Processes and evolution of scour around a monopile induced by tidal currents

Despite offshore wind farms being mostly situated in tidally dominated waters, only limited research on the effects of tidal flow on the scour process around offshore foundation structures has been carried out so far. To further improve the prediction of scour around those structures, systematic laboratory tests were conducted in a closed-circuit flume on the processes and time development of scour around a monopile in tidal flow. The tidal currents were adapted by bidirectionally reversing currents with continuously changing flow velocity. Therewith, multidirectional flow aspects were simplified into flow components in tidal main direction. Tidal cycles and flow intensities were scaled with regard to field measurements at the FINO 1 platform in the North Sea. The model tests were conducted at a length scale of 1:40 and scour depths were measured at multiple positions around the monopile by a camera system. Novel insights on the intrinsic progression of sediment displacement and time scale of the scour process were gained and in the following presented as a function of flow intensity for clear-water and live-bed conditions. In addition, baseline tests with unidirectional currents were conducted, in which the constant flow velocity was either based on the maximum peak or the root mean square velocity of the tidal currents. Significantly slower scour progression and smaller scour depths in tidal currents were found, compared to unidirectional currents based on the maximum peak velocity. In contrast, scour depths in tidal currents exceed those for unidirectional currents by up to 51%, if the unidirectional currents are based on the root mean square value. The comparison further implies that a flow velocity of around 15-20% larger than the root mean square flow velocity of the tide is needed to accurately estimate tidal current induced scour depth by unidirectional current. Thereby, the results underline the importance of selecting suitable conditions for the design process of offshore structures. For design purposes, the evolution of tidally induced scour could be well predicted with a time discretised stepping approach.