The faults, found at up to ten European offshore wind farms, concern the grouting connection between the monopile foundation and the transition piece (TP) which holds the tower (Windpower Monthly, June 2010). The concrete bonding material has failed in a number of cases, causing the TPs to slip downwards by up to 40 millimetres. Monopile foundations are used on over 75% of operating offshore wind turbines in Europe.
The faults came to light as work was about to start on fabricating the foundations for the 317MW Sheringham Shoal project off Norfolk, says project director Rune Ronvik from Scira Offshore Energy, a 50-50 joint venture between Norwegian companies Statoil and Statkraft.
An internal task force set up by Scira proposed brackets inside the TP. Brackets are normally only used to ensure the TP is in a true vertical position while it is grouted into place, but will become a permanent feature.
They will be enlarged, strengthened and welded into place to be able to carry the entire vertical load of the turbine tower. But to prevent fatigue, the TP will rest on an elastomer - a plastic compound which cushions the load during operation.
Ronvik accepts that there are some interesting longer-term design solutions for the problem being proposed by the industry, but explains that Scira did not have time to wait for them to be developed. "We only had six weeks to find a solution, otherwise it would have had significant consequences for our construction programme," he says.
The only part of the programme which was delayed was foundation installation, though construction has now started.
Scira intends to make the fix available to the offshore industry. "Co-operation between developers is to the benefit of the industry. We have much more to gain by co-operating than by competing," Ronvik said at the RenewableUK offshore conference in Liverpool at the end of June.
Meanwhile, a coning design for the transition piece and top of the monopile is emerging as the favoured option for future wind farms. A joint industry project led by standards agency Det Norske Veritas (DNV) is looking for a longer-term solution to the slippage problem.
The preferred design is for the top of the monopile to be tapered slightly and for a corresponding coning of the TP, allowing it to fit over the top. The grouting would then be in compression, rather than subject to shear forces as on current designs. The work of the DNV-led project will eventually lead to an updated industry standard for foundations.