Hydrodynamic characteristics of a 15 MW semi-submersible floating offshore wind turbine in freak waves

Dynamics of the floating offshore wind turbines (FOWTs) in harsh ocean environments gains increasing attention due to the high maintenance costs in case of structural damage. The freak wave occurring more frequently than believed poses a threat to FOWTs, but mechanism and influencing factors of the wave-structure interaction problem have not been fully explained. In this paper, dynamics of the VolturnUS-S semi-submersible platform designed for the IEA 15 MW wind turbine under freak waves is studied using a coupled computational fluid dynamics and finite element mooring line model. First, the hydrodynamic model is validated using the DeepCwind semi-submersible platform. Then, the validated model is employed for the analysis of the VolturnUS-S semi-submersible platform under freak waves described as a focused group of waves. Hydrodynamic behaviors of the semi-submersible FOWT are described in detail, and parametric analysis of the problem including the effects of focusing crest amplitude, focusing location, incident angle, gravity center location is conducted. Important results include the dramatic surge motion of the platform and fairlead tension of the upstream mooring line, and alleviation of these responses by varying the wave angle to 30° and moving the gravity center to the centroid of the platform geometry.