• Nexans has completed qualification of a 525kV mass-impregnated HVDC cable system designed for installation at water depths of 3,000 metres
  • The latest tests covered the flexible repair joint, extending the qualification beyond installation to equipment needed if a deepwater cable section must be repaired

The fact

Nexans has completed qualification of its 525kV mass-impregnated HVDC cable system for operation at water depths of up to 3,000 metres. The system is being developed for the Great Sea Interconnector between Crete and Cyprus, where parts of the planned route cross exceptionally deep water.

The latest stage covered the flexible repair joint, which passed mechanical and electrical type testing. Nexans said the joint was subjected to mechanical tension of up to 842kN. The qualification now covers the cable itself, its flexible factory joint and the flexible repair joint.

The result follows an offshore trial in March, when Nexans installed and recovered a section of its NOVA-L 525kV cable and joint at a depth of 3,000 metres before carrying out electrical testing and inspection. The latest announcement concerns a submarine electricity cable system, not a telecommunications fibre cable.

The Assessment

At 3,000 metres, repairing a cable fault is as much a recovery operation as an electrical one. The damaged section has to be located, brought back to a vessel, cut and joined before it can be returned to the seabed. Nexans' latest qualification shows that the cable and its repair joint can withstand the specified mechanical and electrical conditions at that depth.

What the tests do not show is how quickly a fault on an operating interconnector could be repaired. That would also depend on finding the fault, securing a suitable vessel, recovering the cable, having spare components available and working within the weather conditions at the time. Mobilising those resources could be as important to restoration time as the joint itself.

For BTW readers, Nexans has now shown that the repair joint can meet the same deepwater conditions as the cable itself. The remaining uncertainty is operational: if a fault occurs after the link enters service, restoration will depend on how quickly crews can find and recover the damaged section, secure a suitable vessel and carry out the repair at sea.

What to watch

Watch the Great Sea Interconnector's installation schedule and any disclosures on spare cable, repair joints, vessel access and deepwater recovery procedures. Once installation begins, those arrangements will give a clearer picture of how the project plans to handle a fault at extreme depth and how quickly a damaged section could realistically be returned to service.