Alain Romand, Nexans, France
A consortium formed by Pirelli and Nexans has started work on a g117 million, two-year turnkey contract to construct a submarine power link across the Strait of Gibraltar to interconnect Spain and Morocco’s power grids.
The Spain-Morocco II contract, which is split on a 50:50 basis between Pirelli and Nexans, was the largest contract awarded to the submarine power industry in 2003. It is being carried out on behalf of the Moroccan power company L’Office National de l’Electricité and Spain’s Red Electrica de España. The 400 kV, 30 km link will be partially financed by the European Bank of Investments (BEI) and the African Bank of Development (AFDB) and will add an additional 700 MW to the existing submarine power transmission link between the two countries, taking the total capacity to 1400 MW.
The first submarine interconnection between Spain and Morocco, which was established in 1997, was the first such link between Europe and Africa. Historically, Morocco has tended to import substantial amounts of energy via this link. For example, of the country’s total power consumption of 14 000 GWh in 2000, 2268 GWh was imported from Spain. However, in 2001 Morocco exported power to Spain for the very first time and this trend is expected to increase as Spain, which has some of the highest electricity prices in Europe, seeks to import cheaper power. The new link will significantly improve the interconnection between the power grids of Europe and north Africa, and with the future development of a pan-African grid it will facilitate the export of surplus hydroelectric power from countries in central and southern Africa.
The cable link will run from Tarifa in Spain to Fardioua in Morocco
An extensive investigation of potential cable routes had been carried out for the first link in 1997. This included a detailed marine survey on a 2 km wide corridor in the Strait of Gibraltar to investigate the bathymetry, geomorphology of the seabed, water salinity, temperatures and sea current profiles. At that time it was determined that the optimum routing and alignment of the power cables to provide maximum reliability for the submarine cables and minimal environmental impact would be between Tarifa, Spain and Fardouia, Morocco. The new interconnection will therefore follow the same route, but at a spacing of approximately 1.5 km, over a distance of 30 km and a maximum seabed depth of 615 m.
Three power cables will be laid – a total cable length of 90 km – together with two telecommunication fibre optic cables, both for system control and broadband data transmission. The customers have decided that 400 kV AC operation is the most cost effective approach for both the existing and the new links.
However, the interconnections have been designed to be suitable for both AC and DC transmission, and this offers the possibility of upgrading to a total capacity of 4000 MW in the future via HVDC operation.
In AC use, the new interconnection is designed to transmit 700 MW with athermal overload capability of 900 MW for 20 minutes.
The Skagerrak cable laying ship is capable of loading up to 6500 t of cable
Submarine power cables
The submarine power cables for the new project will be the same self-contained, low pressure oil-filled design as the existing cables with an 80 cm2 copper core. They feature a paper dielectric impregnated with a low-viscosity oil and incorporate a 2.4 cm diameter central cylindrical duct to permit oil flow along the cable. The cables will be lead sheathed, reinforced with bronze tapes covered by a polythene anti-corrosion jacket and double armoured with copper wire. The cables will have an overall diameter of 139 mm and weigh approximately 56 kg/m.
The main reason for selecting more traditional oil filled cables, rather than newer XLPE cable technology, is that lead covered cable offers much greater structural stability at deep water depths. This is vital to ensure that cable joints are strong and flexible enough to withstand both the weight of the cable and water pressure, which can reach 69 bar. Pirelli will manufacture its portion of the submarine link at its Arco Felice (Naples) plant in Italy while the Nexans cables will be manufactured at the specialist Halden plant on the Oslofjord in Norway.
The oil feeding system for the power cables will consist of a pumping station at each end of the circuit. These stations have the capacity to compensate for any oil volume change in the cables that may occur because of variations in temperature as a result of load changes and/or environmental conditions in the Strait of Gibraltar. In addition, the system will have the design capacity to maintain the oil supply for a period of 21 days in the event of a cable being severed and exposed to seawater. During normal operation, the pressure control system will maintain a positive oil pressure with the capability to switch automatically to flow control mode, should the pressure fall below 15 bar, in order to compensate for transient conditions and prevent seawater penetration. The power for the oil pumping stations will normally be provided by the local network, with a diesel generator and DC power supply for emergency backup.
Two submarine optical fibre cables will be installed along the route. Each cable comprises eight single-mode fibres for high capacity transmission in a loose structure reinforced with a steel strand, encased in a welded copper sheath with wire armour protection. These cables will be bundled to two of the three power cables. During deep-sea cable laying, the tension applied on the power cable will be about 26 tons.
The cables for the two land sections of the interconnection will have a copper core cross-section of 1.6 m2 and will be similar in construction to the submarine cables except that they will be unarmoured. The increase in the cross-sectional area of these cables is to compensate for the more severe thermal conditions than the submarine section, ensuring that the transmission capacity of the interconnection is maintained.
The cables will be installed by the Pirelli cable ship, Giulio Verne while the Nexans cables will be installed by its ship Skagerrak. Following manufacture, the cables will be transferred to large storage turntables close to the dockside landing piers adjacent to each plant. The cables will then be transferred directly to corresponding turntables on the cable laying ships – the Skaggerak is capable of loading up to 6500 t of cable. The pulling capacity during cable laying, provided by the cable capstans, is 50 t.
When cable laying commences, the vessels will be positioned approximately 800 m from the landing beach, paying out the cable and supporting it at regular intervals by floats. At the Tarifa (Spain) landing, which has a sandy beach, the floating cable string will be kept in position by rubber boats to maintain the cable as straight as possible between the vessel and shore. On this side, cast iron shells will be used to protect the cable.
On the rocky Fardouia (Morocco) landing, the cables will be pulled into pre-installed steel pipes, each approximately 100 m long. On completion of the cable landing operation, the laying vessels will commence their track along the planned route, laying the submarine cables on the sea bottom. During laying, the vessel position and the cable tension, which is a function of water depth, will be constantly monitored and controlled. ROVs (remote operated vehicles), supported by the DP vessels, will follow the cable touch down point. This real-time video observation of the cable status will enable minor adjustments in the cable route to avoid it resting on top of isolated obstacles or long cable-free spans due to changes in the sea-bed features.
The Nexans cable will be manufactured at Norway’s Halden plant
On completion of the submarine cable laying operation the ROVs will be used to locate and measure the free spans. Any free spans greater than 10 m will be rectified by dumping small rocks beneath the cable, using a fall pipe system capable of operation in deep water.
Different forms of protection for the submarine cables near the shore on both sides of the Strait will be used. On the Spanish side, Nexans will use its remotely operated Capjet 1000 machine. This system uses the water-jetting principle both for trenching and propulsion. As the cable sinks into place, the fluidized material which was removed is used to backfill the trench. The whole process represents no risk of damage to subsea lines or structures. The cable will be buried at a depth below the sea-bed increasing from 1 m at a distance of 80 m offshore to 3 m at the on-shore-jointing bay.
On the Moroccan side, the rocky and uneven sea bottom will require the submarine cable to be protected for the final 1600 m of the route using steel pipes and concrete mattresses, which will extend to a water depth of 30 m. Accomplishing the cable laying operation across the busy shipping lanes in the Strait of Gibraltar, will require careful planning and coordination with the marine traffic control centre located in Tarifa.
The submarine cables will be directly connected to the land cables via transition joints, constituting a single electrical and fluid feeding system. The land sections will be terminated with outdoor sealing ends. These will incorporate porcelain insulators of a design that will permit DC operation of the interconnection in the future.