Bay crossing preferred for UK cable link 18 Sep 2014

Peter Kenyon, TunnelTalk

A 17km subsea crossing of Morecambe Bay in the north-west of England is proposed by UK electricity transmission and distribution company National Grid. The bay is the largest continuous intertidal area in Britain, covering an area of 310km2.

17km cable tunnel alignment
Fig 1. 17km cable tunnel alignment

The approximate 5m diameter TBM-bored tunnel solution is the preferred option for running high voltage cables from the new 3.4GW capacity Moorfield nuclear power station that is planned to replace Sellafield in West Cumbria. Sellafield was closed in 1993 and is currently being decommissioned.

Higher capacity 400kW transmission lines are required to replace the existing 132kW cable infrastructure that runs along the Cumbrian coast for connection with the national power transmission network.

National Grid has just launched a public consultation programme on a route corridor that takes advantage of existing lower voltage pylon infrastructure to form a split capacity “Cumbria ring” comprising two separate high voltage double circuits. By splitting the circuits National Grid would be able to perform maintenance on one circuit while still providing capacity on the other.

The proposal incorporates:

  • A north-running above-ground connection, which would involve replacing the existing lower voltage cables, to the substation in Carlisle in the northern Lakes, and
  • A south-running above-ground connection, using a route utilised by existing lower voltage cables, as far as Barrow-in-Furness. From here the route would then cross under Morecambe Bay for connection with the Middleton substation in Heysham, Lancashire (Fig 1).
Morecambe Bay is 310km2
Morecambe Bay is 310km2

National Grid envisages an alignment that runs 25-35m below the sea-bed, and incorporating a ventilation shaft along the route. Running cables along the sea floor is not considered viable due to the potential for the same sort of tidal damage to the infrastructure that is forcing the same company to relay a vital gas pipeline in a tunnel under the Humber Estuary on the UK’s east coast.

By routing the southern alignment across Morecambe Bay, National Grid would save approximately 60km of land-based cable and pylon upgrade work that would be needed to circumnavigate the estuary, and which would have to be located in the environmentally sensitive southern end of the Lake District National Park, an area of outstanding natural beauty.

The initial consultation exercise ends in November, with National Grid expecting to advance to more detailed design next year (2015).

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