Cranes are an integral part of a vessel's cargo handling system. The way cargo is handled plays a vital role in both the operational cost and the environmental impact of the ship and its cargo. The more efficient the crane, the more efficient the ship's overall operation, which subsequently impacts environmental load.
The new generation cranes for handling heavy cargo such as bales of pulp or reels of paper are electrical with a Variable Frequency Drive (VFD). This is a specific type of adjustable-speed drive system for controlling the rotational speed of an alternating current (ac) electrical motor. This is achieved by controlling the frequency of the electrical power supplied to the motor.
In the older generation of cranes, the electro-hydraulic cranes, the hydraulic system has an internal leakage. To compensate for this the pump for the hook has a slight positive setting called plussing. This makes the cargo in the hook slowly lift at first when the driver lowers the crane, which makes it harder to position the cargo.
In a VFD crane, the start current is low and peak start current is not required, which makes this crane more precise and accurate than the electro-hydraulic.
Bales of pulp can be handled relatively roughly, which permits high rates of loading and unloading. With high rates and bulk load, pinpoint accuracy is desirable at load handling.
Bad sight, heavy rain and wind causes problems for the crane driver when loading and unloading cargo at port. It takes long time for the crane driver to position cargo manually as sight and space is limited and load can start swinging back and forth. Once started, the motion is hard to stop. Reels of paper need to be handled carefully and it is very important not to scratch the reels to the load packed nearby.
MacGregor Cranes in Örnsköldsvik, Sweden has developed an Active Rotation Control system (ARC) to stabilize and automatically turn the load in the air. This is an automatic swivel control that keeps the cargo in the desired direction during the entire operation. The slewing is centre-line located and electrically operated.
A gyroscopic stabilisation cancels out torsional pendulations and keeps the cargo steady. Operation of this function is really smooth and easy for the driver. There is a separate control box with two knobs, one for the starting position and one to set the end position. The load automatically turns while in the air which makes cargo handling faster and safer. With a joystick, the cargo can be fine adjusted to position the cargo correctly.
Cargo positioning time is as much as 30-50 per cent of crane cycle time at port. With shorter cycle time, the port fee for the shipowners will decrease. There are also environmental advantages in shortening cycle time. This means that an electrical VFD crane with the ARC solution will save a considerable amount of time and money for the shipowners as well as decrease the carbon footprint.
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Maria Hellsten is a technical writer. Cargotec Sweden AB is based in Örnsköldsvik, Sweden. www.cargotec.com