In common with polar explorer Roald Amundsen's ship of the same name, Statoil's Gjøa platform has entered the history books with its pioneering technology. When electrical power was connected on 11 July 2010, the platform started to use the world's longest alternating current (AC) cable from land to a floating installation.
This is also a milestone for ABB, which had to simulate a whole range of concepts and dimension systems to ensure the platform receives the quality and stability of power supply it needs.
"Anticipating the load factors (strain) the electrical system might be expected to endure is particularly important for subsea power connections, because they are subject to a great deal of stress and repairs can be complicated," says Asmund Mæland, Global Manager for Electrification within ABB's oil and gas unit.
ABB is ensuring the milestone Gjøa platform project receives a stable supply of quality power. The nearly 100-km long cable will provide Gjøa with reliable power from shore, and means the platform will cut its carbon dioxide emissions by 210,000 metric tons per year at full production. A traditional solution with power generated by gas turbines on board would have generated emissions equivalent to 100,000 cars every year.
The transmission link includes a static AC cable that delivers 40 megawatts of power at 90 kilovolts (kV) from the Norwegian shore, at a depth of 380 meters, over a distance of 98.5 kms. Then a 1.5 km dynamic cable connects the static cable on the seabed to the platform. It is this short dynamic cable that constitutes the ABB innovation and makes the entire project technically and economically feasible.
The dynamic cable weighs about 85 kg per meter and has a total weight of around 130 tons. Suspended in the water between the platform and the seabed, the cable is subject to substantial mechanical stress and fatigue from the sea, wind and waves, which can move the platform as much as 75 meters in any direction.
Traditionally, a lead sheath is used to protect subsea cables from stress and water ingress, but on a dynamic cable like this the lead sheath would wear very quickly and have a short service life. ABB developed an alternative solution that withstands fatigue and the extreme conditions, and is expected to have an operating life of at least 35 years.
In addition to the power cable, ABB has also supplied the high-voltage equipment for the platform. Working closely with Statoil, ABB has developed compact electrical solutions for transformers and engine drives in order to save space on board.
The cable from land makes it possible to supply the platform off the coast of Sogn and Fjordane County with renewable energy from the Norwegian electricity grid. Power from shore is an environmental preservation measure backed by major environmental organisations in Norway.
"This is a necessary environmental measure to reduce emissions on the Norwegian continental shelf, and it is inspirational that we can make a contribution with our technology. ABB is involved in all four electrification projects running here, namely Troll, Gjøa, Valhall and Goliat," states Mæland.
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 for power supply to platforms from land, either via DC (direct current) or AC, has been developed in a close 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 70 kilometer DC cable from Kollsnes. The next installation will be BP's Valhall platform in 2011.
"We have extensive competence in the development of advanced electrical systems, and have long experience with project execution. This means that we are well prepared for the challenges that always arise with such projects, where challenging the technological boundaries is commonplace," concludes Mæland.
Statoil has been operator of the Gjøa field during the startup phase, but will hand over to GDF Suez in Q4 of 2010.
Roald Amundsen's "Gjøa" became world famous as the first ship to sail through the Northwest Passage, early in the 1900s. The ship was built at Rosendal in 1872, just a few miles from Stord, where the modern Gjøa platform was recently completed at the Aker Solutions yard.
