ABB Review Special Report - ABB - ABB Group

On most offshore installations, thepower generators and large compressorsare driven by onboard gas turbinesor diesel engines with total efficienciesthat can be as low as 20–25 %even under ideal conditions. As aresult, fuel consumption and CO 2 emissionsare unnecessarily high. Ever sincethe Kyoto Protocol, which allows tradingof greenhouse gas emissions, highCO 2 emissions have become a costfactor. On top of this, as on the Norwegianshelf, there may be CO 2 taxation,making emissions costly evenwithout trading.If the electrical power for all this equipmentcan be supplied from shore, theCO 2 emissions of offshore installationsare eliminated, saving operators a considerablesum of money. But that isn’tall; transmitting electrical energy fromshore is also more efficient in terms ofequipment maintenance, lifetime andavailability.The overall environmental bonus ofeliminating low-efficiency offshorepower plants is considerable. A landbasedcombined cycle gas powerplant, which utilizes the gas turbine’swaste heat, can have an efficiency ofas much as 75 to 80 %. Even if highlosses of 10 % are assumed for a longtransmission line to an offshore installation,the saving will still be significantfor most installations.1HVDC Light and Motorformerjoin the offshore clubTroll A is the largest gas productionplatform on the Norwegianshelf. Some 40% ofNorway’s total annual gasproduction comes from TrollA, which can produce up to100 million cubic meters ofgas per day. Today, the reservoirpressure drives the gas to the onshoreprocessing plant at Kollsnes, where thecondensate, water and gas are separated.The gas is then compressed andtransported through pipelines to theEuropean continent 1 .As the gas is taken out of the reservoir,the pressure inevitably decreases. Thismeans that to maintain productioncapacity, offshore precompression of thegas will eventually become necessary.ABB has been awarded two contracts aspart of Statoil’s Troll A PrecompressionProject: a US$ 185 million contract forthe compression equipment and a US$85 million contract for the electric drivesystems for compressors. The new installationis due to go into commercial operationin the fall of 2005 as part of a programintroduced to maintain and expandthe platform’s production capacity.Choosing conventional systems for thisproject would have meant that gas turbineswould drive the compressors. Inthat case, it is estimated, annual emis-Gas from Troll A is processed at Kollsnes before being transported to theEuropean continent.sions of some230,000 tons ofCO 2 and 230 tonsof NO x would result.Besides theirimpact on the environment,the CO 2taxation in effecton the Norwegianshelf means that suchemissions would also bea significant cost factor.Working with Statoil, ABB developed analternative system 2 based on two innovativeABB technologies – HVDC Lightand Motorformer. These have beensuccessfully employed on shore since1997 and 1998, respectively, but neverbefore on an offshore installation ortogether as an electric drive system. Thesystem uses power from the onshoreelectrical grid to drive the compressorson Troll A, thus eliminating greenhousegas emissions from the platform.HVDC Light – rectifying, invertingand controllingHVDC Light [1], by using series-connectedpower transistors, enables voltagesource converters to be connected tonetworks at voltage levels higher thanever before for power transmission,reactive power compensation and harmonic/flickercompensation.On Troll A, an HVDC Light converter(inverter) feeds the variable-speed synchronousmachine driving each compressorwith AC power obtained byconverting the incoming DC, which istransmitted from shore over submarinecables. As their speed is variable, thecompressors are supplied with power atvariable frequency and voltage, rightthrough from zero to maximum speed(at 63 Hz) and from zero to maximumvoltage (56 kV), including starting, accelerationand braking. The drive systemsperform equally well at each endof the frequency spectrum. Small filtersat the converters’ outputs keep themotor winding stress at a safe level.The inverter control software is adaptedfor both motor speed and torque control.The motor currents and voltagesand the rotor position are measured and54Special ReportABB Review