Warning that increasingly large offshore wind turbines risk getting too big for their foundations

Engineers currently rely on design codes borrowed from the oil and gas industry to determine the geometry and material properties of foundations.

However platforms designed for the oil and gas industry are very different structures to offshore wind turbines.

“The only thing they have in common with offshore wind turbines is that they are offshore structures but aside from that they are apples and elephants in terms of the types of structure they are,” Prendergast explained.

“The biggest concern for offshore wind turbines is lateral loading and overturning moments - so the forces that try and topple the structure over - as opposed to the vertical deadweight forces that need to be designed against for oil and gas platforms.”

Monopile foundations, which are single, large steel tubes, are the most common foundation for turbines offshore.

“In the original designs monopiles were not so dissimilar to the piles used for oil and gas in terms of the slenderness but now they’re fundamentally totally different structures,” Prendergast said.

Oil and gas piles are long and have low diameters, making them quite flexible, while monopiles are stockier and more rigid. They have larger diameters relative to their length which means they’re more difficult to bend.

“So rather than bending or flexing, monopiles tend to rigidly rotate in the ground with some bending,” Prendergast explained.

“That means the way they mobilise resistance from the soil is different to how the flexible piles do it. So the models that are being used to simulate how flexible piles deform in the ground don’t really capture how rigid piles deform in the ground but they are being applied for the purpose of design.

“The issue is that you end up with potentially incorrect estimates of your movements which means your system is poorly designed to withstand the environment it’s being placed into. That has ramifications for how long these structures can be in service for.”

There has been a push towards offshore wind as a key form of renewable energy, with the government's Net Zero Strategy targeting 1GW of floating offshore wind by 2030.

As such, 10 years ago the average offshore wind turbine could generate 5Mw of energy, four years ago it was 8Mw and now a 15Mw turbine is coming online. As wind turbines have increased in size, their foundations have also increased – which provides an additional challenge in itself.

Typically monopiles had diameters of between 4m to 6m but they are now being fabricated with diameters approaching 10m, and this means that the behaviour of these systems is very different to what the design codes predict.

Prendergast explained: “As these structures get larger they actually mobilise a different form of resistance from the ground. They physically become a different type of system. The models themselves are not accounting for any of this additional resistance that mobilises in the ground.”

In addition to this, many sites near the shore have already been developed so new installations are taking place further from the coast, in more uncertain geological conditions, increasing the technical challenges and uncertainty in behaviour.

This presents a few issues. Firstly, the soil conditions are unknown so advanced testing is needed.

“That testing becomes more problematic in deep waters – it’s physically more challenging to do so the certainty around the data you’re extracting becomes more questionable,” Prendergast says. “There’s a general propagation of uncertainty with moving into these new realms.”

The Pile Soil Analysis (PISA) project developed new design code for monopiles based on North Sea sand and clay conditions, but now windfarms are being built off the US and Taiwan coasts which have different ground conditions again.

Prendergast added: "With PISA, the new code for monopiles in its current form only looks at monotonic loading where you push the structures laterally so it doesn’t do anything in terms of how does the structure react to wave loads repeatedly cycling against it or small movements leading to vibrations in the structures.”

Offshore wind turbines are designed to last up to 30 to 35 years at sea, but there is currently no proof of how large turbines will react to their environment (wind and waves).

“We don’t have any long term examples of this,” Prendergast said. “This is a problem that hasn’t occurred yet. We’re building these now but we don’t know if they’re going to last the 30 or 35 years they’re intended for.

“We could have significant maintenance issues in 10 or 15 years, we could have to decommission windfarms that are not performing, we could get all sorts of damage to the structures themselves that just can’t tolerate the excess movements that might occur based on these poor designs.”

Overall, Prendergast describes the significant increase in turbine technology, the need for larger structures and bigger foundations, along with uncertain ground conditions in deeper waters as “a perfect storm”.

He added: “This is a potential major red flag that needs to be addressed because we’re pretty much putting all our eggs in the offshore wind basket in terms of renewable energy targets and there is a chance we could be decommissioning a lot of these systems before their intended design life if they don’t behave the way we want.”

Like what you've read? To receive New Civil Engineer's daily and weekly newsletters click here.