Keywords: offshore wind turbine, bucket foundation, transition piece, reinforcement, compact reinforced composite.

Going to deeper waters (30-50 m) requires finding new solutions for offshore wind-turbine foundations and transition piece structures connecting the foundation with the wind turbine tower. This paper is focused on optimization of the shape and the material of the transition piece (TP) connecting the offshore wind turbine column with a bucket foundation (suction caisson). While the existing design practice is limited to the use of steel-flange-reinforced shear panels, an alternative material called compact reinforced composite (CRC) is proposed. Optimization of the shape of the TP shape is performed in order to lower the manufacturing costs of the structure without compromising its strength and stiffness. A finite-element model is developed using ABAQUS. Initially, four geometries and two concrete models with various compressive or tensile stress-strain behaviour have been checked for material failure in the ultimate limit state (ULS) to find the required thickness of the section, the amount of reinforcement and the spacing between the bars.

Furthermore, two geometries (Model 2 and Model 3) have been selected for further study as the most representative and dissimilar in their response. The performance of both models has been different. Model 3 has been proposed for further investigation due to the insufficient performance of Model 2 which has shown large force concentrations at the joints and required higher amounts of reinforcement. It has been shown, that Model 3 provides a smooth force transition at the convex and concave parts and, therefore, it has been proposed for further optimization by introducing cutaways to eliminate potential formation of scour.

According to the results of the non-linear finite element simulations, the overall thickness of the substructure could be decreased from the constant thickness of 180 mm to a variable thickness of 150-160 mm for Model 2 with a stronger concrete, whereas for Model 3 the constant thickness could be minimized from 150 mm to a variable thickness of 135-140 mm. The lowest weight of the transition piece is achieved for Model 2 with a constant thickness and for Model 3 with a variable thickness for a material model with high strength of concrete.

Scaled models of the transition pieces of various shapes with variously positioned cutaways will be further tested in the wave flume of Aalborg University and examined for scour formation around them. Simultaneously, computational models will be proposed and verified.

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