Graded approach to scour protection could reduce costs significantly

An innovative form of scour protection that has been tested on a number of windfarms, but about which there is little in-depth data about its performance, could help to further reduce the cost of energy from offshore wind, developer Vattenfall believes. It is so convinced of the benefits of the approach it has decided to use it on three upcoming projects.

Scour is a potential issue for the monopile foundations used to support turbines. Interaction between a foundation and waves and currents amplifies stresses applied to the seabed surrounding a monopile, leading to a scour hole developing, which in turn reduces the bearing capacity of the foundation. To prevent scour holes developing to the point at which the performance of a turbine could be affected, rock armouring is usually installed around it.

Scour protection systems are usually applied in two stages. In the first, a ‘filter layer’ is installed around the base of the monopile. This filter layer consists of fine-grained material. The second stage sees an armour layer of larger rock material installed on top of the filter layer.

However, as DHI Denmark managing director Thor Ugelvig Petersen explained to OWJ, it has long been obvious to the industry that significant cost reduction could be achieved if just one layer of graded scour protection needed to be installed. This has been tested on windfarms, but more data is needed before it could be applied at scale.

Just that kind of approach was tested in the Proteus (Protection of offshore wind turbine monopiles against scouring) project – part of the HYDRALAB-PLUS project funded by the European Union – but such is the potential of the new technique that Vattenfall has already decided to use it to protect the monopiles on three offshore windfarms it plans to build, one in the North Sea and two in the Baltic.

Vattenfall appointed DHI and LIC Engineering to design and optimise the scour protection systems for the windfarms Vesterhav Syd, Vesterhav Nord and Kriegers Flak. The first two are nearshore windfarms in the Danish sector of the North Sea and the latter is in the Danish part of the Baltic.

Mr Petersen said that apart from the cost reduction the new approach would provide, work that DHI and LIC Engineering had carried out – and the work carried out in the HYDRALAB-PLUS project – strongly suggests that, if correctly designed, a single layer of graded scour protection is suitable for use in widely varying conditions.

“The sites that Vattenfall asked us to study are quite different,” he explained. “The Baltic sites experience relatively little wave action but the nearshore sites, although in shallow water, experience much larger waves. The currents are quite different too, but we found that grading the scour protection works well for both.

“Using graded rock material in a single layer combines the filtering effect of a filter layer and the armouring effect of larger material, so only one layer is required. This results in significant cost savings because you only need one installation campaign.”

Vattenfall project manager offshore foundations Victoria Ruiz Gomez said the wide-graded rock solution developed by DHI and LIC Engineering has great potential for cost savings. Mr Petersen told OWJ that using widely-graded material in a single installation campaign could reduce costs by 15-30%.