Ringing response of large-diameter monopile foundations for offshore wind turbines: Recent advances and future trends

The upscaling of offshore wind turbines to the 15-MW class, with prototypes now exceeding 20 MW, has driven the fundamental natural frequency of large-diameter monopile foundations to lower values. At these lowered frequencies, higher-order wave harmonics and breaking-wave impacts can excite a transient resonant response known as ringing, with implications for ultimate and fatigue limit states. To address this multi-physics phenomenon, this review establishes a six-mechanism framework spanning three external wave-driven excitations, namely higher-harmonic wave forces, breaking-wave impact, and secondary load cycle, together with three internal state-dependent coupling mechanisms, namely hydroelastic fluid–structure interaction, aerodynamic damping, and soil–structure interaction. Flow-resolving hydrodynamic models have advanced at the research level, but their computational cost limits their use in routine design analysis. Integrated tools therefore retain simplified hydrodynamic formulations, and the six mechanisms have not been incorporated within a single analysis. The most demanding gaps concern multiphase modeling of aerated breaking-wave impacts, the computational cost of flow-resolving hydroelastic fluid–structure interaction, and accurate identification of aerodynamic damping. To bridge these gaps, four research directions are identified: hybrid multi-scale hydrodynamic modeling, machine learning surrogates for ringing load prediction, real-time hybrid testing for multi-physics validation, and virtual sensing with load inversion for ringing monitoring.