Monitoring critical cable infrastructure to improve offshore efficiencies

Published:  13 January, 2021

Dan Danskin, industry sector manager – Infrastructure at Fotech, a bp Launchpad company, looks at the challenges associated with monitoring subsea power cables and how Fotech’s novel distributed acoustic sensing (DAS) technology enables offshore wind farms to better manage their networks to achieve improved operational efficiencies.

The wind energy market is growing and showing no signs of slowing as governments worldwide seek to increase their capacity for renewable energy. According to the World Wind Energy Association, overall capacity of all wind turbines globally in 2019 was 650.8 Gigawatts – covering more than 6% of global energy demand .

An increasing number of wind farms are also being developed offshore where the wind is considered to be steadier and stronger than onshore. However, these remote offshore locations have substantial construction and maintenance challenges, and their associated costs are considerably higher than onshore sites.

As such, there is increasing demand on operators to significantly enhance the robustness of the supporting infrastructure for their wind farms. In particular, there is a pressing need to optimise the maintenance of the crucial cable networks that control offshore wind farms and transmit power back to shore.

Cabling makes up approximately 9% of the overall cost of an offshore wind farm, but subsea power cable failures are frequently reported issues, accounting for a massive 75%-80% of the total cost of offshore wind insurance claims . Investment in this area will go a long way towards improving operational efficiencies and associated cost savings.

Threats to subsea cables

Cable networks are vulnerable and expensive to repair when something goes wrong. Since the locations are remote, often miles offshore, accessing subsea power cables, and fault finding, is very difficult and time consuming.

The threats to these undersea cables are varied and damage may occur for a number of reasons including strikes from beam/wing trawlers, boat anchors, natural seabed erosion and electrical faults.

Damage caused by fishing trawlers and their nets pose a significant risk in water depths of less than 200m and anchor strikes are problematic in shallow waters where cables come to shore.

Burying cables can help protect them from damage, but seafloor currents can erode the protective seabed layer. This loss of the burial layer and subsequent cable exposure can cause cables to drift or to vibrate, potentially resulting in breakage from strain.

Cables can even breakdown through electrical flashover and arcing, which can happen if the cable insulation fails. These types of internal faults are particularly difficult for operators to find as they are not always obvious.

Delivering real-time information

Fault finding in cables using traditional route surveying methods is expensive, time-intensive and does not provide real-time monitoring. However, Fotech’s distributed acoustic sensing (DAS) technology enables much faster fault detection and repair, as well as opportunities for proactive maintenance programmes.

DAS works by sending thousands of pulses of light along fibre optic cables every second and monitoring the pattern of light reflected back – a process known as Rayleigh backscatter. The fibre optic cable picks up any acoustic or vibrational energy, which changes the light pattern that is reflected back to the hardware, Helios DAS, indicating a disturbance of some kind. Thanks to advanced algorithms and processing techniques, Helios DAS analyses these changes to identify and to categorise the disturbance event. Each type of disturbance has its own signature, and the technology can tell an operator in real-time, what happened, exactly where it happened and when it happened. 

While it is impossible to prevent underwater currents and the damage they can cause, thanks to DAS, operators can monitor cables more accurately, quantify the risk and respond decisively, therefore minimising downtime and repair costs.

Finding faults fast

DAS is proven in the field to identify subsea cable faults fast. In one application, the Helios DAS solution has been used to determine the precise location of a fault along an offshore export cable.

In this case, the Helios DAS technology was connected to an optical fibre integrated inside the three-core subsea power cable, converting it into a highly sensitive acoustic and vibration sensor. A series of 3kV ‘flashover’ tests were carried out in seven and 10 second intervals to locate the fault on the cable. Even though the response from the flashover tests were very weak, the Helios DAS clearly identified each response and pinpointed the fault to a spot 43,044.4m down the optical fibre.

An anchor chain was then dropped to the seabed, offset from the cable but near the fault. The vibrations were picked up by DAS and correlated with GPS. Because of the precise nature of this data, divers were able to target the general fault location and to start "tapping" the cable to generate an acoustic signal.

This signal further refined the location of the fault and, as a result, engineers only had to remove 3.5m of cable. Since Helios DAS was able to identify the exact location of the problem, the cable could be repaired with a single splice and without additional cable.

The technology’s speed advantage compared to conventional fault finding meant the offshore company was able to rectify the issue promptly. In this case the weather deteriorated immediately after the cable was repaired. If traditional repair methods had been used, work on the cable would have needed to be postponed for a month, costing the company millions of pounds in lost productivity.

Easily assess the integrity of subsea cables

As offshore wind becomes an increasingly established source of power for national grids, and investment continues to grow, operators need smarter, faster ways to manage and to reduce the risks on the vital infrastructure and critical cable networks. They need to be able to detect, to locate and to deal with failures immediately.

Subsea cable failures are a very real and significant problem for operators. New monitoring technologies such as DAS are transforming the way operators can assess the integrity of their offshore subsea cables. DAS is enabling offshore wind farms to better manage and monitor their cables more accurately, which ultimately means they can improve efficiencies across their sites.

For more information visit www.fotech.com

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