Accuracy of frequency domain fatigue damage estimation methods for offshore wind turbine support structures

Offshore wind turbines are relatively new structures that are expected to become important contributors to sustainable energy production in the future. In order to make offshore wind energy a reliable source of clean energy as well as a profitable investment, it is essential that the wind turbines operate with very high availability, usually for more than 20 years. The long design lifetime and the intensive environmental loading from wind and waves makes fatigue a key design driver. The support structures must be designed carefully and close to the natural frequency. The increased fatigue damage, due to dynamic amplification, has to be considered. A posteriori changes are extremely expensive in an offshore environment in such tall structures. Therefore, support structures need to be robust to changing parameters, such as the soil parameters and the stiffness of the foundation. In this paper, a methodology for rapid frequency domain fatigue damage estimation is presented. The accuracy of frequency domain fatigue damage estimation methods is evaluated by comparison to rainflow counting using site measurement data from an offshore wind turbine.