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

• Based on “rotor-stop” tests on offshore wind turbines, the structural cross-wind modal properties show distinctly time-dependent behaviour. It is believed that scour development at seabed and varying performance of tower oscillation dampers cause this tendency.
• Using a Winkler approach with an implemented hysteresis loop method predicts reasonable estimates of modal soil material damping when comparing with measured data.
• Spectral analysis on side–side accelerations of operating offshore turbines may serve as an alternative way of determining the dynamic properties. In general, from the investigated offshore turbines it can be concluded that the mean damping value of the first mode is in the range of 0.15–0.18 logarithmic decrement.

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.