Abstract
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.
Highlights
During the last decade more and more wind farms have been erected offshore
The Horns Rev I is located in relatively shallow water (6.5 to 13 m water (MSL)) about 20 krn off the Danish West Coast in the North Sea
This area is exposed to strong tidal currents and large waves from the North Sea
Summary
During the last decade more and more wind farms have been erected offshore. One of the first larger offshore wind farms is the Horns Rev 1. A control survey in 2005 showed that the scour protections adjacent to the mono-piles sank up to 1.5 m. This was unexpected and shortly after the survey in 2005 the holes were repaired by adding additional stones. Scour around unprotected piles have been studied extensively over the last decades. In order to gain an understanding of the mechanisms that cause the sinking of the scour protection, an extensive program of physical model tests with steady current has been carried out in the present study, in an attempt to contribute to the knowledge obtained recently by Chiew and Lim (2000), Lauchlan and Melville (2001), Chiew (2002), De Vos (2008) among others. The model tests showed that the horseshoe vortex, the key element to cause scour around unprotected piles, see e.g. Dargahi (I989) and Roulund et al (2005), is a key flow feature governing the sinking process
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