Norwegians push subsea XLPE cables to new voltage levels

The highest rated submarine transmission links still tend to use the hundred year old technology of paper insulated cables (where the paper is mass impregnated with high viscosity oil for DC applications and with low viscosity oil (requiring pressurising equipment) for AC cables). However, XLPE cable, employing extruded cross linked polyethylene as the insulation, is being used at ever higher voltages and power levels in underwater applications.

A new voltage record for a submarine XLPE line, 420 kV, has recently been set by the cable system Nexans supplied for the mega facility that will process gas from Ormen Lange, the second largest gas field on Norway’s continental shelf (and for which Nexans has also supplied umbilicals as well as submarine dredging services, including development of a special dredging machine to cope with the very difficult subsea topography). The field, which employs no platforms, only highly sophisticated subsea installations, with nothing visible on the surface, is due to start operation on 1 October and its development represents one of the largest industrial projects ever undertaken in Norway.

The XLPE cable system, supplied to Statnett, the Norwegian state owned grid operator, connects the gas processing facility, which is located at Nyhamna on the island of Gossen, to the mainland grid.

Processed gas from Nyhamna will be supplied to Easington in the UK (via Sleipner in the North Sea) by the Langeled pipeline, which at 1200 km is the world’s longest subsea gas line. When fully operational Langeled/Nyhamna/Ormen Lange is expected to meet about 20% of the UK’s gas requirements (with the riser platform at Sleipner giving the project access to gas customers elsewhere in Europe in the future).

All that gas processing (in the region of 20 billion m3 per year) will take a lot of power, about 200 MW in fact. This will be provided through the Nexans submarine XLPE cable system, which is rated at 1000 MW to allow for future expansion in the area.

The cable system, first energised on 1 December 2006, consists of four single core cables each 3.2 km in length. A 2.4 km section of each cable is of submarine design, with a maximum depth of some 210 m.

Before the Nyhamna project, the voltage record for a submarine XLPE cable was held by another Nexans installation, the 170 kV 3-core power export cable for the 160 MWe Horns Rev I offshore wind farm, delivered in 2002. Under a contract with Energinet.dk Nexans is also providing the power export cable for the 215 MWe second phase of Horns Rev, due to be commissioned in 2009. This 3-core XLPE cable, with fibre optics, will also be rated at 170 kV. The length will be 42 km (about twice that for Horns Rev I), making it the world’s longest submarine XLPE cable to date at this voltage. Only slightly shorter, however, is the 40.5 km 132 kV XLPE 100 MW submarine cable link that will take power from the Shuweihat CCGT on the Abu Dhabi mainland to Delma Island, due to be completed by the end of 2008. This will consist of two 3-core cables, a total length of 81 km, the longest 132 kV 3-core XLPE cable ever delivered by the company.

Two of Nexans’ North American projects can claim further records for XLPE in HV submarine power transmission applications.

In Canada the 7.8 m long submarine cable serving the 200 MWe wind farm to be built on Wolfe Island at the eastern end of Lake Ontario, due to go commercial in 2008, will be the first 3-core subsea XLPE cable to achieve a voltage rating of 245 kV.

In the USA the cables being supplied for a new 138 kV submarine link between Norwalk, CT, and Northport, Long Island, NY – the Long Island Replacement Cable (LIRC) project – have the distinction of being the largest (235 mm outer diameter) and heaviest (100 kg per meter) ever supplied by Nexans at this voltage level. Some 58 km of 3-core cable with 24 fibre optics elements will be provided, to be used in three parallel 150 MW circuits each 19 km long. They will replace seven existing 30 year old cables and will be laid in the existing corridor in depths of up to 70 m, with installation to be completed by the end of 2008.

Halden: centre of competence

The XLPE cable designer, developer, and supplier for all these submarine projects is Nexans Norway, specifically its facility at Halden in the south of Norway, about 120 km south west of Oslo, located on a narrow fjord which forms part of the border with Sweden and provides access to the sea.

Nexans describes Halden as its “centre of competence” for submarine power transmission cable as well as umbilicals for the offshore oil and gas industry.

A striking feature of the Halden site is the tower which contains the vertical production line used for XLPE. Built in 1993 the XLPE tower, at over 100 m, is one of the tallest structures in Norway.

But as well as XLPE, which Nexans believes is destined to enjoy a steadily increasing share of the HV submarine cable market, the site also has a very strong track record in the field of paper insulated cables (both AC and DC) for submarine transmission lines. Indeed the site started life in the early 70s as the dedicated manufacturing facility for one such project, the 120 km, 250 kV Skaggerak HVDC link between Norway and Denmark. It has come a long way since then, with constant expansion and upgrade, and over the intervening years has been involved with many other pioneering submarine projects employing paper insulated cable.

A particularly impressive such project is the 450 kV, 700 MW, 580 km Statnett/TenneT NorNed interconnector between Norway and the Netherlands – the world’s longest HV submarine link – due to start operation around the end of 2007/beginning of 2008.

NKT was initially the preferred supplier for the deep (up to 410 m) section but lost the contract in 1999, principally because of delays in qualification of the technology it was proposing to use (subsequently leaving the HV subsea power transmission cable business altogether). The NorNed project itself was then postponed due to utility restructuring and a final decision to proceed was only taken at the end of 2004. In February 2005, Nexans received the contract for the deep sea portion, consisting of two 156 km long cables, the last of which has just been installed. ABB has supplied the two converter stations and cable for the shallower sections under a contract originally awarded in 2000.

Nexans has carried out significant development work on submarine HVDC mass-impregnated cable technology, as used for NorNed, and is today able to supply such cables at voltages up to 500 kV and capacity of of over 800 MW per cable.

Another path-breaking project for which Halden is supplying the submarine cable is the Valhall Power from Shore project, which will enable BP’s Valhall offshore oil an gas field to receive all its power needs from shore, eliminating the need for on-platform electricity generation. The distance from shore, some 300 km, means that Valhall is well within the window of opportunity for HVDC. The project is using ABB’s HVDC Light technology as employed (with ABB’s own cable) to drive compressors at the Troll A platform (see MPS, November 2005, pp 31-34).

For Valhall, Nexans is supplying and laying the 150 kV DC maritime cable, with integral return conductor (IRC), due for installation in 2008 and 2009. Similar IRC cable was delivered by Nexans in 2001 to the Moyle Interconnector, which links Northern Ireland to Scotland.

Other recent notable submarine power transmission projects for which Halden has provided paper insulated cable include the massive GCCIA (Gulf Countries Council Interconnection Authority) project to link Saudi Arabia and Bahrain, due to be commissioned in early 2010, which is being implemented in consortium with Prysmian of Italy (formerly Pirelli). The Nexans scope of supply includes 120 km of 400 kV AC SCFF (Self Contained Fluid Filled) submarine cable, 24 km of SCFF underground cable, 47 km of fibre optic cable, with 48 fibres, and the oil pressurising equipment.

Another collaboration with Prysmian is on the 500 kV 1000 MW HVDC SA.PE.I (SArdegna PEnisola Italiano) link between Sardinia and the Italian mainland, for which Halden is supplying around 325 km of the 840 km of mass impregnated paper insulated cable required.

Cable laying vessel

Halden is also the home port of one of the world’s highest capacity cable laying vessels, C/S Nexans Skaggerak, which can carry 7000 t of cable on its main turntable. At the time of writing Skagerrak had just completed laying the second 156 km continuous section of cable for NorNed.

Skaggerak, which Nexans purchased in 2006 from Bourbon Cable AS, the Norwegian subsidiary of French company Bourbon, having been the exclusive operator of the vessel over many years, is a very busy vessel booked up far into the future – reflecting the high level of activity in the submarine cable sector. Impending future projects include LIRC in the USA, Valhall off Norway and Hainan off Guangdong, China (see below).

Also based at Halden are the trenching/burial machines Capjet and Spider that can be deployed with Skaggerak. Capjet uses water jetting to fluidise the seabed for trenching and simultaneous backfilling. Spider, which uses technology adopted from Swiss forestry machines, was specially developed for travelling on the very steep inclines on the seabed at Ormen (a result of the Storegga landslide of about 8100 years ago).

Growth market

In commercial terms Halden can boast one of the best growth rates of Nexans’ 76 or so sites around the world, and over the period 2005-2007 50 million euros has been invested to increase production capacity there.

As a further reflection of Nexans’ bullish attitude to this market, its HV submarine cable production capacity has being further increased, by about one-third, through a production joint venture with Viscas of Japan called NVC (Nippon High Voltage Cable Corp), 66% owned by Nexans and 34% by Viscas (itself a joint venture of Japanese companies Furukawa Electric and Fujikura). Viscas’ existing Tokyo Bay cable-making plant has been transferred to NVC, which came into being in November 2006. The Tokyo Bay plant, established in 1992, manufactures XLPE and oil filled paper insulated AC cable and will be upgraded for the production of HVDC mass impregnated cables.

NVC will supply oil filled paper insulated cable for the 30 km 600 MW 525 kV AC submarine link that Nexans is providing between Hainan Island and the Chinese mainland (Guangdong). This is scheduled to start operation in July 2009 and is the world’s second 525 kV submarine power cable, the first being the link between Vancouver Island and mainland Canada (BC), also supplied by Nexans. Nexans’ scope of work on Hainan also includes oil pressurising stations, temperature sensing system etc.

Nexans believes the submarine power cable business is likely to continue to expand significantly over the coming years, driven by the need for more international interconnections between power markets, reinforcement of existing grids and new/improved connections to islands. There is also an increasing trend for oil and gas facilities, notably compressors, to be placed on the sea floor (as in the case of Ormen Lange) and for offshore platforms to be powered from shore. Looking a little further ahead another potential driver is likely to be growth of offshore wind generation, which is taking time to gather momentum, but seems destined to be a major renewable energy source in the future.