DeepCwind semi-submersible floating offshore wind turbine platform with a nonlinear multi-segment catenary mooring line and intermediate buoy

In this paper, to improve the mechanical behavior of DeepCwind semi-submersible floating offshore wind turbine (FOWT) platform mooring lines, the nonlinear catenary cables of the platform were divided into multi-segment and intermediate buoys. Mathematical formulations of the boundary element method (BEM) governing the dynamics of mooring line systems with buoy devices were described. This study was applied to the OC4-DeepCwind semi-submersible FOWT platform, which is designed for a 200-meter water depth with mooring lines consisting of three catenary steel chain cables at 120° angles to each other. The dynamic response of the multi-segment catenary mooring lines with different buoy radiuses and different positions along the cables was investigated. The full-scale platform was modeled in ANSYS-AQWA software, and the simulations were performed under harsh offshore conditions. The mooring line’s general arrangement, tension, strain and uplift force for different buoy radiuses and their position along the cable are presented and discussed. Moreover, platform motions in three directions (surge, heave, and pitch) were also analyzed. It was concluded that by correctly selecting the buoy volume and position along the cable, the tension of the cable may be reduced by up to 45%. By incorrectly selecting the buoy, the results caused adverse effects.