Module 1- Structural Integrity Assessment of Offshore Structures (1)

Introduction to structural assessment

1/8/2014 1 Dr. S. NallayarasuDepartment of Ocean Engineering

Indian Institute of Technology Madras-36


1/8/2014 Dr. S. NallayarasuDepartment of Ocean Engineering


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Introduction : Data Collection; Platform classification; Risk Levels; Met-ocean criteria; Platform initiators; Assessment criteria; Long and short term sea state; Estimation of Wave height and period for reduced design life;

Assessment Procedure : Overview of existing assessment procedures from API RP 2A; Initial screening by design level analyses; load reduction; ultimate strength principles; Reserve Strength ratio;

Ultimate Strength Analysis: Basics of ultimate strength analysis; M-P- relationship; Ultimate strength of circular hollow sections; Global plastic collapse analysis; Ultimate strength of Tubular connections; Limit state principles; Ultimate and fatigue limit states;

Risk and Reliability: Introduction to probability distribution functions; Application of probability theory to wave hydrodynamics; Weibull and Gumbel distributions; Wave force modelling; Modelling uncertainties; Load and Resistance Factors; Code Calibration; Reliability Index; Probability of failure;

Fatigue and Risk Based Inspection: Paris law; Crack propagation; Fatigue Reliability; Inspection procedure and interval; Updated inspection methods; Fatigue crack measurement; Mitigation methods;




3

Text Booksa) Nonlinear analysis of offshore structures by Bjorn Skallerud and Jorgen Amdahl,

Research Studies Press Ltd, 2002.b) Handbook of Offshore Engineering by Subrata K. Chakrabarti, Elsevier, 2005.c) Probability Concepts in Engineering Planning and Design, Volume I & II Decision, Risk

and Reliability by A. H. Ang and W. H. Tang, John Wiley & Sons, 2005.d) Structural Reliability Analysis and Prediction by Robert E. Melcher, John Wiley & Sons,

1999.e) Structural Reliability, Analysis and Design by R. Ranganathan, Jaico Publication House,

2000.

Reference Book/Codesa) Assessment of structural integrity for existing offshore load bearing structures,

NORSOK Standard N-006, 2009.b) Recommended Practice for Planning, Designing and Constructing Fixed Offshore

Platforms - Working Stress Design, API RP2A-WSD 21st Edition, December 2000. Errata and Supplement 1, December 2002, Errata and Supplement 2, September 2005, Errata and Supplement 3, October 2007

c) Guidelines for offshore structural reliability analysis Application to jackets, Report No. 95-3203, Rev 01, 5th November 1996, DNV, Norway.


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Dr. S. NallayarasuDepartment of Ocean Engineering


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Major Oil and Gas fields areMumbai High NorthMumbai High South Bassien Tapti HeeraMukta Neelam

Over 200 Fixed jacket type platforms

Many are older than 20 years

Western Offshore Field Map


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MUMBAI HIGH

BASSEIN

HEERA

NEELAM

SCALE0 km 10 km 20 km 30 km 40 km

BS-13 BS-16

BS-12

710000 720000 730000 740000 750000 760000 770000 780000 790000 800000 810000 820000 830000 840000 850000 860000

2060000

2070000

2080000

2090000

2100000

2110000

2120000

2130000

2140000

2150000

2160000

2170000

2180000

PannaField (JV)

MuktaField (JV)

NQO

BHN

ICW

SH

BPA

BPBB-192

B-45

B-46

B-48B-188

B-105

WO-24

WO-16WO-15

WO-5

B-121/119

D-1

D-33

D-18

B-15

B-157B-59 B-127

B-147B-149

B-55

VASAI EAST (BSE)

B-172B-178

B-179B-180

B-173AB-193

B-37

B-134

B-22

B-80

B-23AB-28

B-28A

Marginal Fields around Mumbai High and Bassein Fields

26 " Gas trunk line to Uran

24" Oil tru

nk line to

Uran

26" Gas tr

unk line t

o Uran

30" X 203 km Oil trunk line to Uran

36"

X 2

31 k

m G

as tr

unk

line

Bass

ein

to H

azira

18" o

il and

gas

line

42"

X 2

44 k

m G

as tr

unk

line

BPB

to H

azira

BHS

30" X 142 km SHP - HEERA Line

16" X

77 km

B-55

to SHG

28" X 78 km SH - BPB Line

INDEX OIL LINE GAS LINE

OIL FIELD

GAS FIELD

OIL & GAS FIELD

SOUR FIELD

HIGHLY SOUR FIELD

SWEET OIL/GAS

#

# #

##

####

##

##

##

##

SBM

##

## #

#

#

#

#

BHE-1

14" OIL12" GAS

#

#

#

Field Map Western Offshore, India


1/8/2014 Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of

Technology Madras-36

Structural Integrity Assessment

The assessment can be categorized into four cases

Existing platforms requiring recertification due to design life extension beyond original design life.

Existing platforms that require assessment due to modified loading/strength during service.

Existing Platforms that are damaged during an accident

New Platform Design for extreme accidental loading

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Platform Types and Exposure CategoriesPlatforms can be classified based on usage, type of construction and categorized based on reserve strength

No Usage Types of construction1 Wellhead Platforms Jacket or template structure

Tower StructuresTripod StructuresMonopod Structures

2 Process Platforms Jacket or Template structureFloating StructuresGravity Platforms

3 Living Quarters Platform Jacket or Template structureFloating Structures

4 Flare Towers Jacket or template structureTower Structures

5 Bridge Support structures Tower Structures




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Minimum StructuresMinimum Structures have been successfully used in marginal field developments across the world. The comparison between conventional jacket or template type structure and the minimum structure is given below.

Template Structures Minimum StructuresThree or more legs Single Leg structuresSteel piles driven in to the seabed sufficiently deeper.

No template or unconventional template

Can be designed to suit for various functions such as drilling, processing and living facility

Not feasible for large topsides and hence used for wellhead or drilling facility

Large number of wells can be supported

Can only be designed for minimum number of wells for drilling and tie-backto process platform

Adequate reserve strength and redundant design

Non-redundant frame and hence no reserve strength available




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Minimum structures are defined as structures which have one or more of the following attributes:

Structural framing, which provides less reserve strength and redundancy than a typical well braced, three-leg template type platform.

Free-standing and guyed caisson platforms which consist of one large tubular member supporting one or more wells.

Well conductor(s) or free-standing caisson(s), which are utilized as structural and/or axial foundation elements by means of attachment using welded, non-welded, or nonconventional welded connections.

Threaded, pinned, or clamped connections to foundation elements (piles or pile sleeves).

Braced caissons and other structures where a single element structural system is a major component of the platform, such as a deck supported by a single deck leg or caisson.


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CONVENTIONAL WELL PLATFORM CONFIGURATIONS

Main Pile of 60 INCH 68 INCH 72 INCH 84 INCH 90 INCH

SINGLE PILE CONFIGURATION

SKIRT PILE (ONLY) CONFIGURATION

SKIRT & MAIN PILE CONFIGURATION

Skirt Piles of 84 SKIRT WITH 48 LEG 84 SKIRT WITH 60 LEG FULL BRACING 84 SKIRT WITH 60 LEG

Combinations of 54 SKIRT WITH 48 MAIN PILE 54 SKIRT WITH 54 MAIN PILE 60 SKIRT WITH 54 MAIN PILE




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CONVENTIONAL WELL PLATFORM CONFIGURATIONS


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Minimum Structures

UK Norway Australia


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Mono Pile Concepts

Minimum structures concepts

Braced Conductor legs Guy Supports structure


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Mono Pile Concepts

Mono Pile without Guy Wire

Braced Mono Tower (Inside Conductor)

Braced Mono Tower (Outside Conductor)

Mono pile conceptsinvolve driving of largediameter pile andsupporting the deck fromthe single leg. This canbe augmented byadditional skirt piles inorder to reduce largebending of mono piles.The mono pile houses 3or 4 conductors insidethus reducing the waveloads.

Another alternative tothis is to haveconductors outside themono pile.

1(a) 1(b) 1(c)


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4 Legged Jacket Structure with Batter Piles

4 Legged Jacket Structure

Jacket Type Concepts

3 Legged Jacket Structure

Jacket type conceptsinvolves 3 or 4 legs withconductors inside thejacket framing. Thejacket legs are eitherbattered or vertical.Three alternate schemeare proposed are shownin figure.

The above concepts canbe extended to waterdepths exceeding 30mand has the flexibility ofincrease in number ofwells or topsideconfigurations.

2(a) 2(b) 2(c)


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Braced Conductor Leg Concepts

Braced Leg Jacket (4 Piles)

Braced Leg Jacket (3 Piles)

In this concepts four conductor cumlegs are braced to form frame whichwill be fixed to the seabed by skirtpiles.

The advantage of these concepts isthat the wave loads is reducedconsiderably since the jacket legs andframing near water level is reduced.

3 (a) 3 (b)


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Guy support Structures

4 Legged Jacket with Hollow Base Steel Caisson

4 Legged Jacket with Steel Caisson with Each Legs

Mono Pile with Guy Wires

The slender structure as proposed earlier are transversely supported by guywires to reduce lateral deflection and bending stresses. Further the supportreaction in terms of pile loads will be reduced considerably.

4(a) 4(b) 4(c)


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Jacket Platforms Design life varies from 25 - 30

years. No. of wells varies from 4 16. Water depth ranges from 20m

100m. Two level deck with the dimension

of 20m x 40m. Large space (40 x 20) for CTU

operation Separate Helideck is provided. Platform crane provided. Boat landing is provided. Total topside weight is in the order

of 2000 2500 Tonnes Modular rig such as Sundowner VI

or VII is allowed. Unmanned platform with temporary

two or four man bunk house

Minimum Structures Design life varies from 5 10

years. No. of wells varies from 2 - 4. Water depth ranges from 20m

60m. Two level deck with the dimension

of 20m x 20m. No separate Helideck is provided.

Main deck can be used as helideck. No Pedestal crane provided. V notch ladder type Boat landing is

provided. Total topside weight is less than

750 Tonnes No Modular rig is allowed. Unmanned platform. No temporary bunk house

provided.


1/8/2014 Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of

Technology Madras-36

Exposure CategoriesPlatform exposure can be categorized in to three types based on life safety and consequences of failure.

The exposure category to be used for the design of the platform shall be based on most critical from life safety and failure consequences.

During the design life of the platform, exposure category can be modified based on operational data at that time.

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Exposure category

Life Safety Failure Consequences

L-1 Manned Non-evacuated HighL-2 Manned - Evacuated MediumL-3 Unmanned Low




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Life Safety Categories L-1 Manned Non-evacuated : The platform such as process or living quarters

platform occupied by personnel for the operation of the platform is classifiedunder this category. It may not be possible to evacuate the personnel from theplatform due to practical reasons or it is not intended due to operationalrequirements.

L-2 Manned Evacuated : The platform used for non-production purposessuch as water injection, or non-critical processing platforms can be classifiedin to this category. In case of design environmental event, platform can beshut down and evacuation is feasible. Some process platforms can also comeunder this category as the design can be planned for shutdown in case ofemergency.

L-3 Unmanned : Remote wellhead platforms or satellite platforms connectedto processing platform in the field comes under this category. Normally nomanning is required as all the operations can be controlled from the processplatform or from the control centre on land. This type of platform may haveday shelters for visitors occasionally.




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Failure Consequence Categories L-1 High Consequences : Major process platform or large well platform

with no planned shut down during design environmental event comes underthis category. Further, platforms connected with large trunk pipelines withfor transporting oil or gas to the land shall be included under this group.

L-1 Medium Consequences : Wellhead Platforms planned for shut downduring design event will be coming under this category. Normally providedwith subsea control valves for isolation and will have provision to shut thewells. No oil storage will be available in such platforms.

L-3 Low Consequences : Minimum platform structures planned formarginal field can be grouped under this category. Normally planned to shutduring any design event. Such remotely operated platforms will havecertified subsea valves to isolate the platform from subsea pipelineconnection to the other platforms thus reducing the spread of risk to otherplatforms.




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Platform Assessment InitiatorsAn existing platform even during its service may require to undergo the assessment process if one or more conditions exist. Design life exceeded due to operation beyond planned life. This may happen

due to continuing oil an gas production beyond original reservoir planning during initial estimation.

Change in platform category due to addition of living facilities in a platform originally not designed with living facility.

Addition of facilities due to change in production profile of well fluid during the life time.

Increased loading on structure due to change in environmental loading from wind, wave or current. Better defined recently than earlier.

Inadequate deck height due to increased water levels due to change in environmental parameters such as wave, storm surge and tides.

Damage found during inspection due to collision or fire




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The following information shall be available for assessment: as built drawings of the structure; new information on environmental data, if relevant; permanent actions and variable actions; previous and future planned functional requirements; design and fabrication specifications; original corrosion management philosophy; original design assumptions; design, fabrication, transportation and installation reports which should include

information about material properties (e.g. material strength, elongation properties and material toughness test values or concrete strength development), weld procedure specifications and qualifications, modifications and weld repairs during fabrication, non-destructive testing (extent and criteria used), pile driving records (action effects during pile driving and number of blows);

weight report that is updated during service life;

Data Collection




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weight report that is updated during service life; in-service inspection history including information on marine growth, corrosion,

cracks, dents and deflections, scour, damages due to frost, impact, dents, erosion/abrasion, chloride intrusion, leakages, sulphate attacks;

information on in-place behaviour including dynamic response (measurements and observations);

information and forecast for seabed subsidence; information on modifications, repair and strengthening to the structure during

service; oil conditions, pore pressures and consolidation; experience from similar structures.

Data Collection




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Platform Assessment Category

A-1 - High assessment category

A-2 - Medium assessment category

A-3 - Low assessment category


1/8/2014Dr. S. Nallayarasu

Department of Ocean EngineeringIndian Institute of Technology Madras-

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Assessment Category

Exposure Category Design level analysis

(see notes 1 & 2)

Ultimate strength analysisConsequence

of failureLife safety

A-1 High Manned-Non-Evacuated, Manned Evacuated or Unmanned

High Consequence/design level analysis loading

High Consequence ultimate strength analysis loading

A-2 Medium Manned-Evacuated or Unmanned

Sudden hurricane design level analysis loading

Sudden hurricane ultimate strength analysis loading

A-3 Low Unmanned Minimum consequence design level analysis loading

Minimumconsequence ultimate strength analysis loading

ASSESSMENT CRITERIA-U.S GULF OF MEXICO




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ASSESSMENT CRITERIA- OTHER U.S AREAS

Assessment Category

Exposure Category Design level analysis

(see notes 1 & 2)

Ultimate strength analysisConsequence

of failureLife safety

A-1 High Manned-Non-Evacuated or Unmanned

85% of lateral loading caused by 100-year environmental conditions

(RSR)=1.6

A-3 Low Unmanned 50% of lateral loading caused by 100-year environmentalconditions

(RSR) =0.8

Notes:1. Design level analysis not applicable for platforms with inadequate deck height.2. One-third increase in allowable stress is permitted for design level analysis (all

categories)




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CODES AND STANDARDSFollowing codes can be used for platform reassessment

API RP 2A Recommended Practice for Planning, designing and Constructing Fixed Offshore Platforms, Errata and Supplement 3, October 2007.

N-006 Assessment of structural integrity for existing offshore load-bearing structures.




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ASSESSMENT PROCESSAssessment of existing structures shall be undertaken if any of the initiators specified are triggered. The purpose of such an assessment is to demonstrate that the structure is capable of carrying out its intended functions in all phases of their life cycle.

The assessment process shall include or be based on

Design, fabrication and installation resume and as-built drawings. Documentation of as-is condition, Planned changes and modifications of the facility Updated design basis and specifications Calibration of analysis models to measurements of behavior if such

measurements exists. The history of degradation and incidents Prediction of future degradation of future performance of the structure A documentation of technical and operational integrity Planned mitigations A plan or strategy for the maintenance and inspection




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The assessment of life extension shall conclude on a safe life extension period with respect to technical and operational integrity of the facility.

The assessment shall further identify the circumstances that will limit the life of the faculty without major repairs or modifications, and specify criteria defining safe operation (e.g. permissible cracks lengths, permissible corrosion or remaining thickness, remaining anodes, degrading anodes, degrading of paint protection, subsidence, deteriorating compounds (such as H2S, stagnant water), changed load conditions, deteriorated mechanical outfilling) including appropriate factors of safety.




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DOCUMENTATION OF STRUCTURAL ASSESSMENTThe general requirements to documentation as given in NORSOK N-001 applies also for assessment of existing structures. In addition the following aspects should be documented, if relevant:

Reason for the assessment (assessment initiator) Basis for the condition assessment:

Performance history; As-is condition; Expected future development based on experience.

Reference documents for the assessment including how the integrity of maritime systems and structures relates to regulations and standards.

Assessment analyses and results Maintenance plans for ensuring sufficient integrity including how to monitor and identify

degradation and ageing, and the necessary future mitigations as a result of such degradation

Description of necessary mitigations, including plan for replacement and need for future repairs of structures and maritime systems.

Plans for how to ensure sufficient competence being in place to operate and maintain the facility.




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ASSESSMENT PROCESS

Platform selection Categorization Condition assessment Design basis check Analysis check Consideration of mitigations

The assessment process consists of following steps.




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The structural analysis to be performed on the existing structure to determine its structural integrity and ability to sustain the future loading can be done in stages.

Design level analysis Ultimate strength analysis.

The design level analysis is a simpler and more conservative check, while the ultimate strength analysis is more complex and less conservative. It is generally more efficient to begin with a design level analysis, only proceeding with ultimatestrength analysis as needed.

However, it is permissible to bypass the design level analysis and to proceed directly with an ultimate strength analysis.

If an ultimate strength analysis is required, it is recommended to start with a linear global analysis proceeding to a global inelastic analysis only if necessary.

STRUCTURAL INTEGRITY ANALYSES




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Design level analysis procedures are similar to those for new platform design,including the application of all safety factors, the use of nominal rather than meanyield stress, etc.

DESIGN LEVEL ANALYSES

Category Design Safety Deviations

Loads Load as per API RP 2A Reduction in environmental criteria is allowed as per risk analysis

Structural Behaviour

Linear Elastic Analyses

Member Design

As per section 3 of API RP 2A

K factors may be considered from practiceinstead of stick compliance with code

Joint Design As per section 3 of API RP 2A

50% strength check on tubular connections waived

Fatigue As per section 3 of API RP 2A

Can use the information from latest inspection and survey to update the fatigue assessment

Foundation As per API RP 2A Can use as-built information




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Ultimate Strength analysis procedures are similar to those for new platformdesign, including the application of all safety factors, the use of nominal ratherthan mean yield stress, etc.

ULTIMATE STRENGTH ANALYSES

Static Push Over analysis using non-linear material and geometric properties will be carried out.

Loads corresponding to ultimate limit state shall be used Alternate procedures with static linear elastic analysis is also acceptable with

all the conservative factors in loading is removed including material safety factors.

Global inelastic analysis is intended to demonstrate that a platform has adequate strength and stability to withstand the loading criteria with local overstress and damage allowed, but without collapse.

At this level of analysis, stresses have exceeded elastic levels and modelling of overstressed members, joints, and foundations must recognize ultimate capacityas well as post-buckling behaviour, rather than the elastic load limit.




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SCREENING OF PARAMETERS

DESIGN LEVEL ANALYSIS

ULTIMATE STRENGTH ANALYSIS

FATIGUE ANALYSIS AND INSPECTION

UPDATE

MITIGATION MEASURES

PLATFORM ACCEPTABLE

FOR ADDITIONAL OPERATIONS

Platform assessment procedure

Reduced Design Life




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MITIGATION MEASURESOnce the assessment procedure shows that the platform does not have adequate strength against envisaged loads in future, it may be necessary to activate the mitigation measures including any one or more of the following.

Load shedding to reduce the topside loads (either redundant or removal means reduced functional capability).

Removal of redundant components of substructure to reduce hydrodynamic loading such caissons, risers or boat landing etc.

Removal of marine growth which reduces the hydrodynamic loading considerably. Reduced design life instead of longer expected design life which may require longer

return period for environmental loading, a shorter period can be considered. Strengthening of members and joints which have shown their inability to sustain the

loads. More frequent Inspection and monitoring so that the assumptions made during the

assessment can be verified especially with regards to the fatigue life and associated joint cracks.

Reduce acceptance criteria - Reduced Safety Margin. This may require additional risk analysis and will be a decision by the owner.

Hence, for each platform, combination of the above measures may be required.




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LOAD SHEDDING PROCESSLoad shedding may be initiated if the existing platform does not satisfy the design requirements

Removal of redundant topside facilities Periodical removal of marine growth Removal of redundant caissons, risers and boat landing




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ACCEPTANCE CRITERIA




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Acceptance criteria for AssessmentAcceptance criteria for existing platforms needs to be established considering several factors including

Original Design Criteria Latest Environmental Data History of incidents in the platform Revision codes and procedures

The intention of structural integrity assessment of existing structures is to verify their adequacy against fit for purpose for few more future years. Hence it shall be verified against the latest information!.

Existing platforms designed based on earlier revision of codes such as API RP 2A (18th and 19th Edition) may not pass through the code checks and procedures based on 21st Edition code checks especially for tubular connections as the code has gone through several changes during the past years. Hence a suitable mechanism must be established in such cases of very old platforms (> 20 years).

Similarly, the design environmental data such as wave height and current speed might have changed due to better measurement and awareness of the information and may have to be treated carefully.




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Structures Designed Using ASD Format

Allowable stress design approach uses a concept of Factor of Safety in Design.

In which RD is a Design Resistance of the structure and DD, LD and ED are design load effectsdue to dead, live and environmental loads. FOS is the design factor of safety and variesbetween 1.5 to 2.5 depending on the design category. If the FOS is taken as one, thestresses in the structure reaches yield of the material. This is the reserve strengthavailable in the structure components against failure.

However, it does not guarantee the overall system Behaviour as the structural analyseswere carried out using linear elastic principles and ultimate strength assessment has beenmade.

System redundancy and its Behaviour at increased loads beyond yield of the material hasnot been taken in to consideration.

D D D DR D L EFOS




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Structures Designed using LRFD MethodLRFD method uses partial material and load factors as shown in the equation and are called Partial Safety Factors in Design.

In which Rn is a Nominal Resistance of the structure and Dn, Ln and En are nominal loadeffects due to dead, live and enviornmental loads. is the material factor. The overallMargin of safety can be taken as

n n n nR D L E

in which =n n n nR D L E Typical values of varies from 0.8 to 0.9 and values varies from 1.2 to 1.5. Hence theoverall safety margin available also varies from 1.2 to 1.5.

However, in this method also overall system Behaviour is not considered especially strengthbeyond yield point of the material and redundancy in the system is not evaluated.




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Verification using Plastic Collapse AnalysisThe ultimate strength analyses are useful in assessing the overall capability of the structure against the loads that may be subjected to during its design life. The structure will be subjected to loads from both gravity and the environmental effects until first mode failure is called Collapse Capacity.

Ultimate strength analysis will be used and hence the structural components and connections will be subjected to stresses beyond yield until plastic hinge formation. Instability of the system will be checked and multiple modes of failure may be verified.

Since the dead load effects are considered to be constant throughout the design life and can be considered to be a non-variant and the overall system redundancy is taken as

R D L E Design Environmental load (100 year storm) will be increased until the structure collapse by increasing the factor from 1.0 to a higher value in increments.This factor is called reserve strength ratio or redundancy of the system




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Limiting Displacement and StrainThe ultimate strength analyses are based on plastic collapse method and hence limiting displacement and rotation will be the governing parameters in deciding the ultimate strength of the structure.

A limiting rotation of 0.3 radians and deflection of 1.0 is specified as the failure of the joints and members.

Overall collapse of the structure is considered when the load transfer is unable to proceed when one or more members exceed the limits of end rotation and deflection as specified above.




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ENVIRONMENTAL CONSIDERATIONS


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Dr. S. Nallayarasu Department of Ocean Engineering


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Probability Distributions

Under certain simplified assumptions, the parameters of a random wavefollow known probability distribution functions. Following variables ofinterest shall be assigned with suitable probability distribution function.

Variable Distribution RemarksWave surface elevation

Gaussian Zero mean process for long term sea state

Wave amplitude Rayleigh Fully developed seaWave height RayleighWave period WeibullExtreme wave height Gumbel Type I Short term extremes


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2

1 exp2 2

f

1a

P f dx

1

21 exp

2 2d

Gaussian (Zero mean process, =0) or normal distribution can be used to represent the surface elevation if the mean is zero.

Gaussian Probability Distribution of

The probability of the water surface elevation exceeds the given value of 1 can be determined using


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Rayleigh Probability Distribution of The Rayleigh distribution is used to represent the surface elevation and the corresponding probability density and cumulative probability are given by

22 2exp 2f s s

2where 4

s The probability of the water surface elevation exceeds the given value of a can be determined using (approximating ( = s))

1

1P f dx

1

221

2 21 exp exp

22d


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Rayleigh Probability Distribution of HThe probability of wave crest elevation can be used to express the crest to trough wave height (Hw).

2

2( ) exp 2w

wH

HP H H

The relationship between Hs and standard deviation can be used to relate the Hw and Hs. i.e. Hs = 4H

2

( ) exp 2 wws

HP H HH

N Hmax / Hs

100 1.534200 1.641500 1.7721000 1.8662000 1.9565000 2.09710000 2.15020000 2.22850000 2.328100000 2.401

The storm duration of 3 hours with a minimum of N=1000 waves, will give a probability of 1/1000 and the relationship will give a ratio of 1.86


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Gumbel Type I Probability Distribution of HThe Gumbel type I distribution is used to represent the extreme value of H in a short term data such as storm and the corresponding probability density and cumulative probability are given by

exp H uf H H u e where H u The value of is 0.577215, Euler constant. has to be determined from data recorded earlier.

exp H uF H e 22 2 6H


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ESTIMATION OF WAVE HEIGHT FOR FUTURE

Wave height Hmax100 18 m TR 100 yr

The probbability of exceedance of the above wave height in next 10 years i

TF 10 yr

P 1 expTF

TR

P 9.516 %


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If the structure is to be designed for another 50 years, what will be the maximum wave heighbe considered for the future 50 years.

TF1 50 yr

P1 1 expTF1TR

P1 39.347 %

H 15 m H 2 m

6 H

0.641 1m

uH H0.5772

uH 14.1m

Estimation based on Method 1 HF uH

lnTR

P1 yr

HF 22.736m

Alternate method HF1 Hmax100

lnTRTF1

HF1 19.081m




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Environmental conditions (Wave, Wind and Current) to which the platforms willneed to be designed or verified can be assessed depending on the life of theplatform. Typical design environmental return period of 100 years isrecommended. However, this can be reduced depending on the life of theplatform. In order to assess this, a risk analysis can be carried out by including thefollowing parameters:

Historical experience. The planned life and intended use of the platform. The possible loss of human life. Prevention of pollution. The estimated cost of the platform designed to environmental conditions for

several average expected recurrence intervals. The probability of platform damage or loss when subjected to environmental

conditions with various recurrence intervals. The financial loss due to platform damage or loss including lost production,

cleanup, replacing the platform and Re-drilling wells, etc.

Risk Analysis for Environmental Conditions




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Short Term Sea State

Short Term Sea state describes the sea surface variation during a storm typically over a period of 3 hours. From the measured records of sea surface elevation, wave statistics such as Hmax, Hrms, Hs and associated periods can be calculated. However, this represents only short term wave climate.

This may not represent the design wave for the offshore structure as the structures will be required to survive the future years. This may depend on the design life of the platform. Hence, from the short term wave statistics, long term wave parameters needs to be obtained.

This can be done using wave prediction using Gumbel extreme value distribution (Type I).




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Long Term Sea State

Long Term Sea state describes the sea surface variation during longer exposure period typically a year. From the measured records of sea surface elevation, wave statistics such as Hmax, Hrms, Hs and associated periods can be calculated. The joint distribution of wave height, period and direction will be used to describe the sea state in terms of wave scatter data.

The measured time history of wave record typically for a year can also be used for fatigue analysis for repeated cyclic loading.

Usually it is assumed that the annual exceedance data for each year will be repeated. This may not be true though such assumptions are made in the design process.




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Encounter Probability (Probability of exceedance)Encounter probability is required to determine the probability associated with the risk of consideration of the design loads. It can be calculated using

( ) 1 exp LR

DP E

T Where

L - Design lifetime

TR - Return Period of environment

For Example

- Design lifetime = 30years

- Assume 10% chance that the design wave is exceeded once

- Return period of the environment can be estimated

300.1 1 exp 1 exp

284

L

R R

R

DE

T T

T years

ln 1 ( )LRD

TP E




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Reduced Sea State for Short Design Life

API RP 2A suggests a return period of 100 years for a design life of offshore platform of 20 years.

Most platforms in India are designed for a design life of 20 to 30 years with a 100 year return period environmental criteria.

Typically, extreme waves with 100 year return period is used for design for survival of the platforms.

The encounter probability of the platform with 20 years design life is calculated as 18% or 0.18.

Hence, for future additional design life of 10 years, with the same encounter probability, the reduced return period is calculated as 50 years.

ORIGINAL Design

Original Design Life DL 20 yrReturn period of storm TR100 100 yr

Encounter Probability E 1 expDL

TR100

E 0.181

Encounter probability expressed in terms of % of exceedance

E 18.127 %

Recertification

Original Design Life DL1 10 yr

Return period of storm TR50 50 yr

Encounter Probability E1 1 expDL1

TR50

E1 0.181

Encounter probability expressed in terms of % of exceedance

E1 18.127 %




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Return period for specific encounter probability can be calculated for 30 years design life and 10 years design life extension as illustrated in the table below.

DL 100 50 30 20 10 5

Pe TR TR TR TR TR TR0.00001 9999950 4999975 2999985 1999990 999995 4999970.0001 999950 499975 299985 199990 99995 499970.001 99950 49975 29985 19990 9995 49970.01 9950 4975 2985 1990 995 4970.1 949 475 285 190 95 470.2 448 224 134 90 45 22

0.259 334 167 100 67 33 170.3 280 140 84 56 28 140.4 196 98 59 39 20 100.5 144 72 43 29 14 70.6 109 55 33 22 11 50.7 83 42 25 17 8 40.8 62 31 19 12 6 30.9 43 22 13 9 4 2

1 5 2 1 1 0 0




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Module 1- Structural Integrity Assessment of Offshore Structures (1)

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Transcript of Module 1- Structural Integrity Assessment of Offshore Structures (1)