Cross-wind modal properties of offshore wind turbines identified by full scale testing

According to the Danish wind turbine industry cross-wind vibrations due to wave loading misaligned with wind turbulence often have a significant influence on the fatigue lifespan of offshore wind turbine foundations. The phenomenon is characterised by increasing fatigue loads compared to the fore-aft fatigue and a small amount of system damping since almost no aerodynamic damping from the blades takes place. In addition, modern offshore wind turbines are flexible structures with resonance frequencies close to environmental loads and turbine blades passing the tower. Therefore, in order to avoid conservatism leading to additional costs during the load calculation and the design phase, the structural response must be analysed using reliable estimations of the dynamic properties of the wind turbines. Based on a thorough investigation of “rotor-stop” tests performed on offshore wind turbines supported by a monopile foundation for different wind parks in the period 2006–2011, the paper evaluates the first natural frequency and modal damping of the structures. In addition, fitting of theoretical energy spectra to measured response spectra of operating turbines is presented as an alternative method of determining the system damping. Analyses show distinctly time-dependent cross-wind dynamic properties. Based on numerical analysis, the variation is believed to be caused by sediment transportation at seabed level and varying performance of tower oscillation dampers.