Most of the world’s international telephone conversations are carried by cables laid along the seabed, linking the continents. Communications satellites have not yet removed the need for submarine cables – even the ‘hotline’ between Washington and Moscow uses them. But what happens when a cable fails?
The first transatlantic telegraph cable, laid in 1858, failed within a few weeks. Today the risk of failure has been much reduced by using polythene insulation, and by choosing safer routes that avoid volcanic activity, strong currents and fishing grounds where trawlers may snag their nets on the cables. In shallow sea, cables are often buried.
Despite these precautions, failures still happen. Large telecommunication companies, such as Cable and Wireless, have maintenance ships standing by round-the-clock to carry out repairs.
The job is done by remote-controlled submersibles, as big as a medium-sized van, which are lowered into the water from the maintenance ship, dive to the seabed, locate the fault and attach lines to the surface and repaired on board the ship.
CIRRUS (Cable Installation, Recovery and Repair Underwater Submersible) and it’s even more sophisticated successor ROV128 are controlled through an ‘umbilical’ cable from the ship and powered by hydraulic thrusters.
A submersible’s first job is to find the fault. It follows the line of the cable on the seabed, picking up faint low-frequency signals sent along the cable from the terminal station ashore. If a cable is broken, the water will form a short circuit that will link the individual wires together. When the signal disappears, the submersible settles on the ocean floor and exposes the damaged cable with a powerful jet of water which blows away the layer of sand and silt.
CIRRUS is equipped with powerful lights and television cameras, both colour and black and white, which enable operators on board the ship to see every detail of the seabed. Using the pictures as guidance, the operators extend powerful manipulator arms and grip the cable. CIRRUS uses a special blade to cut the damaged cable, and leaves an acoustic ‘pinger’ on the seabed to mark the spot.
It then rises to the surface, picks up a strong steel line, takes it down to the seabed and clamps it to one end of the cable. The cable is then winched up to the surface. The same process is used to receive the other end of the cable.
Once the cable has been repaired and joined together on board the ship, it is lowered carefully back to the seabed.
Picture Credit : Google