Effect of Local Scour on the Dynamic Response and Fatigue Damage of Offshore Wind Turbine Support Structures

Monopile foundations of offshore wind turbines are susceptible to long-term scour and erosion caused by wave currents, resulting in soil loss and thus the formation of local scour holes. The presence of scour holes significantly impacts the wind turbine support structure. This paper investigates the effect of local scour on the dynamic response of the monopile foundation for offshore wind turbines under both coupled wind and wave loads and coupled wind, wave, and seismic loads. OpenFAST (version 3.2.0) is used to generate random wind and wave loads, and the local scour is simplified as an inverted cone shape. A modified effective unit weight of soil is employed to account for the effect of stress history. ABAQUS (version 2020) is used to establish a three-dimensional finite-element model of the offshore wind turbine and analyze the dynamic response. The results show that the presence of the scour hole reduces the resonance frequency and increases the dynamic response of the wind turbine structure. Additionally, the effect of the scour depth is significantly larger than the scour width. Under the wind and wave loads, scour leads to an increased dynamic response and fatigue damage to the wind turbine structure. Increasing the scour depth causes a lag in the dynamic response of the wind turbine structure, while the increase of the scour width results in an advance in the dynamic response. Under the wind, wave, and seismic loads, there is no clear pattern in the effect of scour on the horizontal displacement of the tower top.