How do we fix a broken submarine fiber cable?

A submarine cable cutting incident happened recently at the Baltic Sea. Another similar incident also happened north of Taiwan. 

I wanted to know more about how to repair a broken submarine fiber cable. I did my research on the Internet. I summarize my findings and list the key fixing steps below.

1. The submarine cable was working.

Initially, the working submarine cable was below the sea water and above the seabed. The submarine cable follows and fits the terrain of seabed.

2. A fault happens.

Examples of faults are punctured insulation, fiber core breaks, and cable cut.

Faults could be natural factors, such as the aging of cable itself, earthquakes, falling rocks, and shark bites. Faults could also be human factors, such as fishing nets dragging, anchoring hits, and malicious sabotage.

3. Use OTDR to pinpoint faulty segments. Send the repair ship to Location A.

Optical Time-domain Reflectometer (OTDR) is a device for us to measure the distance from fiber ends to the fiber core breaks. It will emit laser beams and measure its reflection travel time. With this distance estimation, we can locate the faulty segments on the fiber route map. Now we know faults are happening between Location A and Location B. We can send the repair ship to Location A.

4. Cut the cable at the End A.

Because in the next step we will float End A of this cable to the surface of the sea water, we must cut the cable.

To find the cable and an appropriate spot to cut the cable under the sea water, we can make use of special towed grabbing devices, or undersea robots.

It might surprise you. We do not do undersea repairs of a broken cable. We simply cut the problematic segment away and replace it with spare cable. We do all the splicing at the surface on the repair ship.

5. Attach a buoy to mark and float End A.

Now, we can attach a buoy to float End A to the surface. It also marks the location of End A at the surface so the repair ship can go back to grab End A later.

6. Send repair ship to Location B. Grab End B to repair ship.

The repair ship now can travel to Location B. With the similar approach at Location A, the repair ship can also find and float End B to the repair ship.

We can now grab End B of the cable onto the repair ship.

7. Splice End B to the spare new cable.

We can now splice End B of the cable to spare cable rolls. To splice fiber cables, we usually deploy special splicing and insulation kits to ease up fusion operations of fiber cores on the rocking repair ship.

8. Lay the newly spliced cable way up to Location A.

After the splicing, the repair ship can now lay back the newly spliced cable down to the seabed gradually. Repeat this process all the way back to Location A at the buoy.

9. Splice End A to the newly spliced cable.

At Location A, we can find floated End A of the cable easily because of the buoys. Grab End A to the repair ship and again splice End A to the newly spliced cable.

After this step, the connectivity of the cable restores. We can now also restore the cable back to its original location on the seabed.

10. Release the cable down to seabed.

We can release the repaired cable now from the repair ship and drop it down to the original seabed. We might make cable adjustments when necessary.

One more thing…

We need specialized repair ships first before we can do any repairs of the broken cable. However, few such repair ships are standing by around the globe. Even when the repair ship can travel at a high speed of thirty knots, which is unlikely, it still cannot reach sixty kilometers per hour. It would take days or even weeks just for one repair ship to arrive at the faulty location.

For network operators, we must always have redundant paths and backup cables when we design a submarine fiber cable system.

Next, we also know that the spliced fiber would add up attenuation to the passing signal strength. When we have done enough count of repairing and splicing operations on the fiber cable, the whole fiber cable would become unusable anymore and that is the end of its life. That is, any deployed submarine fiber cables have a limited lifetime. We must be prepared to replace the whole fiber cable when it is out of its lifetime.

Last but not the least, should the broken submarine fiber cable incident be the result of human sabotage, I have two recommendations:

1. Add more repair ships near the troublesome area. This will reduce our waiting time for the repair ship to arrive.
2. Add patrolling guard ships along the submarine cable route. Normally, merchant ships should never be stationary and anchored in the middle of the sea. If they do, they are suspicious of intentional sabotage.

What do we use to measure the distance to the fiber core breaks?

Click to show the answer.

"Optical Time-domain Reflectometer (OTDR)"