Comparison of Ice-induced Vibrations on a Conical and a Cylindrical Offshore wind Turbine Substructure

In the Gulf of Bothnia, the shallow coastline and the consistent wind conditions provide a good environment for the wind energy production. Relatively thick ice fields and low drifting ice field velocities, up to 0.6 m and 0.15 m/s respectively, are the dominating ice characteristics in the same area. The establishing of an offshore wind turbine in the cold regions requires a careful study on the structural response of ice-induced vibrations. Ice-substructure interaction was studied for a conical and a cylindrical monopile. The dominating ice failure mechanism for the conical substructure is bending. On the other hand, various ice interaction processes occur for the cylindrical substructure, e.g. the lock-in vibration, intermittent crushing and continuous brittle ice crushing. A dynamic ice load for the conical substructure was created with an external algorithm and implemented as a time-dependent loading in the finite element procedure. This ice load model is founded on earlier studies of Kärnä et al. (2004). The cylindrical substructure was analyzed by PSSII a special purpose soil-ice-interaction program developed for a vertical offshore substructure. Ice thickness and velocity were parameterized for both substructures. In addition, various configurations were studied by varying the cone angle and the foundation depth. According to the results, the conical substructure introduces a significant reduction in the vibration amplitudes compared to the cylindrical case. However, when the ice field thickness increases, the vibration amplitudes were observed to increase for the both substructures. The different ice interaction processes, depending on the foundation depth and the ice conditions, e.g. the ice thickness and velocity, are addressed.