Author(s): Dawei Guan; Zishun Yao; Bruce W. Melville; Jinhai Zheng
Linked Author(s): Jinhai Zheng, Bruce W. Melville
Keywords: Monopile foundations; Scour; Cyclic loading; Offshore wind turbines
Abstract: Monopile foundations are regarded as the most favorable foundation type for offshore wind turbines constructed in relatively shallow waters with water depth less than 35m (Gavin et al., 2011). It is reported that monopile foundations of offshore wind turbines (OWTs) experience continuous cyclic lateral loads induced by winds and waves during their operation period, which may result in vibration of monopile foundations and changes in the scour process. Previous experimental studies show that the current-induced equilibrium scour depths at vibrating monopiles may be less than those at static monopile foundations (Al-Hammadi and Simons, 2020; Guan et al., 2019). However, scour processes around vibrating monopile foundations under current-only conditions with different flow intensities and under combined wave-current conditions remain unclear. New experimental studies are presented to further investigate the scour characteristics around monopile foundations of OWT subjected to lateral cyclic loadings under current-only conditions and combined current-wave conditions. The experimental results of current-only cases in different flow intensities indicate that the scour process around monopile foundations subjected to lateral cyclic loading is mainly influenced by three factors: vibration-induced sediment subsidence, vibration-induced sediment refill, and current-induced erosion. Based on the experimental data, a “regime transition flow intensity” is obtained, at which the equilibrium scour depth remains constant regardless of any changes in the vibration amplitudes. Furthermore, two distinct scour trends under low and high flow intensities are identified. The increase of vibration amplitude in the low flow intensity regime leads to an increase of equilibrium scour depth. Conversely, the increase of vibration amplitude in the high flow intensity regime leads to a decline of equilibrium scour depth. This reduction is attributed to a more substantial sediment refill induced by the vibrations. Furthermore, the experimental results indicate that equilibrium scour depths at monopile foundations subjected to cyclic lateral loads perpendicular to the flow direction are generally larger than those under cyclic lateral loads parallel to the flow direction.
Year: 2023