Goliat subsea power cable designed for long-distance service in harsh conditions
Nick Terdre
Contributing Editor
ABB has won a contract to supply subsea power cables for Eni Norge’s Goliat development in the Barents Sea. The NOK 676-million ($104 million) assignment covers design, manufacture, and installation, the latter scheduled for summer 2013.
Taking power from shore to run offshore production operations should bring environmental benefits. Even though Eni also will use a gas turbine on the platform, it expects carbon dioxide emissions to be halved compared with a solution based on offshore power generation alone.
It is also looking to the future beyond 2014, when Goliat is due to come onstream, and its long-term plan for the platform to serve as a hub for the tieback of other fields in the area. The cable will be engineered to deliver up to 75 MW at a voltage of up to 123 kV, but, according to Eni, power transmission in the project’s initial stages will be significantly below capacity.
Power will be transmitted as alternating current. Higher power ratings can be achieved through the transmission of direct current, as in ABB’s HVDC (high-voltage, direct current) Light technology. This solution has been supplied to Statoil’s Troll A platform and BP’s Valhall redevelopment in the North Sea, but it requires converter stations at either end, adding to the weight of the offshore equipment and also to the cost.
ABB’s previous delivery of an AC subsea power cable to a Norwegian project was for Statoil’s Gjøa development in the North Sea. That cable, installed this spring, shares common features with the Goliat cable: both will transmit power over a similar distance – 106 km (66 mi) for Goliat compared with 100 km (62 mi) for Gjøa – and both involve sections hung from floating platforms in a water depth of 380 m (1,247 ft).
ABB is a leading player in the design and manufacture of subsea power cables in the Norwegian offshore development sector.
When the Gjøa cable was designed, according to ABB’s Project Manager Magnus Larsson-Hoffstein, its power rating of 40 MW was close to the practical limit for an AC solution over a distance of 100 km. Although the design for Goliat will be similar, improved analysis methods and more favorable thermal conditions allow ABB to increase the power rating.
The Goliat cable will be based on ABB’s high-voltage XLPE (cross-linked polyethylene) design, claimed to exhibit low electrical losses, good resistance to oil, solvents and abrasion, and high strength.
Detailed design is under way. A major challenge is the dynamic section which hangs in the water column between the platform and the seabed, and which will be subject to fluctuating loadings from ocean currents, waves, and the movement of the platform. This section of the cable will be about 1.5 km (0.9 mi) long, and with a weight of some 90 kg/m, will have a total weight in air of around 135 metric tons (149 tons). To reduce the loadings, it will be hung in a wave formation with the aid of buoyancy elements.
Lead will be applied as protection against water ingress in the static section which lies on the seabed, but lead cannot be used in the dynamic section due to the latter’s sensitivity to fatigue loads. Instead, ABB has developed a copper sheath application for this purpose. A prototype section will undergo six months of flex testing to ensure it can withstand the loads foreseen during the cable’s 30-year design life.
The hang-off from the platform will also be different from ABB’s earlier experiences. The Goliat platform will be a Sevan Marine cylindrical FPSO, with the cable passing through an I-tube within the deck structure.
The cable, which will be manufactured at ABB’s Karlskrona plant in southern Sweden, will transmit power to Goliat from a transformer station on Norway’s northern coast near the town of Hammerfest, where the Goliat operations base will be located. Here, power will be taken from the local grid and stepped down, probably to around 110 kV in the initial period. On arrival at the platform it will be further stepped down to 11 kV.
Two fiber-optic cables will be integrated into the cable. These will make it possible to monitor the temperature of the conductor at the most important locations, close to the platform and close to shore. •
For more information contact Magnus Larsson-Hoffstein, ABB AB High Voltage Cables. Tel +46 455 55 949, fax +46 455 82 245, [email protected], www.abb.com
ABB charters cable-lay vessel
The Goliat power cable will be installed in 2013 by Aker Solutions’ new cable-lay vesselAker Connector. ABB has agreed to charter the vessel, currently under construction and due for delivery in 2012, for two years with options to extend the contract a further three years.
The vessel, originally intended to be part of Aker Solutions’ deepwater subsea well intervention fleet, will be outfitted instead for installing heavy power cables over long distances. Aker Solutions also will provide a range of related engineering, project management, and installation services for the execution of marine and offshore projects, according to ABB.