Why Subsea Power Cables Fail

Submarine power cables are a critical asset in the renewable power sector. Yet it's a costly maintenance affair, with its almost inevitable faults. The average cost of repairing a sub-sea cable is around ￡2-5 million per km and has accounted for 77% of the total financial losses in global offshore wind projects. 

Subsea cables success story

Studies project offshore wind farms to grow annually at 15%, with global demand for power cables estimated at 24,103km by 2025.

And the scramble is on for finding, manufacturing and maintaining quality subsea power cables. 

The upsurge in the renewable energy sector's dependence has signalled the onset of a wide variety of subsea power cables too. Although they differ in terms of composition, use and width, the basic function remains the same - conveying electric power. 

Regarding the two types of power cables - the AC or High Voltage Alternating Current (HVAC) Cables and the High Voltage Direct Current (HVDC) Cables, reports suggest the latter is less prone to failures. We had in fact discussed at length about Edge of HVDC over HVAC and other such crucial points in our previous blog in case you want to delve a little deeper.

Why Subsea Power Cables Fail

The common reason cited behind subsea power cable failures is fishing activities.

Trawling and anchors dropped on cables have caused a lot of destruction with a study on Trans-Pacific submarine cables attributing 67% to 72% of failures.

Since subsea power cables are in the saline environment of the sea, their longevity depends on the structural and material integrity of the cables. 47.5% of subsea cable failures were traced to environmental factors (e.g. seabed roughness and tidal flows) leading to corrosion and abrasion of the cables. (The study was conducted by Scottish and Southern Energy plc – over a 15 years period of time, between 1991 and 2006). These failures were due to armour failure, armour abrasion and sheath failure

A large percentage of cable faults point to manufacturing errors. According to a DNV- GL study, 50% of all known types of failures for underground cables are at the production stage including, but not limited to electrical and thermal faults. This can be expected to be similar for submarine cables as well especially if the cables show faults at the initial stage of installation.

The UK’s offshore wind sector power, for instance, had recorded a loss of 1,160GWh due to electrical faults.

Experts and researchers are also eluded by the rising number of instances where the causes of failure are unknown. A recent study by The Institution of Engineering and Technology, UK talks about 33% of faults reported caused by reasons unknown.

How to reduce Faults

Prospero's continued work with energy experts has helped us narrow in on some fault clearing strategies for making your HVDC reliable. To summarize, the way to reduce faults is a three-pronged approach:-

• Early monitoring and reporting
• Increasing Cable Length
• Prompt Maintenance

Monitoring and Early Reporting

?Like many industries, there is inadequate data to help make the right decisions. Previous analysis shows that the present monitoring technologies aren’t able to detect 70% of the failure modes in subsea power cables. There are companies such as ORE Catapult who are at the helm of changing that by investing heavily in research and finding novel ways to reduce subsea power cable failures.

Increasing Cable Lengths

Joints for subsea cables can’t be avoided. By increasing the length of cables and reducing the number of joints wind farm operators can cut costs and minimize errors. The Nemo Link interconnector which runs 140 km through the seabed between the UK and Belgium has displayed a 96% utilisation rate in its maiden year. They have employed an “industry-first” - cross-linked polyethylene (XLPE) technology - to thrive in the subsea environment. Bart Goethals, CCO, NemoLink, (Belgium) will be part of our “Subsea Power Transmission & Cabling 2021” to share more insights on this.

Prompt Repairs

The Western Link HVDC was down due to faults for 18 days costing the National Grid ￡31 million in order to balance mechanism payments. To avoid such costly downtimes, P&Us are ready to invest in technology and companies that could fast track repair. DeepOcean and Osbit are some such companies that are specializing in inspection, maintenance and repair.

Learn from the best

If you are responsible for subsea power transmission or cabling and looking to benchmark your practices with those of the industry leaders, we have just the right platform for you!

We at Prospero Events are bringing together the heads of NemoLink, National Grid, Brit Ned among other leaders in the industry for an exclusive closed-group and business-friendly virtual conference “Subsea Power Transmission & Cabling 2021”, on September 16 & 17.