Abstract:
Wind turbines components undergo large number of loading cycles through the duration of their operational life. Additionally, wind turbines may get exposed to extreme loading conditions from natural hazards, mechanical failures, or emergency stop. Current design practice accounts for the cyclic loading and extreme loading conditions through a pseudo-static approach from structural loading combinations. A rotational stiffness for the foundation stiffness is estimated from the pseudo-static analysis, and then compared to acceptable a rotational stiffness magnitude provided by the manufacturer based on the dynamic response of the tower and hub system. The rotational stiffness is dependent on the stiffness of the foundation elements and foundations soils. The foundation soils stiffness is subjected to degradation based on the number of loading cycles and strain magnitudes, which may change the dynamic response of the system and subsequent change of the foundation rotational stiffness. In this study, the full turbine-tower-foundation-soil system was modeled using finite element methods. Large number of typical operational loads cycles were applied to the model as well as extreme loading events. An advanced non-linear constitutive models and dynamic loading time histories were used to investigate the stiffness degradation of the soil, and the possible impact on the dynamic response of the system. Results were compared to conventional design methodologies to enhance the design procedure to account for foundation stiffness degradation, which will result in lower O & M cost, increase the design-life time for the turbines and hence lowering the LCOE.