development of a structural system ... structural reliability analysis framework for fixed offshore...

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JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms

JIP: Structural reliability analysis framework

for fixed offshore platforms

DEVELOPMENT OF ASTRUCTURAL SYSTEM RELIABILITY

FRAMEWORKFOR OFFSHORE PLATFORMS

May 1998

Document No. JHA003

University of Surrey, Department of Civil EngineeringGuildford, Surrey GU2 7XH

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TABLE OF CONTENTS1. PROJECT SUMMARY ........................................................................................................................................ 4

PHASE 1: REVIEW STUDY.............................................................................................................................................. 4PHASE 1: DEVELOPMENT OF GENERIC ASSESSMENT & PRESENTATION FRAMEWORK .................................................... 4PHASE 2: OFFSHORE STUDY.......................................................................................................................................... 4PHASE 3: DEVELOPMENT OF OFFSHORE FRAMEWORK................................................................................................... 4

2. INTRODUCTION ................................................................................................................................................. 5

2.1 BACKGROUND...................................................................................................................................................... 52.2 NEED FOR A MORE RATIONAL APPROACH TO STRUCTURAL RELIABILITY ANALYSIS .............................................. 5

3. ISSUES IDENTIFIED FROM THE REVIEW STUDY..................................................................................... 6

3.1 SUMMARY OF GENERIC ISSUES ............................................................................................................................. 63.2 SUMMARY OF CONCLUSIONS FOR FIXED OFFSHORE PLATFORMS........................................................................... 73.3 IDENTIFICATION OF TECHNICAL AND PHILOSOPHICAL ISSUES ................................................................................ 8

4. DEVELOPMENT OF THE FRAMEWORK...................................................................................................... 9

4.1 PRESENTATION OF GENERIC FRAMEWORK ............................................................................................................ 94.2 GENERIC FRAMEWORK TABLES .......................................................................................................................... 154.3 OUTLINE GENERIC FRAMEWORK WITH CORRESPONDING REFERENCES................................................................ 164.4 PRESENTATION OF EXAMPLE FRAMEWORK SPECIFIC TO DESIGN OF FIXED OFFSHORE PLATFORMS ...................... 174.5 OUTLINE EXAMPLE FRAMEWORK........................................................................................................................ 19

5. DIFFERENT RELIABILITY ASSESSMENT METHODS............................................................................. 25

5.1.1 “Minimal” analysis approach.................................................................................................................. 265.1.2 Response surface technique...................................................................................................................... 275.1.3 Numerical simulation approach ............................................................................................................... 285.1.4 “System” analysis approach .................................................................................................................... 29

6. BENEFITS AND POTENTIAL APPLICATIONS OF THE FRAMEWORK .............................................. 32

6.1.1 Moving towards “true” reliability............................................................................................................ 326.1.2 Improved preparation............................................................................................................................... 326.1.3 Improved consistency ............................................................................................................................... 326.1.4 Guidelines................................................................................................................................................. 326.1.5 Application tool ........................................................................................................................................ 326.1.6 Management tool ...................................................................................................................................... 326.1.7 Quality assurance tool.............................................................................................................................. 326.1.8 Educational/training tool ......................................................................................................................... 326.1.9 Potential usefulness of framework at each phase of a project.................................................................. 33

7. DISCUSSION AND CONCLUSIONS ............................................................................................................... 34

8. REFERENCES..................................................................................................................................................... 37

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LIST OF TABLES

Table 1: Summary of generic conclusions....................................................................................................................... 6

Table 2: Summary of specific conclusions ...................................................................................................................... 7

Table 3: Main issues to be addressed in the development of a generic framework ......................................................... 8

Table 4: Standard flow chart symbols used ..................................................................................................................... 9

Table 5: Summary outline generic framework presented in tabular format ................................................................... 15

Table 6: Summary outline framework presented in tabular format................................................................................ 16

Table 7: Detailed breakdown table for Stage 1: Modelling of structure........................................................................ 17

Table 8: Summary outline framework specific to design of fixed offshore platforms ................................................... 19

Table 9: Framework Stage 1. Modelling of structure ................................................................................................... 20

Table 10: Framework Stage 2.1 Capacity and load derivation - determination of foundation capacity and stiffness... 21

Table 11: Framework Stage 2.2 Capacity and load derivation - determination of environmental loads....................... 22

Table 12: Framework Stage 3. System analysis model derivation................................................................................ 23

Table 13: Framework Stage 4 Capacity and reliability derivation................................................................................. 24

Table 14: Summary of benefits and potential applications of the framework................................................................ 33

LIST OF FIGURES

Figure 1: Diagram showing the major hazards that can affect offshore structures .......................................................... 5

Figure 2: Top-level generic framework flowchart ........................................................................................................... 9

Figure 3: Generic framework for new (design) and old (reassessment) of structures - complete flowchart .................. 11

Figure 4: Generic framework for design and reassessment of structures - part 1 .......................................................... 12



Figure 7: Specific framework for design of fixed offshore platforms............................................................................ 18

Figure 8: Generic framework for design and reassessment of structures - reliability assessment extract ...................... 25

Figure 9: Framework extract showing steps involved in the “minimal” analysis approach ........................................... 26

Figure 10: Framework extract showing steps involved in the response surface technique ............................................. 27

Figure 11: Framework extract showing steps involved in the numerical simulations approach ..................................... 29

Figure 12: Framework extract showing steps involved in the “system” analysis approach ............................................ 30

Figure 13: Diagram indicating potential usefulness of framework at each phase of a project ....................................... 33

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1. PROJECT SUMMARY

The main objective of this project is to develop a generic framework which will set the basis for achieving

more consistent system reliability assessments. The main steps involved in a system reliability assessment,

together with the key technical and philosophical issues, will be identified and examined. Their inter-

relations and relative significance will be assessed in order to link them together in a rational process that

will provide the basis for consistent reliability assessments. The key underlying question throughout this

project is what changes/improvements can be made to reliability assessments in order to move towards true

reliability. The perceived benefits to the customer of this project include: providing basis for future

working practice/guidance as move towards more consistent reliability, with improved preparation,

improved consistency in results; along with allowing the framework to be used as application,

management, quality assurance and educational/training tools.

Phase 1: Review study

The first report, “A review of system reliability considerations for offshore structural assessments”,presents the findings of a review study which aimed to identify and assess the state of the art in the area ofoffshore structural system reliability, as well as generic aspects of structural reliability. The overallemphasis in this review study was to identify the sensitivities and difficulties associated with reliabilityanalysis that prevents consistent reliability predictions from being obtained.

Phase 1: Development of generic assessment & presentation framework

The second report “Development of a structural system reliability framework for offshore platforms”presents the framework for structural system reliability assessments of offshore platforms. Thebackground to the need for the framework is briefly discussed, along with the main issues arising from thereview study. A number of different formats are used for the presentation of the framework.

Phase 2: Offshore study

This third report, “A parametric and sensitivity offshore study”. Based on the findings of the reviewstudy and experience gained from the framework development phase, a number of sensitivity studies wereidentified. These included: yield strength and foundation capacity parametric studies, foundation capacityassessment sensitivity studies, and comparison studies of the main methods of reliability analysis used inthe offshore industry.

Phase 3: Development of offshore framework

The fourth report, “Presentation of a structural system reliability framework for fixed offshoreplatforms” is an executive summary report. This summarises the key findings from Phases 1 and 2, andpresents the revised framework in context in sufficient detail to enable the reader to apply the flowchartsand tables presented therein.

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2. INTRODUCTION

This report presents the framework for structural system reliability assessments of offshore platforms. Thebackground to the need for the framework is briefly discussed, along with the main issues arising from anextensive review study (see Review Study Report for full details). Three different formats are used for thepresentation of the framework. The potential applications and benefits to be achieved from the applicationof the framework are highlighted, demonstrating the potential of the use of such techniques, to movetowards achieving more consistent and true reliability predictions. It is concluded that new structureswould benefit most from the early application of the framework, but that older structures undergoingreassessment will also see benefits.

2.1 Background

The fundamental design requirement of an offshore platform is that it must satisfy the functional need ofsupport structure for offshore oil and gas operations, and be structurally adequate for both operating andextreme loading. There are many different loads to be taken into account at the design stage, includingdead and live loads, vibration, self weight, ice, ship impacts, wind, wave, tide, current, fatigue, foundationreactions, seismic effects etc. The framework developed is designed to assess extreme weather. In thefuture, other frameworks could address other hazards. The following figure illustrates extreme weather asone of the main offshore hazards:

Fatiguedamage

Foundationsfailure

Extremeweather

FireShip

impact

Corrosiondamage

Aircraftimpact

Major hazards affecting fixed offshore structures in the North Sea Explosion

Figure 1: Diagram showing the major hazards that can affect offshore structures

2.2 Need for a more rational approach to structural reliability analysis

There is a definite need within the field of reliability analysis, especially when used in combination withstructural integrity analysis, to move towards a set of guidelines in order for a more rational approach to beadopted. The use of different models, software and users often means variations in methods andassumptions, and this in turn implies that different modelling and statistical uncertainties are included inthe analysis. There is a genuine need to reduce or better quantify modelling uncertainty, as well as toconsider alternative means of incorporating modelling uncertainty in reliability analysis. A lack ofguidelines or a framework within which such work is undertaken has lead to the development ofinconsistent assessments. Other investigators [62, 118] have also identified the need for setting guidelinesand targets including the benefits that arise from a clear, consistent and efficient approach. A frameworkin which such aspects are included, combined with information on the interpretation and use of the resultsis to be developed here, and the review study described herein aimed to identify all major studies carriedout in the past, and to pull on their combined results to develop such framework and guidelines.

The development of a generic framework which will set the basis for achieving more consistent systemreliability assessments has been undertaken. The main steps involved in a system reliability assessment,together with the key technical and philosophical issues, have been identified and examined. Their inter-relations and relative significance were assessed in order to link them together in a rational process thatprovided the basis for consistent reliability assessments. The key underlying question throughout thisproject is what changes/improvements can be made to reliability assessments in order to move towards truereliability. The perceived benefits to the customer of this project include: providing a basis for futureworking practice/guidance in order to move towards “true” reliability, with improved preparation before areliability analysis is undertaken, improved consistency in results; along with allowing the framework to beused as application, management, quality assurance and educational or training tools.

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3. ISSUES IDENTIFIED FROM THE REVIEW STUDY

A review study was carried out in an attempt to gain a historical appreciation and understanding of thecurrent techniques and the philosophy behind them, as applied to the performance of structural reliabilityanalyses of offshore structures. The emphasis of this study concentrated on the need to move towards more“true” reliability and the increased understanding and hence reduction of uncertainty. The following pointshighlight the main findings of that study. The issues have been segregated into those which are generic,and those which are applicable to the specific example of fixed offshore structures.

3.1 Summary of generic issues

Reliability • Reliability involves dealing with events whose occurrence/non-occurrence atany time cannot be predicted.

• A typical reliability analysis for offshore structures would involve the- generation of directional long term statistics of extreme load,- calculation of ultimate strength of structure for various directions,- estimation of uncertainty in structural strength & then- calculation of the probability of failure.

Probability offailure

• Probability of failure is integration of probability distributions of load/resistance. Reliability results can only be interpreted as absolute values whenphysical uncertainty dominates over model prediction uncertainty.

• Probability of failure, Pf = Φ(-β). Φ() = std normal distribution ftn & β =reliability index.

Uncertainty • Uncertainty is categorised into three main groups: physical uncertainty,statistical uncertainty & modelling uncertainty. However, there is a degree ofuncertainty introduced by the user, which is generally part of the modellinguncertainty.

Relativesignificance

• Sensitivity gives an indication of significance of a parameter in affectingoverall reliability. Investigating relative sensitivity involves a study of theeffect each different parameter has on results of reliability analysis of theoverall structure.

Betterquantification &reduction ofuncertainties

• Reliability results used to be taken as an indication of the notional reliabilityof a structure, but more recently, there has been effort to bring the reliabilityprediction as close to “true” reliability as possible.

• Developments in modelling & software has minimised “error” incurredduring initial stages, & progress in predicting environmental conditions hasenabled more accurate representation of environmental loads.

Improvingconsistency inassessments

• To improve consistency of results between different structures/users,increased awareness of uncertainties/sensitivities & the various philosophicalissues at each step of a reliability analysis is needed.

• Development of a framework to identify main steps, along with justificationsfor these, will go towards improving overall structural reliability &consistency.

Human factors &competence/guidance forusers

• User uncertainty is affected by user’s competence, which becomes higherwhen the activity has high uncertainty or is highly sensitive.

• There is a need to move towards guidelines for a more rational approach - toreduce/better quantify modelling uncertainty, & to consider alternative waysof incorporating this in reliability analysis.

Interpretation ofsystem effects

• There are a number of very different factors that can be studied in order toassess system effects derived from analysis of detailed structural models - keyfactors in such studies are: reserve & residual strength & redundancy.

Table 1: Summary of generic conclusions

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3.2 Summary of conclusions for fixed offshore platforms

Loadinguncertainty

• It has been shown that reliability assessment is generally dominated by theuncertainties in the loading.

• Loading variables account for > 95% of the total uncertainty, & a rigorousmodelling of the uncertainty in these variables is vital for reliability basedintegrity assessments. There is a need for more data to develop joint probabilitydistribution of all relevant environmental parameters.

Foundationuncertainty

• Analyses have shown a significant degree of uncertainty exists about thevalidity of foundation model & of data used for the soil parameters. Thisuncertainty was sometimes found to be of the same order of magnitude as thephysical uncertainty in environmental load.

• Piles studies: for clay & sand NGI recommended API RP2A 20th - conservativefor NC clay, with a modest COV, & slightly higher COV for OC clay, &conservative for dense sand, with a high COV. [188-193].

• Axial capacity: new design approaches were developed at IC for driven piles inclays & sands, is simple to apply; & has advantages over existing APIapproaches. The formulae used to ascertain the pile group interaction for pilesin sand have not yet been widely used in foundation analyses.

Environmentalextremes

• Conventional treatment of waves, current & wind forces was each factorseparately & then combine the independent extremes simultaneously. This isover conservative & overestimates the design loads required. Recently, thedevelopment of more reliable databases of hindcast environmental data hasenabled a joint description of these quantities to be determined.

• Shell carried out studies to assess environmental loads. The New Wavekinematics theory was developed to more accurately represent the wave &current, and to improve the accuracy of the drag coefficient used [7,8,9].

Waveapproaches

• Generally only 1 or 2 wave approaches are used in structural platform analysis.For a full analysis, more wave directions must be carried out.

Treatment ofdrag, inertia &marine growth

• System capacity can be estimated without taking into account randomness ininertia & marine growth coefficients, which can be modelled as deterministic.

• The uncertainty and randomness in drag cannot be ignored, and must beincluded.

System effects • Structural behaviour beyond first member-failure depends on degree of staticindeterminacy, ability of structure to redistribute load, & ductility of individualmembers. For a perfectly balanced structure the system effects for overloadcapacity beyond first member failure, are due to the randomness in the membercapacities. For more realistic structures system effects are both deterministic &probabilistic.

• Deterministic effects are from remaining members in the structure which stillcarry load after one or more members have failed; probabilistic effects are fromthe randomness in member capacities [6]. The system effect is the differencebetween system reliability index & failure of any 1 member [5].

Need forframework

• A number of studies on idealised behaviour of structures identified the need forsome kind of framework or general procedure was needed in order to assessoffshore platforms, with a range of brittle and ductile behaviours, and a varietyof failure modes, but with a more rational and consistent approach. e.g.[52]

Table 2: Summary of specific conclusions

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3.3 Identification of technical and philosophical issues

The initial task undertaken in this project was the review study, whose main aim was to identify the “stateof the art” in the area of offshore structural reliability. The resulting report incorporated an introduction togeneric reliability issues, and then briefly described all major aspects of reliability analysis. During thisreview period, the key findings of the technical and philosophical issues were identified for incorporationinto the subsequent framework. The table below summarises both the key stages in reliability analysis aswell as the related technical and philosophical issues.

Key stages in reliability analysis Related technical & philosophical issues

• Structural model

• Loading model

• Failure modes

• Failure criteria

• Limit states

• Uncertainties in loading &

resistance variables

• Structural resistance prediction

• System Effects

• Reliability methods

• Uncertainties & sensitivities

• Computer programs/tools

• Uncertainties & relevant significance

• Better quantification &/or reduction of uncertainties

• Compatibility of accuracy of sub-models

• Validation of methods in part or in full (experiments,

benchmarking, actual performance)

• Setting target reliability

• Criteria for consistency in assessments

• Criteria for interpreting as absolute values for decision making

• Lessons from other industries (on consistency, interpretation,

actuarial values)

• Human factors relating to competence /guidance for users

• Integrate with design or re-assessment process

• Integrate with other hazards & overall hazard management system

Table 3: Main issues to be addressed in the development of a generic framework

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4. DEVELOPMENT OF THE FRAMEWORK

A generic framework has been developed which will set the basis for achieving more consistent systemreliability assessments. A key underlying question throughout this framework development was whatchanges or improvements could be made to the reliability assessment process in order to move towards truereliabilities (or failure probability that could begin to be interpreted as absolute values for decisionmaking).

The framework was developed using standard flow chart symbols as shown in Table 4 below.

Symbol DefinitionProcess

Input / output

Decision

Document

Terminal (start or end)

Table 4: Standard flow chart symbols used

From the review, the key stages of the assessment process were identified and basic diagrams were drawnup to represent the main steps in the overall process. These diagrams were then augmented and developedto form a more detailed approach. Several different options for presentation were explored, with theflowchart type presentation being the preferred option due to its “visual” impact, and clear presentation ofthe issues and unambiguous representation of their links. The flowchart is a very concise method ofpresentation, with only the key characteristics of each step being described. Standard flowchart symbolswere used in order to help the reader to ascertain the status of each step.

The framework was then studied further and improved in order to allow a more detailed presentation of thekey stages. Tabular formats were therefore developed as an alternative presentation method to theflowchart approach, in order to enable the key background documents at each stage of the framework to beidentified, and clearly presented. The use of the tables allowed a full description of the activity to beincluded, along with an indication of the significance of uncertainty, sensitivity, complexity and level ofuser competency required at each stage.

4.1 Presentation of generic framework

In order to understand the workings of the generic framework flowchart, a top-level flowchart has beenincluded here which introduces the main elements of the generic framework, without going into detail ofall the steps required within each stage of the process. Figure 2 below shows the top-level framework.

Inputs - platformdescription,foundation/

environmentalparameters

Assessmentof fixing ofstructure

Foundationcapacity

derivation

Modellingof

structure

Loadderivation System

analysismodel

derivation

Output - measureof reliability &

comparisons withtargets.

Ultimatecapacity

derivation Reliabilityanalysis

Figure 2: Top-level generic framework flowchart

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The following section describes the above process in more detail:

1. The first symbol used in the top-level generic framework flowchart is that which represents aninput or an output, and shows the main inputs required for a reliability assessment. These includedetails of the platform description, buoyancy effects (if appropriate), details of the foundations andsoil conditions, and details of the environmental conditions to be applied.

2. The second stage is represented by the standard symbol for a process, and in this case, it is for the

assessment of the fixing of the structures i.e. an appraisal of the foundation conditions and thefoundation configuration.

3. The third stage is a process, and is the undertaking of the modelling of the structure - decision

concerning structural members, nodes and elements, along with the parts to be modelled (e.g.decks and equipment) are made here. The result is a sufficiently detailed description of thestructure which meets the precise needs of the study. Decisions as to what software package to beused will also be made at this stage.

4. The fourth stage, again a process, represents the derivation of the foundation capacity and its

stiffness from the foundation capacity and distribution structural configuration and the soilcharacteristics specific to the precise location of the structure.

5. The fifth stage is also a process, and represents the derivation of the loads on the structure. This is

based on an assessment of the statistical distribution of the environmental parameters predicted tobe acting upon the structure.

6. The sixth stage is the process of derivation of the system analysis model, and involves complete

structural analysis using various software options, on the platform model, loads etc. 7. The seventh stage of the top-level generic framework flowchart is the process of derivation of the

ultimate capacity of the structure. The process undertaken in this activity will depend whether a“component” based approach is adopted or whether a system analysis approach is used.

8. The eighth stage of the flowchart is the process of the undertaking of a reliability analysis, and the

determination of the associated uncertainty, using the results of the first seven stages. 9. The ninth and final stage of the flowchart is the output of the whole procedure, and is the

determination of the probability of failure of the structure from a study of the failure surface incombination with the uncertainty descriptions derived at the eighth stage. A determination of thereliability of the structure is also an output and is derived from the probability of failure and theuncertainty analysis.

Figure 3 overleaf shows the flowchart that has been developed for fixed offshore structures. It shows eachstep that needs to be carried out, and the precise sequence for those activities, in order for a full structuralsystem reliability assessment to be undertaken.

The flowchart includes all the activities necessary to collate the inputs required, to perform the processesrequired and to produce all the outputs required.

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Strucuralmembers

determined

What structural partsare to be included in

the model?

Relevance/importanceof parts e.g.Is a detailed

deck necessary

Valid reasonswhy certain parts

not included

How are parts tobe modelled?

Justification formodelling

method chosen

Platformstructure details

What softwarepackage to use?

AssumptionsJudgement

& Knowledgeof user

Assessment ofstructure fixingconditons i.e.fixed/floating

Determineassociateduncertainty


Decide on modellingmethod for "floater"

buoyancy effects


& Knowledgeof user

Inertial anddynamic loading

parameters

Inspection reports, welddefect assessments, specific

damage/ defect reports

Condition assessmentof structure for

reassessment purposes

Platform designdrawings etc. for new

platforms

Design Reassessment

For "floater"structures, determinebuoyancy effects etc.

& their distribution

Deadloadparameters

includingbuoyancy

effects

Fixed Floater

Derivation of loadson the strucutre



Environmental parameters(wave height, wave period,

current, wind,inertia, drag etc.)

Separateenvironment studies

Assessmentof local

environmentalconditons

Foundation parameters(soil conditons, pileconditons, ageing,

group interaction etc.)

Separate foundationstudies

Assessment oflocal foundation

conditons

Determine foundationstiffness & capacity &

associated distributions



Decide onmodelling method

for foundations


& Knowledgeof user


& Knowledgeof user

Determineenvironmental

parameters statisticaldistribution

How are loads applied tostructure (eg. onto everymember or onto bays?)

Systemanalysis model

Does foundationrequire

assessment?

Yes No

Perform a number ofnon-linear pushover analyses

Which reliabilityapproach to

adopt?

Determine distributionof strength (member / structure) &

obtain probability of failure

What factors needto be assessed,

relevant to focus ofthe study

Present,understand &

interpretresults

Measure of reliabilityof structure


Presentassessment

ofuncertainties

Decide on failurecriteria


& Knowledgeof user

Minimal analysesapproach (eg. 8 = one

for each wavedirection)

Identify dominant failuremodes (search algorithms)

Numerical simulationapproach (eg. Monte

Carlo)

Determine ultimate capacity &other failure characteristics &

determine failure surface if reqd.

Build up structural system,series of parallel sub-systems

Perform component reliabilityanalysis

Calculate reliability of systemand sensitivity measures


What factors need to beassessed, relevant to

focus of the study


& Knowledgeof user

Integrate distribution of strengthwith loading on structure

Compare reliability withpre-defined targets &acceptance criteria

Response surfacetechnique

"Component" basedapproach

"System" analysisapproach

Pushover analysis to identifydominant failure modes



& Knowledgeof user


& Knowledgeof user


& Knowledgeof user

Figure 3: Generic framework for new (design) and old (reassessment) of structures - complete flowchart

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Strucuralmembers

determined


the model?


deck necessary


not included



method chosen




& Knowledgeof user

Assessment ofstructure fixingconditons i.e.fixed/floating



Decide on modellingmethod for "floater"buoyancy effects


& Knowledgeof user

Inertial anddynamic loading

parameters

Inspection reports, welddefect assessments, specific

damage/ defect reports

Condition assessmentof structure for

reassessment purposes


platforms

New / design Old / reassessment

For "floater"structures, determinebuoyancy effects etc.

& their distribution

Deadloadparameters

includingbuoyancy

effects

Fixed Floater

Figure 4: Generic framework for design and reassessment of structures - part 1

= Process, = Input / output, = Decision, = Document, = Terminal (start or end)

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JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms of 45







Assessmentof local






conditons






for foundations


& Knowledgeof user


& Knowledgeof user





Does foundationrequire

assessment?

Yes No



_________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________




adopt?






interpretresults



Presentassessment

ofuncertainties



& Knowledgeof user





Carlo)








focus of the study


& Knowledgeof user









& Knowledgeof user


& Knowledgeof user


& Knowledgeof user



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4.2 Generic framework tables

As described earlier, the framework is presented in three forms: generic overview, flowchart and tabularformats. The tabular format allows a much greater depth of detail to be presented. It has also been adaptedto include the main references pertaining to each of the activities. Table 5 shows the outline table whichdescribes the basic stages including main inputs and outputs, and Table 6 shows the outline table with mainreferences.

INPUTS• Description of platform structure (from design drawings, defect/damage/condition reports,

computer models etc.)• Deadload and liveload parameter values (applicable to “floater” structures: incl. buoyancy

effects, inertial/dynamic parameters)• Foundation parameter values (from soil conditions, pile conditions, group interaction etc.)• Environmental parameter values (wave height & period, current, wind, inertia, drag etc.)

Stage 1. ASSESSMENT OF FIXING OF STRUCTURE• Assessment of fixing conditions of structure (to determine whether fixed or “floater”)• For “floater” structures: determination of modelling method for buoyancy effects

Stage 2. MODELLING OF STRUCTURE• Decision as to what software package to use (may be governed by external constraints)• Determination of structural members (i.e. members/parts to model, and in what detail)

Stage 3. CAPACITY AND LOAD DERIVATION• Determination of foundation capacity and stiffness (from capacity and its distribution etc.)• Determination of environmental loads on the structure (from environmental parameters

distribution, statistical distribution etc.)Stage 4. SYSTEM ANALYSIS MODEL DERIVATION

• Complete structural analysis using various software options (platform model, loads etc.)Stage 5. ULTIMATE CAPACITY DERIVATION

• Decision as to which reliability methodology to adopt: either “component” or “system”based (may be governed by external constraints)

• For “component” based approach:⇒ Perform pushover analysis to identify dominant failure modes⇒ Perform either: Minimal analyses/ response surface/ numerical simulation approach⇒ Perform number of pushover analyses (determine load-deformation characteristics)⇒ Decide on failure criteria e.g. determine ultimate capacity & other failure

characteristics, & failure surface if required (from pushover analyses results)⇒ Determination of distribution of strength (dependent upon the focus of the study)⇒ Integrate distribution of strength with loading (e.g. extreme envt loading)⇒ Present assessment of uncertainties to determine uncertainty in strength (on both

member and system level, if required)• For “system” analysis approach:

⇒ Find dominant failure modes from search algorithms (decide failure surface if reqd)⇒ Build up structural system, including series of parallel sub-systems if required

(dependent upon the focus of the study)⇒ Perform “component” reliability analysis, and determine associated uncertainty⇒ Calculate reliability of system, and sensitivity measures

• Present, understand and interpret resultsOUTPUT

• Determination of probability of failure(from study of failure surface, in combination with uncertainty descriptions)

• Determination of measure of reliability of structure(from probability of failure and uncertainty analysis)

Table 5: Summary outline generic framework presented in tabular format

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4.3 Outline generic framework with corresponding references

Framework stage ReferencesINPUTS

• Description of platform structure (from design drawings, defect/damageassessments, condition assessment reports, computer models etc.)

5, 6, 7, 8, 12, 52, 54, 55, 56, 122, 128,129, 194, 195

• Deadload and liveload parameter values (most applicable to “floater” structures:including buoyancy effects, inertial and dynamic parameters)

209, 210, 214

• Foundation parameter values (soil conditions, pile conditions, group interactionetc.)

58, 61, 65, 79, 83, 84, 157, 164, 188,189, 190, 191, 192, 193, 197

• Environmental parameter values (from wave height, wave period, current, wind,inertia, drag etc.)

5, 6, 7, 8, 12, 14, 64, 68, 71, 75, 76,77, 81, 94, 97, 101, 135, 136, 143,153, 162, 163

Step 1. ASSESSMENT OF FIXING OF STRUCTURE

• Assessment of fixing conditions of the structure (in order to determine whetherfixed or “floater”)

213, 214, 215

• For “floater” structures only: Determination of modelling method for “floater”buoyancy effects

211, 212, 216

Step 2. MODELLING OF STRUCTURE

• Decision: what software package to use (may be governed by external constraints) 5, 6, 63, 56, 96

• Determination of structural members (i.e. which members/parts to model, and inwhat detail)

5, 6, 7, 8, 12, 52, 54, 55, 56, 122, 128,129, 194

Step 3. CAPACITY AND LOAD DERIVATION

• Determination of foundation capacity & stiffness (from capacity / distribution etc.) 58, 61, 69, 72

• Determination of environmental loads on the structure (from environmentalparameters distribution, statistical distribution etc.)

5, 6, 7, 8, 135, 136, 138, 143, 149,153, 163

Step 4. SYSTEM ANALYSIS MODEL DERIVATION

• Complete structural analysis using various software options 5, 6, 7, 8, 9, 12, 52, 54, 55, 56, 122

Step 5. ULTIMATE CAPACITY DERIVATION

• Decision as to which reliability methodology to adopt (may be governed byexternal constraints)

1, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 20,21, 23, 60, 62, 122, 133, 155, 160

• For “component” based approach:⇒ Perform pushover analysis to identify dominant failure modes⇒ Perform either: Minimal analyses, response surface or numerical simulation

approaches⇒ Perform number of pushover analyses (determine load-deformation

characteristics etc.)⇒ Decide on failure criteria e.g. determine ultimate capacity & other failure

characteristics, & failure surface if required (from pushover analyses results)⇒ Determination of distribution of strength (dependent upon the focus of the

study)⇒ Integrate distribution of strength with loading on structure (e.g. extreme envt

loading)⇒ Present assessment of uncertainties to determine uncertainty in strength (on

both member and system level, if required)

54, 55, 56, 122, 123, 194, 195

34, 49, 50, 51, 111, 112, 113, 120,121, 132, 141, 146, 186, 2045, 6, 7, 8, 9, 13, 62, 63, 131, 161

1, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 20,21, 23, 60, 62, 122, 133, 155, 1601, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 20,21, 23, 60, 62, 122, 133, 155, 160

5, 6, 7, 8, 9, 13, 29, 62, 63, 131, 160,161, 186, 200, 201, 202, 203, 205

• For “system” analysis approach:⇒ Identify dominant failure modes from search algorithms (decide failure

surface if required)⇒ Build up structural system, including series of parallel sub-systems if required

(dependent upon the focus of the study)⇒ Perform “component” reliability analysis, & determine assoc. uncertainty

5, 6, 7, 8, 9, 13, 62, 63, 131, 161

34, 49, 50, 51, 111, 112, 113, 120,121, 132, 141, 146, 186, 204

OUTPUT

• Determination of probability of failure (from study of failure surface, incombination with uncertainty descriptions)

29, 160, 186, 200, 201, 202, , 205

• Determination of measure of reliability of structure (from probability of failure anduncertainty analysis)

7, 8, 9, 10, 11, 12, 14, 16, 20, 21, 23,29, 62, 122, 151, 154, 155, 160, 165,186, 200, 201, 202, 203, 205

Table 6: Summary outline framework presented in tabular format

___________________________________________________________________________________________________________________________________________________________________________________________________________________________________


Each stage identified in the outline generic framework table has been examined further, and a detailedbreakdown of every single activity required is included. A sample is included here relating to stage 1:modelling of structure.

Stage 1. Modelling ofstructure

Brief description of activity

Input 1 Description of platform structure & fixing conditions (from design drawings,computer models etc.)

Step 1.1 Decision as to what software package to use (may be governed by externalconstraints)

Step 1.2 Determination of structural membersStep 1.3 Assessment of relevance/importance of structural parts (e.g. is a detailed deck

necessary?)Step 1.4 Decision as to what structural parts are to be included in the modelStep 1.5 Presentation of valid reasons why certain parts are not includedStep 1.6 User discretion & interpretation of the environmental loads (User effects are

assumptions, judgment & knowledge)Step 1.7 Decision as to how parts are to be modelledStep 1.8 Presentation of the justification for the modelling method chosenOutput 1 Full model of structure appropriate to & specific to the current assessment being

undertaken

Table 7: Detailed breakdown table for Stage 1: Modelling of structure

4.4 Presentation of example framework specific to design of fixed offshore platforms

In order to examine future potential developments of the generic framework, a framework for a specificapplication suitable for use in the design of fixed offshore structures in the North Sea was derived. Figure 7over-leaf shows the specific framework for the design of fixed offshore platforms as a flowchart. Thisflowchart was based upon that developed for the generic framework.

The generic outline table which described the basic stages for the whole process, including the main inputsand main outputs, was also the basis for the detailed example exercise. The stages identified in this table wereexamined in turn, and a full table of each step to be performed at each stage was described in more detail.See Tables 7 - 12 in the following section.

These detailed tables also show an indication of the following for each step to be performed:

• uncertainty - the amount of uncertainty introduced at each step• sensitivity - the sensitivity to the overall reliability results to each step• complexity - the level of complexity of the actions for each step• user competence - the user competency required, & its perceived importance for each step.

This star scale has been adopted for the four factors. One star indicates “low”, five stars indicates “high”.This scale is an attempt to indicate levels of involvement, but it is not a definitive representation.

At a generic level, all four of the above factors are considered to be of importance. However, during theexamination of the specific example exercise, it was found that the complexity and user competence factorswere often awarded the same level. It was considered important that this should be identified, and eventhough having both factors may not be fully justified in this specific case of the design of fixed offshoreplatforms in the North Sea, it may become more pertinent for different specific examples, and it was decidedthat both factors should be shown here.

___________________________________________________________________________________________________________________________________________________________________________________________________________________________________


Strucuralmembers

determined


the model?


deck necessary


not included



method chosen




& Knowledgeof user


platforms







Assessmentof local






conditons






for foundations


& Knowledgeof user


& Knowledgeof user







adopt?






interpretresults



Presentassessment

ofuncertainties



& Knowledgeof user





Carlo)








focus of the study


& Knowledgeof user









& Knowledgeof user


& Knowledgeof user


& Knowledgeof user

Figure 7: Specific framework for design of fixed offshore platforms

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4.5 Outline example framework

INPUTS• Description of platform structure

(from design drawings, defect/damage/condition reports, computer models etc.)• Foundation parameter values

(from soil conditions, pile conditions, group interaction etc.)• Environmental parameter values

(from wave height, wave period, current, wind, inertia, drag etc.)Stage 1. MODELLING OF STRUCTURE

• Decision as to what software package to use (may be governed by external constraints)• Determination of structural members (i.e. which members/parts to model, in what

detail)Stage 2. FOUNDATION CAPACITY AND LOAD DERIVATION

• Determination of foundation capacity and stiffness(from capacity and its distribution etc.)

• Determination of environmental loads on the structure(from environmental parameters distribution, statistical distribution etc.)

Stage 3. SYSTEM ANALYSIS MODEL DERIVATION• Complete structural analysis using various software options (from platform model,

loads etc.)Stage 4. ULTIMATE CAPACITY DERIVATION

• Decision as to which reliability methodology to adopt: either “component” or “system”based (may be governed by external constraints)

• For “component” based approach:⇒ Perform pushover analysis to identify dominant failure modes⇒ Perform either: Minimal analyses, response surface or numerical simulation

approaches⇒ Perform number of pushover analyses (determine load-deformation characteristics

etc.)⇒ Decide on failure criteria e.g. determine ultimate capacity & other failure

characteristics, & failure surface if required (from pushover analyses results)⇒ Determination of distribution of strength (dependent upon the focus of the study)⇒ Integrate distribution of strength with loading on structure (e.g. extreme envt

loading)⇒ Present assessment of uncertainties to determine uncertainty in strength (on both

member and system level, if required)• For “system” analysis approach:

⇒ Identify dominant failure modes from search algorithms (decide failure surface ifrequired)

⇒ Build up structural system, including series of parallel sub-systems if required(dependent upon the focus of the study)

⇒ Perform “component” reliability analysis, and determine associated uncertainty⇒ Calculate reliability of system, and sensitivity measures

Present, understand and interpret resultsOUTPUT

• Determination of probability of failure(from study of failure surface, in combination with uncertainty descriptions)

• Determination of measure of reliability of structure(from probability of failure and uncertainty analysis)

Table 8: Summary outline framework specific to design of fixed offshore platforms

_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________


Stage 1. Modelling of structure Arbitrary scalesStage of

frameworkBrief description of activity Uncertainty Sensitivity Complexity User

competencyrequired

Input 1 Description of platform structure and fixing conditions(from design drawings, computer models etc.)

*** *** ** **

Step 1.1 Decision as to what software package to use(may be governed by external constraints)

*** *** ** **

Step 1.2 Determination of structural members **** **** *** ****Step 1.3 Assessment of relevance/importance of structural parts

(e.g. is a detailed deck necessary?)***** ***** **** ****

Step 1.4 Decision as to what structural parts are to be included in the model **** **** *** ****Step 1.5 Presentation of valid reasons why certain parts are not included **** **** *** ****Step 1.6 User discretion and interpretation of the environmental loads

(User effects are assumptions, judgment and knowledge)***** ***** **** *****

Step 1.7 Decision as to how parts are to be modeled **** **** *** ****Step 1.8 Presentation of the justification for the modelling method chosen **** **** *** ****Output 1 Full model of structure appropriate to and specific to the current

assessment being undertaken***** ***** **** *****

Table 9: Framework Stage 1. Modelling of structure

_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________


2.1 Foundation capacity and load derivation - determination of foundationcapacity

Arbitrary scales

Stage offramework

Brief description of activity Uncertainty Sensitivity Complexity Usercompetency

requiredInput 1 Description of foundation conditions

(including soil conditions, pile conditions, group interaction etc.)Input 2 Results of foundation studies carried out for specific location

(if available)Step 2.1.1 User discretion and interpretation of foundation and soil data

(User effects are assumptions, judgment and knowledge)Step 2.1.2 Assessment of local conditions

(Decision on values of parameters to be adopted in the study)Step 2.1.3 Prepare deterministic representation of foundation parametersStep 2.1.4 Prepare probabilistic representation of foundation parameters

Step 2.1.5 Determination of foundation capacity & stiffness & its distributionStep 2.1.6 Determination of uncertainty associated with the foundation capacityStep 2.1.7 Decision on modelling method for foundationsOutput 1 Foundation model to be used in the structural analysis model

Table 10: Framework Stage 2.1 Capacity and load derivation - determination of foundation capacity and stiffness

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2.2 Foundation capacity and load derivation - determination of environmentalloads

Arbitrary scales

Stage offramework


requiredInput 1 Description of environmental conditions

(including wave height, wave period, current, wind, inertia, drag)Input 2 Results of environmental studies carried out for specific location

(if available)Step 2.2.1 User discretion and interpretation of the environmental data

(User effects are assumptions, judgment and knowledge)Step 2.2.2 Assessment of local conditions

(Decision on values and methodologies to be adopted)Step 2.2.3 Prepare deterministic representation of environmental parametersStep 2.2.4 Prepare probabilistic representation of environmental parametersStep 2.2.5 Determination of environmental parameters' statistical distributionStep 2.2.6 Determination of uncertainty associated with the environmental

parameters distributionStep 2.2.7 Derivation of loads on the structureStep 2.2.8 User discretion and interpretation of the environmental loads

(User effects are assumptions, judgment and knowledge)Step 2.2.9 Determination of uncertainty associated with the environmental

loading on the structureOutput 1 Environmental loading model to be used in the structural analysis

model

Table 11: Framework Stage 2.2 Capacity and load derivation - determination of environmental loads

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3. System analysis model derivation Arbitrary scales

Stage offramework


requiredInput 1 Description of platform structure and fixing/buoyancy conditions

(design drwgs, inspection/condition reports, existing models etc.)

Step 3.1 Determination of structural members(from computer model - precise elements, nodes etc.)

Step 3.2 Assessment of the relevance and importance of certain structural parts(from focus of study, analysis to be undertaken etc.)

Step 3.3 User / software / modelling restrictions(from use of specialist computer software etc.)

Output System analysis model for structural assessment(from all steps described above)

Table 12: Framework Stage 3. System analysis model derivation

_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________


4. Capacity and reliability derivation Arbitrary scales

Stage offramework


requiredInput System analysis model for structural assessment

Step 4.1 Decision as to which reliability approach to adoptStep 4.2 For “component” based approach:

- Perform pushover analysis to identify dominant failure modes- Perform: Minimal analyses, response surface or numerical simulation Perform

number of pushover analyses (determine load-deformation characteristics etc.)- Decide on failure criteria e.g. determine ultimate capacity & other failure

characteristics, & failure surface if required (from pushover analyses results)Determination of distribution of strength (dependent upon the focus of study)Integrate distribution of strength with loading (e.g. extreme envt loading)Present assessment of uncertainties to determine uncertainty in strength (onboth member and system level, if required)

Step 4.3 For “system” analysis approach:- Identify dominant failure modes from search algorithms (decide failure surface

if required)- Build up structural system, including series of parallel sub-systems if required

(dependent upon the focus of the study)- Perform “component” reliability analysis, determine associated uncertainty

Calculate reliability of “system”, and sensitivity measures

Step 4.4 User discretion and interpretation of the methods for assessment (Usereffects are assumptions, judgment and knowledge).

Step 4.5 Present, understand and interpret resultsOutput 1 Numerical and graphical representation of the performance of the

structure at various stages of environmental load, and under extremeenvironmental conditions

Output 2 Values for the strength of individual members and the overallstructure, and is distribution.

Output 3 Measure of reliability of structure taken from steps 4.12-4.16

Table 13: Framework Stage 4 Capacity and reliability derivation

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5. DIFFERENT RELIABILITY ASSESSMENT METHODS

Due to the complexity of the final stages of the framework that deal with the different options available forperforming reliability assessments, the following section deals with this in detail, and provides examples ofthe different approaches.

This part of the generic framework involves a decision as to which reliability approach to adopt, and thenfollows the procedure required for each option. The methods covered are both the “component” basedapproach and the “system” analysis approach. The term “component” here refers to methods that treat thewhole structure as one component. Within the “component” based approach there are three techniques whichcan be applied;

• “Minimal” analyses approach (e.g. where only 8 pushover analyses are carried out - one for each wavedirection)

• Response surface technique• Numerical simulation approach

This part of the framework includes the performance of non-linear pushover analyses and decisions on failurecriteria, determination of the ultimate capacity and distribution of strength and hence the probability of failure,along with integration of strength with loading.

Within the “system” analysis approach, the dominant failure modes, and reliability analysis, together withcalculation of reliability and sensitivity measures of the system, are required. For both approaches,presentation of associated uncertainties is also required, before the final stage of deriving the measure ofreliability is achieved.

Figure 8 below shows the extract of the framework detailing the options and various steps of the reliabilityassessment stage.



adopt?






interpretresults



Presentassessment

ofuncertainties



& Knowledgeof user





Carlo)








focus of the study


& Knowledgeof user









& Knowledgeof user


& Knowledgeof user


& Knowledgeof user

Figure 8: Generic framework for design and reassessment of structures - reliability assessment extract


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5.1.1 “Minimal” analysis approach

The ‘minimal’ analysis option is shown inFigure 9 where pushover analysis is used to determine the dominant failure modes for each direction anddevelop failure surfaces. An example of this analysis method is the Shell approach. Shell refined thedeterministic pushover approach in the mid-1990s which forms an important element of the methodology theydeveloped for evaluating the reliability of a platform.

Shell’s essential elements to a quantitative reliability analysis are a hindcast database of waves, currents andwinds, a realistic wave load model, a generation of extreme long term statistics and the use of pushoveranalyses [7, 8, 9, 11, 12, 14, 20, 21, 23].







interpretresults



Presentassessment

ofuncertainties



& Knowledgeof user

Minimal analyses approach (eg.8 = one for each wave direction)








& Knowledgeof user

Figure 9: Framework extract showing steps involved in the “minimal” analysis approach

The major phases of this process are described below [11]:• A hindcast database of metocean conditions (magnitudes and directions of winds, waves and

currents generated numerically) is used to produce a representative combination of extremeenvironmental conditions required to generate a reference load set.

• The reference load set is applied to a FE model and increase in increments to obtain the collapsesequence of the structure and its ultimate resistance.

• A failure surface for the structure is developed by applying the reference load in severaldirections.

• To calculate the probability that the loading will fall outside the failure surface it is necessary toreturn to the hindcast and study the extreme response of a similar “generic” structure.

• The probability of failure is obtained by convolution of the cumulative directional distribution oflong-term extreme loads with the distribution of the ultimate capacity of the structure.

___________________________________________________________________________________________________________________________________________________________________________________________________________________________________


Probability of survival, Pθ, under extreme loading in a narrow sector centred on the wave attack direction θ is:Pθ(survival) = Pθ (Lθ < λθ . Sref) where: Lθ = environmental load in direction θ, λθ = collapse load factor indirection θ, and Sref = reference base shear force.

Assuming independence of rare events, probability of long-term survival under extreme loading as the productof probabilities of surviving the extreme loading predicted for each separate wave attack direction:

P (survival) = Πallθ Pθ (Lθ < λθ) where: Πallθ = product over all wave directions θ of the severe sector.

5.1.2 Response surface technique

The second option within the “component” based approach is the use of response surface technique as shownin the figure below. Frieze et al. [165] adopted the response surface technique in a comparison study of thereliability of fixed offshore structures to the reliability of jack-ups. This method generates a failure surface bysystematically varying each of the important basic variables in turn about their mean values and determiningthe ultimate strength in each case via a pushover analysis or similar. By fitting an equation to this surface, astrength model is created. It is a function of the resistance basic variables and so can be readily input into areliability analysis. “The choice of basic variables and modelling accuracies used to create a response surfacewill be influenced by whether their mean values and/or uncertainties (COV) affect the reliability outcome.Where the variable can be treated as deterministic, it need not appear as a variable in the response surface. Itsdeterministic value, however, may be required in the generation of the surface” [16, 165]







interpretresults



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ofuncertainties



& Knowledgeof user









& Knowledgeof user

Figure 10: Framework extract showing steps involved in the response surface technique

___________________________________________________________________________________________________________________________________________________________________________________________________________________________________


5.1.3 Numerical simulation approach

One method of deriving the most dominant failure paths is by performing of pushover analyses. The mostcritical elements are identified in this analysis, but no account is made of the effect of possible variations incomponent strength which could result in different sequences of failure, and different combinations ofelements. The effects of these variations can be explored more fully using simulation techniques which is thethird option identified in this part of the framework (Figure 11), however, its use is limited in the case ofoffshore platforms because of the large scale of the problem in hand.

One of the key studies involving numerical simulations techniques was a study by DNV/SINTEF in 1994 [6].The DNV/SINTEF approach used the programme USFOS for non linear structural collapse analysis, andPROBAN for the probabilistic analysis tool. PROBAN was used to perform the reliability calculation and togenerate outcome of the stochastic parameters in the simulation studies of the ultimate capacity of a structure[6].

The procedure adopted using USFOS and PROBAN can be summarised as follows [6]:

1. Establish wave/current load pattern for a given position of a fixed wave, e.g. the 100 year waveheight and the 10 years current pattern.

2. Choose the number of realisations, NSIM, (here NSIM=100) 3. Sample NSIM sets of outcomes of the stochastic variables, i.e. Yi (yield strength), εi (imperfection

magnitude)and θi (imperfection length) for each structural member i, using PROBAN. 4. For each set of outcome of the stochastic variables:

4.1 Perform a static pushover analysis by scaling up the load profile, established in 1.4.2 Save the results (e.g. displacement of the deck vs. the total base-shear force etc.)

The annual system failure probability was determined as the annual probability that the load exceeds thesystem capacity, thus:

Pf sys,annual = F x f x dxSC Lannual( ). ( ).

0

∞

∫

where: FSC(.) = cumulative annual probability distribution of the system capacityfLannual(.) = probability density function of the annual probability distribution of the load

Investigations performed by DNV/SINTEF found that the system capacity can be related directly to the base-shear force, and can be estimated without taking into account the uncertainty of the load-pattern. It was alsoconcluded that reliability is dominated by uncertainty in Zsea-state, (a vector of random variables modelling theuncertainties in the sea-state description) especially the significant wave height.

___________________________________________________________________________________________________________________________________________________________________________________________________________________________________








interpretresults



Presentassessment

ofuncertainties



& Knowledgeof user


Carlo)








& Knowledgeof user

Figure 11: Framework extract showing steps involved in the numerical simulations approach

5.1.4 “System” analysis approach

Rigorous “system” reliability analysis requires substantial computational undertaking, and research work inrecent years concentrated on the development of efficient methods for identifying the most dominant failurepaths and deriving the combined system probability of failure. An example of this approach is the W S Atkinsmethod. A number of simplified “system” reliability approaches have also been developed examples of whichare briefly outlined in this section.

Figure 12 overleaf shows the framework extract which represents the “system” analysis approach.

___________________________________________________________________________________________________________________________________________________________________________________________________________________________________



interpretresults








focus of the study


& Knowledgeof user





& Knowledgeof user


& Knowledgeof user

Figure 12: Framework extract showing steps involved in the “system” analysis approach

W S Atkins use system reliability analyses to identify dominant failure modes and to calculate systemreliability measures. The method applied is basically stochastic modelling, with the reliability analysis beingbased on the first order reliability method (FORM) approach, with random variable probability models usedfor describing the uncertainty in basic variables. All important environmental parameters, are modelled asexplicit random variables.

W S Atkins use their analysis package, RASOS, which utilises a joint beta-point concept for reliabilityformulation combined with a virtual distortion method (VDM) technique for non-linear structural analysis isused. The VDM concept uses the “superposition principle where any given structural condition is derivedfrom a combination of two states - a fundamental state from the original, linear elastic solution and a virtualstate caused by virtual distortions introduced into the structure to account for the non-linearities” [5]. Thejoint beta-point for a failure sequence, is determined as a solution of a multi-constrained non-linearoptimisation method. This enables the use of more realistic member post-limit behaviour models andcombinations including more than one failure mode per structural element.

Failure-tree enumeration is carried out to obtain close bounds on system reliability. The lower bound onsystem reliability is the reliability index for first failure of any member, and the upper bound is found byanalysis of all the dominant load paths identified [10, 122].

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There are also several simplified “system” reliability methods - these are less commonly applied, and arediscussed briefly in the section below:

• Bea developed several approaches for evaluating the acceptable, tolerable or desirable reliability of astructure [1, 13, 60]. The most recent approach developed by Bea et al [218], reported in 1997, wasapplied to the reassessment and re-qualification of two Gulf of Mexico platforms. The analysisprocedure consisted of three levels of analysis as developed in the API guidelines for reassessmentand re-qualification of steel template-type offshore platforms: i) Screening analysis, ii) Design levelanalysis (DLA), iii) Ultimate strength analysis (USA). The three levels of analysis were performedsequentially, with the checks becoming more detailed and less conservative. Bea et al reported on twotypes of analysis - the DLA using the programme StruCAD*3D and then the USA using theprogramme ULSEA (Ultimate Limit State Equilibrium Analysis) developed in 1995 by Bea.

• In a recent review of contributions to offshore technology, Cornell [62] presented a simple expressionfor the probability of failure, thus: Pf = ∫ H(x) fR(x)dx ≅ H(R) e½(k1δR)² Where Pf = probability offailure, H = complementary cumulative distribution function (CCDF) of the load, S, fR = probabilitydensity function of the capacity or resistance, δR = coefficient of variation of the capacity, R = meancapacity. In 1994 Cornell also worked on the development of a “random-variable level probabilisticmodel of structural demand, behaviour, and capacity”. This work was based on “near-failure,static/dynamic, displacement behaviour of structural systems and exploits an explicit analytical form”[see also 133]. One of the main conclusions of Cornell's review was that “it is desirable to set aquantitative structural reliability level or levels as the objective and starting point for any structuralcriteria” since “many benefits of clarity, consistency and efficiency can follow from that beginning”[62].

• In 1997 AME [155] developed a new method for assessing the strategic level of structural safety. Aneed was identified to have a form of modelling which could accommodate the aspects of structuralsafety with both technical and human factors, in combination with the mechanical aspects ofreliability analysis. This new model “modifies and augments the detailed approach to structural safetyevaluation using reliability analysis. The central concept of the model is called “structuraltoughness.” The toughness aspect reliability analysis is intended to augment the current reliabilityanalysis by assessing if the structure will indeed be safe under conditions which vary from theidealised conditions incorporated in the reliability calculations approaches.” It should be noted thatthis study presented the model in its formative stage, and significant development is envisaged beforethe structural toughness model concept can be transformed in to a working approach.

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6. BENEFITS AND POTENTIAL APPLICATIONS OF THEFRAMEWORK

The perceived benefits and potential applications of the framework can be summarised as follows:

6.1.1 Moving towards “true” reliability

To move towards “true” reliability it is first necessary to identify where uncertainty is introduced, andsecondly to study the uncertainty so as to find ways in which to minimise it significantly. The frameworkdeveloped herein, addresses the first part of the problem, by clearly and concisely presenting the stepsrequired to perform a structural reliability assessment, which precisely identifies those steps which lead to theinclusion of uncertainty.

6.1.2 Improved preparation

The use of the framework would make it easier to see precisely what information is required before anassessment is started. The framework could be easily “filtered” to allow a list of all inputs to be produced.

6.1.3 Improved consistency

If all documentary outputs identified in the framework are produced, then a full set of reports would beproduced which show the assumptions and values used at a particular step during the assessment process.This would also have the benefit of encouraging a more consistent approach to assessments. Similarassumptions could be made for future assessments, and the effect of changes in assumptions could be easilystudied and perhaps quantified. The continued use of the framework would also lead to improvedrepeatability and uniformity of assessments.

6.1.4 Guidelines

The framework has been produced in a format that lends itself to provision of guidelines, which will assistcompetent reliability engineers to perform a proficient reliability assessment. The format also enables theframework to be used as a “register”, in order that an engineer could check whether all activities have beenundertaken correctly, and in the right sequence.

6.1.5 Application tool

The framework was developed to identify each activity necessary to perform a full reliability assessment. It isenvisaged as an application tool for performing reliability assessments, as any competent reliability engineershould be able to follow the steps that are shown on the flowchart and described within the correspondingtables.

6.1.6 Management tool

The framework could be a management tool - to aid project planning, implementation and checking. Therelevant timings and resource allocations needed for each step could be anticipated and predicted in advanceof the process being undertaken.

6.1.7 Quality assurance tool

The framework would allow a simplistic and traceable QA to be developed and performed on the reliabilityassessment process as a whole. QA checkers would be required to examine each step performed, and identifythose areas where either the procedure has not been followed precisely, or where individual activities havebeen carried out with errors.

6.1.8 Educational/training tool

The generic framework shows the step by step process necessary for the adequate performing of a structuralreliability assessment. It should therefore eliminate some of the “mystery” surrounding the field of reliabilityanalysis, and improve understanding of each of the stages within it. It should enhance comprehension of theprocess by showing in a clear, concise and visual presentation what steps are required. The provision of a listof the corresponding references for each stage of the framework allows users to trace the development andreasons behind steps within the framework in context.

___________________________________________________________________________________________________________________________________________________________________________________________________________________________________


Benefit /potential

application

Main aspects

Movingtowards “true”

reliability

• identify where uncertainty is introduced• study uncertainty to find ways to minimise it• framework shows steps required for structural reliability assessment• also identifies steps which lead to the inclusion of uncertainty

Improvedpreparation

• easier to see what information is required to start an assessment• easily “filtered” to show a list of all inputs

Improvedconsistency

• full set of reports produced to show assumptions & values used• encourages more consistent approach to assessments• similar assumptions could be made for future assessments• effect of changes in assumptions can be easily studied• improved repeatability & uniformity of assessments.

Guidelines • format that lends itself to provision of guidelines• “register” - check activities in right sequence & correctly performed

Applicationtool

• competent reliability engineers should be able to follow all the steps• flowcharts & the corresponding tables should provide indication of all steps required

Managementtool

• aid to project planning, implementation & checking• timings & resource allocations needed for each step predicted

Qualityassurance tool

• simplistic & traceable QA• examine each step performed, & identify areas where procedure has not been followed

precisely, or where individual activities have been carried out with errors.Educational/training tool

• step by step process clearly shown• enhance comprehension of process by showing concise presentation what steps are

required

Table 14: Summary of benefits and potential applications of the framework

6.1.9 Potential usefulness of framework at each phase of a project

The following figure summarises the perceived potential usefulness and benefits of applying the framework atthe different phases of a typical offshore development project. The indications of benefits are arbitrary and tosome extent subjective, but give an overall indication of the application of the framework. The usefulness canbe perceived in two aspects: firstly, in terms of providing a more consistent approach to reliabilityassessments, and secondly, in terms of the potential optimisation and cost benefits of using the framework atdifferent stages of an offshore platform project.

Potentialusefulness

Conceptstudy

Feasibilitystudy

Detaileddesign

Installationphase

Operationalphase

Re-asste.g. afterdamage

Re-asste.g. afterinspection

Re-asst e.g.after changesin deck loadsor envt info.

Extensionoforiginaldesignlife

Low

��

��

��

Medium

��

��

��

��

��

High��

��

��

��

��

��

Figure 13: Diagram indicating potential usefulness of framework at each phase of a project

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7. DISCUSSION AND CONCLUSIONS

• The aim of this report was to describe the development of a generic framework for system reliabilityassessment and to concentrate on the main steps in a reliability assessment and the key related technicaland philosophical issues. These issues have been linked together in a flowchart arrangement, in an attemptto present a rational and concise framework.

• A generic framework has been prepared which has been developed for use with both design/new andexisting/reassessment structures, and is applicable to both fixed and “floater” types of installations. Thekey technical issues along with the philosophical issues that need to be addressed during a structuralreliability assessment are shown in the flowchart diagram. The flowchart was developed so that the inter-relations of all the issues was clear and easily understood.

• The flowchart presentation format allows, at a glance, the relations between all the issues raised to bepresented. It is a concise and succinct method of presentation, although it does not allow any detail of thesteps to be presented. It was for this reason, therefore, that the outline tabular presentation was developed.

• In order to study the generic framework in more detail and to break down each stage into individual stepactivities, a specific example had to be adopted before a more detailed framework could be presented. Thisis because different issues are raised for different types of structure, and it was unwieldy to try to includeall structural options within one detailed framework. It was for this reason that the specific example of thedesign of fixed offshore platform within the North Sea was adopted and developed further. A similardevelopment to a more detailed form can be undertaken for different offshore applications.

• The specific example framework that was developed has been presented in both the flowchart and thedetailed tables. This latter method allows for much more detail of each step, as well as an indication of theuncertainty, sensitivity, complexity and level of user competency required at any given stage. The relativesignificance of these factors can be seen at a glance - an arbitrary scale system has been adopted, whereone * is the minimum, and five * is the maximum. This discretionary scale system could be taken further,if required, by sorting and ranking the steps of the framework according to any one of the factorsidentified. All four factors may not be required for every detailed application since, although separate, theissues of complexity and user competence are undoubtedly linked.

• The framework developed has included all the main steps identified through the review study. A top-level framework indicates the main elements of the generic framework starting with the inputs requiredrelating to platform description, foundation and environmental parameters. The next eight stages deal withthe detailed steps required to undertake a reliability assessment and include an assessment of the fixing ofthe structure, modelling of the structure, foundation capacity derivation, load derivation, system analysismodel derivation, ultimate capacity derivation and then the reliability analysis. The final stage is thecollation of outputs of each of the stages, resulting in a measure of reliability and comparisons withreliability targets if required.

• The generic framework was split into three parts for ease of presentation:

1. The first part shows the technical and philosophical issues and their relationships for the initialactivities required. Options are included in this part of the generic framework for both new/designand old/reassessment structures - inspection reports, weld defect assessments and specificdamage/defect reports are included for the old/reassessment structures in order to provided acondition assessment of the overall structure. Options are also included for fixed and floater

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structures - for the floater structures, an assessment of the buoyancy effects and their distribution,along with the associated uncertainty, is required. The determination of structural members,inclusion of structural parts and decisions as to how these are to be modelled are included in thispart.

2. The second part of the generic framework involves the assessment of the local environmentalconditions and the foundation assessment. Determination of the environmental parametersstatistical distribution, and derivation of loads on the structure, combined with the determination ofthe associated uncertainty are performed at this stage. Options are presented in the foundationassessment according to whether or not a foundation assessment is actually required - if it isrequired, then an assessment of the conditions from the local soil conditions, the influence ofageing and group interactions, along with the associated uncertainty.

3. The third part of the generic framework involves a decision as to which reliability approach toadopt, and then follows the procedure required for each option. The methods covered are both the“component” based approach and the “system” analysis approach. Within the “component” basedapproach there are three techniques which can be applied - “minimal” analyses approach (e.g.where only 8 analyses are carried out - one for each wave direction), response surface technique,and the numerical simulation approach. The performance of non-linear pushover analyses anddecisions on failure criteria, determination of the ultimate capacity and distribution of strength andhence the probability of failure, along with integration of strength with loading are all included.Within the “system” analysis approach, the dominant failure modes, and reliability analysis,together with calculation of reliability and sensitivity measures of the system, are required. Forboth approaches, presentation of associated uncertainties is also required, before the final stage ofderiving the measure of reliability is achieved. The final activity is comparing reliability with pre-defined targets and acceptance criteria.

• The key underlying question throughout the framework development was what changes or improvementscould be made to the system reliability assessment process in order to improve consistency and movetowards “true” reliability (or failure probability that could begin to be interpreted as absolute values fordecision making). The framework developed in this report provides a sound basis for more consistentapplication of the reliability techniques. Furthermore, it summarises the whole assessment process so thatindividual steps can be identified, studied and improved, thus leading to an improvement in overallreliability assessments.

• The benefits and potential applications of the use of the framework have been detailed in this report, andinclude moving towards “true” reliability, improved preparation, improved consistency, provision ofguidelines for competent reliability engineers. The framework can also be used as an application tool, amanagement tool, a quality assurance tool and as an educational/ training tool.

• A study of the generic framework has indicated those areas where external constraints were likely toimpinge on the activities. These are mainly shown to affect the choice of software and methodology, andthe provision of different types of data. There may be constraints by the nature or amount of specific dataavailable. Any constraint may affect the inputs and hence performance of the overall reliability procedure.Although these constraints cannot easily be altered, it was felt important that those steps where constraintswould be incurred, should be identified. The following are examples of the areas identified from thegeneric framework where external constraints are likely to impinge:

Step 2. Modelling of structure: Decision as to what software package to use

Step 4. System analysis model derivation: Complete structural analysis using various software options

Step 4. Ultimate capacity derivation: Decision as to which reliability methodology to adopt.

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• The generic framework has enabled identification of areas where future work is needed to improve themethods and converge towards true reliability predictions. These areas were primarily where significantuncertainty is currently incurred or where parameters or processes appear highly sensitive, and hencewhere further work could be focused. Some areas will be addressed in the second phase of the project. Atpresent, these general areas are as follows:

⇒ Determination of environmental parameters statistical distributions

⇒ Derivation of loads on the structure

⇒ Determination of foundation capacity and its distribution

⇒ Decision on modelling method of foundations

⇒ Performance of pushover analysis and determination of ultimate capacity

⇒ Determination of strength distribution (member / structure)

⇒ Derivation of probability of failure.

◊ More specifically, the issue of parameter sensitivity should to be studied, and in particular thefollowing aspects:

⇒ Foundation parameters

⇒ Modelling of wave load

⇒ Wave height

⇒ Drag coefficient

⇒ Imperfection Magnitude

⇒ Imperfection Direction

⇒ Yield strength

◊ The uncertainty of individual steps in the reliability analysis procedure is also an area for furtherwork, including the approach/method adopted, modelling uncertainty, study of foundation effectsand study of system effects. The variability in different methods such as search algorithms,probability criteria, pushover analysis assisted by simulations and simplified system reliabilitymethods should also be studied.

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development of a structural system ... structural reliability analysis framework for fixed offshore...

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