When a subsea internet cable fails, engineers first estimate the fault’s position from shore. A specialist repair ship then locates and recovers the cable, removes the damaged section, joins in spare cable, tests the repair and lowers it back to the seabed. The operation can take days or weeks; the International Telecommunication Union’s 2024 guidance gives a mean repair-time estimate of one to three weeks, not a deadline for any specific fault.
How a subsea cable repair works
Submarine cable repairs combine shore-based fault diagnosis with a marine operation. The exact method depends on where the fault is, how deep the cable lies, whether it is buried, the seabed and weather, and what repair vessel and equipment are available.
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1. Engineers estimate the fault position from shore
Terminal stations monitor the cable system and use electrical or optical measurements to identify abnormalities and estimate where the fault lies. On a repeatered system, supervisory equipment can often narrow the issue to a section between repeaters. Engineers can investigate some end sections from shore using electrical measurements and optical reflectometry. The maintenance authority passes the estimated position to the repair vessel; it is an initial search area, not necessarily the exact point of damage. ITU-T G.971
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2. A repair ship mobilizes and searches at sea
The operator mobilizes a suitable cable repair vessel under the relevant maintenance arrangements. Before sailing, the ship may load spare cable and subsea components and review charts, permits and information about the cable route. At the suspected location, the crew can use methods such as electroding, an ROV inspection or recovering cable for further testing. Electroding can help the ship detect or follow the cable route, but it does not repair the fault. KIS-ORCA’s repair overview
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3. The crew recovers cable and cuts out the failure
For some seabed recoveries, a ship tows a cutting grapnel to catch and sever the cable, then uses a holding grapnel to engage and retrieve an end. A buried cable may instead need an ROV to locate or cut it. The crew manages tension carefully while bringing cable aboard, tests the recovered section and removes the damaged length. Depending on the operation, one end may be buoyed while the crew works on the other. Recovery methods vary with depth and seabed conditions. ITU-T G.971 and KIS-ORCA
4. Spare cable is spliced in and tested
On the ship, technicians prepare the cable ends and join them to a spare section using procedures designed for that cable system. The repaired span is checked for optical-fiber and electrical performance. KDDI’s description of the process includes testing the cable, joining the spare section, recovering both ends and carrying out a final check between the two landing stations. A repair can require extra cable length; on some deep-sea repairs, an additional repeater may be needed to compensate for added attenuation. KDDI Cableships & Subsea Engineering and ITU-T G.971
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5. The repaired cable is returned to the seabed
After testing confirms that communications are normal, the crew lowers the repaired cable back to the seabed. If the route is meant to be buried, an ROV may use water jets to rebury it. KDDI Cableships & Subsea Engineering and KIS-ORCA
How long a repair takes
ITU-T G. Suppl. 41, issued in July 2024, estimates mean time to repair at one to three weeks. That is a broad estimate, not a service-level commitment or a prediction for an individual outage. The guidance identifies fault location, water depth, vessel availability, root cause and weather as factors that affect the time required. A vessel may need to travel to a remote fault, and conditions at sea can delay recovery and redeployment. ITU-T G. Suppl. 41
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Why repairs differ from one fault to another
- Depth: Shallow-water and deep-sea recoveries use different procedures. A deep repair may need additional cable and, in some cases, a repeater to address added attenuation. ITU-T G.971
- Burial and seabed: Buried cable can require ROV assistance to locate or cut it, followed by reburial. Rocky seabed can limit burial. KIS-ORCA and ITU-T G. Suppl. 41
- Access and logistics: Vessel availability, travel distance, permits and weather can all affect response time. KIS-ORCA and ITU-T G. Suppl. 41
- Fault type and position: Engineers localize an end-section fault differently from a fault in a repeatered span, and the amount of cable that must be removed depends on the damage. ITU-T G.971
What causes subsea cable faults—and what limits disruption?
ITU-T G. Suppl. 41 (July 2024) lists external causes including bottom fishing, trawling, ship anchors, ocean currents, geological events such as earthquakes and volcanoes, and thermal failures due to overload. It reports that “Nearly 90 percent of failures are caused by fishing activities and ship anchors’ damage.” This is the figure reported by that 2024 guidance, not a newly calculated or timeless global rate. ITU-T G. Suppl. 41
Prevention and network design can reduce risk or the impact of a break, though neither makes faults impossible. The ITU guidance describes shallow-water burial as a way to reduce exposure where seabed conditions allow, route selection that avoids geological hazards, and diverse network routes that can help traffic reroute while a damaged cable is repaired. ITU-T G. Suppl. 41
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