Author(s): Shun-Yi Wang; Wen-Gang Qi; Biao Li; Fu-Ping Gao
Linked Author(s):
Keywords: Local scour; Tidal current; Pile foundation; Flume experiment; Equivalent velocity; Reduction coefficient
Abstract: There is a lack of research in the existing literature regarding the scour of offshore wind turbine foundations under tidal currents, which primarily relies on laboratory experiments with simplified flow velocity hydrographs and mainly focuses on square tidal currents. To enhance the prediction accuracy of tidal scour for single pile foundations, systematic physical model experiments at a scale of 1:60 have been conducted on local scour of single pile foundations under typical tidal velocity hydrographs (including sinusoidal and square tidal currents) in a specially designed fluid-structure-soil coupling flume. The results demonstrate that the periodic change in flow direction associated with tidal conditions leads to a continuously evolving process of sediment erosion and backfilling around a single pile. The scour depth initially exhibits a high development rate under square tidal current, while it remains zero until the flow velocity surpasses the threshold value for scour under sinusoidal tidal currents. Although the shape of scour hole is similar for both sinusoidal and square tidal currents, there are significant differences in the development process of the maximum scour depth, presenting a shape of "short platform" and "serrated", respectively. The analysis employed the dimensionless effective flow work (DFW) method, revealing a consistent relationship between the dimensionless scour depth and the dimensionless effective work under both tidal and unidirectional current conditions. An equivalent velocity expression for sinusoidal and square tidal currents is proposed and verified using existing experimental data. Furthermore, an empirical expression for the reduction coefficient between square tidal currents and unidirectional currents under semi-diurnal conditions is also proposed. These research results establish a theoretical foundation for simplified tidal current methods in laboratory experiments and offer practical guidance for predicting the scour depth of tidal currents around single pile foundation for in-site offshore wind turbines.
Year: 2023