A preliminary assessment of frozen-in offshore wind turbines

Abstract

The frozen-in scenario-a condition where the offshore wind farm is fully enveloped by a large ice cover-is not typically considered during design. The current study explores this scenario by including a sufficiently large surrounding ice sheet, modelled as a representative linear elastic spring at mean sea level, in a dynamic model of an offshore wind turbine. The effects of the presence of ice on natural frequencies and flexibility of the offshore wind turbine is investigated, as well as its effect on load effects in typical wind-dominated design load cases. Emphasis is placed on cases governing the design of structures above waterline, such as extreme coherent gusts and directional changes. By varying the spring stiffness representing the ice, the load, deformation, and strain rate the modelled ice was subject to, were determined. The study found that the extreme overturning moment and damage equivalent moment reduce when the offshore wind foundations are surrounded by ice, whereas the shear increases from MSL and below. The combined load effects from the frozen-in load case show a higher utilization for a few select elevations below MSL. Depending on the assumed relationship between ice thickness and stiffness, the study evaluates the conditions under which the ice sheet could potentially grow and remain intact during both power production and extreme events. These findings indicate that based on the current methodology, the frozen-in load case cannot be disregarded and should be included in design in regions where there is an increased risk of encountering these conditions. However, it is expected that with improved ice modelling, accounting for viscoelastic behavior and ice failure, utilization levels would not exceed those observed in ice-free conditions.