Coupled-dynamic Response Investigation Of Mooring Systems For Deepwater Semi-submersible Platform

The demand for oil and gas has been increasing, which impulse the world to exploit it under the sea exceeding 1,500 meters. The deepwater semi-submersible platform is widely used in the field of exploitation of offshore oil and gas, due to the advantages of low investment, the capacious deck area and variable deck load, strong ability of anti-stormy waves, wider ranger of operational water depth, the strong ability of drilling and multi-functioned. The deepwater semi-submersible platform is sited in the serious environment by mooring systems, therefore it is important to calculate the response of motion of platform and tension of mooring line, to ensure the platform safe under operating condition and survival condition.In this paper, the sixth generation deepwater semi-submersible drilling platform is studied. The hydrodynamic performance of platform in regular wave is analyzed by AQWA, which is based on the 3D potential theory. The wave load and motion response are foundation to the response analysis of platform in random irregular water wave.According to the criterion of API RP 2SM, the mooring system is designed. Each cable is composed of three parts:chain, polyester rope and chain. Taut-wire mooring type and 8 cables are selected. Each cable is composed of three parts:chain, polyester rope and chain. By simulating the mooring line into hydrodynamic model which consider the inertia and drag force, and coupling the motion of platform, the response of motion of platform and tension of mooring line in survival condition are calculated.On the basis of preliminary design, the parametric studies of coupled motion responses are conducted, such as hydrodynamic coefficients of mooring line, tension dip angle, pretension, arrangement angle of mooring line, total number of mooring line. The research results show that the different parameters have different effects on the coupled motion responses. The motion performances of semi-submersible platform and mooring line dynamic loads can be controlled effectively when these parameters are selected reasonably. Finally, the preliminary design is optimized by increasing tension dip angle and decreasing pretension. The results show that optimize proposal improved performance of mooring system, and meet the requirements.