Finding Subsea Cable Faults Quicker with DAS

All over the world, access to online services during the COVID-19 pandemic has been a saving grace for families and businesses — enabling home-based working, e-learning, communication and e-commerce to thrive.

So imagine the chaos caused when the two main submarine cables carrying internet traffic into and out of Vietnam, Cambodia and Lao PDR — the Asia-Pacific Gateway (APG) and Asia America Gateway (AAG) — suffered a catalogue of problems during April, May and June.

For weeks, internet users in these countries experienced an erratic service with email, messaging, and video conferencing , mobile apps, social media sites and websites all experiencing malfunctions and connectivity issues.

Months of problems

There are six in-service submarine cables coming out of Vietnam and it is not uncommon for there to be damage to one of them. However, simultaneous damage to the APG and AAG — the two largest capacity cables — caused a huge scramble amongst ISPs to redirect their traffic over remaining routes.

Carrying more than 60 per cent of Vietnam’s international internet traffic, the AAG provides data speeds of 5.2 Tbps, The APG has a bandwidth of 54 Tbps — the highest of any network in Asia.

Problems with the 10,400 kilometre long APG cable began on April 20 with three days of disrupted service, then on May 23 the entire system ground to a halt with a problem that wasn’t fully resolved until June 27.

The APG faults came one week after the 20,000 km AAG cable suffered a breakdown on the S1H section connecting Vietnam with Hong Kong. Initially expected to be repaired by June 2 the AAG cable wasn’t repaired until June 6. When AEC News Today journalist Stella-maris Ewudolu inquired into the delay a VNPT Group source — the group responsible for the cable — apparently told her: “they are still looking for it [the fault]” weeks after the issue first became apparent.

Common subsea cable faults

Using traditional route surveying methods this slow speed of fault detection/location is not at all unusual. Because of their location, often miles offshore and deep on the ocean floor, accessing subsea cables and finding faults is naturally very difficult and time-consuming.

Undersea cables face a particularly hostile environment and varied threats including cables being struck by boats and anchors, loss of burial and the breakdown of cables through flashover and arcing.

In shallower water depths of less than 200 metres (where cables come into shore) cables can be easily snagged by fishing trawlers and nets. Large anchors being dragged through the seabed can also damage cables.

Loss of burial is another key issue. Burying cables protects against many external

threats, but seafloor currents can erode the protective seabed layer. An exposed cable can then drift or vibrate, initially causing damage but potentially causing it to break through strain.

Subsea cables are also at risk from flashover or arcing, which can cause internal burning of the cables. These sorts of internal faults are particularly problematic as they aren’t always obvious to operators.

Subsea cable monitoring with DAS

While it is impossible to prevent underwater currents and the damage they can cause, it is possible to monitor cables more accurately, to enable much faster fault detection and repair, as well as opportunities for more proactive maintenance programmes.

Clearly, traditional subsea cable route surveying is expensive, time intensive, and doesn’t provide continuous real-time monitoring. DAS offers a vastly superior alternative.

By providing a way to monitor offshore and subsea cable networks, faults can be detected remotely in real time. DAS can detect disturbances and pinpoint the exact location of the fault – allowing operators to respond decisively and minimise downtime and repair costs.

A good example of how DAS can help operators identify undersea cable faults is Fotech’s work with an offshore cable operator to detect an issue in its export cable from an offshore windfarm.

In this instance, Fotech’s Helios DAS solution was used to determine the precise location of the fault along the cable. An anchor chain was dropped to the seabed offset from the cable but near the fault. The vibrations were then 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 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 3.5 metres of cable. The cable could be repaired with a single splice and without additional cable because Helios DAS was able to identify the exact location of the problem.

Helios DAS’s other advantage is speed compared to conventional fault finding and repair methods. In this case the weather turned for the worst immediately after the cable was repaired. If traditional repair methods had been used, work on the cable would have had to be postponed for a month, costing the company millions of pounds in lost productivity.

Protecting critical infrastructure

Without monitoring technologies such as DAS, operators have a huge struggle on their hands to assess the integrity of offshore subsea cables.

As we continue to rely on the connectivity provided by subsea cables, solutions like DAS need to become an integral part of operations. Doing so will help to protect these critical infrastructure assets all around the world and help prevent the kind of downtime the people of Vietnam have suffered when they needed it the most.

For more detail on how Fotech’s ability to enhance the accuracy and speed of detecting cable faults please read this case study.