Lateral dynamic response of monopile offshore wind turbines in radially softened layered soil

The dynamic response of monopile offshore wind turbine is highly dependent on the pile-soil interaction model and the soil property. However, there is a lack of efficient and reliable method considering both the soil softening effect and stratification property in the existing dynamic response analysis of monopile offshore wind turbine. This work provided an analytical solution of lateral dynamic stiffness of radially softened soil around pile based on the horizontal vibration model of radial multizone plane strain soil firstly. Then the lateral vibration model of pile in radially inhomogeneous soil was established with obtaining the dynamic complex stiffness matrix of pile top using the transfer matrix approach. By introducing this pile-soil system in radially inhomogeneous soil based on apparent fixity model into the wind turbine structure analysis tool FAST V8, the effect of radial softening of the layered soil around the pile on the lateral dynamic response of monopile under wind and wave loads was studied. The results show that the soil softening has a significant impact on the dynamic stiffnesses of pile top. With the decrease of the soil modulus attenuation coefficient, the lateral stiffness and second order natural frequency of structure decreases. Under the wind and wave spectrum action, the response spectrum at the tower top is mainly controlled by the fundamental frequency of tower, and the excitation is not sensitive to the wave peak frequency. The response spectrum of the pile at the mudline are mainly controlled by peak frequency of wave spectrum and the fundamental frequency of tower base, and is not sensitive to the wind frequency. The softening of the soil around the pile has a more significant impact on the pile foundation response at the mudline. A comparison of responses under conditions of wind action only and combined wind and wave actions indicates that the introduction of wave action primarily affects the maximum displacement. This study provides valuable insights into the behavior of monopile offshore wind turbines subjected to dynamic loading.