Dynamic Response Evolutions of Monopile Offshore Wind Turbines Under Wind–Wave Coupling

Offshore wind turbines (OWTs) are subjected to long-term coupled wind–wave loads, and frequently endure extreme loads under wind speeds exceeding the cut-out speed during service. This paper uses the OpenFAST v4.0.0 to conduct a detailed numerical analysis of an offshore monopile wind turbine, investigating its aerodynamic loads, tower deformation, displacement, acceleration, and foundation reactions under cut-in, rated and cut-out conditions, and further explores the influence of reference wind speed. Distinct response discrepancies are identified between directions and operating conditions. Fore–aft (F-A) responses are dominated by axial thrust and the first-order bending mode, reaching their peak under the rated condition. Side–side (S-S) responses are controlled by lateral turbulence; under cut-out conditions, the sharply reduced aerodynamic damping triggers significant higher-order mode participation, resulting in the maximum S-S responses. With increasing reference wind speed, F-A responses rise monotonically, while S-S displacement tends to plateau above a critical wind speed. The aerodynamic loads differ sharply across cut-in, rated and cut-out conditions; F-A thrust fluctuates between 0.25 × 103 and 0.75 × 103 kN at the rated condition and nears zero at the cut-out condition. The nacelle’s F-A acceleration peaks at 0.503 m/s2 under the rated condition, while S-S acceleration peaks at 1.32 m/s2 under the cut-out condition. The OWT’s tower F-A displacement peaks at 0.689 m under the rated condition, while S-S displacement peaks at 0.429 m under the cut-out condition.