Objective
The aim of the project was to generate databases, and algorithms necessary to execute a validated fatigue crack growth/fracture analysis of a welded tubular joint which forms part of a fixed offshore platform. This will enable the consequences of the detection of a surface defect to be determined by tracing crack growth, the consequential reduction in joint stiffness, severance, and failure. The loading applied is representative of service loading including extreme conditions. This is encapsulated within a structural reliability analysis procedure so that both intact and damaged safety levels can be determined.
In turn the procedure is set within an expert system to provide sophisticated data and algorithm file manipulation, and guidance, help and advice as to the options available regarding maintenance and/or repair strategies through the knowledge base.
Innovative features include the use of an Expert System, advanced crack growth calculations.
Databases have been established on environmental data, platform loads, material properties, member strengths, tubular joint stress concentration factors, crack detection and sizing, crack growth, joint flexibility, reliabilities and sample jacket stresses.
An analyses package was developed by adapting and implementing analysis algorithms; these include facture mechanics modelling, tubular member stiffness and strength modelling, modelling of joint behaviour including joint flexibility, yielding and fatigue, reliability analysis models for component analysis, stress concentration factors. For the project, a sample eight leg jacket was selected; the geometry is typical of those found offshore.
Knowledge Bases were derived from available published information, discussions with industry, experience gained from design of offshore structures and solutions of associated problems by engineers within the project team. The topics covered are environmental data and modelling including correlation of events, materials, properties and variabilities, tubular joint stress concentration factors, crack detection sizing and modelling, joint modelling, undamaged tubular member modelling, implications of reliability analysis.
The PLAIM Strategy was developed by simulating practical structures and conditions, to generate safety levels. The studies performed included analysis of a range of in-service conditions and studies of a number of damage scenarios.
The PLAIM System has been developed by integrating software, databases and knowledge bases; this also included prepration of the PLAIM expert system and documentation and compilation of the PLAIM expert system manual.
The project has been executed in 5 phases. Phase I involves the development of databases covering all facets of the project such as stress concentration factors, stress distributions, stress intensity factors, and fatigue crack growth in welded tubular joints, material properties for fracture considerations, joint stiffnesses, reliability levels, inspection results on crack length and depth, and member and joint strengths.
Environmental parameters of wind and wave loads will be considered but to a less extent. Considerable effort will be directed towards database screening to ensure only sound data are incorporated thereby providing a firm basis for statistical characterisation in preparation for reliability analysis.
Phase II involves the development of algorithms to execute substantiated fatigue crack growth life and fracture calculations, elastic 3-D frame analysis which includes joint stiffness and strength modelling, and member modelling. Reliability analysis will be tackledwithin this activity to include multi-path analyses and Bayesian updating. This project will concentrate on an eight leg jacket subjected to typical North Sea conditions. Sub-contractor input to algorithm development and verification will be important here.
Phase III involves the development of knowledge bases each of which relates to a specific facet of the PLAIM system such as environmental description, load assessment, fatigue life analysis, fracture calculation, safety factors, reliability levels, maintenance strategies, repair options, and design in the general and detailed algorithm sense.
Phase IV involves the development of rational inspection, repair, and maintenance strategies. This is based on reviews of existing practice through literature searches and through discussions with relevant personnel from design organisations and operators. Results generated in the course of the work relating to implied reliability levels and costs have been used in developing appropriate strategies.
Phase V involves development of the expert system in its detailed and general form. Shell selection is part of this as is the interfacing to the various software packages, databases, etc. This phase involves implementation of the PLAIM software.
Programme(s)
Topic(s)
Call for proposal
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DEM - Demonstration contractsCoordinator
GU2 5RE Guildford
United Kingdom