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An 800kW drive from Vacon, a provider of AC drive technology, is making the 70,000-tonne liquid natural gas transport ship LNG Abuja much easier to manoeuvre, particularly when it is in port.

The drive, which was fitted and commissioned by marine power plant specialist Wartsila Automation of Montrose, Scotland, is used in conjunction with the vessel’s bow thrusters.

The bow thrusters enhance the manoeuvrability of the huge vessel but, before the modifications were carried out by Wartsila, they had one particularly inconvenient shortcoming.

It was not possible to start the 800kW motors used to power the thrusters unless the ship’s main generators were operating, which meant that the vessel’s turbines had to be running.

The bow thrusters represent a very large electrical and mechanical load to the vessel’s power network and are remote from the main switchboard of the ship.

Over the years, starting the huge thrusters has represented a significant mechanical load to the generators and turbines, which increases wear to bearings and mechanical components, as well as producing a significant voltage drop on the network.

On investigating this problem, engineers from Wartsila discovered that the ship had auxiliary generators with sufficient output to power the bow thruster drive motors while they were running, but that these generators could not supply the much-larger current needed to start the motors.

At that time, the motors were equipped with conventional electromechanical star-delta starters that had an initial inrush of around 300 per cent of the motor’s running current and that also generated another large current peak during the star-to-delta transition.

The Wartsila engineers decided that a variable-speed drive would allow much more controlled starting, with an inrush current limited to little more than the normal running current of the motors.

After carrying out a technical evaluation, they found that Vacon offered a particularly attractive combination of price and performance for the large 800kW that was needed for this application.

The Vacon drive also offered a synchronisation facility that would make it easy to bypass when the bow thruster motors reached full speed.

LNG Abuja has two bow thruster motors mounted on a common shaft.

The variable-speed drive is used to provide controlled acceleration for one of these in such a way that the maximum current demand never exceeds 110 per cent of the motor’s normal running current – well within the capacity of the ship’s auxiliary generators.

When the motor reaches full speed, as confirmed by the synchronisation facility built into the drive, a contactor closes to bypass the drive and connect the motor directly to the supply.

This arrangement means that no special provision has to be made for cooling the drive, as it is only ever in use for short periods.

With the bow thruster system running under the power of one motor, the second motor can then also be switched directly onto the supply.

Since this motor, mounted on a common shaft with the first, is already rotating at close to full speed, this switching operation produces only a minor current peak.

Ian Gordon of Wartsila said: ‘The bow thrusters can now be used whenever they are needed and even started at short notice for maintenance operations, without having to worry about whether the main turbines are running.’

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