Effect of flow incidence angle on vortex-induced motion of a semi-submersible floating offshore wind turbine platform

A computational fluid dynamics study is conducted to investigate the effect of the flow incidence angle (θ) on the three-degree-of-freedom vortex-induced motion (VIM) of a DeepCWind semi-submersible floating offshore wind turbine platform. A scaled model with a scaling factor of 1:72.72 and three θ values, i.e., θ = 0°, 90°, and 180° are considered. The reduced velocity (V_R) is varied from 3 to 15. The response characteristics, hydrodynamic coefficients, motion trajectories, and wake patterns are analysed in detail. Substantial cross-flow VIM is observed in the V_R range of 6–10 while distinct vortex-induced yaw motion appears at higher V_R. Under the diverse effects of the vortex shedding modes at different θ values, the motion trajectories of the platform exhibit figure-eight and raindrop shapes. When θ = 90° and 180°, the wake of the upstream side columns influences the vortex shedding of the downstream side columns, disrupting the vortex field and subsequently suppressing the VIM. In addition, the smaller structural members, namely the pontoons and the cross braces also impose a damping effect on the VIM. The pontoons interfere with the vortices from the upstream side columns at the lower part of the platform and two symmetric vortex streets are formed behind the cross braces inducing a restoring moment against the yaw motion.