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Designing support structures for offshore wind turbines is a complex task as these are highly dynamic systems subjected to long-term cyclic loads with variable amplitude. Long-term cyclic loading may cause stiffening or softening of the soil around the pile foundation of an offshore wind turbine, which leads to variations in the foundation stiffness and accumulated permanent rotation of the pile. Although variations in the foundation stiffness can negatively impact the fatigue life, the long-term variability of the soil conditions is normally not considered in the fatigue damage assessment. The main objective of this study is the investigation of the impact of changes in soil parameters on the fatigue lifetime for an offshore wind turbine founded in loose sand. For this purpose, a generic monopile based offshore wind turbine with flexible foundation model was used. The soil-pile interaction was modeled with a distributed spring model using nonlinear API p-y curves. Integrated analyses in the time domain were performed and fatigue damage was assessed in terms of a damage equivalent bending moment at mudline. The fatigue lifetime varies between -9 percent and +4 percent when considering changes in soil conditions, depending on the assumption of soil softening or stiffening, respectively. These results indicate that changes in soil parameters should be taken into account in the fatigue damage calculations of offshore wind turbines for more precise fatigue lifetime estimation. Moreover, it is emphasized that more accurate modeling of soil-pile interaction is required in the design and optimization of offshore wind turbines

Influence of Soil Parameters on the Fatigue Lifetime of Offshore Wind Turbines with Monopile Support Structure