Keywords: offshore wind turbine, bucket foundation, transition piece, finite element analysis, compact reinforced composite.

In Denmark, the production of renewable energy is focused on offshore wind turbines, since they make little if any inconvenience for residents in inhabited areas. High specifications are placed on the installation of the foundations which can cost about 30% of the total cost of the wind turbine. This paper deals with the transition piece for a relatively novel type of foundation, the so-called suction bucket (caisson), focusing on the design of a transition piece connecting the turbine column with a suction bucket used as a monopod foundation for an offshore wind turbine. Since the current design practice is limited to the use of steel-flange-reinforced shear panels for the transition piece-a production that requires extensive welding work-a desirable solution is to find a material that provides lower cost and easier manufacturing without compromising on the strength and stiffness.

This paper compares the structural behaviour of a transition piece made of steel (the reference case), a compact reinforced composite (CRC) and composite shell elements made of CRC glued to steel sheets. A finite-element model was developed using ABAQUS. Three material models are checked for buckling and material failure in the ultimate limit state (ULS) to determine the required thickness, amount of reinforcement and spacing between the bars.

According to the results of the simulations for the reference material model, 34 mm thick steel sheets are sufficient to withstand failure from the applied loads. As for the CRC material model, a 120 mm thick structure with two layers of rebar of diameter 18 mm (spaced 60 × 60 mm) is adequate. The proposed composite CRC-steel shell consists of a 60 mm thick CRC layer with one layer of reinforcement of diameter 16 mm (spaced 60 × 60 mm) glued to a 12 mm thick steel sheet. Further optimization of the structures can be achieved by reducing the reinforcement ratio in regions of high compressive stresses by cutting some reinforcement bars. This will also decrease the total weight of the transition piece. Recommendations regarding the preferable material model should be based on the economical considerations, including the cost of the materials, production and complexity of fabrication, handling and installation. The simulation results indicate that the amount of ductile steel in the form of reinforcement and steel sheets, carrying the majority of the tensile stresses, is likely to dictate the design of the transition piece. The minimum amount of steel and concrete used can be achieved by the composite CRC-steel shell model. Future research will concentrate on the structural performance of this material.

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