Natural Frequency and Damping Estimation of an Offshore Wind Turbine Structure

During the last years, offshore wind turbines have increased significantly in size with larger rotors and more powerful generators. The costs are kept as low as possible by reducing the overall weight, which leads to very slender and flexible structures. An improper design may cause resonance due to the excitation from wind and waves, leading to immature failure in the fatigue limit state. This in turn necessitates a correct estimate of the basic dynamic properties of the entire wind turbine structure. The paper presents experimental investigations of the first natural bending frequency and damping ratio of an offshore wind turbine located in the North Sea. Simple Fourier Transformation and least square fitting to the vibration decay of ten “rotor stop” tests make it possible to evaluate the dynamic properties of the wind turbine structure. Based on the traditionally p-y curve method (Winkler type approach), a numerical modal analysis of the wind turbine structure is conducted in order to compare the computational first natural bending frequency of the structure and the contribution of soil damping with experimental results.