VORTEX-INDUCED MOTIONS OF FLOATING OFFSHORE WIND TURBINES WITH FOUR CIRCULAR COLUMNS: A REDUCED ORDER MODEL ASSESSMENT OF SMALL-SCALE EXPERIMENTS IN A TOWING TANK

The complex movements caused by ocean currents affecting floating offshore platforms, specifically Vortex-Induced Motion (VIM), are the focus of this study. It examines VIM patterns observed in towing tank experiments involving a scaled model of a Floating Offshore Wind Turbine (FOWT) with a 4-circular-column platform and pontoons. A simplified mathematical model (ROM) was applied to analyze VIM, considering horizontal plane motions such as in-line, transverse, and yaw. The ROM model represented current forces using "wake variables", adapting existing models initially developed for VIM of single-column platforms. It comprised eleven generalized coordinates, covering three for rigid body motion on the horizontal plane and a pair of wake variables for each column, resulting in an eleven-nonlinear second-order system of Ordinary Differential Equations (ODEs). The wake variable pairs followed van der Pol equations, incorporating hydrodynamic coefficients and parameters derived from prior experiments with low-draft cylinders. To simplify the numerical model, the hydrodynamic interactions among columns were not considered. Its accuracy was evaluated through a case study involving a small-scale experimental campaign. Simulations were performed at two different current incidence angles (0 and 45 degrees) across various reduced velocities. The simulations replicated the observed oscillations in the experimental trials, demonstrating fair agreement in transverse and yaw oscillations, including lock-in phenomena.