During recent years a significantly increasing number of offshore wind farms has been developed. The offshore wind farms have become attractive because of larger turbines and the difficulties of finding new good locations onshore. The wind farms are often located nearshore in shallow waters, where they in many cases are exposed to strong currents and large breaking waves. Under these conditions around 30% of the total construction costs are related to the foundation and a significant part of this is the cost of the scour protection.
DHI, Vestas, and Dong Energy have recently finished an evaluation of the scour protection implemented at the Horns Rev I Wind Farm. The Horns Rev is located off the North Sea coast of Denmark. The wind farm is founded on mono-piles located at 6.5 m to 13 m of water (relative to mean sea level). The seabed consists of sand and the scour protection consists of a filter layer and an armour layer of stones.
The construction of the wind farm took place in the summer of 2002 and the scour protections were surveyed shortly after installation. A control survey was made in 2005. Comparison of the bed level in 2002 and 2005 shows a lowering of the seabed around the edge of the scour protection, but not to a critical level. However, close to the foundation relatively large scour holes had developed, often 1 m to 2 m deep, in some cases more. An example of the lowering for turbine 35 can be seen in the figure.
Calculations showed that the failure of the scour protection was caused by the loss of sand through the scour protection proper, especially close to the foundation. For this reason special care must be taken in the design of future scour protections.
A numerical model, WiTuS, for calculating the scour development for an unprotected pile has been developed by DHI during the project. The model calculates the scour development over time for wave, current and tidal forcing.
The wind farm seabed interaction is a highly complex phenomenon and DHI will in the coming years continue the research in the field.
The project was supported by the Danish Public Service Program (PSO) under contract 6508 (FU5102).