Higher-harmonic resonance response of a monopile-supported offshore wind turbine in waves and wind

This study develops a coupled aero-hydro-soil-monopile model to analyse the dynamic response of a monopile-supported offshore wind turbine (OWT) under combined wind and wave loading. The coupled model integrates a nonlinear hydrodynamic model, an aerodynamic model based on blade element momentum theory (BEMT), and an empirical soil-monopile interaction model. The proposed aerodynamic and hydrodynamic models are verified by the aerodynamics of the bottom-fixed NREL 5-MW wind turbine and the higher-harmonic wave forces on a fixed monopile, showing good agreement with reference data and experimental measurements. The dynamic response of a monopile-supported OWT is then investigated under the combined action of regular/irregular waves and steady wind. Regular wave results show that the third-harmonic response is prominent when the wave frequency is close to the one third of the natural frequency. Irregular wave results indicate that higher-harmonic components play a significant role in the dynamic responses of the OWT through the time-frequency characteristic analysis via wavelet transforms. Exceedance probability plots show the present results exceed the corresponding linear estimates by factors of 3.1 and 4.0 for the largest hub velocity and acceleration. This coupled model provides a reliable tool for OWT analysis, emphasizing non-negligible higher-harmonic wave loading effects on structural responses. This is critical for safety design under large wave steepness and higher-harmonic near-resonance conditions.