Offshore wind energy is one of the most promising technologies in the field of renewable energy, especially in cold climate regions. This paper introduces a new way to get a better understanding of ice induced vibrations at offshore wind turbines. Therefore a well established empirical ice model was implemented in the aero-hydro-servo-elastic simulation tool OnWind utilizing the modeling language Modelica. To retain a realistic dynamic behavior, an overall wind turbine model including relevant environmental conditions like soil and wind is used. For the investigations representative ice conditions from the Gulf of Bothnia were chosen.
It has been proved by various measurements that moving ice can induce severe vibrations during an interaction with an offshore structure. According to earlier studies (Kärnä and Turunen (1990)) it is known that the failure mechanisms of ice as well as the ice loads depend on the flexibility of the structure at the ice contact zone. For slender structures like wind turbine support structures, the displacements at the water level and therefore the structural stiffness in ice contact area depends on the dynamic behaviour of the structure. When ice moves against a vertical structure, different failure modes can take place, for instance bending, shearing, buckling or crushing (Kärnä and Jochmann (2003)). From an engineering point of view, crushing failure usually causes higher forces than bending failure and induces severe vibrations which may lead to increased fatigue damage. Therefore, ice-induced vibrations need to be taken into account in the vibration analysis of offshore structure. The dynamic ice forces and related vibrations of compliant structures under crushing failure have been investigated among others by Määttänen (1998), Kärnä and Turunen (1990), Kärnä et al. (2010) and Yue et al. (2009). Määttänen introduced an ice-induced vibration model based on the studies by Blenkarn (1970) and Peyton (1966).