Launched in 1999 in an innovative attempt to harness the expertise of public and private sectors for furthering offshore research, the Dutch Offshore Wind Energy Converter (DOWEC) project has been wrapped up with a final report that raises as many questions as it provides answers.
With the aim of fostering an integrated approach to offshore development, DOWEC brought together Dutch research institutes ECN and TU Delft with former Danish turbine manufacturer NEG Micon (before its merger in March with Vestas), blade builder LM Glasfiber and marine engineering specialists Ballast Nedam and Van Oord ACZ. The objective was to optimise the design of a 500 MW offshore wind farm while designing and constructing a 6 MW offshore wind turbine. After four years, the initiative has come to an end with only the first phase complete.
The final report concludes that while much has been learned, more research is needed on a number of areas including wake modelling, operation and maintenance analysis, grid interaction and aerodynamic and hydrodynamic modelling. More practical experience with offshore operation is also required "to reach the goal of reliable and economic generation of offshore wind energy."
Plans for up-scaling the 2.75 MW DOWEC prototype, now operating at ECN's Wieringermeer test farm, to a 6 MW machine have been curtailed. The merger of NEG Micon with Vestas, which already has a 3 MW model available, means the future of the DOWEC 2.75 MW is uncertain, despite the NEG Micon 2.75 MW being described in Britain as the current "machine of choice" for offshore projects.
"If in June, Shell and Nuon decide to go ahead with their [99 MW] Nearshore pilot project, then we know that 36 of the 2.75 DOWECs will be built, says ECN's Ben Hendriks, DOWEC research manager. "But we will have to wait for the new Vestas R&D strategy statement to see how the 2.75 MW will be integrated into the product range alongside the Vestas 3 MW and the 4.2 MW machine being developed by NEG Micon."
Hendriks is not unduly disappointed by the premature conclusion of DOWEC, regarding it as more or less inevitable given current industry trends. "There is a structural problem with developing large offshore machines," he says. "To take big steps you need big capital."
For the DOWEC project, the wind industry's ongoing mergers and acquisitions meant that what started out as an all Dutch affair quickly took on a Danish flavour with the take over of Rotorline by LM Glasfiber and Nedwind by NEG Micon. "This had advantages and disadvantages," says Hendriks. "Every time a new partner joined it meant restarting to some extent, but the reserves of expertise also increased." In the end, problems of commercial confidentiality grew to the extent that further similar collaborations are unlikely.
Technically, the main challenge in building large offshore machines is modelling, believes Hendriks. Applying current models empirically tuned to smaller machines to large machines offshore will lead to inaccuracies of up to 50%, he says. "We need much better aerodynamic models, otherwise the development lessons might be very expensive -- if you are faced with an in-plane vibration of the blades for example, you'd much rather see that on your computer than on your prototype."
Maintenance is also a key issue. "We are still a long way from the 95% availability rates needed for the commercial operation of true offshore wind farms operating with tiny weather windows," he says. "I think that all the DOWEC partners have a much better appreciation of the importance of testing all offshore components to ensure they are maintenance free."
Hendriks regrets there was little opportunity to evaluate the Jumping Jack installation technique developed by Van Oord ACZ, which only became available once the project was underway. "This might have significant advantages over the floating method using Ballast Nedam's crane ship," he says. A jack-up barge, which extends legs to fix itself firmly on the seabed, adds a larger weather window. Using a floating vessel is faster, but the anchoring is a problem, he adds.
The DOWEC partners still meet informally and each continues to work on the issues outlined in the program. Hendriks is returning to the problem of aerodynamic modelling at ECN.