Anders Wedel Nielsen

Experimental Study on the Scour around a Monopile in Breaking Waves

AbstractThe scour process around monopiles caused by breaking waves is studied experimentally using regular waves. The use of regular waves is conservative, which made it possible to avoid scour phenomena caused by nonbreaking waves such as scour generation and backfilling. The waves were breaking on a flat sand section after shoaling on a mildly sloping ramp. Various monopiles were exposed to plunging breakers that were breaking at various distances from the pile. It was found that the scour was caused by turbulence generated by the breaking and was diverted toward the bottom by the pile. The maximum scour depth found was approximately 0.60D. This was smaller than the scour observed around piles exposed to current; however, in some cases it was an order of magnitude larger than the scour caused by nonbreaking waves. This is apparently especially true for larger piles.

Scour Protection Around Offshore Wind Turbines. Monopiles

The flow processes in a scour protection around a mono-pile in steady current is described in relation to transport of sediment in the scour protection based on physical model tests. Transport of sediment in the scour protection may cause sinking of the scour protection. This may reduce the stability of the mono-pile and change for instance the natural frequency of the dynamic response of an offshore wind turbine in an unfavorable manner. The most important flow process with regard to transport of sediment and sinking of the scour protection is found to be the horseshoe vortex. It is found that a larger pile diameter relative to the size of the protection stones will cause a larger sinking and that two layers of stones will decrease the sinking relative to one layer of stones with the same size.