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Drives that are all at sea

01 March 2009

A Vacon 800kW drive is helping to improve the manoeuvrability of the 70,000 tonne liquid natural gas transport ship, LNG Abuja. The drive, which was fitted and commissioned by marine power plant expert, Wartsila Automation of Montrose, has a key role to play in the operation of the vessel’s bow thrusters

Bow thrusters greatly enhance the manoeuvrability of a large vessel, and are particularly useful when undertaking tricky docking procedures. That was good enough reason for the operator of the 70,000 tonne liquid natural gas transport ship, LNG Abuja to ensure that its vessel was suitably equipped. However, the original installation had one rather inconvenient shortcoming; it was not possible to start the 800kW motors that powered the thrusters unless the ship’s main generators were operating, which meant the vessel’s turbines had to be running.

Bow thrusters impose a very large load on a vessel’s electrical power network and a significant mechanical load on the generators and turbines, leading to premature bearing and power transmission component wear. Moreover, the load causes a very significant voltage drop on the ship’s electrical power network.

Montrose based marine power plant specialist, Wartsila Automation was called in to investigate and quickly discovered that while the ship’s auxiliary generators had sufficient output to power the bow thruster drive motors during normal running, they were unable to supply the large current needed to start them. At the time of Wartsila’s preliminary assessment, the motors were equipped with conventional electromechanical star-delta starters that had an initial inrush of around 300% of the motor’s running current, resulting in a large current peak during the star-to-delta transition.

Wartsila proposed that the star-delta starter be replaced by a variable speed drive, which would allow much more controlled starting, as well as limiting the inrush current to little more than the normal running current of the motors. The next problem was to find a suitable drive. Following a meticulous technical evaluation, Wartsila decided that the Vacon NX series presented a particularly attractive combination of price and performance for this 800kW application. Further, the Vacon drive offered a synchronisation facility that would make it easy to bypass when the bow thruster motors reached full speed.

Mounted on a common shaft
The 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% 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 has another advantage in that no special provision has to be made for cooling, as the drive necessarily operates on a short duty cycle.

With the bow thruster system running under the power of one motor, the second motor can then also be switched directly to 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. Wartsila’s Ian Gordon summarises:

“The versatility and performance of the Vacon drive allowed us to offer a very convenient and cost effective solution for the operators of the LNG Abuja. The results are excellent. 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. That’s a big benefit for the ship’s operators, which has been delivered at a very modest cost.”

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