Effects of Second-Order Wave Load on Floating Offshore Wind Turbines Under Combined Wind and Wave Actions

Abstract In recent years, floating offshore wind turbines (FOWTs) have attracted considerable attention as the leading trend for the future development of offshore wind. This paper investigates the influence of second-order wave load on the motion responses of FOWTs. To accurately predict the motion responses of FOWTs under combined wind-wave actions, a time-domain coupled numerical model is developed, which is based on the unsteady blade element momentum theory, the second-order potential flow theory, and the lumped mass method. In order to explore the specific contributions of second-order wave load to the motion responses, comparisons are made in both the time and frequency domains among three cases. The first case considers only first-order wave load, and the second case adds second-order difference-frequency load to the first-order load, and the third case considers also second-order sum-frequency load. The results show that, during the initial non-equilibrium stage, the FOWT undergoes significant low-frequency resonance motion, deviating considerably from the initial equilibrium position, dominated by aerodynamic forces. In the stable equilibrium stage, minor periodic wave frequency motion is observed, with surge and pitch motions dominated by the first-order wave load and heave motion mainly governed by the second-order difference-frequency load.