Design of Composite Pile Foundations for Offshore Wind Turbines

The current state of the practice of the offshore wind foundation design is the monopiles that are made of large diameter cylindrical steel tubing with a transition piece connecting the pile to the turbine tower. The disadvantages include sub-structure flexibility at greater depths and environmental noise during pile driving with hammers. Monopiles should generally be avoided in deep soft soils and marine life protected area due to the installation limits and impacts. Alternative offshore wind foundations, so called composite pile foundations, with cost and environmental friendly with sufficient stiffness are proposed to make use of offshore wind foundations possible. The composite pile foundations, which are specially used for onshore structure, consists of upper steel tube piles and lower prestressed concrete piles resisting high bending moment in upper part and high compression pressure in the lower part. This paper aims to investigate the lateral behavior of the composite pile foundations taking into account the interaction between pile and the soil for both onshore and offshore purposes with 0.5m and 1.0m diameter piles. The full-scaled pile load tests of composite pile foundations have been performed and both beam spring analyses and finite element analyses are carried out. This paper presents analysis results for the application of composite pile foundations for offshore structure at the Korean western sea, where the first offshore wind farms are about to be installed. This paper recommends proper design processes and methods for the use of composite pile foundations in Korean western sea for economical better practice.