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.

For the design of offshore foundations in regions such as the Baltic Sea, it is paramount that ice-structure interaction is appropriately considered. For the monopile, a common foundation for offshore wind turbines, challenges with ice-induced vibrations and high ridge loads may require ice-mitigating measures to be included in the design. A ‘feasibility map’ showing the necessity for such ice-mitigating measures in the entire Baltic region has been developed for monopiles. The feasibility was considered in technical terms by imposing design, installation, and fabrication constraints, and in economic terms, expressed in weight increase of monopiles when compared to an ‘ice-free’ design. A design assessment of offshore wind turbines across the Baltic Sea was conducted by optimizing foundation designs for the IEA 15 MW reference turbine for nine identified characteristic regions of the Baltic Sea. The assessment was performed via the in-house foundation design software MORPHEUS by Wood Thilsted. MORPHEUS has been coupled to the phenomenological ice model “VANILLA” to capture the dynamic ice-structure interaction for level ice. From the assessment, the following regions are deemed feasible for monopiles without ice-mitigating measures: the Danish Straits, the Baltic Proper South, the Baltic Proper North, the Gulf of Riga and the Archipelago Sea. The Bothnian Sea North and the Bay of Bothnia are deemed infeasible without mitigating measures. For the Bothnian Sea South and the Gulf of Finland, no conclusive answer was found as more research into the cost competitiveness of alternative options is required. The increase in fatigue resulting from ice loading was found to be the main cause for foundation weight increase of monopiles compared to monopiles designed for ice-free waters.