The Engineer

Power for North Sea platform is a shore thing

News

Statoil’s Gjøa platform has become the first floating platform in the North Sea to be supplied with power from shore rather than use power generated by gas turbines.

When electrical power was connected on 11 July 2010, the platform started to use what is claimed to be the world’s longest alternating current (AC) cable from land to a floating installation.

The nearly 100km-long cable will provide Gjøa with power from shore, allowing the platform to cut its carbon-dioxide emissions by 210,000 metric tons per year at full production.

The transmission link includes a static AC cable that delivers 40MW of power at 90kV from the Norwegian shore, at a depth of 380m over a distance of 98.5km. Then, a 1.5km dynamic cable connects the static cable on the seabed to the platform.

The dynamic cable weighs about 85kg per metre and has a total weight of around 130 tons. Suspended in the water between the platform and the seabed, it is subject to substantial mechanical stress and fatigue from the sea, wind and waves, which can move the platform as much as 75m in any direction.

Traditionally, a lead sheath is used to protect subsea cables from stress and water ingress, but on a dynamic cable, the lead sheath would wear very quickly and have a short service life. An alternative solution developed by engineers at ABB is claimed to withstand the extreme conditions, providing an operating life of at least 35 years.

In addition to the power cable, ABB also supplied the high-voltage equipment for the platform. Working closely with Statoil, the company developed both the transformers and the engine drives to save space on board.

The Gjøa platform receives its power via the ABB high-voltage AC cable from Statoil’s Mongstad complex on the Norwegian west coast. Mongstad comprises a refinery, NGL (natural gas liquids) fractionation plant and crude oil terminal.

The technology used to power such platforms from land - either via DC (direct current) or AC - has been developed as the result of a partnership between Statoil and ABB. Gjøa was the second installation on the Norwegian continental shelf to be supplied from land. Compressors on Statoil’s Troll A-platform have been supplied with electricity from land since 2005 via a 70km DC cable from Kollsnes. The next installation will be BP’s Valhall platform in 2011.

Statoil has been the operator of the Gjøa field during the start-up phase, but will hand over to GDF Suez in the fourth quarter of 2010.