Engineers this week uncovered a fundamental design flaw in the design of offshore wind turbine structures.
The problem has hit wind farms across Europe, including UK fleets at Gunfleet Sands off the east coast, and Burbo Bank in Liverpool Bay at the mouth of the River Mersey.
The issue is also being investigated at other sites and potentially affects all offshore turbines that have piled foundations. The UK currently has 336 offshore turbines and plans to create up to 32GW of new generation capacity.
Danish energy giant Dong admitted that grout designed to act as an adhesive between a pile foundation and the transition section of the turbine tower base was failing. The flaw has caused some turbines to slip by up to 25mm.
It said that the problem had been uncovered at three of its sites - Gunfleet and Burbo as well as the Horns Rev II wind farm in Denmark.
Dong learned of the problem from Shell, which revealed it had found the flaw at its wind farm off the coast of Egmond aan Zee in the Netherlands.Centrica is examining its Lynn and Inner Dowsing wind farm offthe Lincolnshire coast, but said its array near Barrow was unaffected.
“The large majority of UK turbines were built to DNV standards and so they could potentially develop this fault”
“Our conclusion so far is that the adhesive between the monopile and transition piece has failed,” Dong lead project manager Hans Jessen Jensen told NCE. Jensen is investigating the problem across the firm’s sites.
“The grout is designed to act as an adhesive, but it is not sticking,” he added. “It would have transferred all loads, except wave loads, down to the monopile [foundation below]. But we have had slips of up to 25mm.”
Offshore turbine towers have frequently relied on these types of foundations, which involve driving piles into the sea bed. A transition piece is attached to the piles and the turbine tower is bolted to that.
Dong said the flaw on its fleets occurs at the 6m overlap between the foundation and transition piece. The 45mm to 90mm annulus between these two is grouted, allowing alignment corrections to be made to compensate for piling inaccuracies. It is this grout that has failed, causing the transition section to slip.
“No safety issues”
Jensen said that no safety issues had been identified and that supporting brackets used during construction and left in place have stopped the turbine towers slipping further.
But he said a more permanent solution had still to be determined, and this would likely cost millions of pounds to fix. “We have identified three possible solutions, but none of them are well developed at this stage,” he said.
“First we are first hoping to leave them as they are, and install a monitoring and alert system. This is the preferred option.
“The second option is to insert shims or wedges under brackets to support the tower on all brackets. But this option would also require a monitoring and alert system. Third would be to weld new brackets.”
A Dong spokesman added: “The most radical solution [the welding option] will cost us up to £13M for all three affected wind farms.
According to RenewableUK (formerly the British Wind Energy Association), standards for building offshore wind turbines are based on the Norwegian Det Norske Veritas (DNV) benchmark.
“The large majority of [UK wind turbines] were built to DNV standards and as such could potentially develop this fault,” said a spokesman.