Subsea Cable Repair
The concept
To design and make an underwater housing to repair one of Northern Ireland's main supplies of electricity.
Project Background
The project centres on the Moyle interconnector- an underwater high voltage cable that runs between Northern Ireland and Scotland and has a capacity of 500MW. It was commissioned in 2001 and was a means by which a reliable and cost effective supply of electricity could be secured for Northern Ireland. The cable had developed a number of faults leading to downtime and reduced capacity. In 2014 ESB International and Mutual Energy approached Marine Technology (MTL) with a concept and asked MTL to set about designing a housing (or habitat) that could be deployed to affect a full repair on the cable.
MTL began work on designing and building a unique subsea housing that could be lowered to the seabed and allow divers to conduct a repair without having to lift the cable to the surface. This method of cable repair has never been tried anywhere in the world, and was the first of its kind for subsea high voltage cables.
Usually in these circumstances, a cable with a fault like this would be lifted from the seafloor by a specialised ship and a repair conducted at the surface. Engineers believed this method would not work however because if used, the lifting and bending stresses caused by bringing it to the surface could potentially damage the sensitive layers in the cable. The decision was made that the cable must be repaired in situ with no disturbance, so Marine Technology was asked to design and manufacture a solution.
The habitat itself is a housing that is split along its length, the lower half is fitted underneath the cable after a suitable trench has been dug in the seafloor. The top part of the habitat is then secured and completely sealed to make a watertight unit. At this stage the unit is full of seawater which is then evacuated from the habitat by pressurised air from the ship above, which blows the water out through sensitive pressure valves. Once this has been done, and other processes have been completed, the habitat now acts as a watertight cylinder in which divers can access the cable through specially designed doors. Two doors on each side allow divers to insert their arms and carry out the repair in full. Once the repair has been completed, the habitat is flooded slowly again and then removed from around the cable.
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