In the rush to cut the levelised cost of energy (LCOE) in offshore wind, some developers may have gone too far, buying steel foundations that are too weak and are unsuitable for the job required, according to renewables insurer GCube Underwriting Ltd.

"We are seeing more foundation claims, and cost-cutting is one possible cause of loss we’ve identified," says GCube.

“When we started covering offshore wind ten or so years ago, they all told us that the maximum water depth that your put a monopile in is 30 metres,” explains GCube underwriter Robert Bates. “A lot of the good shallow-water sites have now gone, so developers are now having to go further offshore and into deeper water — but they are still using monopile foundations for those locations.

“So one theory [for the increase in foundation-related claims] is that they're using monopile foundations when they should've really been using jackets or possibly something else. Another theory is that in the general push for reduced LCOE, they are cutting corners on the fabrication process. So we have seen thinner steel being used on the monopile, so it's more susceptible to failure.”

This has led to bending, denting and “loss of circularity”, Bates tells Recharge. And the cost of repairs can run into millions of euros.

Bates describes one recent insurance claim from an offshore project — not in deep water — where the tips of two monopiles were bent out of shape by the hydraulic hammer being used to drive them into the seabed.

“We thought, ‘well, it’s the just the tip [ie, the top flange] of the monopile that’s damaged, why don’t you just cut it off and weld on a new one?’ And they [the developer] said ‘well, because it's in the North Sea, it's really really cold water all the time so you can never get the metal hot enough to melt it to cut off the tip’.

“So what they had to do was pour concrete into there to create a plug to try to stabilise the foundation so it would be there for 25 years. So it had to be certified by the certification body and all that kind of stuff. So it had to be belt and braces [to ensure that] this is definitely going to last for 25 years. It was incredibly expensive — multi-millions of euros.

“The big cost comes from the vessel," Bates explains. "So there are only a certain number of vessels that can do this work. And they're normally contracted all year round on other projects. So if you have a damage event and you need to get another vessel on site, that vessel owner can charge whatever they like. If it's one of the big foundation or heavy-lift vessels, they can charge £300,000 [$363,600; €328,000] a day, maybe more. So that's where the cost is with offshore wind, probably about two thirds of any offshore wind claim will be in vessel costs."

The developers may have been better off just buying more expensive foundations in the first place, says Bates.

“Our preferred foundation type now for the deeper water is definitely a jacket foundation.”

He continues: “I think a lot of things that go wrong in offshore wind have been down to not really taking a lot of the lessons that the oil & gas industry has learned. A lot of people in the renewable industry don't want to work with oil & gas industry people. So there hasn't been that transfer of knowledge. In the oil & gas industry they've been using jacket foundations for their big structures in deeper water for decades.”

This article has been updated to remove a reference to what has proved to be a misleading figure about the proportion of insurance claims relating to offshore turbine foundations, and another reference to monopiles produced outside Europe has also been removed due to a lack of supporting evidence.