Dynamic response of monopile-supported offshore wind turbines under extreme wind–wave–current conditions and nonlinear soil mechanisms

Abstract As offshore wind power expands into deeper waters and towards larger-capacity turbines, the dynamic behaviour of large-diameter monopile foundations under extreme sea states has become increasingly important for long-term structural safety. This study develops a high-fidelity coupled dynamic analysis framework for an offshore wind turbine (OWT)-monopile–soil system by integrating computational fluid dynamics, the finite element method, a hardening soil constitutive subroutine, and an in-house wind-loading programme. A 6.45 MW OWT is used as a case study to compare structural and soil responses under annual mean conditions and 50 year return-period extreme sea states. The results show that extreme sea states intensify flow disturbances around the pile and increase the nonlinearity of wave–current loading, resulting in stronger higher-order harmonics and marked amplification of displacement, rotation, and internal-force responses. The critical control zone shifts downward, indicating reduced restraint from shallow soils and a stronger influence from deeper embedded sections. Frequency-domain analysis reveals that the response is dominated by low-frequency components, with spectral energy near 0.2 Hz approaching the first natural frequency and increasing the risk of near-resonant amplification. At the mudline, nonlinear coupling, stiffness degradation, and hysteretic energy dissipation in the monopile response become more pronounced. The soil stress, displacement, and depthwise stress distribution further show that, under extreme loading, the pile–soil interaction mechanism evolves from shallow-layer dominance to joint control by both shallow and deeper layers.