How 'magic-like' new tech could curb offshore wind's worst enemy

A new approach to monitoring subsea power cables taken from the telecoms industry could help end their “appalling” current failure rate which is costing offshore wind owners huge sums of money, claims an industry pioneer behind the technology.

Given this issue accounts for four out of five offshore wind insurance claims, it is no surprise that RenewableUK identified cable failures as a key issue that needs to be resolved in its industrial growth plan for the sector last year.

“It’s ridiculous,” said Minto. “An untenable situation” – but one that he aims to solve through Indeximate, a British start-up that launched in 2022.

Currently, Minto said that the offshore wind industry estimates that there will be a failure once per 400km of subsea cable a year. “Which sounds like a big number,” but not if you consider a gigawatt-scale wind farm can have 200-300km of array cable and several export cables of comparable length sending power back to shore.

“You've got around 10,000km of subsea cable in Northern Europe servicing wind farms,” he said. “So that ends up a lot of failures.”

A statistic Minto said he had seen from one insurer found that 95% of subsea power cables “don’t make it to halfway through their design life, which is shocking. Absolutely shocking.”

How costly are cable failures?

In a word: very. Individual subsea cable failures can cause losses of up to £10m ($13.4m), according to Offshore Renewable Energy Catapult.

Minto said that a failure for an export cable could cost £6m in capex at the low end. But this is not the full story.

“The real heartache,” he said, is that although there are often two or three export cables linking a wind farm to shore, there is not 100% redundancy. If a cable breaks, a wind farm will lose “about 30% of its capacity.”

Broken cables will typically be out of action for months and cables “tend to break when they're under the most duress, which is in the winter, and the winter is when you want to do your generation.”

So the lost generation costs are vastly higher than the cost of the cable repair itself, he said – around £50m an incident.

Even for a broken array cable, which might take a few turbines out of action, this cost £3-5m to repair with around 3-5 times that number in lost revenue, said Minto. Not trifling sums.

Why do cables break?

For subsea data cables, Minto said that the vast majority of failures are caused by human activity in boats. A recent UK government report found that 70–80% of data cable faults arise from fishing activity or vessels dragging anchors.

It is a very different picture for power cables, however. Minto said that studies have shown that for these, 75% of the causes of failure are down to longer-term degradation. “And this is absolutely key.”

The most vulnerable point in the cable is the span between its connection to the top of the turbine monopile and the sea floor, he said. This section of cable should be protected by a cable protection system, but it’s always possible that a contractor will not have done a good job during installation.

Even the protection systems can prove a source of danger for cables, he said, citing instances where they have found to be “degrading” cables.

ORE Catapult figures from 2021 say that subsea cables are most commonly caused by issues during installation, at 46% of the total. After that, manufacturing, faults in cable design and external damage are the other causes.

No use to say a cable is ‘about to fail’ – a year’s warning is needed

Minto co-founded Indeximate with Alastair Godfrey and Paul Clarkson, with the trio having previously worked together in senior roles at OptaSense, a company that uses acoustic signals from fibre optic cables to provide real-time intelligence.

This works by installing some hardware on the end of any given cable that Minto said “magically turns the fibre” into series of sensors.

For fibre optic cables, Minto said this innovation was used to monitor everything from road traffic to rockfall onto railway tracks. “It’s become a very successful industry.”

The key innovation of Indeximate, said Minto, has been applying this technology to subsea power cables. Using this same technology, the company can detect where a cable failure has occurred to within a distance of ten metres. “We were the first people to do that,” he claims.

That led to a six-month deployment of Indeximate’s technology at RWE’s 385MW Arkona offshore wind farm in the German Baltic Sea.

In the offshore wind industry, Minto explained Indeximate is less interested in detecting where cable failures are and more in detecting where they are going to be.

“It's no use if we put out an alert that says your cable's about to fail, quick, call the boats,” he said, if it takes “six months” to get one. “We need to be giving a year’s worth of warning of deteriorating conditions.”

To achieve this, Minto said the cables need to be monitored permanently. This is because to understand the petabytes of data that the Indeximate system generates when monitoring cables, it is crucial to establish a long-term baseline for how the cable performs so issues can be detected when they begin to arise.

With such vast quantities of data being generated, Minto said it is also key to “distil this deeply technical information down to something you can put in the client's hands” – something that tells where, when and why a cable failure may occur.

The Indeximate system acts as an early warning system by monitoring what could be described as the seven deadly sins of cable health: Strain, fatigue, abrasion, exposure, vibration, armour integrity and cable skipping.

‘Shift in mindset necessary’

Persuading asset owners to begin monitoring the health of their cables while they are well inside their warranty periods may require a “mindset shift” for the offshore wind industry, said Minto.

But with the industry set to multiply in size, he argues it needs to get on the front foot regarding cable failures.

The current dominant mechanism for monitoring cable health is conducting a burial risk assessment and follow that up with a remotely operated vehicle check every five years, he said.

Indeximate, by contrast, offers a “purely opex based subscription model. There's no capex. The only piece of hardware we have in the field is a computer to pick up this data, compress it, and push it on.”

Indeximate’s first major client – somewhat ironically for a company targeting the offshore wind sector – is the developer of a tidal power project in Scotland.

For that project, Indeximate has helped identify areas where the cables have become dislodged, allowing the operator to put rock bags down to stabilise them.

‘Treasure trove’ of data brings other benefits

Turning power cables into subsea sensors could bring benefits to the offshore wind industry far beyond the health of the cables themselves, said Minto.

“It's a microphone on the seabed,” he said. That means it can listen not just to what’s going on in the cable, but around it, such as the height of waves to “millimetre accuracy,” important when deciding whether to send out a service vessel to a wind farm.

And because the cable stretches all the way up into the turbine, “we can listen to what the turbine is doing,” he said.

“We see all the fundamental resonances of the turbine,” he said, including turbine “nodding” – referring to the movement of the tower in the direction of the wind – and the blade passing frequency.

Minto describes it as a “treasure trove” of data that can be used to produce more measurements and more products to support the industry.

Monitoring vessel noise around offshore wind farms is another use case, said Minto.

“Compared to the standard approach of taking a piece of kit out on a boat and some moorings, putting it down into the hostile sea, coming back and changing batteries and pooling data, all of that is really going to be something of the past.”