Scour Protection of Offshore Wind Farms

One of the first large offshore wind farms is the Horns Rev 1 Wind Farm in the Danish part of the North Sea. It is located around 20 km of the coast in relatively shallow water. The wind farm was installed and commissioned during 2002. In 2005 a control survey of the scour protections around the foundations showed that the scour protections adjacent to the mono piles sank by up to 1.5 m. This was unexpected and shortly after the survey in 2005 the holes were repaired by adding additional stones. The aim of the thesis is to give an explanation of the sinking at Horns Rev 1 Wind Farm and to describe the processes causing the sinking.
In Chapters 2 and 3, a description of the main processes causing the sinking of a scour protection around a pile if exposed to a current is given, based on results of physical and numerical models. Using the results it is possible to make designs of the scour protection to prevent or estimate sinking of scour protections around mono piles. It is shown that the horseshoe vortex at the base of the pile will penetrate into the scour protection and if strong enough mobilize the base sediment and transport it out of the scour protection where it will be removed by the outer ow. The equilibrium sinking of the scour protection is found for various conditions. It is also found that a fine filter layer can prevent the mobilization of the sediment and therefore the sinking.
In Chapter 4, the scour around mono piles in breaking waves is studied. The scour is found to depend on two parameters: (1) The distance between the breaking point and the pile normalized by the wave length and (2) the breaking wave height normalized by the pile diameter. The maximum scour is found to be approximately 0.65 times the pile diameter. It can be larger than the scour generated by non-breaking waves especially for small KC-numbers. The main reason for the increased scour is found to be turbulence generated by the breaking and is forced to the bottom by the pile.
In Chapter 5, the onset of suction from between armour stones under breaking waves is studied. The critical conditions for onset of suction are determined for several different conditions, regarding wave height and period, slope of the bed, sediment and cover stone size and number of cover layers. The oblique descending eddies generated by the breaking waves are found to be the main mechanism regarding suction of the base sediment.
In Chapter 6, the ow in and the bed shear stresses under a stone cover under an oscillatory ow is described. The ow velocities and the turbulence are measured in case of one, two and three layers of stones in several pores. The tests showed that the horizontal velocities in the pores are affected by the outer ow approximately one stone diameter into the stone cover. Under this level the horizontal velocities become constant. The bed shear stresses are much smaller than at a smooth bed without stone cover and large variations are observed.
In Chapter 7, the effect of waves on sinking of the scour protection around a mono pile is studied. It is found that the sinking will increase for increasing KC-numbers for a given diffraction parameter. The magnitude of the sinking is found to be comparable with the scour observed around an unprotected pile exposed the same wave conditions.