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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 …