LONDON -- It might be surprising to learn that Norway’s tallest building is Nexans’ 120-metre extrusion tower at the company’s submarine high-voltage direct current (HVDC) cable factory in Halden. Nexans makes subsea cables that connect offshore wind farms to the grid, transports them around the world and installs them underwater so that the cables can bring clean power from offshore wind farms to onshore substations and from there to our homes.
Several different types of cable are used in offshore wind projects. Low (up to 1 kV) and medium-voltage loop cables transmit the electricity produced in the turbine’s generator to the transformer, usually located at the tower’s base. Then array cables connect the turbines on a wind farm to each other and export cables carry their power to the grid. Finally, underground and overhead line (OHL) cables that make it all work on land.
Offshore wind export and inter array cable types. Credit: Nexans.
On a recent tour of the factory, sponsored by Nexans, guides explained that the high tower at the Nexans factory houses the vertical extrusion machinery that begins the cable-making process. From “clean rooms” at the top of the tower, superclean polyethylene and cross-linkable, super-smooth “semicon” are fed through a closed system of huge tubes back down to an extruder at ground level, where the conductive material and insulation are spit out simultaneously from multiple extruders that feed into a single head. The tower can produce 15 km of cable in one week before the workers have to stop the process to change the enormous receiving baskets.
The height of the tower is important because all of the heat must be removed from the materials before they enter the tube. Curing and cooling takes place in a dry atmosphere of pressurized nitrogen in the building before the materials are fed from the tower to various stations in other buildings through “cable ways” which are little wheeled tracks running across and between buildings.
Using copper, aluminum, lead and wire, the materials are formed into cable lengths weighing up to 400-500 kg. At the end of the process the lengths are combined using proprietary joints to make 60-70 km cables.
After several more processes involving insulation and strengthening of the cables, they are tested for resilience and torsion. As a wind turbine’s nacelle rotates, the cables are severely twisted, so they must be extremely resistant to both torque and vibration. The torsion tests on cables simulate 20 years of use in a wind installation. Nexans said the exact test applied to a given cable depends on the customer’s specifications.
A Challenging Market
The wind industry’s move to deeper waters is challenging, according to Nexans, because transport vessels can only hold so much cable. Nexans’ flagship transport and laying boat, the Skagerrak, holds 50 tons of cable on its built-in turntable. The Skagerrak can accommodate 65 workers and has travelled all over the world. Not many vessels can hold its capacity, according to the company, and there are just one or two others in the world including the Giulio Verne, belonging to Nexans’ main competitor Prysmian.
The Nexans "Capject" can dig trenches in soft or hard sediments, according to the comany, and is able to operate in depths of up to 1,000 meters. Credit: Nexans.
With wind farms moving further offshore, said Vincent Dessale, chief operating officer of the submarine high voltage business line, Nexans’ customers are seeking increasingly higher transmission capacity, which means producing larger and longer cables. The Halden plant ran into problems in 2012, with an invoice delay in submarine cables leading to a drop in Nexans stock and an eventual restructuring of the business. The company has learned some lessons, it said, including that “feeding in more machines and manpower to match market demand is not sufficient” and that “coping with growing complexity and increasing timeline uncertainty requires highly structured organization, robust processes and the right mindset,” said Dessale.
Another challenge is that cables are becoming increasingly important in risk management. “One of the key differences between offshore and onshore wind farms, at the concept and design phase is the need to consider cable failure when designing the electrical architecture,” said David McNaught, senior engineer at consultancy Frazer-Nash. “If a submarine cable fails in service the consequences for the operability and profitability of the wind farm could be dire; especially if there are delays in securing a suitable repair vessel or if weather conditions are severe, likely during the winter months.
“It is essential that the electrical cable systems of wind farms have high reliability – that the system has the ability to withstand unforeseen circumstances,” McNaught continued. Cable risk is a relatively new aspect of wind project financial analyses, he said, but it is increasingly being considered – to the point where new guidelines from GL Renewables Certification, published in January, include on-site and power export cables. To address this growing concern, Nexans said it has scaled up risk analysis at the tendering stage and the company is working to develop and implement risk mitigation before beginning production.
Another challenge is transport for larger and longer cables. The current Skagerrak, the third in its line and 130 km, was built in 1993; the Skagerrak 4, which is expected to be complete in 2014, will be 140 km.
Coils of cable at the base of the Nexans' 120-meter extrusion tower at the company's submarine high voltage direct-current (HVDC) cable factory in Halden, Norway. Credit: Nexans.
The market is growing in complexity, too, said Dirk Steinbrink, executive vice president for high voltage and underwater cables. The project scope of Nexans’ work has expanded to offer not just cables but turnkey interconnection solutions, he said.
On the Northwind project, which is expected to be completed before the end of 2013, Nexans is contracted to supply cables to connect the Belwind 2 offshore wind farm to Northwind, and Northwind to the shore. The project’s scope includes cable design, testing, supply, jointing termination work and on-site testing (called cable witnessing). The company said that it would use the largest cable ever manufactured at the project site, a 1-meter wide, 30-kg behemoth.
Offshore wind farm developers must also consider the social impact of the installation process. “The acceptance level from people living [near a site] is quite low,” Steinbrink said. “They like green energy but don’t want to see us doing the work. So we do micro-tunneling, especially in places with tourism.”
Frédéric Michelland, senior executive vice president for high voltage and underwater cables, North and South America, does not expect the market for wind turbine cables to evolve dramatically over time. Today, he said, Nexans covers 80 percent of the European market, while “tomorrow that will move to North America and China – but we expect our market to remain largely European.” In Europe there are “still plenty of projects where most of the action will take place,” he said.