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Securing subsea cables

Published by , Editorial Assistant
Energy Global,


After years spent proving it can be a commercially and technically viable cog in the world’s energy mix, floating offshore wind is set for a crucial role in the energy transition. It is a sector with almost unparalleled potential, allowing developers to place turbines in deeper waters where the winds are stronger and more reliable. To put the size of the prize into perspective, it is estimated that floating turbines will unlock as much as 80% of the world’s offshore wind resource.

In DNV’s latest Energy Transition Outlook, the company forecast that floating turbines will contribute 5% of total wind generation by 2050, a meteoric rise given its current microscopic market share. As it stands, the levelized cost of energy (LCOE) for floating offshore wind is more than three times that of fixed offshore wind. But despite a recent deceleration in the closure of that gap, DNV remains optimistic – forecasts predict that floating offshore wind will cost only twice the amount of fixed bottom by 2032. By the middle of the 21st Century, economies of scale and improved efficiencies in production mean that disparity will drop to about 30%.

Europe is shaping up to be the global leader in the floating offshore wind market, building on the region’s fixed bottom pedigree to deliver mammoth developments in the North Sea and beyond. But as much as the sector should leverage the successes of fixed bottom wind, floating wind developers must also pay close attention to the challenges that have arisen, many of which they are also likely to encounter.

A familiar technology presenting new problems

One piece of this colossal offshore wind puzzle that is yet to receive the attention it deserves are subsea power cables and the unique challenges they present. Paradoxically, this is something that the industry is well versed in – companies have been laying power cables for decades – but this familiarity means that it has been seen as proven technology. As such, it is often not given the focus it deserves. The scale of offshore wind connections, the various cable entries at any one foundation, and the interface design also means that previously unknown issues have reared their heads. It is a problem that came to the fore in 2021, when Ørsted revealed it had been hit by a spate of cable issues at several of its offshore wind farms, incurring significant financial costs.

Research from Lloyd Warwick, an international specialist in loss adjustment and claim management, has found that power cable failures are to blame for around 83% of all offshore wind related financial losses and insurance claims. A fact which has remained constant since at least the early 2010s, if not earlier. Moreover, with an average interruption period of between 40 – 60 days for repairs to be carried out, the average combined losses (repair and loss of generation) ranges between US$1.2 million (£941 000) – US$12 million (£9.4 million) for array cables and US$10 million (£7.8 million) – US$30 million (£23.5 million) for export cables.

Gazing into the floating wind crystal ball

Owing to the fact that floating wind is still in its infancy, these statistics relate to fixed bottom projects, but they provide a warning as to what can happen if adequate attention is not given. Projects want to procure cables and they must secure manufacturing timelines, so a lot of work is initiated without going through all the necessary steps. For example, cross-sectional cable designs are selected with very little insight into the specific site conditions, the actual cable route, or the interfaces to the foundation or other third parties. After the fact, everyone then has to play catch-up to fill in the gaps, and this is where difficulties arise. When it comes to site specific applications for the cable protection systems (CPS), which shield the unburied portion of the cable where it enters the turbine and is exposed to currents and waves, companies often find failure. There is a disconnect between cable original equipment manufacturers (OEMs), installation contractors, CPS suppliers, and the actual projects. They are all working on their own respective areas, but nobody is looking at the full picture and how it all fits together. Based on DNV’s experience of certifying subsea cables in these markets, it has found that many of the publicly known subsea cable failures in recent years are the result of poor interface design management together with insufficient design for the actual site conditions.

Fortunately for floating wind – where the transition from the seabed to the floater will be significant, reaching hundreds of metres in some cases – this issue is already at the forefront of people’s minds. This might mean stakeholders are pushed to engage earlier and are more aligned on cable integrity, but, with so many interested parties, it is going to be a test.

To date, demonstration and commercial floating wind projects that DNV has been involved in, this challenge has been thrown into sharp focus and companies are having to go above and beyond to prove the safety of their cables. This has typically meant much more rigorous and lengthy design efforts, but it has also required more onerous type and prequalification testing, as well as more onerous O&M strategies.

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Dive into the latest renewable energy insights in the Autumn issue of Energy Global, out now! The issue starts with an insightful guest comment from Cristiano Spillati, Managing Director at Limes Renewable Energy where he discusses the need for European renewable energy suppliers to accelerate the rate of the energy transition. This is followed by a regional report from Cornwall Insights on the battery energy storage industry in Australia. This issue explores key topics including offshore wind subsea cables, offshore wind support vessels, digitalisation, wind turbine components, and more!

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