The number of offshore wind farms is growing rapidly, which increases the likelihood of ship collisions with offshore wind turbines. Additionally, the cost of energy reduction for wind turbines is achieved due to radical design optimizations. Therefore, simple methods should be obtained to determine the offshore wind turbine response during ship impacts in an early stage of wind farm development.

In this research, the response of monopile-supported wind turbines to the impact of a supply vessel is investigated with respect to overturning moments at critical locations; at seabed and tower bottom. The response of the offshore wind turbine is analyzed by modeling a 10 MW wind turbine using the FEM software USFOS. The impact of a 7500-ton supply vessel is represented by a single degree of freedom non-linear spring, of which the force-deformation curve is obtained by using numerical simulations in LS-DYNA. Multiple impact scenarios and two different wall thicknesses at the monopile impact location are considered. Observations from the numerical model are used to develop a method based on analytical equations which can approximate the same maximum overturning moments at seabed and tower bottom during a ship collision. The numerical model is used to verify the analytical model.

Although large bending moments are observed at the tower bottom in the second bend- ing mode directly after impact, the maximum bending moments for both at tower bottom and seabed were found predominantly in the first bending mode. Soil deformation has a significant influence on the permanent displacement of the wind turbine structure. The impact direction relative to the wind direction and the impact speed have significant influ- ence on the maximum overturning moments. The impact force curve is strongly influenced by the second eigenmode with respect to the shape and period. For the analytical model, the force-time curve of the impacting ship is taken as input and has a large influence on the overturning moments. The resulting moments were found to be within a range of 10-40 % of the numerical results, depending on the impact force curve, location, and impact speed.