Horizontal vibration of offshore wind turbines supported by monopile-friction wheel composite foundation in multilayered saturated soil: Theoretical approach

With the continuous development of the offshore wind industry, the design concept of composite foundation has been given attention in the past decade. This paper presents an accurate method for investigating the horizontal vibration of monopile-friction wheel composite foundations in layered saturated soil. Firstly, the three-dimensional continuum mechanics theory with the range of linear elasticity is introduced to calculate the frictional resistance distributed on the upper soil surface. Then, the resistances of multilayered soils and inviscid seawater to the pile shaft under horizontal harmonic excitation are obtained using Novak’s plane strain model, Biot’s porous media theory and radiation wave theory. Thirdly, the expressions for the deformation, bending moment and internal force of the Euler–Bernoulli pile are derived using the boundary conditions with definite physical meaning and transfer matrix method. By comparing with the results of 1 g laboratory test and the idealized formula reported by the literature, the rationality and accuracy of the developed dynamical model can be verified. Finally, this paper conducts a series of worked examples to investigate the influences of the elastic modulus and thickness of three-layer saturated soil and the location of interlayer soil on the horizontal dynamic vibration of composite foundation. The results show that an increase in elastic modulus of the surface soil is an effective way to improve the dynamic stability of the composite foundation in service conditions. The conclusions drawn from the numerical examples can develop some guidelines for the current foundation design of offshore wind turbines.