Global responses analysis of a semi-submersible platform with different mooring models in South China Sea

For semi-submersible platform applications in deepwater oilfield development in the South China Sea, selection analysis for mooring positioning system is one of the key issues in the design of offshore structures. Based on a semi-submersible platform model in three different water depths of 500m, 1000m and 1500m, three types of mooring systems - catenary, semi-taut and taut - are considered, respectively. The three mooring systems have similar static restoring force characteristics and the same number of mooring lines and angle arrangement. The dynamic coupling effects between the semi-submersible platform and its mooring lines are investigated via numerical simulation. A three-dimension hydrodynamic finite element model of the semi-submersible platform is firstly built. The wave forces based on the diffraction theory are calculated using a boundary element method. The wind forces are obtained from wind tunnel tests using a 1:100 scale model of the platform, and the current forces are regarded as steady. The platform motion under the combined action of wind, wave and current is determined using the Runge-Kutta method under one-year and 100-year return period environmental conditions in the South China Sea. The specific numerical results, analyses and conclusions would be helpful in the selection of the mooring system and in the motion performance study of the semi-submersible platform preliminary design.