How a subsea cable is repaired (2021)

It’s Fun Fact Friday and today we’re going to talk about repairing submarine cables. Submarine data cables are located under the ocean, and connect people around the world with data and information, while submarine electrical cables transfer electricity, such as those used on wind farms. Undersea cables for data transmission have been around since the mid-1800s. Today, we’re going to look at how underwater cables can break, how they are repaired, protection methods, and the differences in the two types of cables.

Differences in the two types of cables

Data cables are made up of a small number of fiber optic strands of glass about the width of a human hair. Most of the cable is actually protection for these strands, as well as conductive material to power the cables. The size of the final product varies depending on the amount of protection it needs: larger, more protected cables are usually near the shore in shallow water, and thinner cables are buried deep under the ocean. Repeaters are included every 40-80 km to keep the signal strong.

Submarine power cables vary from about 70mm to over 210mm and can be either AC or DC. According to European Subsea Cables Association, the selection criteria for which type of cable to use is heavily dependent on the route length, voltage, transmission capacity and Grid synchronisation. AC cables are usually used for 80km and shorter, while longer distances require DC cables and higher voltages.

Why do underwater cables need repair?

Damage can be caused by failure in a cable, but this is very uncommon. Earthquakes can damage cables, like the one in 2006 off the coast of Taiwan that caused a loss in international phone and internet data service. Sharks sometimes chew on the cables, but this is also rare and they usually don’t do much damage. The most common failure is boats; trawling by fishing boats and ships dropping anchor can snag and easily damage the cables, which aren’t buried very deeply in the sea bed.

How are underwater cables repaired?

Some of these cables span the ocean to connect continents, and can sometimes be located farther below the surface of the ocean than Mount Everest is tall, making repairs time consuming.

The first step in repairing an underwater cable is figuring out where the damage is located. Data cables are fiber optic, so a light pulse is sent through the cable. Broken fibers bounce the light pulse back, and engineers can measure the time it takes the pulse to return to find the specific location of the break. Undersea power cables use a Remotely Operated Vehicle (ROV) with fault detection instruments to find the damage.

Once the break is located, a cable ship is dispatched. To stay on station during repairs, these vessels are fitted with dynamic positioning systems. These systems keep vessels in a specific location during winds, waves, and even storms.

If the damaged cable is in shallow water, an ROV equipped with cameras and robotic claws is deployed to retrieve the cable. In deep water, the ship has to use a grapnel, which grabs and cuts the cable, dragging the two loose ends to the surface. After the cable is retrieved and on board, engineers repair the cable in a specialized room. Data cables can take up to 16 hours to repair, after which they are lowered back down to the sea bed in an omega or hairpin pattern to accommodate the extra length.

How do we protect undersea cables?

Cables can be protected from shark bites with metal sheathing. For fishing vessels, maps are freely provided to fishermen in the hopes that they do not fish over such cables. Today, the fastest undersea cables can transfer data at speeds upward of 25 terabytes per second. These cables transmit about 95% of voice and data traffic for civilians and governments, making their protection a high priority for global infrastructure.