Structural Optimization of an Offshore Wind Turbines Transition Pieces for Bucket Foundations

Traditionally, offshore constructions are made of steel. The focus of this paper is optimization of a transition piece (TP) connecting the offshore wind turbine column with a suction bucket foundation. Suction caissons, typically used for shallow water depths, have been proved to be adequate in residual soil conditions for depths up to approximately 40 m. The existing design practice is limited to the use of steel-flange-reinforced shear panels. Desirable outcome is proposal of an alternative material which does not require extensive welding work. Compact reinforced composite (CRC) is suggested as an alternative to steel. CRC has an excellent durability and higher compressive strength compared to traditional concrete. This material has also an increased ductility owing to integration of large contents of short, strong and stiff steel fibres. At present, application of high-tension concrete is limited offshore, mainly, to making a grouting connection of a transition piece to a monopile. Lack of standards and norms puts additional restriction on application of CRC. In the earlier work, the structural performance of transition pieces with a conical shape was compared for a 5 MW offshore wind turbine. Three construction materials were proposed: CRC with main reinforcement, composite CRC‒steel shell elements and steel sheets (reference case). The conical shape of the TP structure has been found to provide the smooth transition of forces from the wind turbine tower down to the bucket skirt. Doubly curved segments have been introduced between the conical part and the tubular parts of the structure. While the minimum amount of steel and concrete was required for the composite CRC–steel shell model, the pure CRC model appeared to be the least sensitive to geometrical imperfections, corresponding to deviation of the middle surface from the perfect ideal shape of the shell structure, and was assumed for further investigation. The steel model showed the highest sensitivity to geometrical and loading imperfections. This paper presents optimization of the CRC TP structure to lower manufacturing costs without compromising its strength and stiffness. Several models with variously positioned cutaways are presented and compared to find the one providing the better force distribution, preventing buckling and stress concentration and reducing the amount of material used. Minimization of the material consumption is based on assumed current cost of construction materials. Further investigation includes casting scaled concrete samples of the CRC to monitor the direction of flow and the possibility of mass.