FAST simulations of drifting sea ice loads on offshore wind turbine support structures

The Baltic Sea features a potential for large capacity wind farms because of relatively high and constant wind velocities. Mostly shallow coastal areas enable cost-efficient foundation and grid connection. However, in the northern sea area - Gulf of Bothnia - the sea freezes annually. Sea ice loads and ice-induced vibrations due to drifting ice field introduce the most significant uncertainties in the support structure design for offshore wind turbines. The magnitude and time variation of ice load depends on various factors, like the thickness and velocity of the ice as well as the size and shape of the structure. The ice load magnitude and time variation depends on the failure mechanism of ice, which is strongly governed by the shape of the structure at the water level. A feasibility study of the FAST (Fatigue, Aerodynamics, Structures and Turbulence) simulation software was carried out investigating the structural performance of offshore wind turbines. Various load combinations and operation modes were studied by taking into account coupling between the ice, wind and structural response. The results were compared to Finite Element Method (FEM) simulations implemented with an in-house ice load model. FAST with available IceFloe and IceDyn modules form already a good basis to consider various ice load scenario. In addition, open programming interface in FAST creates a suitable development platform making possible to implement advanced ice load models. Coupled modelling of ice-structure interaction is a necessary step in terms of cost-efficient structural design.