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Interconnector’s engineering feats revealed

interconnector tunnel

National Grid has revealed some of the engineering feats that are being used to build a 720km long interconnector between the UK and Norway.

Tunnelling through fjord rockfaces, building massive avalanche walls and working out how the interconnector will travel across 94 other pipelines are just some of the challenges the team is facing with partners Statnett, the Norwegian transmission system operator.

Called the North Sea Link, the £1.7bn 1400MW subsea electricity cable will go from Blyth in Northumberland to Kvilldal in Rogaland on the Norwegian side.

interconnector map

NS Link map

Source: National Grid

On each side there will be a converter station that will connect the interconnector to the grid. Though energy can be transmitted either way, the lion’s share will be the UK buying Norwegian hydropower.

“Norway has an excess of hydro capacity. It will bring a lot of power into the UK which is low carbon and in supply pretty much 24/7, 365 days per year. The wholesale price of the power is roughly half what it is in the UK,” said NSN Link’s construction director Nigel Williams.

“Norway has capacity to generate more. For them it’s getting an income from something they’ve already built.”

Construction on the Norwegian side stated in 2015 and the substation, on the same site as Norway’s largest hydropower station called Hylen Hydroelectric Power Station, is almost complete. And as Williams explains, the job came with many civil engineering challenges.

The site of the hydropower station is in a mountainous area on the Hylsfjorden fjord coastline.

“We needed to find a good, flat area of land for the substation and grid connection point,” he said.

The converter site is 4ha. National Grid and Statnett had to reduce the land by 16m at some points to get it level. This has meant shifting 30,000m3 of rock so far, which is being used to build huge 40,000m3 avalanche walls, in technical terms called a reinforced earth dam. The rock is layered with a membrane, and finally grass is planted on it so it just looks like a massive grass bank. Two of these avalanche walls are just finishing. By comparison, constructing the converter site at Blythe will be child’s play when it starts next month.

interconnector earthworks

NS Link earthworks

Source: National Grid

The cable from the North Sea will travel along the bed of the Hylsfjorden fjord and then into the mountain. It travels up the inside edge of the mountain through a micro tunnel and then along a 2.3km conventional tunnel. From the other side of the tunnel it comes out onto the bed of a 200m deep mountain lake called Suldalsvatnet. It then travels out of the lake, out of a ravine and onto the landing site.

One of the biggest challenges is the 2.3km tunnel for the cable. The tunnel is 5m high and constructed using blast and drill techniques. There are three blasts a day and each blast makes about 4m progress. Work is done in ten hour shifts. Drilling is carried out ahead of the blasting to test for water. The tunnelling will finish later this month.

Micro tunnels will be used to draw the cable up from the bed of the fjord to the tunnel entrance several hundred metres above – although still below the surface of the fjord’s water. The cable itself is 130mm diameter and weighs 50kg per metre. North Sea Link has contracted with two cable suppliers, Prysmian and Nexans, to deliver the cable which will be 1,450km, as they will be run in parallel.

When it comes to laying the pipeline across the North Sea, there are more challenges. The team is looking to lay the cable in solid, compact clay, not soft and shifting sand. Soil samples are taken every 1km to check the seabed and try to formulate a path which is not disrupted with rocks. April will see detailed seabed surveys undertaken, while the team looks to finalise the route. An important aspect will coming to agreements with third parties so that this cable can cross others – these other cables can be for telecoms, electricity, wind farms and North Sea oil platforms.

There are 94 crossings in total. In order to cross them, a bridge is built on the seabed. The interconnector will then be covered with more rock, to minimise the risk to damage to the cable from, for example, ships anchors.

Once they are ready to lay the cable, a ship can be fitted with a maximum of 130km. The ship has remote operating vehicles that travel down to the seabed and release down arms that look a bit like a chainsaw, but release a extremely high-pressure jet that creates a v-shaped trench, where the cable is laid.

“We do it once, do it well and hope never to see it again,” said Williams. “The cable should be down there for 60 to 80 years.”

Construction is expected to be completed by 2021 when the interconnector comes into operation.




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