Effect of ice drift parameters to estimated fatigue damage in an offshore wind foundation

In the Bothnian Bay, sea ice is present multiple months each winter and ice loads from drifting ice need to be considered in fatigue design of offshore wind turbines. For defining the ice loads, ice thickness together with ice drift speed and direction are required. Ice thicknesses have been reported in ice charts for many decades, whereas ice drift speed and direction are typically calculated from wind data. The aim of this study is to quantify the sensitivity of offshore wind turbine fatigue analysis to these input ice conditions. 42 years of ice chart and ERA5 wind data were used to estimate the relevant loading scenarios and their frequency for the wind turbine lifetime for a site in the Bay of Bothnia. Resulting stress histories and fatigue damage from 25 ice seasons were simulated for one location in the foundation. The simulated structure is the IEA 15 MW reference wind turbine with a monopile foundation and an ice breaking cone. The results showed that changes in input ice drift parameters can double the calculated fatigue damage for ice load events. In the simulations, as the ice load peaks occur relatively infrequently in comparison to wind load peaks, the timing of these load peaks in relation to each other becomes a significant factor defining the highest stress ranges from the loading history.