Recent experience with the consequences of cable tunnel flooding on 132kV XLPE cables and joints

In mid-2024 a major 132kV cable tunnel became flooded with water to a depth of 20m. When the tunnel was drained, jelly was seen leaking from the mid-tunnel joints. This jelly was identified as cable water-blocking material, indicating that the cables had suffered water ingress under the corrugated aluminium sheath via the joint heatshrink seals.

Advice was sought from manufacturers on possible impacts on the cables and joints, and options were considered to ensure safe access to the tunnel for cable repairs. It was recognised that the corrugated aluminium sheath and vertical snaking arrangement made it impossible to remove the water from the cables.

The decision was taken to enact repairs, and the cables were cut back between 30m & 200m until dry cable was found. Water was found inside the joints and had penetrated 30mm along the interface between the premoulded joint body and the XLPE insulation, with water detected in the silicone grease at the core screen cut.

These repairs may be suitable as a permanent solution provided that tunnel access is not obstructed and suitable fire engineering controls are employed.

Recommendations are given for avoiding similar incidents, especially that flood risks be considered thoroughly during tunnel design, and tunnel systems that are damaged by flooding should be quickly repaired or temporary arrangements put in place, to prevent flood recurrence.