Ice-induced vibrations of wind turbines with a jacket support structure

Abstract

The offshore wind industry is a relatively new but fast growing field in the global shift towards renergy to reduce carbon emissions. However, more difficulties arise in the design of an offshore wind turbine (OWT) compared to an onshore wind turbine, as the offshore structures are exposed to more environmental influences. One of those is sea ice. As wind farms are being built in areas where floating sea ice exists, such as the Baltic Sea and Bohai Bay, the effect of interaction of ice with an offshore wind turbine needs to be taken into account in the design phase. Several accidents have taken place in the past, where offshore structures such as bridges, lighthouses and oil platforms were exposed to severe ice loading, leading to human discomfort and damage or even failure of the structure. Recent research has shown that ice-induced vibrations (IIV) can develop in wind turbines with a monopile foundation. This research studies IIV in a wind turbine with a jacket foundation, which is used in deeper waters. These IIV can be categorized into three regimes: intermittent crushing (IC), frequency lock-in (FLI) and continuous brittle crushing (CBR), depending on both structural and ice properties. During IC, which occurs for low ice velocities, large forces act on the structure. For higher velocities, the failure of ice ‘locks in’ with one of the structural eigenmodes, leading to the FLI mode, where cyclic loading of the structure sustains. This periodic movement affects the fatigue of the structure. When ice is moving at even higher velocities, the CBR regime begins, in which the ice load does not show a periodicity but fluctuates around a relatively low mean value, resulting in small, random oscillations of the structure around the mean deflection. In this research, the effect of these IIV on offshore wind turbines with a jacket foundation was investigated. To that end, a structural model for a 5MW wind turbine has been implemented and a preliminary study indicated that the second eigenmode is most susceptible to FLI. Next, the structural model has been coupled to a phenomenological ice model which simulates the three regimes of IIV. Case studies have shown that all IIV regimes do occur in the OWT when ice is acting on the structure; large displacements of the structure occur in IC and sustained oscillations take place in the second bending mode during FLI. Moreover, it was shown that the angle of incidence of ice has no effect on the IIV due to symmetry in the model. On the other hand, the chosen damping values have a large influence on the occurrence of IIV. Finally, it was found that a wind force solely can increase the displacements during IIV, but the aerodynamic damping involved reduces them.