Offshore substation platforms connect the array cable system of an offshore wind farm to the export cables and are often designed based on the jacket support structure concept with almost vertical legs. The size of these platforms, and the number of cables arriving at the platform through j-tubes, make that these have many structural elements crossing the waterline. For design of such multi-leg structures to ice loading, it is important to account for sheltering and interference effects as well as potential jamming of ice between closely spaced members. Guidance on these topics can be found in design standards; however, it mostly concerns four-legged structures with equal leg diameters for which experience has been obtained in full-scale and model-scale. In this paper we present results from a pre-study for a recent offshore substation design. A preliminary method for defining the sheltering and interference factors for multi-member structures with more than four vertical members and members of different diameters crossing the waterline is presented. The method is based on the original work on this topic by Saeki. The sensitivity of the global ice load on the platform support structure to the placement of cable j-tubes is investigated with the proposed method. The results are discussed in relation to design of substation support structures with a focus on dynamic interaction between ice and the platform, the potential benefit of lay-out optimization and extending the approach to include jamming and ice ridges interaction. This study highlights the need for further model-scale or full-scale testing to validate key assumptions required to develop these kinds of approaches for dealing with sheltering and interference on multi-member structures.

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.