Vacon gives thrusters a boost
Until recently, the 70,000tonne LNG Abuja needed its main turbines running in order to start up the motors to power its bow thrusters. However, a replacement drive from Vacon demonstrates a more effective way of generating this significant mechanical load.
A
n 800kW drive produced by the Finnish-based Vacon used in conjunction with bow
thrusters is making the manoeuvring of the 1980-built liquefied natural gas carrier LNG Abuja significantly easier, particularly in port. Bow thrusters create a very large
mechanical load on a vessel’s power network, with attendant wear on mechanical components, being remote from main switchboards, and also produce a significant voltage drop on the network. On investigating the problem onboard
the 285m long by 44m across the beam LNG Abuja, engineers from Wärtsilä Automation, of Montrose, Scotland, discovered that auxiliary generators onboard the vessel had sufficient power output for the bow thruster drive motors whilst running, but could not supply the initial large current needed to start the motors. The motors original ly f itted
were equipped with conventional electromechanical star-delta starters that had an inrush of around 300% of the motors’ running current; also causing
Vacon drive system.
current of the motors. Vacon’s drive offered a further operational advantage, according to Wärtsilä, in that it included a synchronisation facility to bypass the motor 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 control the acceleration of one of these, so that the maximum current demand never exceeds 110% of the motors normal running current - well within the capacity of the ships auxiliary generators. Once the motor has reached full
another current peak during the star to delta transition. To control the inrush, Wärtsilä concluded that a variable speed drive could be fitted to limit the inrush current to that of a normal running
speed, which is confirmed by the synchronisation facility built into the drive, a contactor closes to bypass the drive and connect the motor directly to the supply. No special cooling facility is needed with this drive as it will be used for short periods only. With the bow thrusters system
running under the power of one motor, the second motor can then be switched directly onto the supply. This switching will produce a minor power peak as the second motor is mounted on the common shaft. Ian Gordon, of Wärtsilä, 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. That’s a big benefit for the ship’s operators, and it has been delivered at a very modest cost.” NA
LNG Abuja, where the Vacon drive has been installed.
The Naval Architect February 2009 71
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