This paper focuses on spectral-based fatigue analysis for deepwater semi-submersible rig in South China Sea. The radiation/diffraction theory is used to calculate wave load acting on semi-submersible rig. The 3D FEA model of the rig is made using shell element, beam element, and mass element. According to long-term wave distribution of South China Sea, stress response of the global structure is calculated. The fine FEA model of typical joint is made, the result of global FEA is taken as the load boundary condition, the hotspot stress transfer functions are calculated accounting for hydrodynamic load and gravity load. Based on the Miner's rule, the fatigue life of the typical joint are calculated using a spectral-based fatigue analysis method.
Semi-submersible drilling rig has small water plane area compared to ship structures with same dimension. So it has long natural resonance period in heave motion and has a merit that can avoid from resonance with wave period range. As projected area subjected by wave is small and pontoon with relatively large volume is under water, semisubmersible drilling rig has small wave load and the heave motion compared to ship's heave. Because semi-submersible drilling rig can be used under more severe sea states, regardless of shallow water or deepwater, and it has favorable motion characteristic. So a large number of semi-submersible drilling rigs are used in offshore oil and gas exploitation. Because the structure of rig suffer from cyclic wave load during operation, the fatigue failure caused by cyclic stress will be dominating mode of structural damage. Therefore, fatigue strength is one of the most important factors should be consider in design(Yuren Hu,1996; Haixia Liu, 2003; Gang Liu, 2002). HYSY981 platform is the 6th generation semi-submersible drilling rig designing and constructing in China. It is designed with an advanced dynamic positioning system.