Numerical Investigation of Vortex-Induced Motions of a Scaled OC4 Floating Offshore Wind Turbine

Vortex-induced motion (VIM) is a critical issue for floating structures composed of one or more columns. This phenomenon can excite large oscillatory motions on floating structures, which can severely affect the stability of the structure and the fatigue life of mooring lines. In the present work, the VIM of the OC4 semi-submersible floating offshore wind turbine (FOWT) platform is studied numerically over a wide range of flow velocities by using an open-source Computational Fluid Dynamics (CFD) toolbox OpenFOAM. Comparisons are made between results for models with and without the connecting structures, including delta pontoon and cross braces, to assess their impacts on the platform VIM and the effect the yaw motion is investigated. Results show that the “lock-in” phenomenon in the transverse direction lies within the range of 8 ≤ Vr ≤ 12. By including the connecting structures in modelling the FOWT platform, its transverse motion amplitude is significantly reduced. Analysis reveals that the connecting structures exert considerable negative work on the platform motions and thus effectively mitigate the VIM phenomenon. Meanwhile, when the yaw motion is considered, large motion amplitude is excited in the transverse direction even at Vr ≥ 18, clearly indicating the coupling effects between the transverse and yaw motions. This work will contribute to the understanding of motion responses of FOWTs subjected to environmental loads from current.