Effects of Soil Profile Variation and Scour on Structural Response of an Offshore Monopile Wind Turbine

This paper presents an engineering approach to study the effects of soil profile variation and scour on structural response of an offshore monopile wind turbine. A wind-wave model for finite water depth is proposed to obtain the corresponding sea-state based on the incident wind. Different wind, wave, and current loads on the wind turbine for the operational conditions are considered. The interaction between the foundation and the soil is simulated by nonlinear springs, for which stiffness properties are obtained from the axial load transfer curve, the tip load-displacement curve, and the lateral load-deflection curve. Four types of soil conditions are considered, i.e., 100% sand layer, 50% sand layer (top) and 50% clay layer (bottom), 50% clay layer (top), and 50% sand layer (bottom), as well as 100% clay layer. For a given current speed, the variations of the structural response of the wind turbine due to the effects of different wind-wave load combinations, soil conditions and scour have been investigated. Different wind-wave load combinations directly affect the mean internal bending moment and mean displacement vertically along the support structure. Different soil conditions change the eigenfrequency of the structure significantly. The top layer of the soil appears to have a strong influence on the mean internal bending moment and the mean shear force distribution along the foundation. Moreover, the effect of scour alters the eigenfrequency of the structure significantly. The maximum mean bending moment and displacement increase for the cases with a scour hole as compared to the cases with scour protection.