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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. Practical Applications: Monopile foundations of offshore wind turbines experience cyclic lateral vibration caused by winds and waves. The cyclic lateral vibration of monopile foundations potentially can affect the current-induced scour process around the foundations. The experimental results of this study revealed that the scour depth around vibrating monopile foundations under clear-water scour conditions is influenced by a coupling effect of flow intensities and vibration amplitudes and frequencies. The findings emphasize the influence of vibration on the scour process around monopile foundations of offshore wind turbines. These results provide valuable insights to optimize the design and manufacturing processes of monopile foundations. Moreover, the findings contribute to the development of more-effective scour protection measures in offshore wind engineering. [ABSTRACT FROM AUTHOR]

Accurate estimation of local scour depth has long been considered to be an important step for the design of bridge piers. Methods currently serving in different countries are generally based on equilibrium equations, which have been found to either overestimate or underestimate the local scour depth. In this study, we proposed a new design method for clear-water local scour depth based on temporal evolution characteristics. We collected data from 67 experiments from the referenced literature and conducted four in-house experiments. Using factor analysis and trial calculation, we derived a new temporal equation for local scour depth evolution. Comparison showed that the maximum predicting error of the total 71 experiments was reduced from 162% to 534% using four existing temporal equations to 34% using the new equation. The corresponding design method was illustrated by an idealized scenario. In addition to its understandability and usability, the new method had better flexibility and fault tolerance, because it did not involve the concept of equilibrium scour depth, which lacked an objective and universal criterion. [ABSTRACT FROM AUTHOR]

Xiao, Y.; Jia, H.; Guan, D.; Liang, D.; Yuan, S., and Tang, H., 2021. Modeling clear-water scour around the high-rise structure foundations (HRSF) of offshore wind farms. Journal of Coastal Research, 37(4), 749–760. Coconut Creek (Florida), ISSN 0749-0208. The high-rise structure foundation (HRSF) has been used in building the Donghai offshore wind farm in Shanghai, China. So far, few published studies can be found on the local scour around an HRSF. This study presents the results of a physical modeling investigation of the clear-water scour around two typical HRSF models consisting of eight piles and 10 piles, respectively. In the study, the flume bed was covered with uniform noncohesive sediment and the incoming flow was steady. The effects of the alignment angle of the HRSF, water depth, and relative sediment size on the equilibrium scour depth were analyzed. Results showed that the maximum scour depth always occurred around the middle piles on the shoulders of the HRSF. The alignment angle of the HRSF had little influence on the final equilibrium scour depth. For permeable multipile foundations, the scour depth increased when the water depth increased within the range of the testing conditions, which indicates a similar trend between a multipile foundation and a monopile. An empirical formula is proposed for estimating the equilibrium scour depth. [ABSTRACT FROM AUTHOR]