Onshore power link cuts weight and cost of Gjøa platform

The project was initiated by Statoil to gain substantial environmental benefits - power generated offshore with very low efficiency is replaced by power generated onshore with very high efficiency and carbon dioxide removal. The solution also reduces platform weight and total cost, making it very interesting in a broader perspective, according to Statoil.

The Gjøa cable will be 98 km (61 mi) long and transmit 40 MW of power at 90 kV. In addition to engineering, procurement, and manufacture, ABB is also responsible for installation, scheduled for 2009. Gjøa is due onstream in late 2010.

The cable will be based on ABB’s high-voltage XLPE (cross-linked polyethylene) subsea cable, which has low electrical losses, is resistant to oil, solvents, and abrasion, and has excellent tensile strength, according to the company. More than 2,300 km (1,429 mi) of this cable have been delivered to projects around the world.

ABB is a leading force in the supply of power-from-shore systems in the North Sea. In 2005, the company supplied its HVDC (high-voltage, direct current) Light technology to Statoil’s Troll A to provide power for a pre-compression project, and currently it is supplying converter stations for a power-from-shore system to BP’s Valhall redevelopment project.

For Gjøa, power will be transmitted in alternating current form, so that there will be no need for AC-DC conversion either at shore or on the platform.

Power for Gjøa will come from a new combined heating and power plant that Statoil is building at the site of its Mongstad refinery in western Norway. When Gjøa is operating at full capacity, the combination power from shore with the Mongstad CHP plant will eliminate the production of about 230,000 metric tons (253,531 tons) a year of carbon dioxide emissions.

Transmission to Gjøa will start at a sub-station at Mongstad, where power arriving at around 132 kV will be converted to 90 kV. On arrival at the platform, it will pass through transformers for distribution at 11 kV and in high voltage. ABB has other substantial deliveries of electrical equipment to Gjøa, including drives, drive transformers, and HV and medium-voltage switch-gear.

The platform will be in a water depth of 380 m (1,247 ft). While the water depth presents no special problems for the static part of the cable, which lies on the seabed, there are serious challenges for the dynamic part, the section that hangs in the water column between the seabed and the platform, Jonsson says.

This section will be about 1,500 m (4,921-ft) long, and given that the cable weighs about 85 kg/m, will have a weight in air of 130 metric tons (143 tons). Hanging in the water column, it will be subject to loadings from sources such as sea currents, waves, and the movement of the platform itself, which will affect its fatigue life. It is also essential to ensure that the cable is watertight.

Finding an acceptable solution took a year of studies, Jonsson says. Buoyancy elements will be used to configure the cable in a wave formation, thus reducing the loadings.

Traditionally, lead is used to protect a subsea cable from water ingress, but in a dynamic application, it could be subject to excessive fatigue loading. A new (confidential) solution has therefore been specified for the Gjøa cable, Jonsson says. Laboratory testing of a prototype section of cable was carried out to verify both its fatigue life and watertightness.

Statoil considered both AC and DC power supply for Gjøa. While most of the platform’s requirements will be met by power from shore, the gas export compressor will be powered by an offshore turbine. Heat from the exhaust stream will be captured and used, giving high-efficiency use of the turbine, according to Statoil. This solution also means that the imported power requirement is reduced and can be met by AC power.

Transmission of 40 MW over a distance of nearly 100 km (62 mi) is close to the practical limit for an AC solution, according to Magnus Larsson-Hoffstein, ABB’s project manager for the Gjøa cable. The limits can be extended by means of reactive compensation, but this adds considerably to the cost and complexity, making the AC solution less competitive.

Following the signing of the contract in June, detailed engineering for the Gjøa cable is now under way. The cable will be manufactured early next year at ABB Power Technologies’ cable manufacturing facility at Karlskrona in southern Sweden. Installation will be subcontracted to OceanTeam, with which ABB Power Technologies has a long-term agreement.

The environmental benefits constitute an important driver for power-from-shore solutions, not only for the offshore oil and gas sector but also offshore wind farms, according to Jonsson. ABB has several ongoing deliveries to offshore wind farms both in the North Sea and Baltic Sea.

Interest in such solutions for offshore oil and gas projects is also growing in other parts of the world, such as West Africa, the Middle East, and the Far East. ABB already has one subsea power cable installed in the Arabian Gulf, which serves Saudi Aramco’s Abu Safah field 50 km (31.07 mi) offshore.

For more information, contact Hans Åke Jonsson, ABB. Tel +46 455 556 00, fax +46 455 556 55,[email protected], www.abb.com