Dynamic Responses of Mono-piles in the Presence of Scour Holes

Mono-piles are widely used in offshore wind industry, and it will be necessary to consider the influences of scour for mono-piles under combined waves and currents. There are about two main effects on the mono-piles when scour occurs: the first is to reduce the ultimate resistances and the other is to reduce the impedances (stiffness and damping ratio). While impedances play a dominant role on the natural frequencies and dynamic responses of offshore wind turbines, the risk of resonance and fatigue will increase dramatically due to the presence of scour holes. In this paper, the evolution of dynamic characteristics of model mono-piles under different scour depths and excitation frequencies are studied by model tests. The scour depths are calculated by laboratory tests or field observations in the literature. By means of Fourier transform and modal analysis, the dynamic responses of mono-piles in time domain and frequency domain are obtained and analyzed. The important parameters for mono-piles are obtained, including the resonant frequencies and resonant amplitudes. The experimental results are compared with a dynamic numerical analysis. The results of this paper will be helpful to understand the dynamic characteristics of offshore mono-piles in complex ocean environment.