Subsea Cable Installation & Repair: Step-by-Step

Updated 14 November 2026 · Standard formula, shown below

Subsea fiber optic cables carry over 95% of intercontinental data traffic, connecting continents through cables laid on the ocean floor. This guide explains how these cables are installed and repaired, covering the ships, tools, and procedures involved. You’ll learn the key steps from route planning to final burial, and how faults are located and fixed.

What You’ll Need

Cable ship with dynamic positioning and cable laying equipment
Submarine cable (armored or lightweight depending on depth)
Plow or jet trencher for burial
Remotely operated vehicle (ROV) for inspection and burial
Submersible grapnel and cutting tools for recovery
Fusion splicer and cable jointing kit
Optical time-domain reflectometer (OTDR) for fault location
Spare cable sections and repeaters

Step-by-Step

Route planning and survey: The cable route is chosen to avoid fishing grounds, shipping lanes, seismic zones, and steep terrain. A survey ship maps the seabed using sonar and sub-bottom profilers to identify hazards and determine burial feasibility.
Manufacturing and testing: The cable is manufactured in continuous lengths, often with optical fibers wrapped in steel and copper layers. Repeaters (amplifiers) are spliced in at regular intervals (every 50-100 km) and each section is tested for optical loss and signal integrity.
Cable loading: The cable is loaded onto the cable ship in large tanks, coiled in figure-eight patterns to prevent twisting. The ship then sails to the landing station or the start of the cable route.
Shore end installation: The cable is brought ashore using a technique called “surf zone installation.” A smaller vessel or the cable ship itself pays out the cable to a beach manhole, where it is connected to the terrestrial network. The shore end is often buried deeper to protect against anchors and storms.
Main lay: The ship moves along the planned route at 5-8 knots, paying out cable at a controlled tension. The cable passes over sheaves and through a linear cable engine that controls speed and tension. Repeaters are launched carefully, and the ship’s dynamic positioning keeps it on course.
Burial: In shallow water (up to 1,500 m), a plow or jet trencher is towed behind the ship. It cuts a trench (typically 1-3 m deep) and lays the cable in it, which is then covered by natural sediment. In deeper water, an ROV with a jetting tool buries the cable. In very deep water, the cable is left on the seabed.
Post-lay inspection and burial: An ROV surveys the cable path to ensure it lies correctly and is buried to the required depth. Any exposed sections are re-buried using the ROV’s jetting tool.
Final testing and commissioning: After landing both ends, the entire cable is tested with an OTDR to verify splice losses and overall attenuation. Repeaters are powered and signal transmission is tested. The cable is then handed over to the operator.
Fault detection: When a cable fails, the operator uses an OTDR from shore to measure the distance to the fault based on reflection and loss. The fault location is pinpointed to within a few meters using the cable’s known fiber length.
Repair ship dispatch: A cable repair ship is sent to the fault location. The ship uses dynamic positioning to hold position over the fault.
Recovery: The ship lowers a grapnel (a hooked device) to hook the cable. Once hooked, the cable is cut and brought to the surface. In shallow water, a grapnel may be towed; in deep water, an ROV may attach a lifting line.
Jointing: The cable ends are brought aboard and cleaned. The damaged section is removed, and new cable (with a repeater if needed) is spliced in. Each fiber is fusion spliced and protected with a splice tray. The cable is then re-jacketed and pressure-tested.
Re-lay and burial: The repaired cable is lowered back to the seabed. In shallow areas, it is re-buried using a plow or ROV. The ship then performs a post-repair survey and tests the cable from shore.

Common Mistakes

Underestimating the importance of route survey: poor seabed data can lead to cable exposure and future faults.
Incorrect tension control during laying: too much tension can strain the cable, too little can cause kinks.
Improper handling of repeaters: they are heavy and delicate; mishandling can damage internal electronics.
Not accounting for thermal expansion: cable lengths change with temperature, so slack must be managed.
Rushing the burial process: shallow burial in rocky areas can leave the cable exposed to fishing gear.
Inadequate fault location before repair: cutting the wrong section wastes time and money.

Tips

Use a cable route that avoids known trawling areas and anchorages to reduce fault risk.
During burial, monitor the plow’s depth continuously to ensure the cable is buried to the required depth.
Keep spare cable and repeaters on the repair ship to speed up repairs.
Use an ROV for post-lay inspection to catch any exposed sections early.
Coordinate with local authorities and fishing communities to avoid conflicts.
Document every splice and repair with accurate coordinates and fiber length measurements.

Troubleshooting

If the OTDR shows a fault but the ship cannot find the cable: The cable may have drifted or been buried deeper than expected. Use a side-scan sonar to locate it, or use an ROV with a cable-finding sensor.
If the cable breaks during recovery: The grapnel may have cut it. Re-grapnel with a more robust tool, or use an ROV to attach a lifting line to the remaining end.
If splicing fails repeatedly: Check for contamination in the splice trays or moisture in the cable. Re-clean the fibers and use a fresh splice sleeve.
If the repaired section has high loss: The splice may be bad, or the new cable may have been damaged during laying. Re-test with an OTDR and redo the splice if necessary.
If the cable is not buried properly after repair: Use an ROV to re-jet the area, or if the seabed is too hard, consider using a mechanical trencher.

Frequently Asked Questions

How long does it take to install a subsea cable?

A transatlantic cable can take 2-3 months to lay, depending on length and weather. Shore end installation and burial add extra time.

How deep are subsea cables buried?

In shallow water (less than 1,500 m), cables are typically buried 1-3 meters deep. In deeper water, they are often left on the seabed.

What causes most subsea cable faults?

Fishing trawlers and ship anchors cause about 70% of faults. Other causes include earthquakes, underwater landslides, and cable degradation.

Can subsea cables be repaired at sea?

Yes, repair ships can lift the cable, splice in a new section, and re-lay it. Repairs typically take 1-3 weeks depending on depth and weather.

How is a fault located exactly?

An OTDR sends light pulses down the fiber and measures reflections. The distance to the fault is calculated from the time delay, giving accuracy to within a few meters.

Do subsea cables have a lifespan?

Most cables are designed for 25 years, but many last longer. Repeaters are the main limiting factor; they can be replaced, but it’s often cheaper to lay a new cable.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.