Validated Loads Prediction Models for Offshore Wind Turbines for Enhanced Component Reliability

To improve the reliability of offshore wind turbines, accurate prediction of their response is required. Therefore, validation of models with site measurements is imperative. In the present thesis a 3.6MW pitch regulated-variable speed offshore wind turbine on a monopole foundation is built in the aero-servo-hydro-elastic code HAWC2. The results are validated with full scale measurements from a 3.6MW Siemens offshore wind turbine installed in the Walney Offshore Wind Farm 1 at the west coast of England. Damping estimation, wind-wave misalignment cases and storm loads are analyzed. The findings are used for the modification of the sub-structure/foundation design for possible material savings.
First, the background of offshore wind engineering, including wind-wave conditions, support structure, blade loading and wind turbine dynamics are presented. Second, a detailed description of the site is given and the metocean conditions are analyzed. The joint wind-wave distribution and the probability of the misalignment angles are estimated. Third, the calibration process of the different components is thoroughly depicted. The turbulence intensity implemented in the simulations is extracted from a nacelle mounted cup-anemometer.
The model setup is based on the downscaled 5MW NREL reference wind turbine. Modifications on the downscaled model to match the actual full-scale wind turbine (mass and natural frequency) are applied. Extreme and mean measured loads from the free wind and the wake sectors, as well as 1Hz equivalent loads are used for the validation of the model. The uncertainties both in the model and full-scale wind turbine are quantified.
The main contribution of the current thesis is presented in the final three chapters. The support structure net damping is estimated from the impulse response of a boat impact. The first and second modal damping of the system during normal operation both from measurements and simulations are identified with the implementation of the Enhanced Frequency Domain Decomposition technique. The effect of damping on the side-side fatigue of the support structure due to wind-wave misalignment cases is examined. The higher measured net damping is then used in the design process of the sub-structure/foundation for material savings. A detailed ultimate strength, stability strength and fatigue analysis are performed in the baseline and the modified designs to ensure structural integrity of the system.