Electric motors to cut emissions, maintenance on Troll compression project

The supply of ABB electric motors to Statoil's Troll compression project follows the recent trend offshore of using electric drives in place of mech-anical drives.

The Troll delivery comprises two very high voltage synchronous motors manufactured by ABB Automation Technologies at its plant in Västerås, Sweden. They are identical 40-MW AMT 1600 units with a rated voltage of 56 kV and a speed of 1,260-1,890 rpm. Both units weigh 110 tonnes. The first was shipped in January and the second was due to be delivered this month.

The motors will be installed on the Troll A gas platform. Early in 2005, they will undergo spin tests, and should start operating later that year. By then the Troll gas reserves will have been in production for 10 years, and a boost will be required to help the gas flow 70 km to shore, from where the motors will be powered.

Power to drive the motors will be provided in direct current via four subsea cables. On arrival at the platform it will be converted to alternating current at an HVDC Light inverter station. The HVDC Light technology has been developed by ABB to enable voltage source converters to be connected to networks at higher voltage levels than ever before for power transmission. The subsea cables involve a novel extruded cable which also forms part of the HVDC Light development.

Near-100% efficiency

Electric motors provide numerous advantages over the gas turbines that power most North Sea platforms, according to Johannes Ahlinder, business development, AC Machines, Motors and Machines. For instance, they can operate for several years without a major overhaul, whereas a gas turbine needs more frequent maintenance.

Traditionally gas turbines have been used as the mechanical drive for large compressors in the offshore industry. However, they are much less efficient than electric motors, with a rating of 35-40% compared with 97-99% for synchronous motors. Another relevant factor is that producers nowadays see gas as a valuable commodity in its own right, which may yield greater value through being sold than used as fuel.

Electric motors are also environmentally beneficial in that they generate no damaging emissions, unlike gas turbines. In Norway there is a penalty to pay in the form of a dedicated tax on carbon dioxide emissions.

These three factors – maintenance, efficiency, and environmental benefits – can be seen as the main reasons behind the current trend in the offshore industry of replacing mechanical drives with electrical drives, Ahlinder says.

There is also a trend among electrical drives, he points out, for variable-speed drives to be chosen over fixed speed motors. With a variable-speed drive the speed can be optimized from a process point of view, in other words it can be adjusted to match the need. This is a benefit that Statoil will receive with its new AMT motors.

Soft, smooth start

Another benefit, in the case of local power generation offshore, is that the motor can be soft-started. Given that offshore power networks are often relatively weak, start-up of a large motor can be difficult for a fixed speed motor, but in contrast can be performed gently with a variable- speed motor. This is turn implies that a voltage drop in the local power network is avoided when starting a large motor.

The Troll motors employ four poles. In general this is more efficient than a two-pole solution, and significantly lower cost, as it is less complicated to produce. This results in a smaller and much lighter motor, which is an important consideration for an offshore application.