Damping Estimation of a Prototype Bucket Foundation for Offshore Wind Turbines Identified by Full Scale Testing

Wave loading misaligned with wind turbulence often introduces large fatigue loads on offshore wind turbine structures. In this particular case, the structure is sensitive to resonant excitation acting out of the wind direction due to a small aerodynamic damping contribution in the cross-wind direction. Therefore, in order to assess the fatigue damage accumulation during the lifetime of the offshore wind turbine structure, a correct estimation of the cross-wind modal damping is necessary. This paper describes the cross-wind modal damping of the lowest eigenmode of a fully operational Vestas V90-3.0 MW offshore wind turbine installed on a prototype bucket foundation. The foundation and the turbine tower are equipped with a monitoring system with 15 Kinemetrics force balance accelerometers and a Digitexx acquisition system. Using free vibration decays from “rotor-stop” tests and operational modal techniques, the cross-wind modal damping is estimated on a regular basis. Analyses show maximum cross-wind damping at rated wind speed. For higher wind speeds decreasing damping is observed, mainly due to blade pitch activation. In addition, a high structural acceleration level is needed to activate the soil damping.