Analysis on Fatigue Damage and Time Dependent Reliability for Components of Offshore Jacket Platform

Abstract: With the development of offshore oil & gas exploitation, there have been almost 200 fixed platforms constructed in china offshore. Offshore platform structure is the infrastructure for ocean ail exploitation, and it’s the basis for engineering activity and staff residence in the sea. At the meantime, offshore platform is a kind of complex structure, with large capital investment, in harsh environment. Therefore, the safety of offshore platforms has long been the focus of attention of scholars both at home and abroad. Fatigue damage is one of the main reasons that affecting the life of the platform components, so it is very important significance to study failure mechanism of fatigue damage and life prediction of ultimate bearing capacity on fatigue coupling for ensuring the safety of offshore platform structures and achieving the maximum economic benefits. The method of fatigue damage analysis, time-dependent reliability calculation and life prediction of ultimate bearing capacity on fatigue coupling are studied in this paper. The main works in this study are as follows:(1) For existing fatigue damage analysis method of offshore platform components doesn’t meet the real process of fatigue damage caused by environ-ment loads, it is developed an improved method of fatigue damage analysis that calculates fatigue damage in the load stress levels. By stress data of the simple structure model with simulating correlated wind and wave loads, the result of fatigue analysis implies that the existing method is insecure while the improved method is reasonable.(2) Based on monitoring data from monitoring system established in CB32A platform in Chengdao of Shengli Oil field, the cumulative fatigue damage of jacket joint are calculated, and the attenuation model of resistance on fatigue coupling are obtained. Combined with time-dependent reliability theory, the time-dependent reliability index of jacket joints are obtained.(3) Combining with the method of time-dependent reliability, it is summa-rized the algorithm for the life prediction method of ultimate carrying capacity. Based on target reliability index for failure state of structure that is advised in this paper, and associating with the resistance attenuation model and the extreme value distribution of loads, the residual life of jacket joints of CB32A platform are obtained.This paper presents a new idea of calculation for the cumulative damage in complex fatigue loads, it is of great significance for the improvement and development of cumulative fatigue damage calculation theory and promoting fatigue analysis methods to the engineering application…
Key words: Offshore jacket platform structure; fatigue cumulative damage; time-dependent reliability; life prediction

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