................................................16B 100 General............................................................................ 16C. Local requirements to plates and stiffeners ..........................16C 100 Plates............................................................................... 16C 200 Stiffeners......................................................................... 16D. Local requirements to simple girders ...................................17D 100 General............................................................................ 17D 200 Minimum thickness ........................................................ 17D 300 Effective flange............................................................... 17D 400 Effective web .................................................................. 17D 500 Strength requirements for simple girders ....................... 17E. Complex girder systems .......................................................18E 100 General description......................................................... 18E 200 Loads............................................................................... 18E 300 Impact from connecting structure ................................... 18F. Buckling stability..................................................................18F 100 Bars, beams, columns and frames................................... 18F 200 Flat plated structures and stiffened panels...................... 18F 300 Tubulars .......................................................................... 18F 400 Capacity checks according to other codes ..................... 18

Sec. 6 Assessment of Hull – Topside Interface ........... 19A. Introduction...........................................................................19A 100 General considerations.................................................... 19B. Strength assessment ..............................................................19B 100 General............................................................................ 19B 200 Requirements to the FE model ....................................... 19B 300 Loads............................................................................... 19B 400 Combination of loads ..................................................... 19B 500 Acceptance criteria ......................................................... 19C. Fatigue assessment................................................................19C 100 General............................................................................ 19

Sec. 7 Fatigue Capacity Assessment ........................... 20A. Introduction...........................................................................20A 100 General............................................................................ 20B. Principles and methodology .................................................20B 100 Assessment principles..................................................... 20B 200 Methods for fatigue capacity .......................................... 20C. Structural Details and Stress Concentration Factors ............20C 100 General............................................................................ 20D. Design Loads and Calculation of Stress Ranges ..................20D 100 Local and global loads .................................................... 20

Sec. 8 Accidental Conditions ........................................ 21A. General..................................................................................21A 100 General............................................................................ 21B. Design Criteria......................................................................21B 100 General............................................................................ 21B 200 Dropped objects .............................................................. 21B 300 Fires ................................................................................ 21B 400 Explosions....................................................................... 21

Sec. 9 Welding and Weld Connections........................ 22A. Introduction...........................................................................22A 100 General requirements...................................................... 22B. Size of Welds........................................................................22B 100 Double continuous fillet welds ....................................... 22B 200 Fillet welds and deep penetration welds subject to

high tensile stresses......................................................... 22B 300 Full penetration welds..................................................... 22B 400 Direct calculations .......................................................... 22

Sec. 10 Corrosion Control .............................................. 24A. Hull and hull structural elements..........................................24A 100 General............................................................................ 24B. Topside structure ..................................................................24B 100 Void spaces and elements in the atmospheric zone........ 24B 200 Tanks .............................................................................. 24

App. A Cross Sectional Types .......................................... 2

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Offshore Standard DNV-OS-C107, April 2008 – Contents

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Offshore Standard DNV-OS-C107, April 2008 Sec.1 –

SECTION 1INTRODUCTION

A. GeneralA 100 Objectives101 The objectives of this standard are to:

— provide an internationally acceptable standard for designof ship-shaped Drilling and Well Service Units

— serve as a technical reference document in contractualmatters between purchaser and manufacturer

— serve as a guideline for designers, purchaser, contractorsand regulators

— specify procedures and requirements for units subject toDNV classification services

— base the design of the hull and topside on the same principlesand methodology for all transit and operational scenarios

— provide, as far as possible, consistent loads for both top-side and hull design.

The hull strength may be assessed according to DNV Rules forClassification of Ships Pt.3 Ch.1 for all transit and operationalconditions.

A 200 Classification201 Classification principles related to classification of off-shore units are given the DNV Offshore Service Specificationsgiven in Table A1.

202 Documentation for classification shall be in accordancewith the NPS DocReq (DNV Nauticus Production System fordocumentation requirements) and Guideline No.17.

B. Assumptions and ApplicationsB 100 General101 It is assumed that the units will comply with the require-ment for retention of the Class as defined in the DNV-OSS-101.102 This standard is applicable to hull and topside of ship-shaped drilling and well service units, such as well stimulationand well intervention vessels, constructed in steel for both non-restricted and restricted operations.

C. DefinitionsC 100 Verbal forms101 Shall: Indicates a mandatory requirement to be followedfor fulfilment or compliance with the present standard. Devia-tions are not permitted unless formally and rigorously justified,and accepted by all relevant contracting parties.102 Should: Indicates a recommendation that a certaincourse of action is preferred or particularly suitable. Alterna-tive courses of action are allowable under the standard whereagreed between contracting parties but shall be justified anddocumented.103 May: Indicates a permission, or an option, which is per-mitted as part of conformance with the standard.

C 200 Terms201 Standard terms are given in DNV-OS-C101.202 Transit: Moving the unit from one geographical locationto another.203 Drilling vessel: A unit used for drilling in connectionwith exploration and/or exploitation of oil and gas. The unit isgenerally operating on the same location for a limited period oftime and is normally equipped with dynamic positioning sys-tem with several thrusters. The unit follows the normal classsurvey program.204 Well stimulation vessel or well intervention vessel: Aunit equipped for performing wire-line intervention (withoutriser) of subsea wells and or coiled tubing of subsea. The unitis generally operating on the same location for a limited periodof time and is normally equipped with dynamic positioningsystem with several thrusters. The unit follows the normalclass survey program.

C 300 Symbols301 The following Latin characters are used in this standard:

302 The following Greek characters are used in this standard:

C 400 Abbreviations401 The abbreviations given in Table C3 are used in thisstandard. Definitions are otherwise given in DNV-OS-C101'Design of Offshore Steel Structures, General' (LRFDmethod).

D. ReferencesD 100 DNV Offshore Standards, Rules and Classifica-tion Notes101 The offshore standards and rules given in Table D1 arereferred to in this standard.

Table A1 DNV Offshore Service Specifications Reference TitleDNV-OSS-101 Rules for Classification of Offshore Drilling and

Support Units

Table C1 Latin characters used V Speed in knots CW Wave coefficient as given in DNV Rules for Classification of

Ships Pt.3 Ch.1 Sec.4 av Vertical accelerationsat Transverse accelerational Longitudinal accelerationsMwv Vertical wave bending momentMwh Horizontal wave bending momentQwv Vertical wave shear force

Table C2 Greek characters used η0 Basic usage factorβ Coefficient depending on type of structureηp Permissible usage factor

Table C3 Abbreviations Abbreviation In fullDFF Design fatigue factor NDT Non-destructive testing SCF Stress concentration factors WSD Working Stress Design

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Offshore Standard DNV-OS-C107, April 2008 – Sec.1

Table D1 DNV Offshore Standards, Rules, Classification Notes and Recommended Practice Reference TitleDNV-OS-C101 Design of Offshore Steel Structures, General

(LRFD method) DNV-OS-C401 Fabrication and Testing of Offshore Structures DNV-OS-B101 Metallic Materials DNV-RP-C201 Buckling Strength of Plated Structures Classification Note 30.5

Environmental Conditions and Environmental Loads

Classification Note 30.7

Fatigue Assessment of Ship Structures

DNV-RP-C203 Fatigue Strength Analysis of Offshore Steel Struc-tures

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SECTION 2STRUCTURAL CATEGORISATION, MATERIAL SELECTION AND

INSPECTION PRINCIPLES

A. Selection of Material

A 100 General101 The material grade shall be selected according to DNVRules for Classification of Ships Pt.3 Ch.1.102 The Design Temperature is by default -15°C based onlowest mean daily air temperature.103 Lower Design Temperatures than -15°C may be speci-fied. The DNV DAT(-X°C) notation is mandatory in suchcases.104 In structural cross-joints where high tensile stresses areacting perpendicular to the plane of the plate, the plate materialshall be tested according to DNV-OS-B101 Sec.6 to prove theability to resist lamellar tearing (Z-quality).105 The steel grades selected for structural elements shallcomply with the requirements given in the DNV-OS-B101.106 For stiffeners, the grade of material may be determinedbased on the thickness of the web.107 The grade of materials for Offshore Crane pedestals andsupporting structure shall not be less than NVE.108 Structural elements used only in temporary conditions,e.g. fabrication, are not considered in this standard.

A 200 Design temperature for elements not specified by the DAT(-X°C) notation201 When the DAT(-X°C) is relevant, the design tempera-ture is used for selection of materials, ref. DNV Rules for Clas-sification of Ships Pt.5 Ch.1 Sec.7.202 The topside structures shall be regarded as ExternalStructure according to the definition given in the DAT(-X°C)notation.203 Materials for structural members which are not definedas External Structure, may be selected according to DNVRules for Classification of Ships Pt.3 Ch.1.

A 300 Structural categorisation301 In DNV Rules for Classification of Ships Pt.3 Ch.1materials are categorised into Material Classes. The purpose ofthe structural categorisation is to ensure adequate material andsuitable inspection to avoid brittle fracture, and to ensure suf-ficient fracture resistance of a material (stress intensity factor)to avoid crack sizes which may develop into brittle fracture atcertain stress situations.

Guidance note:Conditions that may result in brittle fracture should be avoided.Brittle fracture may occur under a combination of:

- presence of sharp defects such as cracks- high tensile stress in direction normal to planar defect(s)- material with low fracture toughness.Sharp cracks resulting from fabrication may be found by inspec-tion and repaired. Fatigue cracks may also be discovered duringservice life by inspection.High stresses in a component may occur due to welding. A com-plex connection is likely to provide more restraint and largerresidual stress than a simple one. This residual stress may bepartly removed by post weld heat treatment if necessary. Also a

complex connection shows a more three-dimensional stress statedue to external loading than simple connections. This stress statemay provide basis for a cleavage fracture. The fracture toughnessis dependent on temperature and material thickness. Theseparameters are accounted for separately in selection of material.The resulting fracture toughness in the weld and the heat affectedzone is also dependent on the fabrication method.Thus, to avoid brittle fracture, first a material with suitable frac-ture toughness for the actual design temperature and thickness isselected. Then a proper fabrication method is used. In specialcases post weld heat treatment may be performed to reduce crackdriving stresses. Inspection is carried out to detect unacceptableplanar defects. In this standard selection of material with appro-priate fracture toughness and avoidance of unacceptable defectsare achieved by linking different types of connections to differentstructural categories and inspection categories.

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302 Structural members not covered by the DNV Rules forClassification of Ships Pt.3 Ch.1 shall be categorised accord-ing to A400.

A 400 Material Class for structural member not cov-ered by the DNV Rules for Classification of Ships Pt.3 Ch.1.401 Structural members are classified into Material Classesaccording to the following criteria:

— significance of member in terms of consequence of failure— stress condition at the considered detail that together with

possible weld defects or fatigue cracks may provoke brittlefracture.

Guidance note:The consequence of failure may be quantified in terms of residualstrength of the structure when considering failure of the actualcomponent.


402 The principles for determination of Material Classes aregiven in Table A1.

Table A1 Material Classes

Material Class

Principles for determination of structural category

Equivalent structural category in the

DNV OS- standards

I and IIStructural parts where failure will be without significant con-sequence.

Secondary

IIIStructural parts where failure will have substantial conse-quences

Primary

IV

Structural parts where failure will have substantial conse-quences and are subject to a stress condition that may increase the probability of a brittle fracture. 1)

Special

1) In complex joints a tri-axial or bi-axial stress pattern will be present. This may give conditions for brittle fracture where tensile stresses are present in addition to presence of defects and material with low fracture toughness.

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403 The material class for specific structural members isgiven in Table A2.

Figure 1 Minimum requirements to topside stool with soft nose brackets

Figure 2 Offshore Crane pedestal

B. Inspection PrinciplesB 100 General101 The purpose of inspection is to detect and removedefects that may grow into fatigue cracks during service life. 102 When determining the locations of required non-destructive testing (NDT), consideration should be given torelevant fabrication parameters including;

— location of block (section) joints — manual versus automatic welding — start and stop of weld.

B 200 Hull structure201 The extent of non-destructive testing during fabricationof the hull shall be in accordance with DNV Rules for Classi-fication of Ships Pt.2 Ch.3 Sec.7.

B 300 Topside structure301 Fabrication and testing of topside structure shall complywith the requirements in DNV-OS-C401. The requirementsare based on the consideration of fatigue damage and assess-ment of general fabrication quality. 302 The inspection categories are related to the structuralcategories as shown in Table B1.

303 The weld connection between two components shall beassigned inspection category according to the highest of thejoined components. For stiffened plates, the weld connectionbetween the plate and stiffener, stringer, and girder web to theplate may be inspected according to inspection category III.

Table A2 Material Classes Material

Class Structural member

I

Outfitting steelMezzanine decks, platformsPipe support structureLetdown platforms.Doubler plates, closer plates and support infill steels in topside structures. 1)

II

Stair towers.Module decks plates, stiffeners and girders.Bulkheads structure (plate, web frames, and stiffeners) in modules. Longitudinal bulkheads in way of moonpool. Offshore Crane boom rest support structure.

III

Main girders and columns in truss work type modules.Topside support stools with brackets of soft nose design 2) Ref. Figure 1Pipe rack stanchions. Drill-floor substructure. Helicopter deck substructure. Main girders in drill-floor.

IVDeck and bottom corner plates in moonpool.Topside support stools with brackets without soft noseDerrick support structure.

1) To have the same minimum yield strength as the material to which they are attached.

2) Length 'a' to be 0.35l, minimum 120 mm. 'a' need not to be bigger than 500 mm.

al

Uplift expected No uplift expected

Partly penetration weld .IC = I

Fillet or partly penetration weld .IC = II

Full penetration weld .IC = I



Full penetration weld.IC = I

Partly penetration weld .IC = I

Full penetration weld .IC = I

Table B1 Inspection categories Inspection category

Material Class

Equivalent structural category in the DNV OS- standards

I IV Special II III Primary III I and II Secondary

IC = II except as shown

IC = IIC = II

IC = I500 mm each side

Deck plate

Full penetration weld

Z- Quality

Full penetration weld

Z- Quality

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304 If the fabrication quality is assessed by testing, or wellknown quality from previous experience, the extent of inspec-tion required for elements within Material Class III may bereduced, but not less than for inspection category III. 305 Fatigue critical details within Material Class II and IIIshall be inspected according to requirements in inspection cat-egory I.306 Welds in fatigue critical areas not accessible for inspec-tion and repair during operation shall be inspected according torequirements in inspection category I.

307 The extent of NDT for welds in block joints and erectionjoints transverse to main stress direction shall not be less thanfor inspection category II. 308 Topside stools, or topside - hull connections, similar toFigure 1, Material Class III, shall be inspected according to therequirements in inspection category I for the areas shown inFigure 1. 309 Inspection categories for Offshore Crane pedestals andthe supporting structure are given in Figure 2.

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SECTION 3DESIGN PRINCIPLES

A. Introduction

A 100 Overall design principles101 This section defines the principles for design of the hulland topside structures.102 The overall principles are based on the following:

— the safety of the structure can be demonstrated by address-ing the potential structural failure mode(s) when the unit issubjected to loads scenarios encountered during transit,operation and in harbour.

— the structural requirements are based on a consistent set ofloads that represent typical worst possible loading scenar-ios

— the unit has inherent redundancy. The unit’s structureworks in a hierarchical manner and as such, failure ofstructural elements lower down in the hierarchy should notresult in immediate consequential failure of elementshigher up in the hierarchy

— structural continuity is ensured. The hull, topside structureand their elements should have uniform ductility

— permanent deformations are minimised. Local yieldingand permanent deformations of local panel or individualstiffened plate members may be acceptable provided thatthis does not affect the structural integrity, containmentintegrity or the performance of structural or other systems

— the unit has adequate structural redundancy to survive inthe event that the structure is accidentally damaged, forexample, minor impact leading to flooding of any com-partment or dropped objects from crane operations.

103 Topside structural elements shall be fabricated accord-ing to the requirements given in DNV-OS-C401.

A 200 Operational modes201 All relevant modes of operation shall be considered.Typically, the assessment of the unit shall be based on the fol-lowing operational modes:

— all operating conditions, intact and damaged, at the designlocation(s)

— all transit conditions — dry-docking condition.

A 300 Local design loads301 The local design loads for design of decks for within thehull, accommodation and deck houses are given in the DNVRules for Classification of Ships Pt.3 Ch.1. Local loads for top-side facilities are given in Section 4.

A 400 Still water loading conditions401 Still water loading conditions shall be given in the load-ing manual. All still water loading conditions in transit (at sea),for operation and for harbour situations shall be less, or equalto, the maximum permissible bending moments and shearforces given in the Class Certificate (limit curves.) The globalweight of the topside facilities shall be included. The curves forpermissible bending moments and shear forces are used as basisfor the still water loads in the longitudinal strength assessment.

B. Hull Strength

B 100 Hull girder and hull girder structural members101 The hull girder and it's structural members may bedesigned according to DNV Rules for Classification of ShipsPt.3 Ch.1. Permissible still water curves for bending momentsand shear forces shall be calculated considering all relevantload conditions in transit and operation.102 The stress distribution in areas with global stress con-centrations, such as moonpool openings, shall be derived fromFinite Element analysis and used as basis for buckling andyield capacity assessment. 103 For units intended to operate in regions exposed toexceptional environmental conditions, e.g. typhoons or hurri-canes, the longitudinal strength of the hull shall be assessed asa normal operating condition. The wave bending moments andshear forces shall be derived from direct calculations based onthe environmental data for the exceptional wave data based on100 years return period. The basic utilisation factor η0 is thus0.8 according to load combination b) in Table C2.104 For unit not intended to stay on location during theexceptional environmental conditions, the longitudinalstrength of the hull unit is regarded as an accidental conditionand shall be assessed according the load combination d) inTable C2.

C. Topside facilities and supporting structure

C 100 General design principles101 For world wide operation of the unit, the hull girderbending moments, shear forces and accelerations defined inDNV Rules for Classification of Ships Pt.3 Ch.1 may be usedin the assessment of the topside structure. Alternatively thevalues may be derived from direct calculations according toC602, and used in the assessment of topside structure and top-side support structure.102 In the operating conditions, the topside loads are nor-mally different from the transit conditions and direct calcula-tions of the accelerations may be carried out. The assessmentshall comply with the following principles:

— the heading profile of the ship shall to be taken into con-sideration

— operational limitation profile to be established— loading conditions for each operational restriction and cor-

responding mass distribution to be established— direct calculations of the accelerations may be carried out.

The accelerations need not exceed the accelerations calcu-lated according to the DNV Rules for Classification ofShips Pt.3 Ch.1.

103 The deformations due to hull girder bending and stiff-ness variations of the supporting structure shall be accountedfor in the structural analyses.

C 200 Load combinations201 Each structural member shall be designed for the mostunfavourable of the loading conditions given in Table C1.

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For each of the load combinations in Table C1 and for eachstructural element, the combination of loads, positions anddirections giving the most unfavourable load effect shall beused in the analyses.

C 300 Working Stress Design method (WSD)301 In WSD the target component safety level is achieved bycomparing the calculated stress for different load combinationswith maximum permissible stress. The maximum permissiblestress is defined by multiplication of the characteristicstrength, or capacity, of the structural member with a permis-sible usage factors. 302 The permissible usage factors are a function of loadingcondition, failure mode and importance of strength member.303 The maximum permissible usage factor, ηp, is calcu-lated by:

ηp = βη0where:

304 Stresses shall be calculated using gross thicknesses, pro-vided the corrosion protection system prevent structural dimi-nution throughout the design life.

C 400 Basic usage factors401 For the topside facilities and the supporting structure,including the supporting elements within the hull, the permis-sible utilisation factors for structural strength are given inTable C2.

The basic usage factor η0 accounts for:

— possible unfavourable deviations of specified or expectedloads

— uncertainties in the model and analysis used for determi-nation of load effects

— possible unfavourable deviations in the resistance of mate-rials

— possible reduced resistance of the materials in the struc-ture, as a whole, as compared to the values deduced fromtest specimens

— deviation from calculated responses due to fabrication.

C 500 Yield check501 Structural members shall be cheeked for excessiveyielding.502 Individual stress components and the von Mises equiva-lent stress for plated structures shall not exceed the permissiblestress specified in Section 5.

Guidance note:For plated structures the von Mises equivalent stress is defined asfollows:

where σx and σy are membrane stresses in x- and y-directionrespectively, τ is shear stress in the x-y plane, i.e. local bendingstresses in plate thickness not included.


503 Local peak stresses by FE analysis in areas with pro-nounced geometrical changes, such as in moonpool corners,frame corners etc., may exceed the permissible usage factor in303 provided plastic mechanisms are not developed in theadjacent structural parts.

Guidance note:Linear peak stress (von Mises) of is gener-ally acceptable. fyNS and fy are the yield stresses for normal steel (235 MPa) andthe minimum specified yield stress of the actual material, respec-tively.


C 600 Design accelerations, bending moments and shear forces601 The basic responses vertical accelerations av, transverseacceleration at, longitudinal accelerations al, wave bendingmoments and shear forces shall be determined according to theDNV Rules for Classification of Ships Pt.3 Ch.1. The rollradius of gyration kr and metacentric height GM used in thecalculation of roll acceleration shall be based on representativeglobal distribution of masses in the hull and topside.602 Alternatively direct calculations may be used. If directcalculations are carried out, the wave load analysis shall becarried out based on the principles given in DNV-OS-C102. 603 The ship motions, accelerations, moments and shearforces shall be given as extreme values (i.e. probabilitylevel = 10-8 for North Atlantic scatter diagram assuming omni-directional waves with equal probability of occurrence.

C 700 Combination of hull responses The basic accelerations, hull bending moments and shearforces may be combined accounting for joint probability ofoccurrence. In principle each response parameter is in turnmaximised and combined with fraction of the other responses.

C 800 Capacity models for strength801 The model used for yield and buckling strength assess-ment of the topside structure shall be capable of describing thestress distribution in the structure to the required degree ofaccuracy.802 The following aspects are the basis for selection ofstrength capacity models:

— simplified models may be used for elements which areanalysed at a later stage by means of more accurate meth-ods.

— simplified models where some of the stress componentsare neglected are to always give conservative results.

— capability of response calculations to represent the physi-cal behaviour of the structure up to the given load level

Table C1 Load combinationsCombination Descriptiona) Static loadsb) maximum combined static and dynamic loads c) accidental loads and associated static loads d) maximum combined operational static loads and

dynamic loads from exceptional environmental situ-ations, e.g. hurricane or typhoon

Notes:

c) represent accidental conditions with little probability of occur-rence such as explosions, fire, dropped objects etc.

d) represent an exceptional environmental condition, e.g. hurri-cane or typhoon situation, with return period of 100 years. The load combination is applicable to units not intended to stay on location during the exceptional environmental condition.Units intended to stay on location during the exceptional envi-ronmental condition shall be assessed according to b).

η0 = basic usage factorβ = coefficient depending on type of structure, see Table B1

in section 5

Table C2 Basic usage factors η0 Load combination

a) b) c) d) η0 0.60 0.80 1.00 1.00

222 3τσσσσσ +−+= yxyxj

400fy

fyNS----------- N/mm2

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— complexity of structure— complexity of loads.

C 900 Capacity models for fatigue901 The fatigue capacity shall be documented according tothe principles and methods given in DNV Classification Note30.7 or DNV-RP-C203. 902 Simplified fatigue methods may be used when the longterm distribution of stresses can be described by a stress rangeand a Weibull shape parameter.

Guidance note:In cases where the total stress range comprises stresses from sev-eral load responses, a combined Weibull parameter should beused.


903 The accumulated fatigue damage from the transit andoperating conditions shall be calculated according to the oper-ational characteristics of the unit. The fatigue life shall be cal-

culated considering the combined effects of global and localstructural responses. 904 The resistance against fatigue is normally given as S-Ncurves, i.e. stress range (S) versus number of cycles to failure(N) based on fatigue tests. Fatigue failure should be defined aswhen the crack has grown through the thickness. 905 The required fatigue life of new units shall be minimum20 years assuming that the unit complies with the DNVrequirements for dry-docking inspection. A design fatigue fac-tor (DFF) of 1.0 is thus acceptable for all structural elementswhich are accessible for inspection and repair during docking.Higher DFF according to DNV-OS-C102 Appendix A shouldbe used in case the structure is not accessible for inspection.906 The effect of mean stresses may be accounted foraccording to guidelines given in CN.30.7.907 The stresses may be based on gross thicknesses (i.e.without deducting the corrosion additions).

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SECTION 4DESIGN LOADS

A. Introduction

A 100 General101 The accelerations from the DNV Rules for Classifica-tion of Ships Pt.3 Ch.1 shall be used for design of the topsidefacilities with loads present in the transit conditions. 102 In the operational conditions, the structure shall beassessed for a set of loading conditions containing operationalrestriction and corresponding loads, ref Sec.3 Design Princi-ples.103 The static and dynamic loads acting on the topside facil-ities are determined according to the following paragraphs inthis section. 104 The combination of accelerations of drilling units ofconventional hull form used in the structural assessment of thetopside facilities and hull-topside interface are given in thissection. The combination of accelerations may alternatively bedetermined by direct calculations.

A 200 Definitions201 Symbols:

p = design pressure in kN/m²

202 The load point for which the design pressure shall be cal-culated is defined for various strength members as follows:

a) For plates:midpoint of horizontally stiffened plate field. Half of thestiffener spacing above the lower support of verticallystiffened plate field, or at lower edge of plate when thethickness is changed within the plate field.

b) For stiffeners:midpoint of span.When the pressure is not varied linearly over the span thedesign pressure shall be taken as the greater of:

pm, pa and pb are calculated pressure at the midpoint andat each end respectively.

c) For girders:midpoint of load area.

B. Local static loads in topside structure

B 100 Local loads on decks and bulkheads101 The local static loads for decks and bulkheads in topsidefacilities, which are not part of a tank, are given in Table B1below. For areas not specifically mentioned in Table B1, rele-vant values in the DNV Rules for Classification of Ships Pt.3Ch.1 may be used.

Notes:

— wheel loads to be added to distributed loads where rele-vant. (Wheel loads can normally be considered acting onan area of 300 x 300 mm.)

— point load may be applied on an area 100 x 100 mm, andat the most severe position, but not added to wheel loadsor distributed loads

— the factor f may be taken as:

where A is the loaded area in m2.

B 200 Liquid in tanks201 The local strength requirements to plates, stiffeners andsimple girders in tanks shall comply with the requirements inDNV Rules for Classification of Ships Pt.3 Ch.1. The allowa-ble stress for longitudinal members need not be less than 160MPa.

C. Global static loads in topside structureC 100 General 101 The static loads to be applied for the global analysis ofthe topside facilities or in the still water loading conditions ofthe unit are in principle determined by considering the perma-nent loads and realistic values for simultaneously acting varia-ble loads.102 The total static load of a module, excluding tank loads,

pm and2

ba pp +

Table B1 Local static loadsPlates and stiffeners

Girders(kN/m2)

Evenly distributed

load (kN/m2)Point

load (kN)

Decks— Storage areas in

modules 2) q 1.5 q f * q

— Lay down areas 2) q 1.5 q f * q— Lifeboat platforms 9.0 9.0 9.0*f— Area between

equipment 5.0 5.0 5.0*f

— Walkways, stair-cases and plat-forms, crew spaces

4.0 4.0 4.0*f

— Walkways and staircases for inspection only

3.0 3.0 3.0*f

— Minimum values for areas not given above 1)

2.5 2.5 2.5

1) The minimum values shall be determined considering the weights of the equipment and bulks, which may be located on the area. The minimum values shall not be less than 2.5 kN/m2

2) The distributed loads, q, to be evaluated for each case. Lay down areas should not be designed for less than 15 kN/m2.

f min 1. 0 0.5 3A

--------+⎝ ⎠⎛ ⎞;

⎩ ⎭⎨ ⎬⎧ ⎫

=

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is determined according to:

1) Typical values are between 0.5 and 0.7

103 The tank loads within a module shall be added, if rele-vant.104 The load used should include all equipment over 50 kNplus the sum of all realistic deck loads accounting for the jointprobability of occurrence.

D. Global static and dynamic loads in topside structure

D 100 General101 The dynamic loads to be combined with the global staticloads are determined by multiplying the masses with thedesign acceleration.

E. Combination of accelerations, bending moments and shear forces

E 100 Basic responses101 The basic hull girder responses according to the DNVRules for Classification of Ships Pt.3 Ch.1 used for design ofthe topside facilities are:

The sign convention is according to the coordinate systembelow:

Positive vertical bending moment gives longitudinal tensionstress in deck. Positive horizontal bending moment gives longitudinal tensionstress at starboard side.

Positive shear force act down at aft end and up at forward endof a part of the ship102 For units with double side, the horizontal bendingmoment can be ignored for design of topside structures. 103 The vertical shear force can normally be ignored, unlessthe vertical relative shear deformation of the support stools ofthe module are significant.

E 200 Transit conditions201 Referring to Table E1 one load case should be generatedfor each of the maximum basic responses for the head sea,beam sea and oblique sea. For symmetrical structures about alongitudinal and transverse plane through the centre of gravityof the topside structure, load combination 4 and 7 may be omit-ted.

where:

L = Length of unit (m), shall not be taken higher than 200 norless than 100.

E 300 Operating conditions301 The basic hull girder responses shall be determined forloads present in the operating conditions provided the effect ofthese loads has not been considered in the transit analysis.302 The following heading profile of the ship shall be con-sidered, unless documented otherwise:

Head sea : 60%+15 degrees : 15%-15 degrees : 15%+30 degrees : 5%-30 degrees : 5%A cosine square energy distribution may be considered.Based on the heading profile in 302 the load cases given in

qS = Static global weight of module (kN)Fs = Total steel weight of decks (kN)Fe = Weight of equipment (kN)n = Total number of heavy equipment (>50kN)K 1) = Global load reduction factor for the deck considered

to account for simultaneous acting module loadsPv = Evenly distributed design load (kN/m2) for the deck

considered, ref Table B1.m = Total number of decksA = Loaded area of deck considered (area covered by

equipment may be excluded)

av = vertical accelerationsat = transverse accelerational = longitudinal accelerationsMW = wave bending momentQW = wave shear force

qS = ∑∑==

++m

kkk

n

kk APvKFeFs

11

Table E1 Combination of dynamic responses in transit

Heading Loadcase

Maximum response

Combination with fraction of responses

Mwv Qwv MWh av at al

Head Sea

1 Mwv -1.0 1.0 0.0 0.5 0.0 -r2 Mwv 1.0 -1.0 0.0 -0.5 0.0 +r

Beam Sea

3 at +a +a -b 1.0 1.0 -c4 at +a +a -b 1.0 -1.0 -c

Oblique Sea

5 al +h -h -i -j 0.4 1.06 at -k +k -l +m 1.0 0.97 at -k +k -l +m -1.0 0.9

Values for L > 200 m

Values for L < 100 m

a = -0.003 L + 1.3 0.7 1.0b = -0.006 L + 1.5 0.3 0.9c = -0.003 L + 0.7 0.1 0.4h = 0.002 L + 0.5 0.9 0.7i = 0.003 L + 0.4 1.0 0.7j = -0.002 L + 0.4 0 0.2k = -0.003 L + 0.7 0.1 0.4l = -0.001 L + 1.1 0.9 1.0m = -0.004 L + 1.1 0.3 0.7r = -0.004 L + 1.4 0.6 1.0

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Table E2 shall be analysed.

where:

L = Length of unit (m), shall not be taken larger than 200 m norless than 100 m.

F. Hull deformationF 100 General101 The Tables E1 and E2 give combination factors for

applied loads in an integrated hull-topside model. If the topsidemodule is analysed separately from the hull, the hull deforma-tion caused by the bending moments shall be applied to themodel. The deformations should be determined by finite ele-ment analysis. Within regions with no global stress concentra-tions, the longitudinal deformation in deck may alternativelybe determined by:

1) The design bending moment in both a), b) and d) load combinations to beconsidered, ref. Sec.3 Table C2.

Table E2 Combination of dynamic responses in operating conditions

Heading Load case

Maximum response

Combination with fraction of responses

Mwv Qwv Mwh av at al

Operation8 al 0.9 -0.9 -a -b -c 1.09 at -d +d -1.0 0.8 1.0 -e10 av -1.0 1.0 1.0 1.0 +f -g

Values for L > 200 m

Values for L < 100 m

a = 0.002 L + 0.6 1.0 0.8b = 0.003 L + 0.2 0.8 0.5c = -0.002 L + 0.8 0.4 0.6d = 0.004 L + 0.2 1.0 0.6e = 0.004 L + 0.2 1.0 0.6f = -0.005 L + 1.3 0.3 0.8g = 0.004 L + 0.2 1.0 0.6

δ = longitudinal deformation between sections 1 and 2M = design bending moment at sections 1 and 2 1)

Z = section modulus at the deck at the interface with top-side structure

E = Young’s modulus of elasticityl1 = distance between sections 1 and 2

121 )(5.0 l

EZMM

⎟⎟⎠

⎞⎜⎜⎝

⎛ +=δ

1 2l1

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SECTION 5STRENGTH OF TOPSIDE STRUCTURES

A. IntroductionA 100 General101 This section gives provisions for checking of ultimatestrength for typical topside structures such as:

— drill-floor and substructure — derrick— modules — deck houses which carry loads from risers, mud, brine etc.

102 Local requirements to plates, stiffeners and simple gird-ers in tanks are given in DNV Rules for Classification of ShipsPt.3 Ch.1 and thus not covered by this section.103 Deck houses, accommodation or superstructure, whichis not part of the load-bearing structure for typical offshore ele-ment loads, may be designed according to DNV Rules forClassification of Ships Pt.3 Ch.1.104 Topside structures of truss work type of structure as theprimary load-bearing elements and where the plates are notincluded in assessment of the global strength, the plates withstiffeners may comply only with the local requirements.105 When the plates with stiffeners are part of the primaryload-bearing structure, both local and global requirementsmust be complied with.

B. Permissible stressesB 100 General101 The maximum permissible usage factor, ηp, is calcu-lated by:

ηp = βη0

C. Local requirements to plates and stiffenersC 100 Plates101 The local requirements to end connections of stiffenersand design of brackets are given in DNV Rules for Classifica-

tion of Ships Pt.3 Ch.1 Sec.3 C.102 The plate thickness t shall not to be less than:

where:f1 = See DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.2.

103 The thickness of plating subjected to lateral pressureshall not be less than:

C 200 Stiffeners201 The section modulus Zs for longitudinals, beams, framesand other stiffeners subjected to lateral load shall not be lessthan:

Minimum 15000 mm3

202 The requirement in 201 applies to an axis parallel to theplating. For stiffeners at an oblique angle with the plating, therequired section modulus shall be multiplied by:

ϕ = angle in degrees1) between the stiffener web plane andthe plane perpendicular to the plating.

1) ϕ is to be taken as 90 degrees if the angle is greater or equal to 75 degrees.

203 Stiffeners with sniped ends may be accepted wheredynamic stresses are small and vibrations are considered to beof minor importance, provided that the plate thickness t sup-

η0 = basic usage factor as given in Sec.3 C400β = coefficient depending on type of structure, see Table B1.

Table B1 Multiplication coefficient β

ItemsLoad combination

(ref Sec.3)a) b) c) d)

Local requirements to plates and stiffeners 1.14 1.0 NA NALocal requirements to web area of girders and stringers 0.71 1.0 1.0 1.0

Local requirements to section modulus of girders and stringers 1.0 1.0 1.0 1.0

Global strength of topside load-bearing elements in general 1.14 1.0 1.0 1.0

Global strength of drill-floor, substruc-ture, flare, derrick 1.0 1.0 1.0 1.0

Global strength of support structure for modules, over and under deck 1.0 1.0 1.0 1.0

Buckling stability check in general 1.0 1.0 1.0 1.0

ka = correction factor for aspect ratio of plate field = (1.1 - 0.25 s/l)2

maximum 1.0 for s/l = 0.4 minimum 0.72 for s/l = 1.0

s = stiffener spacing in ml = stiffener span in mp = local design load in Sec.4 B and EηP = permissible utilisation factors as given in Sec.3fy = minimum yield strengthtk = corrosion addition according to the Ship Rules,

Pt.3 Ch.1 Sec.2 Table D1.tk = 0 for elements which are not part of a tank.

l = effective stiffener span in m s = stiffener spacing in m, measured along the plating p = local design load in Sec.4 B and Ekm = bending moment factor, see Table D1 ηP = permissible utilisation factors as given in Sec.3fy = specified minimum yield stress of the material in N/mm2

)(5

1

mmtf

t k+=

( )mmtfpsk

t kyP

a +=η

8.15

( )362

10 mmfk

pslZyPm

S η=

1ϕcos

------------

DET NORSKE VERITAS


ported by the stiffener is not less than:

In such cases the required section modulus in 201 shall bebased on the following parameter values: km = 8The stiffeners should normally be snipped to an angle of max-imum 30°.

Guidance note:For typical sniped end details as described above, a stress rangelower than 30 MPa can be considered as small dynamic stress.


D. Local requirements to simple girdersD 100 General101 The requirements in this sub-section give minimumscantlings to simple girders with respect to yield. When bound-ary conditions for individual girders are not predictable due todependence of adjacent structures, direct calculations shall becarried out.102 The local requirements to end connections of girders anddesign of brackets are given in DNV Rules for Classificationof Ships Pt.3 Ch.1 Sec.3 C.103 The requirements for section modulus and web areagiven in D500 apply to simple girders supporting stiffeners, orother girders, exposed to linearly distributed lateral load. It isassumed that the girder satisfies the basic assumptions of sim-ple beam theory, and that the supported members are approxi-mately evenly spaced and similarly supported at both ends.Other loads should be specially considered based on the samebeam-theory.104 The section modulus and web area of the girder shall betaken in accordance with particulars as given in D500. Struc-tural modelling in connection with direct stress analysis shallbe based on the same particulars when applicable. 105 Dimensions and further references with respect to buck-ling capacity are given in sub-section F.

D 200 Minimum thickness201 The thickness of web and flange plating shall not be lessthan:

— for longitudinal girders located lower than 4.0 m above theupper continuous deck of the hull or up to the first deck inmodules or topside deck houses: t = 5 + 0.01 L (mm),maximum 8 mm

— for longitudinal girders at higher locations or transversegirders:t = 4 + 0.01 L (mm), maximum 7 mm, minimum 5 mm.

D 300 Effective flange301 The effective plate flange area is defined as the cross-sectional area of plating within the effective flange width. Thecross section area of continuous stiffeners within the effectiveflange may be included. The effective flange width be is deter-mined by:

be = Ceb (m)

Figure 1 Graphs for the effective flange parameter Ce

D 400 Effective web401 Cut-outs in the web of girders are generally accepted,provided the shear stress level, buckling capacity and fatiguelife are acceptable.

D 500 Strength requirements for simple girders501 Simple girders subjected to lateral loads and which arenot taking part in the overall strength of the unit, shall complywith the following: 502 Minimum section modulus Section modulus Zg:

503 Minimum web area after deduction of cut-outs:

The web area at the middle of the span is not to be less than0.5 AW.

Ce = parameter given in Figure 1 for various numbers of evenly spaced point loads (Np) on the girder span

b = full breadth of plate flange in m, e.g. span of the sup-ported stiffeners, or distance between girders

t 1. 25 l 0.5 s–( ) spf1

-------------------------------- (mm)=

l0 = distance between points of zero bending moments in m = Sg for simply supported girders = 0.6 Sg for girders fixed at both ends

Sg = girder span as if simply supported.

Sg = girder span in m. The web height of in-plane girders may be deducted. When bracket(s) are fitted at the end(s), the girder span Sg may be reduced by two thirds of the bracket arm length(s), provided the girder end(s) can be assumed clamped, and that the section modulus at the end(s) of the girder is satisfactory. The brackets may be included in the calculation of section modulus.

b = breadth of load area in m (plate flange), b may be deter-mined as:

= 0.5 (l1 + l2) where l1 and l2 are the spans of the supported stiffeners on both sides of the girder, respectively, or dis-tance between girders

p = local design load in Sec.4 B and Ekm = bending moment factor, see Table D1 kτ = shear force factor, see Table D1ηp = permissible utilisation factors as given in Sec.3τp = permissible shear stress in N/mm2

0.39 fy for load combination a) 0.46 fy for load combination b)

Ns = number of supported stiffeners on girder spanPp = average “point load” from stiffenerfy = specified minimum yield stress of the material in N/mm2

( )362

10 mmfkpbS

Zypm

gg η

=

)(10 23 mmPNpbSk

Ap

psgW τ

τ −=

DET NORSKE VERITAS


504 The km value in 501 may be calculated according to gen-eral beam theory. In Table D1, km-values are given for somedefined load and boundary conditions. Note that the smallestkm-value shall be applied to simple girders. For girders wherebrackets are fitted or the flange area has been partly increaseddue to large bending moment, a larger km-value may be usedoutside the strengthened region.

E. Complex girder systems

E 100 General description101 For girders that are parts of a complex 2- or 3-dimen-sional structural system, a complete structural analysis shall becarried out to demonstrate that the stresses are acceptable withrespect to yield and buckling.102 The method used in the analysis shall be capable ofdescribing the physical behaviour of the structure whenexposed to the required load levels in the limit states consid-ered. 103 For girder systems consisting of slender girders, theassessment for all load combination Sec.3 Table C1 can nor-mally be based on elastic beam theory. Due attention shall begiven to:

— shear area variation, e.g. due to cut-outs — moment of inertia variation — effective flange — lateral buckling of girder flanges.

E 200 Loads201 Both global and local loads as defined in Sec.4 shall beconsidered. The relevant load combinations given in Sec.3Table C1 shall be addressed.

E 300 Impact from connecting structure301 The impact of structures connected to the part coveredby the capacity model shall be included in the assessment ofthe girders.

F. Buckling stability

F 100 Bars, beams, columns and frames101 Elements with cross sections which do not satisfy therequirements to cross section type III defined in Appendix A,shall be checked for local buckling. 102 Buckling analysis shall be based on the characteristicbuckling resistance for the most unfavourable buckling mode.103 The characteristic buckling strength shall be based onthe 5th percentile of test results.104 It shall be ensured that there is conformity between theinitial imperfections in the buckling resistance formulas andthe tolerances in the applied fabrication standard.

Guidance note:If buckling resistance is calculated in accordance with Classifi-cation Note 30.1 for bars and frames, the tolerance requirementsgiven in DNV-OS-C401 should not be exceeded, unless specifi-cally documented.


F 200 Flat plated structures and stiffened panels201 The buckling stability of plated structures may bechecked according to DNV-RP-C201.

F 300 Tubulars301 Tubular members may be checked according to Classi-fication Note 30.1 or API RP 2A - WSD. For interactionbetween local shell buckling and column buckling, and effectof external pressure, DNV-RP-C202 may be considered.302 Cross sections of tubular member are divided into differ-ent types dependent of their ability to develop plastic hingesand resist local buckling. Effect of local buckling of slendercross sections shall be considered.

Guidance note:

a) Effect of local buckling of tubular members without exter-nal pressure, i.e. subject to axial force and/or bendingmoment) are given in Appendix A, cross section type IV.Section 3.8 of DNV-RP-C202 may be used.

b) Effect of local buckling of tubular members with externalpressure need not be considered for the following diameterDm to thickness t ratio:

where

E = modulus of elasticity andfy = minimum yield strength.


303 Tubular members with external pressure, tubular jointsand conical transitions may be checked according to API RP2A-WSD.

F 400 Capacity checks according to other codes 401 Stiffeners and girders may be checked according to pro-visions for beams in recognised standards such as AISC-ASD.

Guidance note:The principles and effects of cross section types are included inthe AISC-ASD.


Table D1 Values of km and kτLoad and boundary conditions Bending moment and shear

force factorsPositions 1 2 3

1Support

2Field

3Support

km1kτ1

km2-

km3kτ3

120.5

24 120.5

-0.38

14.2 80.63

-0.5

8 -0.5

150.3

23.3 100.7

-0.2

16.8 7.50.8

-0.33

7.8 -0.67

Dmt

-------- 0.5 Efy----≤

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SECTION 6ASSESSMENT OF HULL – TOPSIDE INTERFACE

A. IntroductionA 100 General considerations101 The overall principles for assessment of hull-topsideinterface are given in Sec.3, both for an integrated hull-topsideanalysis and for separate capacity models for the topside struc-ture. This section gives provisions for checking of ultimatestrength for typical hull - topside interface structure.102 Topside supporting structure is the structural elementsof which the strength and fatigue capacities may be affected bythe presence of the topside structure. This includes elementslike support stools for topside modules, and the parts of the hullstructure where the additional stresses from the topside struc-ture is of such a magnitude that the yield, buckling and fatiguecapacities need to be assessed.103 In case of separate local models for hull and topsidestructures, part of the topside structure may be required to beincluded in the hull model to ensure that the reaction forcesfrom the topside model will be applied to the hull model at alocation which will have negligible impact on the stress distri-bution in the hull model.

B. Strength assessmentB 100 General101 The structural strength of the supporting structure oftopside structures shall be documented by means of Finite Ele-ment (FE) analyses, or equivalent methods. Typical supportingstructures to be analysed are:

— drill floor substructure — module supports — support structure for rail for cranes and other heavy equip-

ment like BOP, X-mas trees, etc.

B 200 Requirements to the FE model201 The extent of the model shall be based on requirementsto determine the stress distribution from:

— hull girder bending moments and shear forces— local loads from equipment, lateral pressures in tanks and

such.

202 The boundary conditions applied to the model shall notintroduce significant errors in the structural response.203 The mesh size shall be sufficient to determine the stressdistribution in relation to the acceptance criteria used.

204 When peak stress criterion is applied in the assessment,the mesh shall be such that the area which exceeds the yieldstress of the material is determined to such a degree that it ispossible to evaluate the impact on adjacent elements.

Guidance note:Element size of 50 by 50 mm is typically required in such areas.


B 300 Loads301 The hull girder bending moments and shear forces in thesagging and hogging conditions shall be applied. The stillwater values shall not be smaller than the Permissible StillWater Bending moment values. The still water shear forcesshall be corrected according to the DNV Rules for Classifica-tion of Ships Pt.3 Ch.1.302 Tank pressure on elements within the model and loadsfrom heavy equipment shall be included provided the responsefrom these loads will increase the stresses in the topside sup-porting structure. 303 Both static loads and inertial forces from hull accelera-tions shall be applied. In case of separate models for the top-side structure, all six reaction load components shall be appliedat the interface.

B 400 Combination of loads 401 The dynamic loads combination may account for jointprobability of occurrence. Unless direct analysis is carried outto determine the phases between the dynamic responses, theloads may be combined according to Table E1 and E2 in Sec.4.

B 500 Acceptance criteria501 The yield and buckling capacity for the structural ele-ments below and including the deck on which the topside isconnected shall comply with the requirements in DNV Rulesfor Classification of Ships Pt.3 Ch.1. Alternatively the assess-ment may be based on the design principles given in Sec.3 inthis standard.

C. Fatigue assessment

C 100 General101 The fatigue life of the topside supporting structure shallbe documented according to the principles and requirementsgiven in Sec.7 Fatigue Capacity Assessment.

DET NORSKE VERITAS


SECTION 7FATIGUE CAPACITY ASSESSMENT

A. Introduction

A 100 General101 This section gives provisions for assessment of fatiguecapacity of the structural details in the unit. The assessmentshall account for all significant loads contributing to fatiguedamage.102 In the assessment of fatigue life, consideration shall begiven to the stress concentration factors from fabricationimperfections which exceed the values included in the S/Ncurves.

B. Principles and methodology

B 100 Assessment principles101 The accumulated fatigue damage from the transit andoperating conditions shall be calculated according to the oper-ational characteristics of the unit. The fatigue life shall be cal-culated considering the combined effects of global and localstructural response. 102 The resistance against fatigue is normally given as S-Ncurves, i.e. stress range (S) versus number of cycles to failure(N) based on fatigue tests. Fatigue failure should be defined aswhen the crack has grown through the thickness. 103 The required fatigue life of new units shall be minimum20 years assuming that the unit complies with the DNVrequirements for dry-docking inspection. A design fatigue fac-tor (DFF) of 1.0 is thus acceptable for all structural elementswhich are accessible for inspection and repair during docking.Higher DFF according to DNV-OS-C102 Appendix A shouldbe used in case the structure is not accessible for inspection. 104 The effect of mean stresses may be accounted foraccording to guidelines given in CN.30.7.105 The stresses may be based on gross thicknesses (i.e.without deducting the corrosion additions).

B 200 Methods for fatigue capacity201 The fatigue analysis should be based on S-N data, deter-mined by fatigue testing of the considered welded detail, andthe linear damage hypothesis. When appropriate, the fatigueanalysis may alternatively be based on fracture mechanics.202 Acceptable analysis methods for calculation of the accu-mulated damage are given in DNV Classification Note 30.7and DNV-RP-C203.

Guidance note:Requirements to fatigue assessment may be given by additionalDNV Class Notations like CSA-2 of FMS.


C. Structural Details and Stress Concentration Factors

C 100 General101 Fatigue sensitive details in the hull shall be documentedto have sufficient fatigue strength. Particular attention shouldbe given to the following details:

— main deck, including deck penetrations, bottom structureand side shell

— hull longitudinal stiffener connections to transverseframes and bulkheads

— shell plate below the draught in full load condition— hopper tank knuckles and other relevant discontinuities— attachments, foundations, supports etc. to main deck and

bottom structure— topside and hull connections including substructure for

drill floor— openings and penetrations in longitudinal members— transverse frames.

102 Stress concentration factors of local details may bedetermined according to Classification Note 30.7. For detailsnot covered by Classification Note 30.7, or documented inother recognised publications, detailed finite element analysisshould be carried out for determination of SCFs, according tothe procedure given in Classification Note 30.7.

D. Design Loads and Calculation of Stress Ranges

D 100 Local and global loads101 The accumulated damage in transit may be based on theWorld Wide scatter diagram as given in CN 30.7. 102 In the operating conditions the site specific scatter dia-gram(s) should be used as basis for the calculations.103 Typical local load effects to be considered are:

— vortex shedding— external sea pressure— tank pressure.

104 Typical global loads to be considered are:

— wave bending moments and shear forces— horizontal hull deformations due to wave bending moment— vertical deformation at stools due to differences in stiff-

ness of hull supporting structure — wave induced accelerations (inertia loads).

105 The global and local load effects shall be combinedaccording to the procedures given in CN 30.7.

DET NORSKE VERITAS


SECTION 8ACCIDENTAL CONDITIONS

A. GeneralA 100 General101 Safety assessment is carried out according to the princi-ples given in DNV-OS-A101 for relevant accidental scenarios.102 The overall objective for design with respect to acciden-tal conditions is that unit's main safety functions shall not beimpaired by accidental events. Satisfactory protection againstaccidental damage may be achieved by two barriers:

— reduction of damage probability — reduction of damage consequences.

103 The design against accidental loads may be done bydirect calculation of the effects imposed by the loads on thestructure, or indirectly, by design of the structure as tolerableto accidents.

B. Design CriteriaB 100 General101 Structures shall be checked for accidental loads in twosteps, according to the loading conditions presented in Sec.3Table C1:

— resistance of the structure against design accidental loads,i.e. loading condition c)

— post accident resistance of the structure against environ-mental loads after accidental damage, i.e. loading condi-tions d) and e).

The unit shall be designed for environmental condition corre-sponding to 1 year return period after accidental damage. Typical accidental loads are:

— impact from dropped objects— fires— explosions.

102 Generic values of accidental loads are given in DNV-OS-A101.103 The different types of accidental loads require differentmethods and analyses to assess the structural resistance. Localexceedance of the structural capacity is acceptable providedredistribution of forces due to yielding, buckling and fractureis accounted for.104 The inherent uncertainty of the frequency and magni-tude of the accidental loads, as well as the approximate natureof the methods for determination of accidental load effects,shall be recognised. It is therefore essential to apply soundengineering judgement and pragmatic evaluations in the

design.105 If non-linear, dynamic finite element analysis is appliedfor design, it shall be verified that all local failure modes (e.g.strain rate, local buckling, joint overloading, and joint fracture)are accounted for implicitly by the modelling adopted, or elsesubjected to explicit evaluation.

B 200 Dropped objects201 Critical areas for dropped objects shall be determined onthe basis of the actual movement of potential dropped objects,e.g. crane or other lifting operation mass, relative to the struc-ture of the unit itself. Where a dropped object is a relevant acci-dental event, the impact energy shall be established and thestructural consequences of the impact assessed.202 Critical areas for dropped objects should be determinedassuming a minimum drop direction within an angle of 10°with the vertical direction.

B 300 Fires301 The structure that is subjected to a fire shall maintainsufficient structural strength before evacuation has occurred.The following fire scenarios shall be considered:

— jet fires — fire inside or on the hull — fire on the sea surface.

Assessment of fire may be omitted provided fire protectionrequirements made in DNV-OS-D301 are met.

B 400 Explosions401 One or more of the following main design philosophieswill be relevant:

— ensure that hazardous locations are located in unconfined(open) locations and that sufficient shielding mechanisms(e.g. blast walls) are installed

— locate hazardous areas in partially confined locations anddesign utilising the resulting, relatively small overpressure

— locate hazardous areas in enclosed locations and installpressure relief mechanisms (e.g. blast panels) and designfor the resulting overpressure.

402 As far as practicable, structural design accounting forlarge plate field rupture resulting from explosion actionsshould be avoided due to the uncertainties of the actions andthe consequences of the rupture itself.403 Structural support of blast walls and the transmission ofthe blast action into main structural members shall be evalu-ated when relevant. Effectiveness of connections and the pos-sible outcome from blast, such as flying debris, shall beconsidered.

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SECTION 9WELDING AND WELD CONNECTIONS

A. Introduction

A 100 General requirements101 The technical requirements for the welding and weldconnections shall, as a minimum, comply with the Rules forClassification of Ships Pt.3 Ch.1 Sec.11.102 Full penetration welds shall be used if weld improvements(e.g. grinding) is needed to achieve required design fatigue life,unless the fatigue life at the weld root is documented.103 Deep penetration welds are acceptable in areas where thedesign load is primarily static or shear. As a guide, if the staticcompression stress constitutes more than 35% of the yield stress,deep penetration welds may be used with a root face of t/3.

B. Size of Welds

B 100 Double continuous fillet welds101 Double continuous fillet welds shall be dimensionedaccording to principles given in DNV Rules for Classificationof Ships Pt.3 Ch.1 Sec.11. The Table C1 has been extended toinclude C factors for typical offshore members, and are shownin Table B1 below.

B 200 Fillet welds and deep penetration welds subject to high tensile stresses201 Fillet welds and deep penetration welds subject to hightensile stresses shall be dimensioned according to principlesgiven in DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.11.

B 300 Full penetration welds301 In addition to the full penetration welds required forjoints specified by the Rules for Classification of Ships Pt.3Ch.1 Sec.11, full penetration welds shall be used for the fol-lowing connections:

— crane pedestal to deck plating — topside support stools to main deck 1) — flare to hull structure — drillfloor support structure to main deck 1).

1) Ref. Section 2 for details

B 400 Direct calculations401 The distribution of forces in a welded connection may becalculated on the assumption of either elastic or plastic behav-iour.402 Residual stresses and stresses not participating in thetransfer of load need not be included when checking the resist-ance of a weld. This applies specifically to the normal stressparallel to the axis of a weld.403 Welded connections shall be designed to have adequatedeformation capacity.404 In joints where plastic hinges may form, the welds shallbe designed to provide at least the same design resistance asthe weakest of the connected parts.405 In other joints where deformation capacity for joint rota-tion is required due to the possibility of excessive straining, thewelds require sufficient strength not to rupture before generalyielding in the adjacent parent material.

Guidance note:In general this will be satisfied if the design resistance of the weldis not less than 80% of the design resistance of the weakest of theconnected parts.


406 The design resistance of fillet welds is adequate if, atevery point in its length, the resultant of all the forces per unitlength transmitted by the weld does not exceed its designresistance.407 The design resistance of the fillet weld will be sufficientif both the following conditions are satisfied:

and

Table B1 Weld factor C

Item 60% of span

At ends

Local buckling stiffeners 0.14 0.14Stiffeners, frames, beams or longitudinals to shell, deck, oil tight or water tight girders or bulkhead plating, except in after peaks. Secondary stiffeners in topside structures.

0.16 0.26

Web plates of non-watertight girders except in after peak. 0.20 0.32

Girder webs and floors in double bottom. Stiffen-ers and girders in after peaks. Main girder system in topside structures of stiff-ened plate design type.Horizontal stringers on transverse bulkheads.

0.26 0.43

Watertight centre line girder to bottom and inner bottom plating.Boundary connection of ballast and liquid cargo bulkhead:

— longitudinal bulkheads— transverse bulkheads.

0.52Hatch coamings at corners and transverse hatch end brackets to deck.Top horizontal profile to coaming.Strength deck plating to shell scuppers and dis-charges to deck.Main girder system in topside structures of frame-work design type. Fillet welds subject to compressive stresses only. 0.25All other welds not specified above. 0.43

σ⊥ = normal stress perpendicular to the throat τ⊥ = shear stress (in plane of the throat) perpendicular to the

axis of the weld τ|| = shear stress (in plane of the throat) parallel to the axis

of the weld, see Table B1 fu = nominal lowest ultimate tensile strength of the weaker

part joined βw = appropriate correlation factor, see Table B1 η0 = basic usage factor, see Sec.3 C400

( ) 022

||2 3 η

βττσ

w

uf≤++ ⊥⊥

0ησ uf≤⊥

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Figure 1 Stress components in a fillet weld

Table B1 The correlation factor βw Steel grade Lowest ultimate tensile strength

fu Correlation factor

βw NV NS 400 0.83 NV 27 400 0.83 NV 32 440 0.86 NV 36 490 0.89 NV 40 510 0.9 NV 420 530 1.0 NV 460 570 1.0

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SECTION 10CORROSION CONTROL

A. Hull and hull structural elementsA 100 General101 The corrosion protection of the hull and it’s structuralmembers shall comply with the requirements in DNV Rulesfor Classification of Ships Pt.3 Ch.1.

B. Topside structureB 100 Void spaces and elements in the atmospheric zone101 Steel surfaces in topside structure except tanks shall be

protected by a suitable coating system proven for marineatmospheres.102 Tanks for fresh water shall have a suitable coating sys-tem. Special requirements will apply for coating systems to beused for potable water tanks.

B 200 Tanks Tanks for liquids shall have a corrosion protection systemaccording to DNV Rules for Classification of Ships Pt.3 Ch.1.

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Offshore Standard DNV-OS-C107, April 2008 App.A –

APPENDIX A CROSS SECTIONAL TYPES

A. Cross Sectional TypesA 100 General101 Cross sections of beams are divided into different typesdependent on their ability to develop plastic hinges as given inTable A1.

Figure 1 Relation between moment M and plastic moment resistance Mpand rotation θ for cross sectional types. (My is elastic moment re-sistance.)

102 The categorisation of cross sections depends on the pro-portions of each of its compression elements, see Table A3.

103 Compression elements include every element of a crosssection which is either totally or partially in compression, dueto axial force or bending moment, under the load combinationconsidered.104 The various compression elements in a cross sectionsuch as web or flange, can be in different classes.105 The selection of cross sectional type is normally quotedby the highest or less favourable type of its compression ele-ments.

A 200 Cross section requirements for plastic analysis201 At plastic hinge locations, the cross section of the mem-ber which contains the plastic hinge shall have an axis of sym-metry in the plane of loading.202 At plastic hinge locations, the cross section of the mem-ber which contains the plastic hinge shall have a rotationcapacity not less than the required rotation at that plastic hingelocation.

A 300 Cross section requirements when elastic global analysis is used301 When elastic global analysis is used, the role of crosssection classification is to identify the extent to which theresistance of a cross section is limited by its local bucklingresistance.302 When all the compression elements of a cross section aretype III, its resistance may be based on an elastic distributionof stresses across the cross section, limited to the yield strengthat the extreme fibres.

Table A1 Cross sectional typesI Cross sections that can form a plastic hinge with the rotation

capacity required for plastic analysisII Cross sections that can develop their plastic moment resist-

ance, but have limited rotation capacityIII Cross sections where the calculated stress in the extreme com-

pression fibre of the steel member can reach its yield strength, but local buckling is liable to prevent development of the plas-tic moment resistance

IV Cross sections where it is necessary to make explicit allow-ances for the effects of local buckling when determining their moment resistance or compression resistance

Table A2 Coefficient related to relative strainNV Steel grade 1) ε 2)

NV-NS 1NV-27 0.94NV-32 0.86NV-36 0.81NV-40 0.78NV-420 0.75NV-460 0.72NV-500 0.69NV-550 0.65NV-620 0.62NV-690 0.58

1) For steels of improved weldability the required specified mini-mum yield stress is reduced for increasing material thickness, see DNV-OS-B101

2) ε 235fy

--------- where fy is yield strength=

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Offshore Standard DNV-OS-C107, April 2008 – App.A

Table A3 Maximum width to thickness ratios for compression elementsCross section part Type I Type II Type III

d = h - 3 tw 3)

d / tw ≤ 33 ε 2) d / tw ≤ 38 ε d / tw≤ 42 ε

d / tw ≤ 72 ε d / tw ≤ 83 ε d / tw ≤ 124 ε

when α > 0.5:

when α ≤ 0.5:

when α > 0.5:

when α ≤ 0.5:

when ψ > -1:

when ψ ≤ -1:

Tip in compression Tip in compression Tip in compression

Tip in tension Tip in tension Tip in tension

d / tp ≤ 50 ε 2 d / tp ≤ 70 ε 2 d / tp ≤ 90 ε 2

1) Compression negative2) ε is defined in Table A23) Valid for rectangular hollow sections (RHS) where h is the height of the profile4) C is the buckling coefficient. See e.g. Classification Note 30.1, Table 3.2, No. 4 and 7 or Euro code 3 Table 5.3.3 (denoted kσ)5) Valid for axial and bending, not external pressure.

d tw⁄ 396ε13α 1–----------------≤

d tw⁄ 36εα

--------≤

d tw⁄ 456ε13α 1–-------------------≤

d tw⁄ 41.5εα

--------------≤

d tw⁄ 126ε2 ψ+-------------≤

d tw⁄ 62ε 1 ψ–( ) ψ≤

Rolled: c tf⁄ 10ε α⁄≤

Welded: c tf⁄ 9ε α⁄≤

Rolled: c tf⁄ 11ε≤

Welded: c tf⁄ 10ε≤

Rolled: c tf⁄ 15ε≤

Welded: c tf⁄ 14ε≤





Rolled: c tf⁄ 23ε C 4 )

≤

Welded: c tf⁄ 21ε C≤

Rolled: c tf⁄ 10εα α------------≤

Welded: c tf⁄ 9εα α------------≤

Rolled: c tf⁄ 11εα α------------≤

Welded: c tf⁄ 10εα α------------≤

Rolled: c tf⁄ 23ε C≤

Welded: c tf⁄ 21ε C≤

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DNV-OS-C107: Structural Design of Ship-shaped Drilling … · STRUCTURAL DESIGN OF SHIP-SHAPED...

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