Fatigue life sensitivity of monopile-supported offshore wind turbines to damping

Offshore wind energy is an important renewable electricity source in the UK and Europe. Monopiles are currently the most commonly used substructures to support offshore wind turbines. The fatigue life of offshore wind turbines is directly linked to the oscillatory bending stresses caused by wind and wave loading. The dynamic response of the structure is highly dependent on the combined aerodynamic, hydrodynamic, structural, and soil damping present. The fatigue life sensitivity of a reference 5 MW wind turbine under operational and non-operational conditions has been investigated using time-domain finite element simulations. The model uses beam elements for the monopile and tower and includes nonlinear p-y curves for soil-structure interaction. The effects of the wind turbine operation, environmental loads, and variable damping levels on the fatigue life were investigated systematically. The fatigue life increases significantly as a result of reductions in the bending stress caused by increased damping. From a practical point of view, significant cost-savings could be achieved in the design of a wind turbine by fitting supplemental damping devices. An efficient approximate method is proposed to assess the influence of damping, by scaling the vibration amplitudes around the first natural frequency of the system.