Subsea Cables

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Submarine Cable Maintenance

Under a Cable Maintenance Agreement and on notification from a Maintenance Authority, a cable repair ship operator will mobilise one of its vessels to conduct a repair on a subsea cable. The maintenance authority will provide an estimated position of the cable fault based on tests carried out from the cable Terminal Stations located either end of the marine cable. Permitting, Charting, and existing cable information is assessed prior to the vessel departing.

On arriving at the suspected cable fault location, the repair vessel will attempt to localise the cable fault further by using one of three different techniques:

  1. Electroding - The terminal station passes a low current signal along the cable and the vessel trails two electrodes behind the ship a specific distance apart and sails over the fault area. Signals of the frequency on the cable lead into a tuned amplifier on board and where the input voltage signal is lost a clean break is indicated or a reduced signal indicates a damaged cable;
     
  2. Remotely Operated Vehicle (ROV) - The terminal station places a tone on the cable and the ROV tracks the cable (where depth allows) detecting for a drop in tone and using its cameras to search for physical damage; or
     
  3. Side Scan Sonar - Positioned behind the vessel, a pulse is emitted from the equipment to create an image of the seafloor. This can be used to support the methods above to look for physical damage to the cable on the seabed, such as scars.

In the event the cable being damaged but not broken, the cable will need to be cut. This is carried out from the ROV or by using a cutting grapnel. A grapnel is a device with multiple hooks (attached to the vessel by rope) and dragged along the seabed in order to cut or hook the cable. The essence of grappling is that sufficient rope be streamed to ensure the grapnel is in continuous contact with the seabed without reducing sensitivity in tension due to excessive rope. The ROV on the other hand uses one of its manipulator arms to attach a gripper to the cable which is connected to the ship via a rope or wire for recovery.

Remotely Operated Vehicle (ROV)

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The first cable section recovered to deck will be sealed and buoyed off. The second cable end will then be recovered from the seabed, cleaned and prepped ready for a new section of cable to be spliced in. This operation is a delicate process and is carried out under strict health and safety and hygiene procedures. The armour wires are removed and the cable is stripped back to the individual fibres. These fibres are then spliced together with the fibres in the new cable section (the fibres are coloured or mapped to ensure the correct fibres are spliced together). On completion of all the fibres being spliced together, the cable joint is sealed, moulded and a protected casing attached. Once the first joint has taken place the vessel will lay out the new cable to the location of the earlier dispatched cable buoy, which is then recovered. Again the cable ends are prepared, the fibres spliced and the joint sealed together. The cable is rigorously tested before the vessel slowly moves away from the site and simultaneously lowers the cable to the seabed, thus avoiding the cable looping and sitting back on itself.

For maximum cable protection the cable should then be immediately reburied. Using the ROV, a high pressure water jet liquefies the seabed beneath the cable which then drops into the trench created. Cutting tools on the ROV can be used in areas of hard seabed.

 

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