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

Introduction to structural assessment

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Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

Introduction to structural assessment 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; 1/8/2014

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Introduction to structural assessment Text Books a) 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/Codes a) 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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Introduction to structural assessment Western Offshore Field Map

 Major Oil and Gas fields are  Mumbai High North  Mumbai High South  Bassien  Tapti  Heera  Mukta  Neelam  Over 200 Fixed jacket type platforms  Many are older than 20 years

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Introduction to structural assessment Field Map Western Offshore, India #

B-48

2170000

B-46

NQO

B-157#

SBM

" 16

B-59

X

77

km

B-

55

to

SH

G

Gas trun k lin e Ba ssein Gas to H trun azira k lin e BP B to Haz ira

2180000

B-188

B-55

B-15

B-127

2150000

36" X 2 31 k m

BHN

2160000

MUMBAI HIGH 14" OIL

WO-24

ICW

BHS

12" GAS

Mukta Field (JV)

BHE-1

2140000

Panna Field (JV)

SH

B-45 2130000

B-149 B-147 B-192

2 8"

2120000

X 78 km

VASAI EAST (BSE)

BPA B-22# BPB

SH B

PB L in

e

30

BASSEIN

BS-12

2110000 30

#

"

B-80 X

2100000 #

WO-15

##

WO-16

14

2

km

##

B-178 ##

B-28## B-193 -H

2090000

D-33

EE RA

26

"

X

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as

tru

nk lin et oU Oi l tr ra n un kl i ne B-172## to Ur an

20

3

km

B-173A#

B-23A SH P

#

BS-13 BS-16

B-121/119

WO-5

42" X 2 44 k m

##

B-105

##

B-179

B-180## Li

ne

INDEX

B-28A##

OIL LINE GAS LINE

B-37#

2080000

OIL FIELD GAS FIELD

B-134

2060000

SCALE

710000

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720000

730000

740000

750000

760000

5

0 km

10 km

20 km

30 km

40 km

770000

780000

790000

800000

810000

HEERA 820000

OIL & GAS FIELD

NEELAM

Marginal Fields around Mumbai High D-18 and Bassein Fields

D-1

18 "o il a nd ga s li ne

2070000

830000

840000

n Ura ne to nk li r an il tru U O " to 24 e k lin trun G as 26 "

850000

#

SOUR FIELD

##

HIGHLY SOUR FIELD

860000


SWEET OIL/GAS

Introduction to structural assessment 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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Introduction to structural assessment Platform Types and Exposure Categories Platforms can be classified based on usage, type of construction and categorized based on reserve strength No

Usage

Types of construction

1

Wellhead Platforms

Jacket or template structure Tower Structures Tripod Structures Monopod Structures

2

Process Platforms

Jacket or Template structure Floating Structures Gravity Platforms

3

Living Quarters Platform

Jacket or Template structure Floating Structures

4

Flare Towers

Jacket or template structure Tower Structures

5

Bridge Support structures

Tower Structures

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Introduction to structural assessment Minimum Structures Minimum 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 Structures

Three or more legs

Single Leg structures

Steel 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-back to process platform

Adequate reserve strength and redundant design

Non-redundant frame and hence no reserve strength available

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Introduction to structural assessment 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, nonwelded, 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. 1/8/2014

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Introduction to structural assessment CONVENTIONAL WELL PLATFORM CONFIGURATIONS SINGLE PILE CONFIGURATION

SKIRT PILE (ONLY) CONFIGURATION

SKIRT & MAIN PILE CONFIGURATION

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

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

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

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Introduction to structural assessment CONVENTIONAL WELL PLATFORM CONFIGURATIONS

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Introduction to structural assessment Minimum Structures

UK 8/28/201 4

Norway 12

Australia


Introduction to structural assessment Minimum structures concepts Mono Pile Concepts

Braced Conductor legs

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Guy Supports structure


Introduction to structural assessment 1(a)

Mono Pile Concepts

1(b)

1(c)

Mono pile concepts involve driving of large diameter pile and supporting the deck from the single leg. This can be augmented by additional skirt piles in order to reduce large bending of mono piles. The mono pile houses 3 or 4 conductors inside thus reducing the wave loads. Another alternative to this is to have conductors outside the mono pile. 8/28/201 4

Mono Pile without Guy Wire 14

Braced Mono Tower (Inside Conductor) Dr. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

Braced Mono Tower (Outside Conductor)

Introduction to structural assessment 2(a)

Jacket Type Concepts

2(b)

2(c)

Jacket type concepts involves 3 or 4 legs with conductors inside the jacket framing. The jacket legs are either battered or vertical. Three alternate scheme are proposed are shown in figure. The above concepts can be extended to water depths exceeding 30m and has the flexibility of increase in number of wells or topside configurations. 8/28/201 4

4 Legged Jacket Structure 15

4 Legged Jacket Structure with Batter Piles


3 Legged Jacket Structure

Introduction to structural assessment Braced Conductor Leg Concepts

3 (a)

3 (b)

In this concepts four conductor cum legs are braced to form frame which will be fixed to the seabed by skirt piles. The advantage of these concepts is that the wave loads is reduced considerably since the jacket legs and framing near water level is reduced.

Braced Leg Jacket (4 Piles) 8/28/201 4

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Braced Leg Jacket (3 Piles)

Introduction to structural assessment Guy support Structures 4(a)

4(c)

4(b)

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 guy wires to reduce lateral deflection and bending stresses. Further the support reaction in terms of pile loads will be reduced considerably.

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Introduction to structural assessment Jacket Platforms           

Minimum Structures

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 8/28/201 4

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          

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.


Introduction to structural assessment Exposure Categories Platform exposure can be categorized in to three types based on life safety and consequences of failure. Exposure category

Life Safety

Failure Consequences

L-1

Manned – Non-evacuated

High

L-2

Manned - Evacuated

Medium

L-3

Unmanned

Low

 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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Introduction to structural assessment 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 classified under this category. It may not be possible to evacuate the personnel from the platform due to practical reasons or it is not intended due to operational requirements.  L-2 Manned Evacuated : The platform used for non-production purposes such as water injection, or non-critical processing platforms can be classified in to this category. In case of design environmental event, platform can be shut down and evacuation is feasible. Some process platforms can also come under this category as the design can be planned for shutdown in case of emergency.  L-3 Unmanned : Remote wellhead platforms or satellite platforms connected to processing platform in the field comes under this category. Normally no manning is required as all the operations can be controlled from the process platform or from the control centre on land. This type of platform may have day shelters for visitors occasionally. 1/8/2014

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Introduction to structural assessment Failure Consequence Categories  L-1 High Consequences : Major process platform or large well platform with no planned shut down during design environmental event comes under this category. Further, platforms connected with large trunk pipelines with for transporting oil or gas to the land shall be included under this group.  L-1 Medium Consequences : Wellhead Platforms planned for shut down during design event will be coming under this category. Normally provided with subsea control valves for isolation and will have provision to shut the wells. No oil storage will be available in such platforms.  L-3 Low Consequences : Minimum platform structures planned for marginal field can be grouped under this category. Normally planned to shut during any design event. Such remotely operated platforms will have certified subsea valves to isolate the platform from subsea pipeline connection to the other platforms thus reducing the spread of risk to other platforms.

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Introduction to structural assessment Platform Assessment Initiators An 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 1/8/2014

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Introduction to structural assessment Data Collection 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;

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Introduction to structural assessment Data Collection  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.

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Introduction to structural assessment Platform Assessment Category

A-1

- High assessment category

A-2

- Medium assessment category

A-3

- Low assessment category

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Introduction to structural assessment ASSESSMENT CRITERIA-U.S GULF OF MEXICO Assessment Category

Exposure Category

Design level analysis (see notes 1 & 2)

Ultimate strength analysis

Consequence of failure

Life safety

A-1

High

Manned-NonEvacuated, Manned Evacuated or Unmanned

High Consequence/des ign level analysis loading

High Consequence ultimate strength analysis loading

A-2

Medium

MannedEvacuated or Unmanned

Sudden hurricane design level analysis loading

Sudden hurricane ultimate strength analysis loading

A-3

Low

Unmanned

Minimum consequence design level analysis loading

Minimum consequence ultimate strength analysis loading

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Dr. S. Nallayarasu 26 Department of Ocean Engineering Indian Institute of Technology Madras36

Introduction to structural assessment ASSESSMENT CRITERIA- OTHER U.S AREAS Assessment Category

Exposure Category

Design level analysis (see notes 1 & 2)

Ultimate strength analysis

Consequence of failure

Life safety

A-1

High

Manned-NonEvacuated or Unmanned

85% of lateral (RSR)=1.6 loading caused by 100-year environmental conditions

A-3

Low

Unmanned

50% of lateral (RSR) =0.8 loading caused by 100-year environmental conditions

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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Introduction to structural assessment CODES AND STANDARDS Following 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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Introduction to structural assessment ASSESSMENT PROCESS Assessment 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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Introduction to structural assessment 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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Introduction to structural assessment DOCUMENTATION OF STRUCTURAL ASSESSMENT The 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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Introduction to structural assessment ASSESSMENT PROCESS The assessment process consists of following steps.      

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

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Introduction to structural assessment STRUCTURAL INTEGRITY ANALYSES 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 ultimate strength 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.

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Introduction to structural assessment DESIGN LEVEL ANALYSES 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 mean yield stress, etc. 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 practice instead of stick compliance with code

Joint Design


50% strength check on tubular connections waived

Fatigue


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

As per API RP 2A

Can use as-built information

Foundation 1/8/2014

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Introduction to structural assessment ULTIMATE STRENGTH ANALYSES 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 capacity as well as post-buckling behaviour, rather than the elastic load limit. Ultimate Strength analysis procedures are similar to those for new platform design, including the application of all safety factors, the use of nominal rather than mean yield stress, etc.  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. 1/8/2014

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Introduction to structural assessment Platform assessment procedure SCREENING OF PARAMETERS

DESIGN LEVEL ANALYSIS

ULTIMATE STRENGTH ANALYSIS

FATIGUE ANALYSIS AND INSPECTION UPDATE

Reduced Design Life

MITIGATION MEASURES 1/8/2014

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PLATFORM ACCEPTABLE FOR ADDITIONAL OPERATIONS


Introduction to structural assessment MITIGATION MEASURES Once 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. 1/8/2014

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Introduction to structural assessment LOAD SHEDDING PROCESS Load 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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Introduction to structural assessment Acceptance criteria for Assessment Acceptance 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. 1/8/2014

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Introduction to structural assessment Structures Designed Using ASD Format Allowable stress design approach uses a concept of “Factor of Safety” in Design.

RD   DD  LD  E D  FOS In which RD is a Design Resistance of the structure and DD, LD and ED are design load effects due to dead, live and environmental loads. FOS is the design factor of safety and varies between 1.5 to 2.5 depending on the design category. If the FOS is taken as one, the stresses in the structure reaches yield of the material. This is the reserve strength available in the structure components against failure. However, it does not guarantee the overall system Behaviour as the structural analyses were carried out using linear elastic principles and ultimate strength assessment has been made. System redundancy and its Behaviour at increased loads beyond yield of the material has not been taken in to consideration.

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Introduction to structural assessment Structures Designed using LRFD Method LRFD method uses partial material and load factors as shown in the equation and are called “Partial Safety Factors” in Design.

Rn   Dn  Ln  En   In which Rn is a Nominal Resistance of the structure and Dn, Ln and En are nominal load effects due to dead, live and enviornmental loads.  is the material factor. The overall Margin of safety can be taken as

Rn



  Dn  Ln  En 

 in which  = 

Typical values of  varies from 0.8 to 0.9 and  values varies from 1.2 to 1.5. Hence the overall safety margin available also varies from 1.2 to 1.5. However, in this method also overall system Behaviour is not considered especially strength beyond yield point of the material and redundancy in the system is not evaluated.

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Introduction to structural assessment Verification using Plastic Collapse Analysis The 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 1/8/2014

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Introduction to structural assessment Limiting Displacement and Strain The 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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Introduction to structural assessment Probability Distributions Under certain simplified assumptions, the parameters of a random wave follow known probability distribution functions. Following variables of interest shall be assigned with suitable probability distribution function. Variable

Distribution

Remarks

Wave surface elevation

Gaussian

Zero mean process for long term sea state

Wave amplitude

Rayleigh

Fully developed sea

Wave height

Rayleigh

Wave period

Weibull

Extreme wave height

Gumbel Type I

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Short term extremes


Introduction to structural assessment Gaussian Probability Distribution of η Gaussian (Zero mean process, =0) or normal distribution can be used to represent the surface elevation if the mean is zero. 2    1    f    exp         2  2    

The probability of the water surface elevation exceeds the given value of “1” can be determined using 

P   1   f   dx a 2    1     exp       d      2 1     2   

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

  2  f    2 exp    2   s   2s  



where

s  

2 4 

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

P   1   f   dx 1

2    1       12    2   exp      d  exp    2       1   2        2   

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Introduction to structural assessment Rayleigh Probability Distribution of H The probability of wave crest elevation can be N used to express the crest to trough wave height (Hw). 100

  H w2 P ( H w  H )  exp    2   2 H

    

Hmax / Hs

1.534

200

1.641

500

1.772

The relationship between Hs and standard deviation can be used to relate 1000 the Hw and Hs. i.e. Hs = 4H 2000

1.866

  H w2   P ( H w  H )  exp  2   H   s  

5000

2.097

10000

2.150

20000

2.228

50000

2.328

100000

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 8/28/201 4

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1.956

Introduction to structural assessment Gumbel Type I Probability Distribution of H The 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



f  H    exp   H  u   e



F  H   exp  e

  H u 

  H u 



where



H  u 

 

2   2 6 2

H

The value of  is 0.577215, Euler constant.  has to be determined from data recorded earlier.

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Introduction to structural assessment ESTIMATION OF WAVE HEIGHT FOR FUTURE Hmax100  18 m

Wave height

TR  100 yr

The probbability of exceedance of the above wave height in next 10 years i TF  10 yr

 TF  P  1  exp  TR   P  9.516 %

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Introduction to structural assessment If the structure is to be designed for another 50 years, what will be the maximum wave heigh be considered for the future 50 years. TF1  50 yr

 TF1    TR 

P1  1  exp

 H  15 m

 

P1  39.347 %

H  2 m

   0.641

6 H

uH   H 

1 m

0.5772 uH  14.1 m



 TR   P1 yr   HF  uH   ln

Estimation based on Method 1

 TR   T  F1  HF1  Hmax100  

HF  22.736 m

ln

Alternate method

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HF1  19.081 m


Introduction to structural assessment Risk Analysis for Environmental Conditions Environmental conditions (Wave, Wind and Current) to which the platforms will need to be designed or verified can be assessed depending on the life of the platform. Typical design environmental return period of 100 years is recommended. However, this can be reduced depending on the life of the platform. In order to assess this, a risk analysis can be carried out by including the following 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. 1/8/2014

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Introduction to structural assessment 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). 1/8/2014

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Introduction to structural assessment 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. 1/8/2014

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Introduction to structural assessment 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

 DL  P ( E )  1  exp    T  R  

Where L - Design lifetime TR - Return Period of environment For Example

DL TR  ln 1  P ( E ) 

- Design lifetime = 30years - Assume 10% chance that the design wave is exceeded once - Return period of the environment can be estimated

 DL   30  E  0.1  1  exp    1  exp     T   T  R    R TR  284 years 1/8/2014

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Introduction to structural assessment Reduced Sea State for Short Design Life ORIGINAL Design

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.

Hence, for future additional design life of 10 years, with the same encounter probability, the reduced return period is calculated as 50 years. 57

DL  20 yr

Return period of storm

TR100  100 yr

Encounter Probability

E  1  exp

Encounter probability expressed in terms of % of exceedance

E  18.127 %

 DL    TR100 

E  0.181

Recertification

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

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Original Design Life

Original Design Life

DL1  10 yr

Return period of storm

TR50  50 yr

Encounter Probability

 DL1  E1  1  exp   TR50 

Encounter probability expressed in terms of % of exceedance

E1  18.127 %


E1  0.181

Introduction to structural assessment Reduced Sea State for Short Design Life

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Introduction to structural assessment Reduced Sea State for Short Design Life 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 TR

Pe

1/8/2014

50 TR

30

20

10

5

TR

TR

TR

TR

0.00001

9999950

4999975

2999985

1999990

999995

499997

0.0001

999950

499975

299985

199990

99995

49997

0.001

99950

49975

29985

19990

9995

4997

0.01

9950

4975

2985

1990

995

497

0.1

949

475

285

190

95

47

0.2

448

224

134

90

45

22

0.259

334

167

100

67

33

17

0.3

280

140

84

56

28

14

0.4

196

98

59

39

20

10

0.5

144

72

43

29

14

7

0.6

109

55

33

22

11

5

0.7

83

42

25

17

8

4

0.8

62

31

19

12

6

3

0.9

43

22

13

9

4

2

1

5

2

1

1

0

0

59


Introduction to structural assessment Reduced Sea State for Short Design Life

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

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