Three-degree-of-freedom vortex-induced motions of an OC4 floating offshore wind turbine platform under different current incidences

For column-type floating structures operating in current flow, significant oscillatory responses known as vortex-induced motions (VIM) can be excited, greatly impacting structure stability and safety of mooring lines. In this work, three-degree-of-freedom (DoF) VIM, including in-line, transverse and yaw motions, of a 1:72.72 scaled OC4 semi-submersible floating offshore wind turbine platform is investigated numerically across a wide flow velocity range, using a Computational Fluid Dynamics approach together with a Detached-Eddy Simulation model for turbulent flows. To analyze the impacts of current incidence on platform VIM responses, three representative current incidence angles of 0°, 60°, and 90° are examined. Results reveal that the “lock-in” phenomenon appears for the transverse motion within the reduced velocity (V_r) range of 8–12 at 0° and 90°, but the “lock-in” range narrows down to 8 ≤ V_r ≤ 10 at 60° incidence with smaller response amplitudes, suggesting that three-column platforms can be installed with two columns placed upstream to minimize VIM impacts. Meanwhile, comparing results between two models with and without the yaw motion demonstrates that the yaw DoF can induce significant in-line and transverse motions even at V_r ≥ 18 and thus must be taken into consideration in VIM simulations to accurately capture the platform motion characteristics at high flow velocity. Additionally, for the yaw motion V_r should be redefined with its natural period to yield a “lock-in” range similar to that of the transverse motion. Furthermore, connecting structures can apply substantial negative work upon the platform, demonstrating their important role in VIM suppression, and therefore should not be neglected in geometrical models. Additionally, the downstream column experiences significantly smaller mean drag due to the shielding effect exerted by the upstream column.