Local Scour Mechanism around Dynamically Active Marine Structures in Noncohesive Sediments and Unidirectional Current

This paper sheds light on the mechanism of post equilibrium sea bed scour around dynamically active marine structures such as wind turbines. Exposure of a fully developed scour hole (at equilibrium state) around a wind turbine mono-pile to the cyclic movement of the structure leads to the backfilling and deformation of the scour hole. The existing approaches to scour prediction for foundation design of offshore wind turbines generally consider wind turbines as static structures and ignore the physical impact of the cyclic movement of the pile on the supporting soil and, hence, on the scour process. Through an experimental program, this paper explains the influence of the cyclic movement of the pile on the local scour in noncohesive sediments. A series of flume tests at two scales were conducted. Simple hydrodynamic conditions and bed sediment configurations were adopted to highlight the effect of pile movement. The results obtained indicate that a mechanism exists by which the scour hole can be significantly deeper and wider in extent than that predicted by conventional methods. This arises through a multistage process consisting of periodically alternating cyclically loaded and unloaded stages simulating a sequence of storms.