Onset of Motion of Sediment underneath Scour Protection around a Monopile

The stability of scour protections is, potentially, an important issue during the design of fixed foundations for offshore wind turbines. One of the failure mechanisms observed at placed scour protection around offshore foundations is suction of sediment through the scour protection and subsequent sinking of the scour protection. Incipient motion of sediment and the initiation of suction underneath scour protections around piles in the marine environment were studied under waves, current and combined waves and current conditions. The motion of a thin layer of sediment underneath the scour protection was studied through the glass bottom of the test flume, which provided a clear view of the initiation of the motion of the sediment. The results show that the mobility depends on the Keulegan-Carpenter number for the pile, the ratio between waves and current flow and the ratio between the thickness of the scour protection and the base sediment. The critical mobility number is smaller for the wave-dominated situation compared to current-dominated conditions, which again are smaller than for combined waves and current conditions. Consequently, larger -numbers cause larger critical mobility numbers than smaller -numbers. Design diagrams are presented for the threshold of incipient motion of sediment underneath a scour protection in waves, current and combined waves and current.