DEVELOPMENT OF A STRUCTURAL SYSTEM RELIABILITY FRAMEWORK FOR OFFSHORE PLATFORMS

Transcription

1 JIP: Structural reliability analysis framework for fixed offshore platforms DEVELOPMENT OF A STRUCTURAL SYSTEM RELIABILITY FRAMEWORK FOR OFFSHORE PLATFORMS May 1998 Document No. JHA003 University of Surrey, Department of Civil Engineering Guildford, Surrey GU2 7XH JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 1

2 TABLE OF CONTENTS 1. PROJECT SUMMARY... 4 PHASE 1: REVIEW STUDY... 4 PHASE 1: DEVELOPMENT OF GENERIC ASSESSMENT & PRESENTATION FRAMEWORK... 4 PHASE 2: OFFSHORE STUDY... 4 PHASE 3: DEVELOPMENT OF OFFSHORE FRAMEWORK INTRODUCTION BACKGROUND NEED FOR A MORE RATIONAL APPROACH TO STRUCTURAL RELIABILITY ANALYSIS ISSUES IDENTIFIED FROM THE REVIEW STUDY SUMMARY OF GENERIC ISSUES SUMMARY OF CONCLUSIONS FOR FIXED OFFSHORE PLATFORMS IDENTIFICATION OF TECHNICAL AND PHILOSOPHICAL ISSUES DEVELOPMENT OF THE FRAMEWORK PRESENTATION OF GENERIC FRAMEWORK GENERIC FRAMEWORK TABLES OUTLINE GENERIC FRAMEWORK WITH CORRESPONDING REFERENCES PRESENTATION OF EXAMPLE FRAMEWORK SPECIFIC TO DESIGN OF FIXED OFFSHORE PLATFORMS OUTLINE EXAMPLE FRAMEWORK DIFFERENT RELIABILITY ASSESSMENT METHODS Minimal analysis approach Response surface technique Numerical simulation approach System analysis approach BENEFITS AND POTENTIAL APPLICATIONS OF THE FRAMEWORK Moving towards true reliability Improved preparation Improved consistency Guidelines Application tool Management tool Quality assurance tool Educational/training tool Potential usefulness of framework at each phase of a project DISCUSSION AND CONCLUSIONS REFERENCES JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 2

3 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 Table 6: Summary outline framework presented in tabular format Table 7: Detailed breakdown table for Stage 1: Modelling of structure Table 8: Summary outline framework specific to design of fixed offshore platforms Table 9: Framework Stage 1. Modelling of structure Table 10: Framework Stage 2.1 Capacity and load derivation - determination of foundation capacity and stiffness Table 11: Framework Stage 2.2 Capacity and load derivation - determination of environmental loads Table 12: Framework Stage 3. System analysis model derivation Table 13: Framework Stage 4 Capacity and reliability derivation Table 14: Summary of benefits and potential applications of the framework 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 Figure 4: Generic framework for design and reassessment of structures - part Figure 5: Generic framework for design and reassessment of structures - part Figure 6: Generic framework for design and reassessment of structures - part Figure 7: Specific framework for design of fixed offshore platforms Figure 8: Generic framework for design and reassessment of structures - reliability assessment extract Figure 9: Framework extract showing steps involved in the minimal analysis approach Figure 10: Framework extract showing steps involved in the response surface technique Figure 11: Framework extract showing steps involved in the numerical simulations approach Figure 12: Framework extract showing steps involved in the system analysis approach Figure 13: Diagram indicating potential usefulness of framework at each phase of a project JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 3

4 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 interrelations 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 of offshore structural system reliability, as well as generic aspects of structural reliability. The overall emphasis in this review study was to identify the sensitivities and difficulties with reliability analysis 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. The background to the need for the framework is briefly discussed, along with the main issues arising from the review 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 review study and experience gained from the framework development phase, a number of sensitivity studies were identified. These included: yield strength and foundation capacity parametric studies, foundation capacity assessment sensitivity studies, and comparison studies of the main methods of reliability analysis used in the offshore industry. Phase 3: Development of offshore framework The fourth report, Presentation of a structural system reliability framework for fixed offshore platforms is an executive summary report. This summarises the key findings from Phases 1 and 2, and presents the revised framework in context in sufficient detail to enable the reader to apply the flowcharts and tables presented therein. JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 4

5 2. INTRODUCTION This report presents the framework for structural system reliability assessments of offshore platforms. The background to the need for the framework is briefly discussed, along with the main issues arising from an extensive review study (see Review Study Report for full details). Three different formats are used for the presentation of the framework. The potential applications and benefits to be achieved from the application of the framework are highlighted, demonstrating the potential of the use of such techniques, to move towards achieving more consistent and true reliability predictions. It is concluded that new structures would benefit most from the early application of the framework, but that older structures undergoing reassessment will also see benefits. 2.1 Background The fundamental design requirement of an offshore platform is that it must satisfy the functional need of support structure for offshore oil and gas operations, and be structurally adequate for both operating and extreme loading. There are many different loads to be taken into account at the design stage, including dead and live loads, vibration, self weight, ice, ship impacts, wind, wave, tide, current, fatigue, foundation reactions, seismic effects etc. The framework developed is designed to assess extreme weather. In the future, other frameworks could address other hazards. The following figure illustrates extreme weather as one of the main offshore hazards: Ship impact Extreme weather Fire Fatigue damage Major hazards affecting fixed offshore structures in the North Sea Explosion Foundations failure Aircraft impact Corrosion damage 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 with structural integrity analysis, to move towards a set of guidelines in order for a more rational approach to be adopted. The use of different models, software and users often means variations in methods and assumptions, and this in turn implies that different modelling and statistical uncertainties are included in the analysis. There is a genuine need to reduce or better quantify modelling, as well as to consider alternative means of incorporating modelling in reliability analysis. A lack of guidelines or a framework within which such work is undertaken has lead to the development of inconsistent assessments. Other investigators [62, 118] have also identified the need for setting guidelines and targets including the benefits that arise from a clear, consistent and efficient approach. A framework in which such aspects are included, combined with information on the interpretation and use of the results is to be developed here, and the review study described herein aimed to identify all major studies carried out 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 system reliability 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 interrelations and relative significance were assessed in order to link them together in a rational process that provided 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 a basis for future working practice/guidance in order to move towards true reliability, with improved preparation before a reliability analysis is undertaken, improved consistency in results; along with allowing the framework to be used as application, management, quality assurance and educational or training tools. JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 5

6 3. ISSUES IDENTIFIED FROM THE REVIEW STUDY A review study was carried out in an attempt to gain a historical appreciation and understanding of the current techniques and the philosophy behind them, as applied to the performance of structural reliability analyses 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. The following points highlight 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 Probability of failure Uncertainty Relative significance Better quantification & reduction of uncertainties Improving consistency in assessments Human factors & competence/ guidance for users Interpretation of system effects Reliability involves dealing with events whose occurrence/non-occurrence at any 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 in structural strength & then - calculation of the probability of failure. Probability of failure is integration of probability distributions of load/ resistance. Reliability results can only be interpreted as absolute values when physical dominates over model prediction. Probability of failure, Pf = Φ(-β). Φ() = std normal distribution ftn & β = reliability index. Uncertainty is categorised into three main groups: physical, statistical & modelling. However, there is a degree of introduced by the user, which is generally part of the modelling. Sensitivity gives an indication of significance of a parameter in affecting overall reliability. Investigating relative sensitivity involves a study of the effect each different parameter has on results of reliability analysis of the overall structure. Reliability results used to be taken as an indication of the notional reliability of a structure, but more recently, there has been effort to bring the reliability prediction as close to true reliability as possible. Developments in modelling & software has minimised error incurred during initial stages, & progress in predicting environmental conditions has enabled more accurate representation of environmental loads. To improve consistency of results between different structures/users, increased awareness of uncertainties/sensitivities & the various philosophical issues at each step of a reliability analysis is needed. Development of a framework to identify main steps, along with justifications for these, will go towards improving overall structural reliability & consistency. User is affected by user s competence, which becomes higher when the activity has high or is highly sensitive. There is a need to move towards guidelines for a more rational approach - to reduce/better quantify modelling, & to consider alternative ways of incorporating this in reliability analysis. There are a number of very different factors that can be studied in order to assess system effects derived from analysis of detailed structural models - key factors in such studies are: reserve & residual strength & redundancy. Table 1: Summary of generic conclusions JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 6

7 3.2 Summary of conclusions for fixed offshore platforms Loading Foundation Environmental extremes Wave approaches Treatment of drag, inertia & marine growth System effects Need for framework It has been shown that reliability assessment is generally dominated by the uncertainties in the loading. Loading variables account for > 95% of the total, & a rigorous modelling of the in these variables is vital for reliability based integrity assessments. There is a need for more data to develop joint probability distribution of all relevant environmental parameters. Analyses have shown a significant degree of exists about the validity of foundation model & of data used for the soil parameters. This was sometimes found to be of the same order of magnitude as the physical in environmental load. Piles studies: for clay & sand NGI recommended API RP2A 20th - conservative for NC clay, with a modest COV, & slightly higher COV for OC clay, & conservative for dense sand, with a high COV. [ ]. Axial capacity: new design approaches were developed at IC for driven piles in clays & sands, is simple to apply; & has advantages over existing API approaches. The formulae used to ascertain the pile group interaction for piles in sand have not yet been widely used in foundation analyses. Conventional treatment of waves, current & wind forces was each factor separately & then combine the independent extremes simultaneously. This is over conservative & overestimates the design loads required. Recently, the development of more reliable databases of hindcast environmental data has enabled a joint description of these quantities to be determined. Shell carried out studies to assess environmental loads. The New Wave kinematics theory was developed to more accurately represent the wave & current, and to improve the accuracy of the drag coefficient used [7,8,9]. Generally only 1 or 2 wave approaches are used in structural platform analysis. For a full analysis, more wave directions must be carried out. System capacity can be estimated without taking into account randomness in inertia & marine growth coefficients, which can be modelled as deterministic. The and randomness in drag cannot be ignored, and must be included. Structural behaviour beyond first member-failure depends on degree of static indeterminacy, ability of structure to redistribute load, & ductility of individual members. For a perfectly balanced structure the system effects for overload capacity beyond first member failure, are due to the randomness in the member capacities. For more realistic structures system effects are both deterministic & probabilistic. Deterministic effects are from remaining members in the structure which still carry load after one or more members have failed; probabilistic effects are from the randomness in member capacities [6]. The system effect is the difference between system reliability index & failure of any 1 member [5]. A number of studies on idealised behaviour of structures identified the need for some kind of framework or general procedure was needed in order to assess offshore platforms, with a range of brittle and ductile behaviours, and a variety of failure modes, but with a more rational and consistent approach. e.g.[52] Table 2: Summary of specific conclusions JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 7

8 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 state of the art in the area of offshore structural reliability. The resulting report incorporated an introduction to generic reliability issues, and then briefly described all major aspects of reliability analysis. During this review period, the key findings of the technical and philosophical issues were identified for incorporation into the subsequent framework. The table below summarises both the key stages in reliability analysis as well as the related technical and philosophical issues. Key stages in reliability analysis 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 Related technical & philosophical issues 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 JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 8

9 4. DEVELOPMENT OF THE FRAMEWORK A generic framework has been developed which will set the basis for achieving more consistent system reliability assessments. A key underlying question throughout this framework development was what changes or improvements could be made to the reliability assessment process in order to move towards true reliabilities (or failure probability that could begin to be interpreted as absolute values for decision making). The framework was developed using standard flow chart symbols as shown in Table 4 below. Symbol Definition Process 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 drawn up to represent the main steps in the overall process. These diagrams were then augmented and developed to form a more detailed approach. Several different options for presentation were explored, with the flowchart type presentation being the preferred option due to its visual impact, and clear presentation of the issues and unambiguous representation of their links. The flowchart is a very concise method of presentation, with only the key characteristics of each step being described. Standard flowchart symbols were 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 the key stages. Tabular formats were therefore developed as an alternative presentation method to the flowchart approach, in order to enable the key background documents at each stage of the framework to be identified, and clearly presented. The use of the tables allowed a full description of the activity to be included, along with an indication of the significance of, sensitivity, complexity and level of user 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 been included here which introduces the main elements of the generic framework, without going into detail of all the steps required within each stage of the process. Figure 2 below shows the top-level framework. Inputs - platform description, foundation/ environmental parameters Assessment of fixing of structure Modelling of structure Foundation capacity derivation Load derivation System analysis model derivation Ultimate capacity derivation Reliability analysis Output - measure of reliability & comparisons with targets. Figure 2: Top-level generic framework flowchart JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 9

10 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 an input or an output, and shows the main inputs required for a reliability assessment. These include details of the platform description, buoyancy effects (if appropriate), details of the foundations and soil 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 the foundation 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 the structure which meets the precise needs of the study. Decisions as to what software package to be used 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 soil characteristics 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 to be 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, 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 in combination with the descriptions derived at the eighth stage. A determination of the reliability of the structure is also an output and is derived from the probability of failure and the analysis. Figure 3 overleaf shows the flowchart that has been developed for fixed offshore structures. It shows each step that needs to be carried out, and the precise sequence for those activities, in order for a full structural system reliability assessment to be undertaken. The flowchart includes all the activities necessary to collate the inputs required, to perform the processes required and to produce all the outputs required. JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 10

11 Platform structure details Design Reassessment Inspection reports, weld defect assessments, specific damage/ defect reports Platform design drawings etc. for new platforms Condition assessment of structure for reassessment purposes Deadload parameters including buoyancy effects Assessment of structure fixing conditons i.e. fixed/floating Inertial and dynamic loading parameters Fixed Floater For "floater" structures, determine buoyancy effects etc. & their distribution Decide on modelling method for "floater" buoyancy effects What software package to use? Strucural members determined Relevance/importance of parts e.g.is a detailed deck necessary What structural parts are to be included in the model? Valid reasons why certain parts not included How are parts to be modelled? Justification for modelling method chosen Environmental parameters (wave height, wave period, current, wind, inertia, drag etc.) Assessment of local environmental conditons Separate environment studies Yes Does foundation require assessment? No environmental parameters statistical distribution Foundation parameters (soil conditons, pile conditons, ageing, group interaction etc.) Assessment of local foundation conditons Separate foundation studies Derivation of loads on the strucutre foundation stiffness & capacity & distributions How are loads applied to structure (eg. onto every member or onto bays?) Decide on modelling method for foundations System analysis model Which reliability approach to adopt? "Component" based approach "System" analysis approach Pushover analysis to identify dominant failure modes Minimal analyses approach (eg. 8 = one for each wave direction) Response surface technique Numerical simulation approach (eg. Monte Carlo) Identify dominant failure modes (search algorithms) Decide on failure criteria Perform a number of non-linear pushover analyses Build up structural system, series of parallel sub-systems What factors need to be assessed, relevant to focus of the study Decide on failure criteria ultimate capacity & other failure characteristics & determine failure surface if reqd. Perform component reliability analysis What factors need to be assessed, relevant to focus of the study distribution of strength (member / structure) & obtain probability of failure Present assessment of uncertainties Calculate reliability of system and sensitivity measures Integrate distribution of strength with loading on structure Present, understand & interpret results Compare reliability with Measure of reliability pre-defined targets & of structure acceptance criteria Figure 3: Generic framework for new (design) and old (reassessment) of structures - complete flowchart JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 11

12 Platform structure details New / design Old / reassessment Inspection reports, weld defect assessments, specific damage/ defect reports Platform design drawings etc. for new platforms Condition assessment of structure for reassessment purposes Deadload parameters including buoyancy effects Assessment of structure fixing conditons i.e. fixed/floating Inertial and dynamic loading parameters Fixed Floater For "floater" structures, determine buoyancy effects etc. & their distribution Decide on modelling method for "floater" buoyancy effects What software package to use? Strucural members determined Relevance/importance of parts e.g.is a detailed deck necessary What structural parts are to be included in the model? Valid reasons why certain parts not included How are parts to be modelled? Justification for modelling method chosen Figure 4: Generic framework for design and reassessment of structures - part 1 = Process, = Input / output, = Decision, = Document, = Terminal (start or end) JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 12

13 Environmental parameters (wave height, wave period, current, wind, inertia, drag etc.) Assessment of local environmental conditons Separate environment studies Yes Does foundation require assessment? No environmental parameters statistical distribution Foundation parameters (soil conditons, pile conditons, ageing, group interaction etc.) Assessment of local foundation conditons Separate foundation studies Derivation of loads on the strucutre foundation stiffness & capacity & distributions How are loads applied to structure (eg. onto every member or onto bays?) Decide on modelling method for foundations System analysis model Figure 5: Generic framework for design and reassessment of structures - part 2 = Process, = Input / output, = Decision, = Document, = Terminal (start or end) JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 13 of 45

14 Which reliability approach to adopt? "Component" based approach "System" analysis approach Pushover analysis to identify dominant failure modes Minimal analyses approach (eg. 8 = one for each wave direction) Response surface technique Numerical simulation approach (eg. Monte Carlo) Identify dominant failure modes (search algorithms) Decide on failure criteria Perform a number of non-linear pushover analyses Build up structural system, series of parallel sub-systems What factors need to be assessed, relevant to focus of the study Decide on failure criteria ultimate capacity & other failure characteristics & determine failure surface if reqd. Perform component reliability analysis What factors need to be assessed, relevant to focus of the study distribution of strength (member / structure) & obtain probability of failure Present assessment of uncertainties Integrate distribution of strength with loading on structure Calculate reliability of system and sensitivity measures Present, understand & interpret results Measure of reliability of structure Compare reliability with pre-defined targets & acceptance criteria Figure 6: Generic framework for design and reassessment of structures - part 3 = Process, = Input / output, = Decision, = Document, = Terminal (start or end) JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 14 of 45

15 4.2 Generic framework tables As described earlier, the framework is presented in three forms: generic overview, flowchart and tabular formats. The tabular format allows a much greater depth of detail to be presented. It has also been adapted to include the main references pertaining to each of the activities. Table 5 shows the outline table which describes the basic stages including main inputs and outputs, and Table 6 shows the outline table with main references. 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 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 Calculate reliability of system, and sensitivity measures Present, understand and interpret results OUTPUT Determination of probability of failure (from study of failure surface, in combination with descriptions) Determination of measure of reliability of structure (from probability of failure and analysis) Table 5: Summary outline generic framework presented in tabular format JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 15 of 45

16 4.3 Outline generic framework with corresponding references Framework stage INPUTS Description of platform structure (from design drawings, defect/damage assessments, condition assessment reports, computer models etc.) Deadload and liveload parameter values (most applicable to floater structures: including buoyancy effects, inertial and dynamic parameters) Foundation parameter values (soil conditions, pile conditions, group interaction etc.) Environmental parameter values (from wave height, wave period, current, wind, inertia, drag etc.) References 5, 6, 7, 8, 12, 52, 54, 55, 56, 122, 128, 129, 194, , 210, , 61, 65, 79, 83, 84, 157, 164, 188, 189, 190, 191, 192, 193, 197 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 whether 213, 214, 215 fixed or floater ) For floater structures only: Determination of modelling method for floater 211, 212, 216 buoyancy effects 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 in 5, 6, 7, 8, 12, 52, 54, 55, 56, 122, 128, what detail) 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 environmental 5, 6, 7, 8, 135, 136, 138, 143, 149, parameters distribution, statistical distribution etc.) 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 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 in strength (on both member and system level, if required) 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. OUTPUT Determination of probability of failure (from study of failure surface, in combination with descriptions) Determination of measure of reliability of structure (from probability of failure and analysis) Table 6: Summary outline framework presented in tabular format 1, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 20, 21, 23, 60, 62, 122, 133, 155, , 55, 56, 122, 123, 194, , 49, 50, 51, 111, 112, 113, 120, 121, 132, 141, 146, 186, 204 5, 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, 160 1, 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 5, 6, 7, 8, 9, 13, 62, 63, 131, , 49, 50, 51, 111, 112, 113, 120, 121, 132, 141, 146, 186, , 160, 186, 200, 201, 202,, 205 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 JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 16 of 45

17 Each stage identified in the outline generic framework table has been examined further, and a detailed breakdown of every single activity required is included. A sample is included here relating to stage 1: modelling of structure. Stage 1. Modelling of structure Input 1 Step 1.1 Step 1.2 Step 1.3 Step 1.4 Step 1.5 Step 1.6 Step 1.7 Step 1.8 Output 1 Brief description of activity Description of platform structure & fixing conditions (from design drawings, computer models etc.) Decision as to what software package to use (may be governed by external constraints) Determination of structural members Assessment of relevance/importance of structural parts (e.g. is a detailed deck necessary?) Decision as to what structural parts are to be included in the model Presentation of valid reasons why certain parts are not included User discretion & interpretation of the environmental loads (User effects are assumptions, judgment & knowledge) Decision as to how parts are to be modelled Presentation of the justification for the modelling method chosen 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 specific application suitable for use in the design of fixed offshore structures in the North Sea was derived. Figure 7 over-leaf shows the specific framework for the design of fixed offshore platforms as a flowchart. This flowchart 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 inputs and main outputs, was also the basis for the detailed example exercise. The stages identified in this table were examined 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: sensitivity complexity user competence - the amount of introduced at each step - the sensitivity to the overall reliability results to each step - the level of complexity of the actions for each step - 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 the examination of the specific example exercise, it was found that the complexity and user competence factors were often awarded the same level. It was considered important that this should be identified, and even though having both factors may not be fully justified in this specific case of the design of fixed offshore platforms in the North Sea, it may become more pertinent for different specific examples, and it was decided that both factors should be shown here. JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 17 of 45

18 Platform structure details Platform design drawings etc. for new platforms What software package to use? Strucural members determined Relevance/importance of parts e.g.is a detailed deck necessary What structural parts are to be included in the model? Valid reasons why certain parts not included How are parts to be modelled? Justification for modelling method chosen Environmental parameters (wave height, wave period, current, wind, inertia, drag etc.) Assessment of local environmental conditons Separate environment studies Foundation parameters (soil conditons, pile conditons, ageing, group interaction etc.) Assessment of local foundation conditons Separate foundation studies environmental parameters statistical distribution foundation stiffness & capacity & distributions Derivation of loads on the strucutre Decide on modelling method for foundations How are loads applied to structure (eg. onto every member or onto bays?) System analysis model Which reliability approach to adopt? "Component" based approach "System" analysis approach Pushover analysis to identify dominant failure modes Minimal analyses approach (eg. 8 = one for each wave direction) Response surface technique Numerical simulation approach (eg. Monte Carlo) Identify dominant failure modes (search algorithms) Decide on failure criteria Perform a number of non-linear pushover analyses Build up structural system, series of parallel sub-systems What factors need to be assessed, relevant to focus of the study Decide on failure criteria ultimate capacity & other failure characteristics & determine failure surface if reqd. Perform component reliability analysis What factors need to be assessed, relevant to focus of the study distribution of strength (member / structure) & obtain probability of failure Present assessment of uncertainties Integrate distribution of strength with loading on structure Calculate reliability of system and sensitivity measures Present, understand & interpret results Measure of reliability of structure Compare reliability with pre-defined targets & acceptance criteria Figure 7: Specific framework for design of fixed offshore platforms JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 18 of 45

19 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 in strength (on both member and system level, if required) 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, and determine Calculate reliability of system, and sensitivity measures Present, understand and interpret results OUTPUT Determination of probability of failure (from study of failure surface, in combination with descriptions) Determination of measure of reliability of structure (from probability of failure and analysis) Table 8: Summary outline framework specific to design of fixed offshore platforms JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 19 of 45

20 Stage 1. Modelling of structure Arbitrary scales Stage of framework Brief description of activity Uncertainty Sensitivity Complexity User competency required 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 JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 20 of 45

21 2.1 Foundation capacity and load derivation - determination of foundation Arbitrary scales capacity Stage of framework Brief description of activity Uncertainty Sensitivity Complexity User competency required Input 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 User discretion and interpretation of foundation and soil data (User effects are assumptions, judgment and knowledge) Step Assessment of local conditions (Decision on values of parameters to be adopted in the study) Step Prepare deterministic representation of foundation parameters Step Prepare probabilistic representation of foundation parameters Step Step Step Output 1 Determination of foundation capacity & stiffness & its distribution Determination of with the foundation capacity Decision on modelling method for foundations 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 JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 21 of 45

22 2.2 Foundation capacity and load derivation - determination of environmental Arbitrary scales loads Stage of framework Brief description of activity Uncertainty Sensitivity Complexity User competency required Input 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 User discretion and interpretation of the environmental data (User effects are assumptions, judgment and knowledge) Step Assessment of local conditions (Decision on values and methodologies to be adopted) Step Prepare deterministic representation of environmental parameters Step Prepare probabilistic representation of environmental parameters Step Determination of environmental parameters' statistical distribution Step Determination of with the environmental parameters distribution Step Derivation of loads on the structure Step User discretion and interpretation of the environmental loads (User effects are assumptions, judgment and knowledge) Step Determination of with the environmental loading on the structure Output 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 JIP: Structural Reliability Analysis Framework For Fixed Offshore Platforms Page 22 of 45