Heavyweight cable threads through environmental demands
Creating an HDVC link between Caithness and Moray was an outsize project writes Stevie Knight.
At over 113km long it necessitated deploying subsea cable with an in-air weight “the equivalent of 143 double-decker buses” explained Drew Boa of SSEN Transmission.
It’s really a bundle of two power cables and one communication cable, he explained; however, while the optic communication cable element has an outer diameter of 22mm and weighs only 1kg per metre, by contrast “the subsea HVDC cable design has an outer diameter of 132mm and weighs approximately 50kg/m – in air”. He pointed out that it adds up to “around 11,413,000kg” in all.
There is a renewable element to this project: David Gardner, SSEN director of transmission explained that not only will it help transfer the power from wind farms to the regions that need it, but there’s an element of “future-proofing” as it will also help tie in wave and tidal power developments around Orkney.
However, both its size and location threw up challenges. The project utilised the newly built, 140m long, 29.6m broad NKT Victoria; a purpose-built laying vessel designed to hold 9,000 tonnes of cable between its main deck basket and the below-deck carousel. But this particular link still couldn’t be laid in one round as it was simply too heavy for a single load.
Therefore the NKT Victoria had to carry out the lay on in two sections. Its first campaign brought the cable from Noss Head to the centre point of the Moray Firth, the second coming from Moray to the drop point, joining them together in the middle. A specialised subsea plough, carefully chosen to minimise sediment transport, initially cleared the route and subsequently trenched the lay path.
However, connecting up with the shoreside infrastructure also required particular care: low-impact, trenchless Horizontal Directional Drills (HDDs), lined with steel ducts were needed at not just one, but both ends of the link.
It wasn’t easy: the HDD at Noss Head required “a landfall to be made along an area of cliffs”, explained Richard Baldwin, SSEN Transmission’s head of environment. This meant drilling from the cliff top downward, emerging on the seabed approximately 500m offshore.
Marine Mammal Observers (MMOs) were also stationed to watch over an area of rock foreshore south of the drill path, these so-called ‘haul outs’ are used by seals to rest, moult or breed, so it was important to note any change of behaviour.
The next issue to negotiate was the Noss Head Nature Conservation MPA, an area with a horse mussel biogenic reef explained Baldwin. Rather than trenching, the cable was surface laid and protected with rock till it reached the mussels, then Tekmar TekDuct plastic protection shells were used for the crossing. Baldwin added that SSEN will continue to monitor this particular length: the aim being to “build on our understanding of how the horse mussel bed and cable interact”.
The other end of the link at Portgordon was no less testing. The nature of the seabed plus a Site of Special Scientific Interest (SSSI) at the landfall presented the project with some unique challenges. It was decided that to avoid impacting the SSSI and to cope with the practical issues presented by geology and wave dynamics, the HDD had to be much longer, with bores of around 1,600m.
More, the seaward end was topped with around 5m of loose overburden – sand, gravel and cobbles – which were at risk of collapsing back into the bore. The specialist contractor, LMR Drilling, came up with a novel solution: protect it by installing a duct similar to the casing below as the drill string passed through.
Finally, both sides of the link were pulled through into the underground joint bay by the lay vessel, where each end was married with its respective landside connections.
In conclusion, it may have been a picky, demanding operation, but it’s made the £1.2bn, four year Caithness Moray project possible, helping further the reach of renewable power and showing how other, ecologically sensitive links might be achieved.
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