How tsunami-proof are offshore wind turbines?

After a tsunami swept the Pacific Ocean today following an earthquake off Russia, we asked experts: How resistant are offshore wind turbines to such events? Answer: 'It depends.'

With a provisional magnitude of 8.8 on the Richter scale, the earthquake that struck near Russia’s Kamchatka Peninsula earlier today is one of the most powerful on record, although fortunately there do not appear to be any reported casualties as yet.

Countries all around the Pacific have been put on high alert, with millions told to evacuate in Japan and warnings in place in countries as diverse as the US, Colombia, China and New Zealand.

These countries and many other Pacific neighbours affected boast offshore wind industries in various stages of development, meaning there will likely be more offshore wind farms exposed to such events in the future.

Resilient, but not equally so

So how resilient are current offshore wind turbines to such events?

The good news, according to Suby Bhattacharya, a founder of UK renewables consultancy Renew-Risk who has conducted extensive research into the impact such events can have on offshore wind turbines, is that they are very resistant.

These latter foundation types are “more vulnerable,” he said, simply because they rely on one singular structure to keep the turbine in place. In other words, you have “all the eggs in one basket.”

Compare that to a jacket foundation that has three or four legs where the load is distributed.

He put it another way: Imagine you need to reach something high up in the kitchen cabinet. What would you rather stand on, a three-legged stool or a chair?

How do fixed and floating turbines compare?

Floating offshore wind turbines are of course another kettle of fish entirely, how would they hold up against a tsunami?

Fixed-bottom wind turbines are forced to withstand whatever load the tsunami acts on them and absorb this through their foundations, said Bhattacharya. Floating systems by contrast “don’t resist” against a tsunami or a wave and are therefore “less vulnerable.”

He compared a floating wind turbine to a ship that is hit by a tsunami. It might be displaced by five, ten metres but will remain perfectly intact. “You can think of a floating turbine as a turbine you're putting on a ship.”

Sunjung Kim, South Korea country manager at consultancy OWC, agreed that fixed bottom turbines are “more exposed to tsunami induced horizontal forces” given they are locked to the sea floor.

“Floating turbines with mooring systems may be better than fixed bottom by their ability to move with water to reduce stress on the structure – mooring integrity and dynamic stability will be critical factors.”

‘Turbines built for one in a hundred year waves’

Another important factor to consider is where the wind turbines are relative to the shore, said Bhattacharya. The force of a tsunami becomes greater nearer the shore, as water is forced upwards. Wind farms farther out to sea will meanwhile be less affected.

Because there will generally be some warning before a tsunami hits any given area, Bhattacharya noted that this also gives the operators of any turbines in its path a chance to put the machines on shutdown mode.

This is because a turbine with its blades spinning will already be experiencing strain on its structure.

A turbine is however built with “reserve capacity” when it comes to withstanding forces on it and with a “one in hundred year wave” in mind, he said. “We don't expect it to collapse.”

Tsunamis not specifically covered in current IEC standards

Perhaps because they have not been something that the industry has had to think about too much to-date, tsunami resilience is not spefically covered in International Electrotechnical Commission (IEC) standards for offshore wind turbines.

Kim at OWC said that design codes like the IEC 61400-3 cover extreme weather conditions but tsunamis are “not explicitly considered” because of their rarity.

However, after the Tohoku Tsunami that devastated Japan’s eastern coast in 2011, she said “it is true that more attention has been paid to near shore infrastructure resilience.”

Research is now progressing on Tsunami modelling for near shore and shallow water structures, she said, as the industry moves towards more “comprehensive hazard modelling.”

Peter Brun, Global Segment Leader Offshore Wind, Energy Systems at DNV, highlighted that the Norwegian classification society back in 2021 issued a new recommended practice for seismic design for wind farms, along with a technical note to address wind farm design in tropical cyclone areas.

“This was deemed necessary after broad dialogue with leading industry actors as offshore wind power technology was starting to be installed in new more demanding site conditions outside Europe.”

This recommended practice and technical note do not however cover specific conditions for tsunamis, he said.

“Offshore wind designs for wave heights up to 15 meters are already covered in other offshore wind standards, and has thus so far been deemed to cover the risk of tsunamis.”

What examples are there of offshore wind farms being hit by tsunamis?

Possibly as few as one – and there are caveats.

That was at least the assessment of Bhattacharya and two co-researchers in a 2022 paper on the design of monopiles of offshore and nearshore wind turbines in “seismically liquefiable soils” – those that can become liquid in the right conditions, such as a subsea earthquake.

They investigated how the Kamisu semi-offshore wind farm – a site boasting a handful of small turbines located just 50 metres from the shoreline in Japan – withstood the aforementioned Tohoku earthquake.

Bhattacharya and his colleagues concluded that despite “strong shaking and soil liquefaction,” the Hitachi turbines in use performed well. Just a few days after the incident, the turbines were restarted.

The waves that struck the turbines were around 5 metres high – so certainly tall, but not apocalyptic, especially by the standards of modern-day machines that tower above sea level.

Studying that incident allowed Bhattacharya and his colleagues to develop a numerical model for such events, he said. Now that model is calibrated, he said they can use it to "extrapolate" to how larger turbines would perform in such events.