Wave Forces on Transition Pieces for Bucket Foundations for Offshore Wind Turbines

Since offshore wind turbines continually increase in size and move to deep water depths (>25 m), significant wave loading becomes a major concern for their design. A monopile foundation is one of the most commonly used types of the offshore wind turbine foundations today. This solution, however, becomes not very cost-effective in deep water depths. Therefore, it is essential to find some alternative economically efficient solutions. This paper presents an experimental study of wave loading on several shapes of transition pieces (TPs) used to transfer the loads from a 5 MW offshore wind turbine to a bucket foundation (suction caisson) located at 35 m water depth in the North Sea. Several models of the TPs (wedge-shaped steel flange-reinforced shear panels, conical and doubly curved with or without cutaways) are tested in a wave flume and compared with respect to wave loading. Due to a larger size of the suggested TPs compared to a typical slender monopole foundation, wave loads acting on these TPs are also expected to be considerably higher and much more difficult to predict. The results of the present investigation can be further applied for other wind turbine types.