Investigation of the dynamic behavior of a composite pile foundation for offshore wind turbines

This paper presents an analytical methodology that provides the vibration characteristics of monopile–wheel composite foundation embedded in homogeneous saturated soil, when the top of the foundation is subjected to a harmonic horizontal load. In the proposed frame, the horizontal resistances along the pipe pile due to the vibrations of the outer and inner elastic soil and compressible seawater are considered by using the Biot porous medium theory, plane strain model and radiation wave theory. The closed-form expression of the frictional force caused by the wheel vibration is calculated through the three-dimensional continuum mechanics theory. Based on the Euler beam model, the dynamic governing equations of different pile segments in the composite foundation are simulated as a one-dimensional linear elastic rod. Analytical solutions of dynamic impedances of composite pile in the frequency domain can be derived by virtue of the boundary and continuity conditions. Following the validation of the proposed methodology, the sensitivity of the dynamic response and natural vibration frequency of this innovative foundation to the main geometrical problem parameters is studied. The results show that increasing the wheel radius, wheel thickness and embedded length can improve the dynamic stiffness of the composite foundation, and increase the natural frequency of foundation–soil system simultaneously. Finally, the differences of the dynamic responses between the composite foundation and single pile with the same fabricating cost are discussed in detail, and the corresponding analysis proves the superiority of the composite pile under offshore loading conditions. Meanwhile, the contribution of the friction wheel to the dynamic behavior of composite foundation under different parameters is also investigated.