Dynamic response of offshore wind turbine monopile foundations in layered soils under wind, wave and earthquake actions

Earthquakes occur frequently in China’s southeast coastal areas. Therefore, in this region, offshore wind turbine (OWT) structures are highly likely to be simultaneously affected by wind, waves and earthquakes. Firstly, based on the wave theory and Snell’s law, this paper deduces the formulas for the equivalent seismic nodal force of layered soils suitable for viscoelastic artificial boundaries. Compared with a method that employs an average modulus to calculate the seismic motion input of layered soils, the proposed formulas enable more accurate simulation of seismic wave propagation in layered soils. Based on the formulas, an integrated numerical model incorporating a nacelle, tower, monopile and layered seabed is established to analyse the dynamic response of large-diameter monopiles of OWTs under stochastic wind, wave and seismic loads. The analysis results show that the displacement and stress responses under combined wind-wave loads are larger than those under the individual wind/wave load, showing obvious amplification effects, but the maximum acceleration at the mudline indicates an inhibitory effect, which is not consistent with a superposition principle. Under the combined wind-wave-seismic loads, the monopile acceleration is primarily attributable to the seismic load, with wind and wave loads mitigating the acceleration response caused by the seismic load. Moreover, the seismic load has little effect on the monopile’s stress but increases its displacement, especially at the mudline. Thus, additional treatment for foundation deformation is needed for the monopile design in seismic areas.