Sheltering and interference effects for multi-leg offshore substation platforms interacting with sea ice
Florian L. van der Stap (TU Delft - Offshore Engineering)
Martin B. Nielsen (Wood Thilsted Partners Limited)
Hayo Hendrikse (TU Delft - Offshore Engineering)
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Abstract
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.