The success of offshore wind auctions in Europe, and recently the U.S., has spurred leaseholders to offer new areas of seabed up for development. But a catch awaits bidders in the next generation of offshore wind leases.

Almost all offshore development has happened in waters up to around 100 feet deep. Areas now under consideration could be up to twice that depth, stretching the limits of what can be achieved with today’s foundation designs. 

In April this year, the Bureau of Ocean Energy Management (BOEM) announced a high-level assessment of all waters off the U.S. Atlantic Coast for potential future offshore wind lease locations. Among its selection criteria was a water depth up to 60 meters, or around 200 feet.

And in November, the Crown Estate, which manages offshore wind leases in the U.K., extended a preselection of areas for the country’s fourth round of auctions to include waters up to 60 meters, a 10-meter (33-foot) increase on previous proposals. 

The leasing body said the increase came “after feedback from industry confirmed appetite for development of fixed foundation turbines beyond the 50 meters initially proposed.” 

But it is still not entirely clear how the offshore wind industry will plumb such depths. The water columns envisioned in upcoming lease areas could be equivalent in height to a 20-story building. 

If the preferred foundation for use in these depths is a monopile, which is currently used in four out of five offshore wind projects, then it may have to go much deeper into the seabed to maintain stability. 

In 2017, a study of 30 monopile-based offshore wind farms conducted by the Polytechnic University of Madrid found that only a couple had water depths of more than 100 feet, and the maximum was only 105. 

However, some of the monopiles used in the plants surveyed were already more than 200 feet long.

The U.K.’s 25-meter (82-foot) deep London Array, for example, was estimated to have foundations buried 60 meters (197 feet) into the seabed, giving its monopiles a total length of 85 meters (almost 280 feet).

At least four other projects had monopiles of 70 meters (230 feet) or longer. And monopile makers are now gearing up to take on depths roughly twice as great as the accepted maximum at the beginning of the decade. 

Companies such as EEW of Germany are pinning their deep-sea hopes on so-called XL monopiles, which are up to 33 feet in diameter.

There is merit in trying to extend these as far as possible, because even though their total weight increases with length, it decreases in proportion to the rated power of the turbine, essentially offering more watts per ton at greater depths. 

At the Borssele I and II Offshore projects, monopile manufacturer Sif has been commissioned to build piles for water depths of up to 38 meters (125 feet), with production due to start next year.

But research in 2015 by the University of Strathclyde in Glasgow concluded that even these massive monopiles would not be cost-effective in water depths of more than 40 meters or 130 feet. 

And many experts believe that at greater depths it will make sense to move to an alternative design, such as jacket foundations.

“You’re going to see a switch because the water depths are increasing,” said Søren Lassen, offshore wind analyst at Wood Mackenzie Power & Renewables. “Jackets are coming into this space.”

This does not mean monopiles are out of the question for depths beyond 130 feet, though. Manufacturers are continuing to optimize the XL concept, said Lassen, potentially extending the range of the technology. 

“They said monopiles would never go up to 30 meters, then 40 meters, now maybe 50 meters,” he said. 

And one thing that favors continued use of monopiles at greater depths is that the offshore wind industry supply chain, in Europe at least, is well set up to deliver the products. 

This, ultimately, could be what keeps monopiles alive even when wind farms move out into 160-foot-plus waters in the latter half of the 2020s. “It’s not a technical challenge,” said Lassen. “It’s more of a cost thing.”