A Norwegian application of cross-linked polythene insulated (XLPE) submarine cable has broken the record for the highest operating voltage used with this technology. Gunnar Evenset of Nexans and Jan Erik Larsen of Statnett detail the background to this project that carries power at 420 kV AC from the mainland to an island plant that processes gas from the Ormen Lange field.

Gunnar Evenset, Nexans & Jan Erik Larsen,Statnett, Norway

Ormen Lange gas field is one of the most challenging developments in the world. This pioneering project takes gas from the 40 km by 8 km reservoir that lies 3000 m below the difficult climatic and oceanographic conditions of the Norwegian Sea to the Nyhamna facility on Gossen Island on the west coast of Norway.

Here, a processing and compressing facility consumes a huge amount of power and exports 20 billion m3 of gas per year to the UK – around 20 per cent of that country’s requirement – through a 1200 km subsea pipeline. With other large power consumers nearby and the area lacking electrical generating capacity, Norwegian state-owned transmission system operator Statnett decided to build a 100 km long 420 kV overhead line to a new substation at Fraena on the mainland, from where a radial extension takes electricity to the gas processing plant.

CS Skagerak, Nexans’ specially designed vessel laying the XLPE subsea cable
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This extension includes 6 km of overhead line, a transition compound, 420 kV submarine cables, short sections of land cable and a new Nyhamna substation on Gossen Island.

Statnett awarded Nexans the €15 million ($23.3 million) contract to manufacture and install the submarine power cable system. The link was specified at 1000 MW to ensure that there is ample capacity to meet demand from Nyhamna – which is expected to increase to around 200 MW when large compressors are installed to compensate for the fall off in gas pressure that will occur as the gas in the reservoir is exploited – and to meet the requirements of the general industrial development of the region.

Statnett originally invited suppliers to offer fluid-filled cables as its base case, with XLPE as an alternative to promote the development of that technology, because expectations are that development of Norway’s grid will include a number of 420 kV submarine cables to cross fjords.

During contract negotiations with Nexans, Statnett decided to move ahead with XLPE as the base case and fluid-filled cables as a backup as long as the technology achieved certain milestones during the test programme. This was a major leap in the world record operating voltage for submarine XLPE cables, previously 170 kV, set by Nexans in 2002 at the Horns Rev wind farm in Denmark.

PoWER LINk choice

The choice of technology for this type of submarine power link is effectively between two options: fluid-filled cables or XLPE insulated cables. The first has been a proven performer since the late 1930s. The paper-insulated cables employ extruded insulation and have been refined constantly to deliver high voltages in a host of environments, including underground, in tunnels, in shafts and at deep ocean depths.

Fluid-filled cables use a tube-like stranded copper or aluminium conductor that carries oil under pressure that permeates the surrounding paper layers. This pressurization of the insulation ensures that its pores and gaps are filled by fluid under all operating conditions. For submarine cables, a lead sheath is the dominant method of preventing water penetration, with a semiconducting tape layer or semiconducting polythene jacket to prevent the lead sheath being punctured by over-voltages.

Submarine cables normally have armour protection of steel wire with an external layer of bitumen coating and polypropylene yarn layers to prevent corrosion. For deep-water applications, the armour consists of two cross-wound layers to provide a torsion-balanced design.

Normal submarine cable designs must be regarded as bonded at both ends, thereby incurring significant armour losses. For high-capacity applications, copper or aluminium armour may be specified where they meet mechanical design requirements. The armour has to be designed for the specific application.

The proven characteristics of fluid-filled cables include high reliability at very high voltages, a homogeneous insulation system ensured by fluid-impregnated paper and the ability to be custom-designed for tough environments.

Development of XLPE Cables

High voltage XLPE cables have been under development since the 1960s. They comprise a stranded copper or aluminium conductor with an extruded insulation system and various shields, sheaths and armouring. In common with fluid-filled cable the normal application has a lead sheath and the same basis for the choice of armour material.

Just like fluid-filled cables, they can be equipped with optical fibres integrated into the cable design/armour to allow utilities to perform thermal monitoring or even to optimize the true carried ampacity.

Cable joint ready for mechanical testing
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XLPE technology has several distinct advantages over traditional paper-insulated, fluid-filled cables, including the lack of need for an auxiliary fluid-pressure system, low maintenance, pre-made accessories for land applications, as well as repair joints for shallow water depths and less need for reactors for compensation at cable ends in the grid.

The use of XLPE cables is well established in land-based, underground applications, and maximum service voltages are well over 500 kV. However, despite the technical advantages, adoption of this technology for undersea applications has been gradual, mainly because of the much higher cost of intervention and eventual repair – in addition to the risk of a long outage – should a problem develop with a submarine cable compared with an underground cable. It is important for potential customers of submarine cable projects to understand the technical risk scenario related either to new technology or an upgrade of existing technology. Consequently, an extensive qualification programme that includes long-term testing is often required.

Golden Opportunity

For both Statnett and Nexans, the Ormen Lange project was a golden opportunity to gain experience with XLPE at 420 kV. Normally, the major obstacle to applying XLPE at extra-high voltage is the feasibility of creating flexible joints. However, for this project, the cable length is relatively short. Depending on the voltage and cross-section, the Nexans factory in Halden, Norway, can manufacture submarine XLPE cables in long single lengths of more than 20 km. In this installation, no flexible factory joint or field joint was required. But to provide a repair strategy, a rigid repair joint was designed and qualified as part of the project.

A steep incline at the cable landfall site at Nyhamna had to be accommodated
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Using XLPE cables in submarine applications presents a number of challenges. They need to be watertight and resistant to the corrosion and abrasion caused by sea currents and waves. In deep water, they must also withstand high pressures. The Ormen Lange cables are deployed at a maximum depth of 210 m, where they are subjected to a pressure of 2000 kPa. In contrast, when a fluid-filled, paper-insulated cable is used, the fluid pressure will nearly balance the water pressure. Laying cable at this depth also subjects it to great mechanical strain because of the long unsupported weight.

The previous world-record voltage XLPE installation of 170 kV was set with three-core cable in much shallower water. The main technical challenge for Nexans in the Ormen Lange project was to develop a cable capable of operating at a much greater voltage at increased depth. This required particular attention to the correct selection of sheathing and insulating materials with the right combination of strength and coefficients of expansion to maintain the water integrity of the cable, while accommodating the bending ratios required during installation.

Technical Solution

The Ormen Lange cables feature a single 1200 mm2 copper core and double armour of flat copper wires with a cross section of about 1930 mm2. The conductor is of traditional design for submarine XLPE cables. Its round wires are stranded in several layers and filled with a semiconducting water-blocking compound in the interstices.

The insulation system consists of a super-smooth conductor shield, insulation made of super-clean XLPE and an insulation shield. This system was applied in a quadruple cross head in a vertical extruder line. Both curing and cooling were carried out in an atmosphere of dry nitrogen. Water swellable tapes were applied between the insulation system and the lead sheath to prevent water penetration should the lead sheath be punctured.

The lead sheath is an F3 alloy applied in a discontinuous ram press. A plastic sheath of semiconducting polythene was applied on top of the lead sheath. The armour consists of two layers of copper wire embedded in bitumen.

The lay length of the two layers was selected to create a torsion-balanced cable design. Two layers of polypropylene yarn were applied outside the armour as corrosion protection.

Cable Design and Transition Joints

The design of the underground cables is identical to the submarine cables up to and including the lead sheath. The semiconductor polythene sheath has been replaced by an insulating sheath with a nominal thickness of 4.7 mm. This insulating sheath has a thin extruded semiconductor layer outside that enabled it to be voltage tested to verify single-point bonding performance.

Transition joints were installed close to each landfall. At Nyhamna, the steep incline meant that the jointing pit was some 70 m from the landfall, close to the top of the slope. The transition joints comprise a prefabricated EPDM body with a heavy outer protection that contains a pre-installed shield break.

The shield break insulation was designed for an impulse voltage of 125 kV since the separation between the transition joints was larger than 3 m. On Nyhamna, the cables have fluid-filled terminations in a gas insulated switchgear (GIS) substation. In the Hamneset transition compound, the cables are connected to an overhead line.

The cables are terminated with SF6-filled composite outdoor terminations installed in a concrete bunker. Since the submarine cable route is only around 2.5 km long, the cable lengths could be produced without factory joints. However, repair joints were included to enable the submarine cable system to be repaired should it be damaged at some time during its life.

Qualification Testing, Installation and Protection

An extensive test programme was established at the tender stage to qualify the 420 kV XLPE cable system for this project. This included mechanical and electrical testing according to appropriate standards. Tests were also performed to verify the fatigue life of the lead sheath for a design life of 40 years.

The following components were type tested: land cable, submarine cable, transition joint, GIS termination, outdoor termination and submarine repair joints.

A total of four submarine repair joints and two transition joints were type tested to prove reproducibility. After successful completion of the type test programme, a long-term test programme featuring the same components in a test loop was initiated at the outdoor test facility in Calais, France.

Nexans’ own cable ship the CS Skagerrak installed the submarine cables. This vessel has a capacity of 7000 tonnes of cable and is equipped with a dynamic positioning system. A remotely operated vehicle was used for touch-down monitoring during the laying operation.

The submarine cables were pulled into 200 m long pipes in the wind and wave exposed landfalls at Hamneset on the mainland. Due to a steep slope from 30 m to 200 m water depth on the Nyhamna side, the cables were mechanically secured by a hang-off arrangement to the sea floor on the top of the slope. The cables were protected by jetting with the Capjet system to a depth of up to one metre depending on the bottom soil conditions.

The cable system consists of four cables, totaling 13.2 km in length, with around 2.1 km of each length deployed at depths of up to 210 m below sea level. Nexans also supplied a fibre optic cable for control and telecommunications, and this was strapped to one of power cables during installation.

To verify that the installation of cables and accessories had been successful, the whole cable system was subjected to an AC voltage test using variable frequency test equipment. KEMA supplied the equipment and performed the test. The sheaths on the underground cables were also subjected to a 10 kV DC test. The link was energized in December 2006.

Meeting the Needs of Norway

The world’s first 420 kV XLPE submarine cable system was successfully qualified, produced, installed and commissioned between mainland Norway and the Island of Gossen. Repair joints for the submarine cable system have also been designed and qualified. The 1000 MW rating of the link will cover the needs of the Ormen Lange onshore process plant, as well as future development in the region and is also designed to become part of the future 420 kV main grid.