Monopile offshore wind turbines (MOWTs) are increasingly being built in earthquake prone areas, facing the challenges of strong seismic loading that may cause foundation stiffness degradation, resonance fatigue and structural instability. This study investigates the dynamic response and fundamental frequency evolution of a MOWT under sequential seismic loading through centrifuge tests. A scaled model of a 4.2 MW MOWT is tested under progressively increasing peak seismic acceleration (0.05g–0.40g), with measurements of acceleration, bending moments, pore water pressure and displacement responses. The main results reveal that the peak acceleration response of MOWT increases with elevation, and the spectral peak shifts from high frequency to low frequency. Bending moments along the monopile exhibit a bow-shaped distribution, peaking at 15 m below the mudline. Excess pore pressure dissipates the fastest in the surface silt layer, the slowest in the middle clay layer, and exhibits long-term dissipation in the bottom silt layer due to the low permeability clay interlayer. The model experiences a permanent horizontal tilt of approximately 1° under sequential seismic loading, exceeding the design standards. Natural frequencies of the MOWT model initially increase but decrease after high-intensity seismic damage. It indicates that the dynamic response of the MOWT model and the shift in its fundamental frequencies are primarily influenced by the peak intensity of seismic waves in the sequential seismic conditions. Low-intensity seismic excitations densify the soil, increase foundation stiffness and amplify the structural acceleration response, resulting in a shift of the model's natural frequencies toward higher frequencies. Conversely, high-intensity excitations induce transient liquefaction in the foundation, reducing soil strength, which in turn decreases the dynamic response of the model and causes the natural frequencies to shift toward lower frequencies.

Monopile offshore wind turbine; Sequential seismic loading; Centrifuge tests; Dynamic response; Fundamental frequency evolution