That short but critical distance

Yard gantry cranes offer significantly denser stacking than straddles but at a higher cost per box move. Straddles allow quay cranes to work uninterrupted as they lift boxes directly form the stack or from the quay, and therefore act independently.

Terminal automation lends itself very well to harmonising the flow of containers between the quay and the stack. The automation of straddles, which is proving successful in Patrick's Fisherman Islands terminal in Brisbane, has opened up new possibilities. "One of the problems with straddle carrier operations is you can get congestion where too many machines are trying to work in one bit of the stack, " says Richard Clarke, market sector director for containers at Halcrow. "With an automated straddle operation, you can change where you're going to put a box right up to the last minute."

Automation is an option which is particularly appealing where labour costs are high as we have seen with the few terminals using AGVs. A requirement of automated terminals is that no people or manually driven equipment can be in the operation zone. That works well with a straddle operation because there are no road trucks moving around the stack anyway. The machine automatically goes to an interchange point to transfer containers to road trucks. The natural layout of straddle operations means they can be transferred to an automated operation without doing much to the layout.

Automating end-fed RMGs is a relatively easy prospect with road trucks handled at one end and internal movement vehicles at the other end. And there is the option of manually taking over the operation for interfacing with road trucks. RTGs though are a very difficult prospect in comparison as there are inherently road vehicles and internal movement vehicles using the same paths.

Going for fast travel speeds on RMGs in order to bring productivity in line with the quay crane may not always be a smart solution. Rail wear and the wheel wear on RMGs goes up dramatically as the long travel speed goes up. Although operators want to make machines go faster, it is not worth putting in very fast RMGs only to find that they could have been 30% slower without any difference to the output.

MINI STRADS FOR MEGA TERMINALS The recently developed one over one mini-straddle carriers are designed to be used with automated stacking cranes in mega terminals. According to Ilkka Annala, vp for straddle carriers at Kalmar, we will see their version, the Shuttle Carrier, used in a mega terminal from 2006.

"AGVs are the main competitor to mini straddles but while five to seven AGVs are required per quay crane, only two or three shuttles are required, " says Annala. So there may be three times as many AGVs operating in the same area between the quay and the stack which inevitably makes things more congested and slower with AGVs.

Mini-straddles keep down the number of vehicles which is particularly relevant for handling bigger ships with six or more quay cranes.

Because each piece of equipment acts independently in a ministraddle operation, the waiting times inherent with AGVs and tractor/trailers are eliminated. Buffer zones at the stack allow containers to be deposited so the mini-straddle and the yard gantry crane can work continuously. "This means the sequence that the yard crane works in can be slightly different from the sequence the quay crane works in, and that can provide huge benefits, " says Clarke. "If you have a buffer it allows a whole different group of algorithms to improve that delivery."

Having a buffer zone also means the operation can tolerate less than perfect stacking. "If you have to move two boxes in the stack to get at the one you want, it may be that you can take those boxes to the end of the stack because they're going to be wanted soon anyway, " suggests Clarke. "If you put them in a buffer zone, rather than somewhere else in the stack, you can avoid doing another move later on."

Another version of the buffer zone was developed at Thamesport where RMGs operate on a 400 metre long general stack. Road trucks feed export boxes into one end of the stack and at the other end is the buffer, known as the magazine. When the load plan for the ship is known, those boxes are moved out of the main stack into the magazine so that when ship loading starts, boxes are peeled out of the magazine very quickly with minimal movements of the crane.

The system has been very successful but has achieved about 10% less than the anticipated throughputs. There was an assumption the load plan of the ship would be known twenty hours before the vessel sailed but in practice alterations to the load plan may not be finalised until as little as three hours before departure.

A new challenge for synchronising quay and yard productivity is looming with the rapid development of multi-lift technology. Bromma's Tandem telescopic spreader and RAM's FlexiLift spreader, each capable of twin-lifting 40ft or 45ft containers, have been ordered for Yantian in China. Meanwhile, the Split Headblock design from Stinis, which links two extendable spreaders to twin-lift 40ft or quad-lift 20ft containers, has just begun operations in a testing phase at the APM Terminal in Rotterdam. "If we can make it work so that 70% of the boxes are handled with multi-lift, it'll be a huge breakthrough in productivity, " says Clarke.

Twin-trailer systems for transporting two 40ft containers side by side have been successfully used in Dubai which was the first terminal to routinely use twin 40ft spreaders. Mini-straddles lend themselves to handling these loads provided the spreader systems are designed to leave sufficient space between them for the machine's legs. The real problem though is going to be the ability of the yard to keep up with the loading and discharge rates, a synchronisation problem which is likely to take several years to overcome.