Ice Loads for Offshore Wind Turbines in Southern Baltic Sea

Wind turbine structures shall resist ice loads in the southern areas of the Baltic Sea. This paper describes a method of determining site-specific load values for structures to be built in this area. This method is based on the forthcoming standard ISO 19906, which emphasizes the use of field data and site-specific considerations. Historical data on the occurrence of ice in the sea area concerned are reviewed. This data shows that competent ice sheets can be met once in eight years in the southern Baltic Sea, as an average. A recommended design ice force formula is given for Southern Baltic conditions with only competent ice sheet once per 8 years and with a freezing index less than 500. The highest ice thickness in open waters has been recorded as 0.48 m in winter 1941-42, when the associated freezing index was 495. In subsequent winters the ice thickness of fast ice has not exceeded 0.3 m in conditions where the freezing index has been lower than 300. But for the links across Great Belt and Oresund a design ice thickness of 0.032*(0.9*480-50) = 0.63 m was applied for a 1/100 years event. The formula for design ice thickness is too conservative for open waters. A design ice thickness of 0.48 m (for a 1/50 years event) for open water in the Southern Baltic is recommended. Field measurements that were conducted at the lighthouse Norstromsgund in 1999-2003 have been analyzed elsewhere, and a probabilistic model for the ice strength has been obtained. It is appreciated herein that the number of ice action events is significantly smaller in the southern Baltic Sea than in the Gulf of Bothnia. Therefore, the probabilistic model can be used to justify a reduction in the ice strength. This is helpful in attempts to develop economic designs for the wind turbine structures.