In recent months, Deltares has tested a new foundation structure for offshore wind turbines in the Atlantic Basin in Delft and optimised a design for scour protection.
The study showed that scour protection can be a lot cheaper than was thought, which makes it more likely that the idea will be used in practice. Gravity-based structures (GBS) are support structures for wind turbines that can be floated to the site and then submerged with ballast.
Necessity
The GBS approach has been used for many years in the oil and gas sector but it has only recently come into the picture for offshore wind farms. In a joint research project with the contractors Van Oord and BAM, and the RWE energy company, Deltares looked at the practicality of this type of foundation for the offshore wind industry. Tim Raaijmakers, the Deltares project leader: ‘A major increase in offshore wind capacity is expected in the coming years. However, because of shipping routes, oil and gas platforms, the visual impact and ecological considerations, wind farms will increasingly be located further offshore in deeper waters. The familiar monopiles, which are driven into the seabed, are no longer a practical option in those locations. GBS foundations are expected to be much more suitable for deeper waters and so this alternative is in the picture now.’
Benefits
However, GBS foundations have other benefits. Installation does not require the large and expensive crane vessels that are needed for monopiles and this represents a potential saving. In addition, the submersion approach means that installation generates almost no noise. That means there will be less underwater noise, a source of potential harm for marine mammals. As a result, installation can take place at any season, simplifying the planning process for offshore wind farms considerably.
Scour protection
The load generated by a wind turbine is very different from an oil platform and so there are a lot of unanswered questions. The research in the Atlantic Basin generated a lot of new knowledge. Tim Raaijmakers: ‘We looked at what scour protection was needed. In the test design, the rock size and the rock volume were based on the prevailing guidelines. However, the tests showed that these values were much too conservative for this design and that the required rock volume could be halved easily. In addition, smaller rocks can be used. As a result, these new design rules will result in major cost reductions for scour protection.’
Wave loads
To refine the design even further, Deltares also looked at the wave loads on the GBS structure. On the basis of the physical tests, a range of numerical models were validated, widening the scope of applicability to different types of offshore structure.
Geotechnical load capacity
As well as the hydraulic issues, there are also interesting questions relating to the geotechnical behaviour of the subsoil. These structures are installed in shallow trenches directly on the seabed and so the load capacity of the top layer of the seabed is the determining factor. Wave loads can affect this top layer, leading to the subsidence of the foundations. To understand this process better, shear stress tests were conducted and a new calculation method is being developed.
FLOW
This GBS study is part of the FLOW research programme (Far and Large Offshore Wind energy, flow-offshore.nl). The programme is intended to accelerate the construction of wind farms far offshore (>50 km) by using new, innovative technologies and carving out a leading position for Dutch companies on the international market for offshore wind farms.
[mappress]
Press release, June 24, 2013; Image: deltares