A £130 million ABB contract with Statoil is the first in the world to bring power from the mainland to an offshore platform using HVDC Light technology. It offers cost and space savings, as well as environmental advantages
The Troll A pre-compression programme, run by production operator Statoil, has just been completed. The total project, which cost around £269 million, is part of a long-term plan to expand the capacity of the platform that supplies some 40 per cent of Norway’s total gas output and is a major source of supply to continental Europe.
The Troll gas development consists of a processing plant at Kollsnes, north of Bergen, and the Troll A platform some 70km offshore in the North Sea. This platform is the only installation on the Norwegian continental shelf powered by electricity transmitted from the mainland.
Troll A, which is the tallest moveable structure ever built, started production in 1996. In 2002 a programme was begun to expand the compressor capability on the platform, raising processing from 85 million cubic metres to 100 million cubic metres per day. As wellhead pressure declines, further investment will be necessary. Two more phases are likely in 2010 and 2024.
The pre-compression installation contract with ABB includes the HVDC Light connection plus two 40MW compressors with associated utilities located on the platform.
LAND-GENERATED POWER SAVES MONEY, SPACE AND THE ENVIRONMENT
The decision to take power from the mainland rather than install generators on the platform was taken for economic and environmental reasons.
Conventional platform-based generation, with gas turbines or diesel generators, usually achieves just 25 per cent efficiency compared with land-based combined-cycle generation of 75-80 per cent.
On-platform power generation produces large amounts of CO2. In the case of Troll A it would result in an estimated 230,000 tons of CO2 and 230 tons of NOx and potentially substantial climate tax liabilities.
According to Asmund Maeland of ABB, Oslo, 'Transmitting high voltage electricity as alternating current (AC) becomes impractical beyond 50km or so with power above a few tens of megawatts'.
'Conventional high voltage direct current (HVDC) systems have also been considered. These are well established on shore as conventional load-commutated transmission systems, but the size and weight of a typical HVDC station and the complexity of control during start-up have prohibited their use on offshore platforms'.
Over the past eight years, ABB has developed an alternative solution for high voltage transmission known as HVDC Light, based on state-of-the-art solid state power transmission technology. The main attractions of the system have already been demonstrated in several commercial onshore applications - namely good controllability, low maintenance, high efficiency and long lifetime in installations ranging from 40MW to 330MW.
Other characteristics make HVDC Light particularly suitable for offshore applications - notably that the equipment is relatively compact and lightweight and there is no physical limit on transmission distance. A new generation of subsea power cables is making underwater connections from the shore a more attractive option for economy and installation flexibility. ABB has an extruded polymer DC cable, which is highly reliable, cost-effective and flexible, and an important part of the HVDC Light concept.
Most critically, with HVDC Light the voltage variation in the cable is smaller, and the DC-AC inverter at the platform receiving end is able to compensate for this small variation so that AC voltage is effectively constant.
In February 2005, the two HVDC Light systems were successfully commissioned and tested ready for scheduled start-up of the Troll A gas precompression project in the last quarter of 2005.