2.2 Special Vessels - Cable Layers
Cable laying ships are designed to load, lay, lift and maintain submarine cables for the transmission of telecommunications or electrical power across the oceans.
Cable layers feature massive cable storage holds and deployment equipment capable of handling immense cable weights and lengths. The largest cable-laying vessels reach 20,000 tons.
Repair vessels, around 5,000 tonnes, are equipped with lifting equipment and underwater robots capable of cutting, recovering and re-welding buried cables, as well as carrying out short layouts of around 200 km.
Mixed vessels, of intermediate size, lay medium-length cables of up to 600 km.
Specificities: Cable laying operations require precise navigation and control to ensure accurate cable placement. Sophisticated positioning systems and dynamic positioning technology are crucial to maintain the vessel's position during deployment.
Equipment: Key equipment onboard includes:
Cable Storage Carousels: These giant spools hold kilometers of cable, ensuring smooth and controlled unwinding during deployment.
Laying Tensioners: These maintain constant tension on the cable as it's payed out, preventing damage during the process.
Plows or Trenchers: Depending on the seabed conditions, cable layers may deploy plows or trenchers to bury the cable beneath the seabed for added protection.
Laying a cable involves several successive operations:
Loading : This consists of loading and coiling the cable on board the vessel in specially designed cylindrical tanks, along with repeaters to amplify the signal, placed every 50 or 100 km to maintain signal strength.
Landing : This is the cable's journey from its underwater section to its installation on land. The ship is anchored as close as possible to the coast, and the cable, carried by floats, is towed to the beach. It is anchored and connected to the terrestrial network, then released from its floats and laid on the seabed.
Laying : The cable can be buried in sensitive areas or in shallow waters, at a depth of around 1m below ground. The cable-laying vessel tows a plough which digs a furrow. This technique is used at depths of between 20 and 1,500 m.
Outside coastal areas (the continental shelf), we use the classic "deep-bottom" installation method. A cable machine controls the length of cable required according to depth, ship speed (between 3 and 6 knots) and bottom profile.
Repairs: There are many causes for cable failure. The cable operator dredges the seabed perpendicular to the cable to be repaired, using a grapple, until the cable is hooked.
Once the cable is back on board, it is cut on deck and tested at each end until the fault location is found. The faulty section is then replaced.