Fatigue Analysis Of Deepwater Semi-submersible Platform And Fatigue Test On Key Joint

With the continuous development of offshore oil and gas to deepwater, deepwater semi-submersible platform (Semi) has become one of the major platforms in offshore oil industry, and the structural analysis technology of deepwater semi-submersible platform appears particularly important. In order to accelerate the exploration of oil and gas resources in deepwater area of South China Sea and insure the safe and reliable service of semi-submersible platform in deepwater, the key technology for deepwater platform have been brought into the eleventh five-year plan of "National Science and Technology Major Project". In this thesis, surrounding fatigue analysis technology of deepwater semi-submersible platform structures, the fatigue life and fatigue reliability for deepwater semi-submersible platform structures are analyzed, and fatigue tests and monitoring investigation on key joints of deepwater semi-submersible platform are carried out. The major contents are summarized as follows:1. The study of fatigue analysis method for deepwater semi-submersible platform. Traditional sub-model technique, multiple interpolation method and improved multiple interpolation method are analyzed in detail. The multiple interpolation method and improved multiple interpolation method are proposed for the fatigue analysis on deepwater semi-submersible platform structures and can be satisfied with the requirement of accuracy and the condition of beam element through the cut-boundary. The application of sub-model technique is broadened.2. Fatigue life analysis on deepwater semi-submersible platform structures. The global finite element model and local finite element sub-models of key parts of deepwater semi-submersible platform are built. Fatigue strength of the global finite element model of deepwater Semi-submersible platform is analyzed under different sea states to determine the fatigue key parts. Stress response analyses on local finite element sub-models are carried out by using the improved multiple interpolation method. The hot spot stresses of fatigue key nodes are calculated by using the extrapolation of geometric stress. Based on the cumulative fatigue damage and fatigue crack growth, the fatigue lives of key nodes of deepwater semi-submersible platform are investigated.3. Fatigue reliability and sensitivity analysis on deepwater semi-submersible platform structures. Based on the S-N curve approach and the fracture mechanics approach, the fatigue reliability analyses on key parts of deepwater semi-submersible platform are investigated. In connection with wave scatter diagram of12sea areas in South China Sea, fatigue reliability indexes and failure probabilities of key nodes of deepwater semi-submersible platform in different sea areas are calculated. Sensitivity analyses of the parameters in S-N curve approach and the fracture mechanics approach are analyzed to study the influences of the parameters on fatigue reliability of deepwater semi-submersible platform.4. Fatigue test on key joint of deepwater semi-submersible platform structures. The fatigue lives of two types of key joints under bending loads are investigated, and the analyses of fatigue failure phenomenon, fatigue failure procedures and fatigue crack growth law are carried out. Through the scanning electron microscope, microstructures of different regions in fatigue fractures are analyzed to reveal the fatigue crack growth rates, directions and the fatigue failure mechanism.5. Residual fatigue strength test on key joint of deepwater semi-submersible platform structures. Based on the damage tolerance design principle, the fatigue loading cycles are determined in connection with the detailed periodic inspection of deepwater semi-submersible platform during the service life. The residual fatigue strength tests on two types of key joints with grinding weld toe and no grinding weld toe are investigated, and the influences of the weld grinding or not on the fatigue performances are presented by the comparative analysis.6. Fatigue monitoring test on key joint of deepwater semi-submersible platform structures. Based on the monitoring principle of fiber Bragg grating (FBG). fatigue monitoring test and residual fatigue strength monitoring test on key joints of deepwater semi-submersible platform structures are investigated, and the changing laws of strain and displacement are analyzed by comparing test results and monitoring results. The rationality of fatigue monitoring on key joints with FBG is discussed.