Hybrid FRP-concrete-steel double-skin tubular monopile foundation: An innovative offshore wind turbine support

The main aim of this study is to propose an innovative hybrid fiber-reinforced polymer (FRP)-concrete-steel double-skin (HyFRP-CSDS) tubular monopile foundation for supporting offshore wind turbines. The HyFRP-CSDS design is specifically intended for the high-stress region of the turbine's support structure between the mudline and water level. The construction of the HyFRP-CSDS section involved filling an ultra-high-performance cementitious layer between an inner steel tube and a concentric FRP tube at the periphery of the cross-section. A 3D nonlinear finite element model was developed to assess the performance and effectiveness of the proposed HyFRP-CSDS as a support structure. The model investigated different diameter ratios, which represent the correlation between the outer FRP tube and the prototype monopile diameter. Through a comprehensive analysis of maximum applied horizontal force, bending moment, lateral displacement, pile rotation, and global buckling ratio, it was determined that the HyFRP-CSDS support structure demonstrates sufficient lateral stability, reduces total external loads, and mitigates damage in the high-stress region of the turbine's support. Consequently, due to the synergistic effect of FRP, steel, and concrete, the proposed cross-shaped section holds the potential for enhancing support structure design. Investigation of pile displacement and rotation shows that the new hybrid foundation improves the reliability of power production and availability by reducing the displacements and rotations in comparison with conventional and other composite foundations. This can limit excessive vibrations and the need for costly repair and interruptions in power production. Additionally, the implementation of HyFRP-CSDS allows for stress redistribution within the monopile by strengthening critical segments.