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Corran Narrows

Project Location:

Loch Linnhe, Scotland

Client Name:

SSE

Project Description:

OCU delivered a critical 33kV underground cable installation beneath Loch Linnhe to replace ageing seabed cables that had repeatedly faulted due to harsh tidal and geological conditions.


The project involved installing three HDD ducts, each approximately 470m in length, to accommodate new trefoil cable bundles, armoured fibre-optic cables and a spare duct for future capacity. Working beneath solid bedrock and variable glacial deposits, our teams engineered bespoke drilling solutions to provide a safe, resilient and environmentally sensitive alternative to a 70km trenched diversion route.

Key Stats:

  • 3 x 470m HDD ducts beneath Loch Linnhe

  • Trefoil 33kV aluminium cable layout

  • Spare duct included for future resilience

  • Fibre-optic cable installed in bundle

  • Drilling through Psammite and Schist bedrock

8 Months

Duration

Challenge

  • Replacing faulted seabed cables in an area with strong tidal currents

  • Drilling through extremely hard Psammite and Dalradian Schist bedrock

  • Managing bore instability in glacial till with cobbles and boulders

  • Coordinating two active work fronts during HDD operations

  • Restricted vehicle and plant access in a remote Highland location

  • Complex traffic management and environmental constraints

Solution

  • Constructed a floating stone road to safely support heavy HDD equipment on deep peat

  • Developed bespoke HDD methods following R&D to adapt tooling for hard rock conditions

  • Deployed a modular percussive drill mole to navigate boulder-rich deposits

  • Adjusted exit strategy to utilise a narrow viable corridor between the coast road and high-water mark

  • Installed ducts using punch-out and trenching to place them precisely to SSE’s requirements

  • Worked with local suppliers and contractors to reduce vehicle movements and environmental impact

Outcome

  • Project delivered safely with no harm and no environmental incidents

  • Successful installation of all HDD ducts despite complex and variable ground conditions

  • Long-term, resilient replacement for seabed cables with a history of repeated failures

  • Installation of a reliable electricity supply encourages renewable energy uptake and decarbonisation

  • Eliminated the requirement for a 70km alternative cable route around the loch

  • Enhanced the natural landscape by removing overhead cabling and pylons

  • Delivered community benefits, including school visits and reuse of surplus materials for local wildlife projects

  • The new cables are readily available standard “Off the Shelf” items, rather than long lead time specialist marine cables

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