Borkum Riffgrund: Making Offshore Wind Power Cheaper
All eyes are on Borkum Riffgrund 1, the 78-turbine DONG Energy wind farm in the German North Sea which had the industry’s first suction bucket jacket foundation installed late last month. The technology is just one demonstration of how Borkum Riffgrund 1, located 37km north of the island of Borkum, is leading the offshore wind industry in becoming increasingly cost competitive with other forms of electricity generation.
Unlike the other 77 conventional jackets installed on the project, the suction bucket foundation is not anchored to the seafloor with hammered-in piles. Instead, the 850-ton steel foundation is 57-metres tall and has cylindrical-shaped feet that are pulled down into the seabed by a system of high-pressure pumps. Each suction bucket has a diameter of 8m and a total height of 8m.
The foundation is suitable for water in the range of 30-60m. It is a lightweight structure that only requires a single operation when the foundation is installed offshore, reducing the time of the installation and hence contributing to lowering the costs of electricity from offshore wind.
The new foundation concept was developed by DONG Energy in cooperation with Carbon Trust Offshore Wind Accelerator (OWA) in the UK and marks the beginning of a new technology generation in the construction of turbines offshore. Borkum Riffgrund 1 was selected for testing the foundation due to its sandy seabed, which makes the installation of this concept a challenge, thus providing a solid case for the evaluation of the concept.
Tove Feld, vice president in DONG Energy, said: “The reduction in cost of electricity and our ability to utilize some of the more challenging sites further from shore and in deeper water show some of the potential that this concept has. I’m very excited to see that the cooperation between the partners in this project, not least with Carbon Trust in the UK, is paying off with specific results.”
The foundation is quieter to install than conventional systems, a key environmental concern for DONG Energy on this project. Installation of the other foundations for the project was carried out using a noise mitigation system to lower the impact of pile driving operations on adjacent marine life. The system was another industry first for the project.
A 3.6MW Siemens turbine will be installed on the bucket jacket foundation in October and A2SEA will use its second second-generation installation vessel, Sea Installer, for the job. Sea Installer incorporates many innovations – all aimed at making installation faster, safer and less costly. It has already proved capable of carrying and installing up to eight 3.6 MW turbines at a time.
As a second-generation vessel, Sea Installer was purpose-built to install turbines and to be highly fault tolerant. With the redundancy built into the design, the vessel is more reliable than first-generation vessels and quite capable of continuing with installation tasks, albeit at a slower pace, even if a sub-system fails.
The deck layout being used for the turbine installations on Borkum Riffgrund 1 is based upon West of Duddon Sands, the project Sea Installer finalized just prior to the Borkum project. During mobilization, which was carried out in Frederikshavn, Denmark, during June and July, minor changes were made to the layout in order to accommodate the increased weight of the larger turbines. At the same time, some of the nacelle positions were relocated, and a new nacelle-lifting beam was installed, along with a new lifting yoke for the towers. The lifting yoke’s design is particularly interesting because it enables Siemens Wind Power technicians to perform fewer manual tasks, which promotes safety.
Sea Installer is using the Gemini quay at the Port of Esbjerg as the load-out port for the project. For Borkum Riffgrund 1, the quayside has received extra attention with the construction of a stone bed upon which to jack up for loading, as soil conditions in the area vary greatly and the seabed isn’t strong enough to support a heavy installation vessel.
Borkum Riffgrund 1 is located 114 nautical miles from the Port of Esbjerg, or around 15 hours for Sea Installer in a straight line. Getting there is plain sailing but vessel project operating conditions are a little more difficult. Some of that difficulty centres around the fact that the site lies relatively far out to sea, in an area where there are no islands or reefs to break the pattern of the swells. So, even though wind speeds may be within the limit for the marine operations, wave heights or periods may still approach or exceed the maximum. In addition, the risk of scour around the legs at the wind farm site will require more attention than usual.
Originally, the plan for Borkum Riffgrund 1 was to install seventy-seven 3.6 MW Siemens Wind Power wind turbine generators. During the planning phase, performance of the turbines was boosted to be able to deliver the equivalent of 4 MW in effect, raising the wind farm’s power output to a total capacity of 312 MW. Also during this phase, transport and installation of the 78th turbine, which will serve as a sea trial for a new suction bucket foundation, was added to A2SEA’s scope, so Sea Installer is now preparing for installing this turbine.
“Projects for which we have been preparing for several years are now well underway. During those years, our company and the wider industry have taken many steps forward in virtually all aspects of offshore wind energy,” says Jens Frederik Hansen, CEO of A2SEA. “We’ve matured technologies, methods and safety to bring the levelized cost of energy further down than might have been the case had wind farm plans moved more quickly. These advances are in place for coming projects to bank on, helping to deliver viable sustainable energy through offshore wind”
When fully operational in 2015, Borkum Riffgrund 1 will be able to supply green power corresponding to the annual consumption of approximately 320,000 German households.
Source: A2SEA and Dong Energy