Clear-Water Scour Mechanism at Vibrating Monopile Foundations

Cyclic lateral loads on monopile foundations of offshore wind turbines induce vibrations that affect the clear-water scour process around the foundations. Previous experimental studies showed that the current-induced equilibrium scour depths at vibrating monopiles may be less than those at static monopile foundations. However, this observation may not be comprehensive when applied to the cases of clear-water scour around vibrating monopile foundations subjected to different flow intensities. This paper presents an experimental study that investigated the clear-water scour mechanism at vibrating monopile foundations and obtained new findings. The experimental results indicate that the scour process is influenced by three major factors: vibration-induced sediment subsidence, vibration-induced sediment refill, and current-induced erosion. Based on the three major influencing factors and experimental data, a dimensionless equation for equilibrium clear-water scour depth at vibrating monopile foundations was derived. Furthermore, a regime transition flow intensity is defined, at which the equilibrium scour depth remains constant regardless of any changes in the vibration amplitudes. Two distinct scour trends under low and high flow intensities were identified. The two different trends were found to be induced by different dominant factors. The vibration-induced subsidence effect dominates in the low flow intensity regime, whereas the vibration-induced sediment refill effect dominates in the high flow intensity regime. The new findings of this paper help to understand the characterization of clear-water scour process around vibrating monopile foundations.