Array cables

How does Array cables work, step by step?

1. 1

   Step 1: Survey and fix the routes between positions

   Before anything is manufactured the seabed between every pair of positions is surveyed. Geophysical survey maps the surface and the shallow layers, sampling and cone testing establish what the ground will do when a burial tool is dragged through it, and the results are combined into a route for each section. Boulders, sand waves, wrecks, archaeology, existing cables and pipelines and any unexploded ordnance are all identified and either avoided or cleared. Routes are kept as short and as straight as the constraints allow, because every extra metre is cost and every bend is a place where the cable will want to move.

2. 2

   Step 2: Set the cable schedule and order manufacture

   Each section has its own length, taken from the route and then increased to allow for the slack the installation needs, the rise up each foundation, the terminations and a margin for the unexpected. Those lengths go into a cable schedule that tells the manufacturer exactly what to make, and getting it wrong in either direction is expensive: too short and a section has to be remade, too long and the surplus is wasted and the handling becomes harder. Cable is then made in continuous runs, tested at the factory and wound onto carousels or drums ready for shipment. Manufacturing slots for offshore cable are booked long in advance and are commonly the earliest thing on the project programme.

3. 3

   Step 3: Load out at the marshalling port

   Cable is transferred from the factory onto the installation vessel, either directly from a barge or through a marshalling quay where sections are staged. Loading is a controlled operation because the cable is being coiled into a carousel or a tank in a way that must not damage it, and the loading rate governs how long the vessel sits alongside instead of working. The vessel is loaded with a planned batch of sections in the order they will be installed, along with the accessories each one needs. Quay space, crane capacity and the reliability of the supply chain feeding the port all shape how many sections go out per trip.

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   Step 4: Prepare the foundation entries and run the messenger wires

   Ahead of the lay vessel arriving, a support spread prepares each foundation. The entry tube is inspected and cleared, any temporary caps are removed, and a messenger wire is run down through the entry and out onto the seabed so that the cable end can later be attached and pulled up. A winch and the pull-in equipment are set on the platform inside the foundation. This preparation work is deliberately decoupled from the lay vessel because the lay vessel is the most expensive part of the spread and the campaign is planned so that it never waits for something that could have been done earlier.

5. 5

   Step 5: Pull in the first end and start the lay

   The vessel comes onto position at the first foundation and the cable end is prepared and connected to the messenger. It is then pulled up through the entry and into the structure while the vessel pays out cable and holds position, with a remotely operated vehicle watching the cable at the seabed and reporting how it is lying. Once enough cable is inside, it is clamped off and made secure. The end is capped and left ready for the termination crew who will follow later, and the vessel then moves off along the route paying cable onto the seabed behind it.

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   Step 6: Lay the section and pull in the second end

   The vessel tracks the surveyed route while the cable is laid, with the position of the cable on the seabed monitored throughout rather than assumed from the position of the vessel. Currents, water depth and lay speed all move the cable away from the ship track, and the crew adjust for it as they go. At the far foundation the process reverses: the vessel approaches, the second end is cut to the planned length, prepared, connected to the messenger and pulled up into the structure. The surplus, if any, is recovered and the section is complete as a laid but unburied cable.

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   Step 7: Bury the cable in a separate pass

   Burial is usually a separate operation from laying, done by a trenching machine that follows the laid cable and works it down into the seabed. Different tools suit different ground, and the choice belongs to the installation contractor working to what the ground model says: jetting tools that fluidise sand, mechanical cutters for stiffer material, and ploughs that displace it. Progress is recorded continuously so that the project knows what was achieved along every metre. Where the target cannot be reached, the position is logged and dealt with under the protection and remedial regime rather than left as an open question.

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   Step 8: Terminate, test and record

   A separate termination crew works inside each foundation, preparing the cable end and making it off into the equipment it serves. This is skilled, slow, clean work carried out in a controlled environment inside the structure, and it is one of the few offshore activities that is genuinely weather independent once the crew are aboard. After termination each section is tested before it is ever taken live, and the results are compared against the factory records for the same cable. The as-laid position, the burial record, the termination records and the test results are assembled into a file for that section, which is what the operator will use for the rest of the life of the asset.