As offshore wind turbines get heavier there are now more questions than ever about what could take the monopile’s place, writes Stevie Knight.
First of all, Siemen’s 3.6MW nacelles and hubs weighed in at just 225t. But jump to the 12MW Haliade-X and the head reaches a massive 825 tonnes.
What’s interesting is that the technology beneath is very site specific “and given shallow waters with good soil you could still see bigger turbines on 8m or 10m diameter monopiles”, said Karl Davis of Empire Engineering. However, as Henning Carlsen of DNV GL pointed out, “if you are to put 12MW monopiles in deeper waters, it could, depending on the site, require huge penetration into the seabed... I’d expect these projects could meet some tough issues”.
Further, there’s signs the industry is about to begin looking at 14MW turbines – and it will likely move on from there. “Some are now saying that we should be mentally prepared for 20MW capacity turbines,” said Carlsen’s colleague Arnstein Eknes.
Therefore, given the mixed pressures of deeper water, less than ideal soils and heavier turbines, other foundations may simply “prove more cost effective”, said Carlsen. Certainly jackets, suction buckets, gravity-based or hybrid varieties are already established alternatives, and some offer time advantages: Vattenfall’s triple suction-bucket bases for Scottish 8.4/8.8MW turbines were each installed inside a couple of hours.
Other ideas are being explored: float-and-submerge, 7,500t gravity foundations were used at Blyth by BAM, while the Elican project towed out a concrete, telescopic installation, again ballasting down onto the seabed. It doesn’t always work out: snags have delayed the mono-buckets destined for the Deutsche Bucht, and BAM has exited the foundations business altogether.
However, according to Eknes, “the weight of these submerged structures is increasing significantly”; for example, Vattenfall’s suction bucket support structures weighed in at 1,800t each. Therefore, deployment of the new 12MW to 14MW varieties further offshore may well entail foundations over 2,000t. But, according to Jan de Nul, current installation vessels are already experiencing difficulties handling the new components.
It’s also worth noting that floating foundations won’t provide a solution to the challenge, at least when there’s an alternative. Despite media interest, Davis is sceptical about the comparative cost – even given deployment reductions associated with towing out a prefabricated structure: “The engineering is far, far more demanding, and this is not readily levelled out by economies of scale,” he explained. As for viability in deeper water, jackets still win: “They are already proven at 150m by the oil and gas industry... and may still be competitive against floating solutions at 80m or 90m depths.”
While lift innovations such as fibre ropes “could dramatically reduce the weight of the total system” said Eknes, he pointed out the second challenge “is how high do you need to reach?” These 12MW turbines will likely have nacelles 130m or 140m from the sea surface.
Could this provide an entry point for completely different approaches? Dynamic positioning vessels are already answering some of the challenges: Parkwind’s Arcadis Ost 1 farm will see the 9.5MW turbines assembled onboard, saving time and avoiding problems on the seabed because the vessel will mount both tower and then completed nacelle, all without putting its feet down.
Likewise, last year a slip-joint connection allowed the fully assembled Delft Offshore Turbine (DOT) to be lifted, complete, onto a monopile by Hereema’s heavy-lift Aegir under DP. Possibly in anticipation of the trend, Jan De Nul’s latest order, Les Alizés, is also a DP vessel with a lifting capacity of 5,000t.
But the scale calls into question the established economics, which have to be considered against the volatile nature of the market: “To be honest, in the past the wind industry has been able to turn over the risks of creating new solutions to the supply chain,” says Eknes: “But costs are going up and up with the size, so I think it could force a discussion about collaboration with windfarm developers.”
It may result in some consolidation among contractors, but it could also open up the design horizon, as well as giving this cyclic industry a little more certainty.