A CFD Study of Vortex-Induced Motions of a Semi-Submersible Floating Offshore Wind Turbine

Vortex-induced motion (VIM) is a critical issue for floating structures made of one or more columns, due to its significant impacts on their operational stability. Supported by column-type floating platforms, floating offshore wind turbines (FOWTs) may also experience large-amplitude VIM responses in current flow. Existing research on FOWTs has mostly focused on their wind/wave induced responses, yet less attention has been paid to their responses in current flow. In this paper, the VIM of the OC4 semi-submersible FOWT platform is studied numerically over a wide range of flow velocity. Three incidence angles, i.e., 0 degrees, 90 degrees, and 180 degrees, are considered and the effect of current incidence on platform VIM is analysed. Results show that the so-called lock-in phenomenon is present and that a large transverse response amplitude of more than 0.3D persists until V-r = 30, with its maximum reaching over 0.8D at V-r = 8. Meanwhile, the transverse response amplitude for cases with the incidence angle of 180 degrees is generally smaller, with a narrower lock-in regime, than those under the other two incidence scenarios. Flow field visualisation reveals that upstream vortices continuously interact with the downstream side column when the incidence angle turns to 180 degrees, impacting the vortex shedding process and consequently fluid forces of the downstream column.