Structural Integrity Assessment And Risk Management For Ageing Offshore Platforms

At present, there are a large number of ageing platforms that are needed to extend life for continual service in the world. To ensure the continued safe performance of these ageing platforms, it's necessary to research on the dynamic structural integrity assessment and risk management technologies. The dissertation focuses on structural integrity management (SIM) for ageing offshore platforms, which is a part of the"863"High Technology Research and Development Program of China (No. 2006AA09Z355) of"Research on Life Extension Technology for Ageing Offshore Platforms", the National Natural Science Foundation Program (No. 50679083) of"Research on Life Prediction Methods and Management Theory of Ageing Offshore Platforms for Life Extension"and the Fundamental Research Funds for the Central Universities (No. 09CX05008A) of"Fundamental Research on Risk Control Techniques Application for Deepwater Oil and Gas exploitation in South China Sea". The main investigate issues include the time-dependent reliability, dynamic fatigue reliability, risk assessment and management technologies, and risk-based inspection and repair planning for ageing platforms. And based on the theoretical achievements, the ageing platform structural integrity management system software APSISM is developed. The main work is summarized as follows:1. Time-dependent reliability analysis of ageing platformsAs ageing effects would imperil the safety and serviceability of the structures, the global time-dependent reliability of ageing platforms is investigated. The stressors of resistance degradation were analyzed, and within these factors, structural deterioration of the platforms by corrosion and crack are studied. According to the function of corrosion process and crack propagation with time, the resistance degradation models with polynomial expression, power function expression and exponential expression are proposed. The probabilistic characteristics of the extreme loads including extreme ice load, typhoon load and seismic load are analyzed. The methods for determination of the load effect and its probabilistic characteristics are presented. Based on the resistance-load interference theory, the global time-dependent reliability analysis model of ageing platforms is proposed through discrete time method, equivalent load method and incremental time method. On the based of the time-dependent model, reliability updating based on Bayesian approach and proof load is studied.2. Dynamic fatigue reliability analysis of ageing platformsWith the service time increasing, the fatigue resistance of ageing platforms degrades for fatigue damage accumulation. Based on the S-N curve approach and fracture mechanics approach, the models of fatigue damage accumulation and crack propagation are established. The time domain analysis and frequency domain analysis are employed to calculate the nominal stress in the fatigue analysis model. The tubular joint is taken as the basic element of fatigue analysis, by virtue of path mapping technique, a new numerical method on stress concentration factors (SCFs) calculation of welded tubular joints is proposed, and the SCF distribution of tubular joints under combined loadings is studied. Taking fatigue damage accumulation and crack propagation as stochastic processes, the dynamic fatigue reliability analysis models are proposed based on fatigue damage interference and crack parameters interference theories. Considering the effect of corrosion, the fatigue stress under corrosion condition is investigated, and the dynamic reliability analysis model including the fatigue- corrosion interaction is proposed. The reliability is updated based on inspection and repair data, and the updating models under different results of inspection and repair are presented. According to the system-level reliability theory, the methods for failure modes determination and system reliability calculation are presented, and the dynamic fatigue system reliability of ageing platforms is studied.3. Dynamic risk assessment and risk managementApplying risk assessment to SIM of ageing platforms, the failure mode and effect analysis (FMEA) method is used for risk identification. By virtue of finite element analysis, the criticality of hazardous factors is classified, and the structural risk is assessed semiquantita -tively by failure modes and effect and criticality analysis (FMECA) method. For quantitative expression of risk, risk estimation and risk acceptance criteria are investigated. The failure probability of ageing platform is calculated by fault tree analysis method. According to the ALARP ("As Low As Reasonably Practicable") principle, the quantitative risk assessment is carried out. As the risk of ageing platform is varied with time, the dynamic risk is studied. Mitigation and risk reduction measures are considered if the risk of the platforms does not meet the fitness-for-purpose acceptance criteria, and four methods for risk reduction are proposed in this paper. The validity of these measures is verified by finite element method.4 Risk-based inspection and repair planningInspection and repair are the main ways to ensure structural integrity of ageing platforms, and at the same time they bring about economic burden if the times of inspection and repair increase. Therefore, it is necessary to develop the optimal inspection and repair planning. The planning programme is formulated as an optimization problem where the expected lifetime costs are minimized with a constraint on the minimum acceptable reliability index, and the expected inspection, repair and failure costs models are proposed. The genetic algorithm in Matlab toolbox is employed to solve the optimization problems. The safety margins are presented for the inspection events, the repair events and the failure events of ageing platforms. Moreover, based on event tree analysis, the formula is derived to calculate inspection probability, repair probability and failure probability at the time of each inspection. Based on the optimization model, the optimal results of single factor including inspection times, inspection intervals, inspection methods and repair methods are obtained. Then all the indeterminate factors are considered in the optimization problem, and 10 candidate strategies are determined by combining different inspection methods and repair methods. In each strategy, the inspection times and inspection intervals are optimized, and by comparison between the different inspection and repair strategies, the best inspection and repair planning is determined.5 Software development of ageing platform structural integrity management systemFor convenient application of the ageing platform SIM theory porposed in the paper, the ageing platform SIM software system APSIMS is developed by Visual Basic 6.0 program. The framework of ageing platform SIM is constructed according to the processes of fixed offshore structures SIM in API standard. And based on the framework, the software structure of APSIMS is designed. The database system is established by Microsoft Office Access for data storage and transmission. And three kinds of database, including platform structure database, environmental load database and inspection database, are designed. ANSYS program and MATLAB program are employed as application programming interface for structural analysis and optimization solution. The main functions of APSIMS consist of dynamic reliability analysis, risk analysis, risk management and assessment, and risk-based inspection and repair planning, which can implement SIM of all the life extension period for ageing platforms.