DHI evaluates scour protection for wind turbines

The Horns Rev wind farm is founded on monopiles located in 6.5m to13m 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.

As DHI noted, in recent years a large number of offshore windfarms have been built, which are often located in shallow waters where they may be exposed to strong currents and breaking waves. Under these conditions around 30% of total construction costs are related to the foundation and a significant part of this is the cost of the scour protection.

The construction of the Horns Rev wind farm took place in the summer of 2002 and the scour protection was surveyed shortly after installation. A control survey was made in 2005. Comparison of the bed level in 2002 and 2005 showed 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 1m to 2m deep, and in some cases more.

Calculations showed that the failure of the scour protection was caused by the loss of sand, particularly close to the foundation. For this reason, notes DHI, particular care needs to be taken in the design of future scour protection structures.

A numerical model, called WiTuS, which can be used to calculate scour development for an unprotected pile was developed by DHI during the project. The model calculates the scour development over time for varying wave, current and tidal forces.