Fatigue Strength Assessment Of Moonpool Structure Of Deepwater Drillship

With the offshore gas and oil development constantly proceeded in deeper water, the structural fatigue has become one of the most important reasons of offshore structure damage or even accidents. The expensive deepwater drillship, which has a quite complex structure because of its large openings, moonpool, always works in deep sea area in which the ocean environment is very severe. Severe casualties and great property losses will be caused in case of an accident. Therefore, It is extremely significant to make a fatigue strength assessment of the moonpool structure of deepwater drillship.The paper takes a deepwater drillship which is working at South China Sea as an example to analyse the influence of the moonpool plate thickness to the structural fatigue strength through the simplified fatigue assessment method and assess the fatigue strength of the moonpool structure by spectrum ananlysis. The main contents arc shown as follows:After summarizing the fundamental knowledge of ship structure welding and fatigue theory in SECTION 2, the thesis expounds the basic principle and concrete computational procedure of the simplified method and the spectrum ananlysis method of fatigue assessment of shipping and marine structures.The second part describes the concrete implementation process of fatigue stress analysis using the finite element method. And meanwhile the rational finite element model of deepwater drillship structure is established by MSC/Patran accordance to the concrete structural plan. On top of this, the mesh at fatigue check points where the fatigue assessment is need to be developed according to the specification is refined.Thirdly, the thesis makes a fatigue assessment of moonpool structure of deepwater drillship with different plate thickness using the simplified method in SECTION 4 so as to analysis the influence of the moonpool plate thickness variation to the structural fatigue strength. Additionally, the load and motion responses of the drillship in regular waves are calculated by the linear wave-induced loading calculation software HstarWV6.4 to obtain the corresponding fatigue load. Follow on calculating the transfer function of the stress response by carrying out the whole-ship FEM analysis of drillship, the last part makes a spectral-based fatigue strength assessment of the moonpool structure to obtain the fatigue life of all the hot spots and does some analyses on the result.