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  • Writer's pictureSarah Whiteford

How is subsea cable repaired?

Updated: Apr 26, 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, and you can learn more about the history of data cables and how they send information all around the globe in our article How do undersea data cables work? 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.

“NKT Victoria – one of the world’s most advanced cable laying vessels”. Image from

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 is usually used for 80km and shorter, while longer distances require DC cables and higher voltages. AC cables are 3 core in a bundle or laid as separate cables. Depending on the system, DC cables are mono-polar or bi-polar with the two conductors laid separately or as one.

“3 Phase AC Submarine Power Cable”. Image from European Subsea Cables Association.

“Co-Axial HVDC Submarine Power Cable”. Image from European Subsea Cables Association.

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, as seen in the article above, 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 that can be thousands of miles long is figuring out where the damage is located. The general location is found by internet and phone outages, or in the case of electrical cable, the power outage. Data cables are fiber optic, so a light pulse is sent through the cable that would normally go all the way to the other side. 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 get a rough estimate of where the break is, then send a Remotely Operated Vehicle, or ROV, with fault detection instruments to find the damage. Some causes, like an anchor strike, may be easy to see, but others take more time to find. Once the break is located, a cable ship is dispatched to repair. All they have to do is brave the weather, pirates, and sea creatures for a successful fix. To stay on station during repairs, cable lay and repair vessels are fitted with dynamic positioning systems. These systems keep vessels in a specific location during winds, waves, and even storms, and are essential to the repair process.

“(Image Credit: Yannick Le Bris – Image Source:”. Image from GeoTel.

Cable ships are loaded with enough cable for repairs, maybe 5-10 km, which can be loaded in a few hours. Fun fact: this is just a fraction of the cable capacity of some of the newer ships like Orange Marine's Le Pierre de Fermat, which can hold 9,000 km of cable and take weeks to load. Cables are at the highest risk of damage during the loading and installing process. Cable ships have systems that can send power through the cable to make sure it is tested before install. Breaks are not uncommon, according to WIRED: “The breaks happen surprisingly frequently. ‘There's a cable fault somewhere in the world twice a week,’ Tim Stronge, vice president of Telegeography, which creates giant undersea cable maps.” Crews have to be vigilant, as these vessels are sometimes attacked by pirates who steal parts of the cables that connect them to land. Most cable breaks aren’t big news because there are so many cables that data can be rerouted fairly easily. Wind farms could be in greater danger due a lack of redundancy in the main power cables that deliver the electricity to shore. There have been large outages, such as in 2008 when Egypt lost 70% of its nationwide internet network and India had disruptions in 60% of theirs.

Check out the short video below of cable being loaded onto the La Pierre de Fermat.

If the damaged cable is in shallow water, an ROV equipped with cameras and robotic claws is deployed to find the fault and retrieve the cable. Sometimes, the cable is hard to find due to displacement, such as being dragged off course by a fishing vessel. The ROVs can’t operate in deep water due to the increased pressure, so to fix a deep water cable, the ship has to use a grapnel, which grabs and cuts the cable, dragging the two loose ends to the surface. If needed, one end can then be hooked to a buoy and the other end brought on board. Cable has to be added to make the repair, since there is not enough slack to bring the cable up and cut a piece out. After the cable is retrieved and on board, in a repair room that looks like a laboratory, engineers repair the cable. 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. There is new technology in development that would make in situ repair possible for power cables, preventing the need to pull the cables up to the surface.

“Cable welding during a repair operation”. Image from

How do we protect undersea cables?

Cables can be protected from shark bites with metal sheathing or a metal tube. For fishing vessels, maps are freely provided to fishermen in the hopes that they do not fish over such cables. But, the cables themselves aren’t what’s valuable, especially with data cables, the information that flows through them is the most valuable part. How do we protect the data within the cables? Countries take steps to prevent certain companies from controlling too much data, but there are 750,000 miles of undersea data cables around the world, without which people wouldn’t be able to function. According to Defense News, “In 1858, when the first submarine cable was installed, sending a message across the Atlantic took nearly 18 hours. Today, the fastest undersea cables can transfer data at speeds upward of 25 terabytes per second — more than twice the amount of data generated by the Hubble Space Telescope each year.” These cables transmit about 95% of voice and data traffic for civilians and governments. Cables can be tapped for information, or cut to drastically slow communication between countries. A greater emphasis on government protection of the cables may be in the future.

We hope you enjoyed this Fun Fact Friday about cable repair. Be sure to check out our blog How do undersea data cables work? where we talk more about the history of data cables and how they are laid in the ocean. Check out our Services page to learn more about how we test dynamically positioned vessels so they can run safely and efficiently.


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