Numerical modelling and PTO damping optimization of an IEA-15-MW-VolturnUS-WEC hybrid system in real sea states

Based on the coupling framework between OpenFAST and WEC-Sim (OWS), this study proposes a numerical model for a floating offshore wind turbine (FOWT) and wave energy converter (WEC) hybrid energy system and develops a multi-objective, multi-parameter configuration optimization solver to find the optimal power take-off (PTO) damping. The hybrid system consists of an IEA-15-MW reference wind turbine (RWT), a UMaine-VolturnUS-S semisubmersible platform, and three toroidal heaving WECs installed on the side columns of the platform. By introducing an artificial viscous damping coefficient tuned from the computational fluid dynamics (CFD) results, a corrected potential flow (PF) model is employed to avoid the overestimation of hydrodynamic coefficients caused by the gap resonance between the WECs and the side columns. The permanent magnet linear generators (PMLGs) for the direct-drive WECs are modelled as linear-damping PTO. Aiming at maximum wave energy extraction, the PTO damping is optimized in real sea states using the optimizer that integrates a global population-based metaheuristic scatter search algorithm and several local large-scale nonlinear programming methods. Compared with the single FOWT, the WECs provide additional power gain while positively contributing to the platform response in pitch. Moreover, the study reveals that the time difference in the relative heave motion between the platform and the WECs, determined by the environmental conditions, is a key factor that affects the overall power production of the WEC array.