Today’s international cargo system is based on the standardised use of shipping containers. These stackable steel boxes, mostly 20 or 40 foot in length, travel day and night along the world’s shipping lanes, railways and roads, keeping commercial freight continuously on the move. Within these logistics, ports – and port cranes – stand at a crucial juncture.
In every cargo-handling seaport, great ship-to-shore (STS) gantry cranes tower over the dockside. Trollies shuttle back and forth along their overhead booms, lifting and lowering containers by means of powerful hoists and specialised gripping attachments known as spreaders.
Under constant pressure to complete transfers as quickly and efficiently as possible, crane trollies travel up to about 350 m/min, while hoists can move at about 150 m/min unloaded or 90 m/min with a load of, say, 60 or 70 tonnes. These speeds must be achieved without sacrificing either safety or precision of control.
A swift, smooth-operating crane benefits from a power system in which AC motors (or, less commonly, DC motors) are controlled by drives fitted with modules that have been programmed with specific software packages and are connected to PLCs via a Fieldbus network.
As valuable and hard-won as a crane operator’s skill at operating his machine is, it is significantly enhanced with the assistance of such systems, especially where they relieve the operator of tedious or repetitive tasks.
The basic use of torque in a load-bearing hoist motor brings a significant improvement to the motion control involved in lifting and lowering containers. Rather than just turning it on or off, being able to accelerate and decelerate the hoist’s action takes the stress off the mechanism when the brakes are opened or applied.
The weight of a crane’s load, moreover, may be sensed and measured, usually through load cells or in some cases through the hoist drives’ internal torque measurement. In up-to-date systems, this information is acted on by the hoisting motors in order to effect the optimum lifting speed.
Similar technology can be used for motion control in luffing. Instead of using a trolley, a luffing crane lifts its load with one long, hinged arm and then rears up, keeping the hook moving horizontally at a steady height by closing the angle between jib and boom. Here, thanks again to the drives’ dynamic response to the load’s weight, motion can be kept smooth and level.
Two particular problems that beset crane movement are sway and skew. Containers suspended from hoists naturally start to sway like pendulums both when the lateral transporting motion starts and when it stops. The problem can be compounded by the skewing, or uneven tilting, of the load if, for instance, it is of uneven weight.
An automated anti-sway function depends on information derived from both lasers and weight sensors. This information is fed back and translated into compensatory motor behaviour, such as acceleration or deceleration, so that the ropes supporting the container remain almost completely vertical while at work.
As with all well-designed power systems, achieving appropriate levels of motor use, eliminating disproportionate force and stress, goes hand in hand with energy conservation – a gain that can be furthered when opportunities for regenerative braking (such as those presented by crane hoists) are exploited.
An alternative anti-sway technology has been developed which utilises only mathematical calculations (based on such data as length and angle of rope) to determine the corrective forces required. When it comes to port cranes, however, sensor-based systems have the advantage of being able to take into account environmental variables such as buffeting winds.
Other site-specific factors present significant challenges to electronic systems generally. The digital drives’ enclosures must be designed to withstand extreme temperature fluctuations and the saltiness of sea air and spray. And, given their unique structure, cranes bring with them an unusual number of grounding and interference issues. Fibre-optic cables, for example, tend to be used to protect against electrical crosstalk, an inevitable consequence of the cable festooning associated with moving trollies.
Given the heavy loads and high stakes fundamental to the container-handling industry, safety standards are naturally also high. When it comes to the power system, it is advisable – and, given enough panel space, straightforward – to install and/or configure secondary drives to act as backups if called for.
As container ships have become bigger over the years – there are vessels now carrying well over 10,000 TEUs (twenty-foot equivalent units) – port cranes have grown with them. A so-called Super-Post-Panamax crane (the name refers to a size of ship able to travel through the Panama Canal) now stands over twenty stories high and weighs over 1,500 metric tons.
It is becoming increasingly unrealistic for these constructions to be fitted with cabins from which an operator would struggle to judge distances and operate controls with the required level of precision. Instead, within the last ten years, the specially designed control room has become the natural vantage point from which to supervise an increasingly automated set of operations.
In any case, the age of the fully automated port is already upon us. Australia’s Victoria International Container Terminal, opened last year, deploys automatic STS and stacking cranes alongside automatic container carriers. Shanghai’s Yangshan Deep Water Port has been conducting trial operations since last year and eventually expects to integrate over 100 remotely controlled gantry cranes into its fully automated system.
It will, of course, be some time before these pioneering sites represent any kind of norm. In the meantime, as ports around the globe continue to upgrade their power structures – and to downgrade the purely manual aspects of crane control – it is the management systems available through the latest programmable drives that are delivering enhanced levels of safety, efficiency and productivity.