Fatigue Sensitivity of Monopile-supported Offshore Wind Turbines

Offshore wind energy is increasing in the UK to meet renewable energy targets. A literature review on the most commonly used offshore wind turbine support structures (i.e. monopiles) has been compiled. The dynamic behaviour of offshore wind turbines is highly dependent on the stiffness and damping. Scour and damping have been researched recently, but their long-term influence on the fatigue life are not completely understood. This research aims to better understand their effect using finite element (FE) simulations. The influence of soil-structure interaction was studied using beam element and 3D solid element models and a good agreement was found. The natural frequency and dynamic amplification of the wind turbine were found to be sensitive to soil stiffness. The influence of the transition piece on the offshore wind turbine was examined using 3D models, and its effect on the static and dynamic responses was found to be minor. A full fatigue analysis was performed using a combination of different simulation packages (TurbSim, FAST, ABAQUS and MATLAB). Damping and operational regime influences were studied using time-domain implicit dynamic analysis of beam element models. The fatigue life of the non-operational turbine was found to be significantly lower than the operational turbine. Almost linear increase in the fatigue life of operational turbines was observed with increasing damping. In addition, a simplified method to predict the fatigue life for variations in the damping was proposed. Scour and backfilling influences were studied using the beam element model. The magnitude and location of the maximum bending moment changed depending on the scour level. The modal properties experienced minor changes, while fatigue life showed a considerable reduction. The backfilling process led to the recovery of the first natural frequency. However, fatigue life recovery was dependent on the backfilling timing. Finally, it was demonstrated that fatigue life variations due to scour could be predicted from the static response of offshore wind turbines