Yard Conundrums

In recent years, much of the emphasis by the quay crane and spreader manufacturers has been on the development of multi-lift systems. Originally this was simply twin-lift, where two 20′ containers are lifted at the same time as if they were a single 40′ box. Now the equipment is available to do tandem-lift, two 40′ containers side by side, and double twin-lift, four 20′ containers arranged as though they were two 40s. There are also technologies available to handle several containers one above another but structural limitations in the container itself means that this can only be done with empty boxes.

The first challenge in all this is to have sufficient lift capacity as the traditional quay cranes with a 40 or 60 ton capacity are at best usable for twin-lift. Some of the major ports have however decided that the multi-lift approach is the way forward and they have invested in cranes able to lift up to 120 tons. This significantly increases the weight of the container crane and also increases the crane rail loadings. These cranes also need special rope arrangements to allow for differences in the container weights.

If multi-lift systems are to be used efficiently, the ship loading pattern must be planned with this in mind and this is an area which will need considerable development.

The main issues which have yet to be resolved with multi-lift systems are how the containers are transferred from the quay crane to the yard and how they are handled when they arrive at the stack.

With twin-lift operations, transportation to the stack is fairly simple as the two 20′ containers can be put onto a standard trailer.

If the yard is operated with RTGs or RMGs, the containers can be lifted off singly or, if the equipment and yard planning systems are in place, the two containers can be lifted into the stack using a twin-lift spreader. Where a straddle carrier system is used, again the containers can either be picked up singly or taken to the stack using a twin-lift spreader. It is unlikely that a terminal seeking the productivity benefits of twin-lift would be worked with reachstackers but their use is technically possible. Twin-lift would be difficult to implement with a US style chassis operation as it is very unlikely that the two 20′ containers would be leaving the yard on the same trailer.

Obviously the greatest benefit from tandem-lift or double twin-lift comes if the crane can put down all the containers in a single operation. Operationally, if you can solve the problems for tandem-lift, the same approach can be taken for the double twin-lift as described above. Technically the crane can put down one container and then trolley further across to lower the second but this takes time, largely negating the benefit of using a multi-lift system in the first place.

COMPLICATIONS Most of the tandem-lift spreaders in the market can move the two containers apart but as the separation distance gets larger, the weight and complication of the spreader goes up considerably. The current state of the technology is that a separation of up to one metre can be provided within a reasonable spreader weight but any significantly larger gap carries a large penalty.

With a one metre separation, it is possible to position two trailers side by side and place the containers directly onto them. This either requires the trailers to be positioned with great accuracy or the crane driver to “spot” both vehicles. The very accurate placing of two trailers is relatively easy to achieve with automated vehicles but it is potentially a source of disruption if the vehicles are hand-driven.

Getting the crane driver to make an adjustment for each trailer takes time and again takes away the point of the multi-lift. The other complication of using two trailers is that if either vehicle is delayed it will disrupt the entire operation.

Using the one metre separation it is possible to pick up each container with a straddle carrier but they have to be lifted one at a time as there is insufficient space between the boxes for two machines to pass. This is not a particular problem but it does extend the time before the ground space is available for the next pair of containers. The effect of this limitation depends on the availability of straddle carriers and the time before the ground space is needed again. This is a classic problem to make the subject of a simulation exercise and at least one operator is about to do this.

ONE TRAILER FOR TWO 40’s Dubai Ports Authority are one of the leaders in the development of multi-lift systems and they have come up with the solution of building a single trailer which can carry two 40′ containers side by side (or four 20′ boxes in the double twin-lift configuration). This trailer can operate at an all-up weight of over 100 tons but this is only manageable with a large terminal tractor as used with trailer trains.

There are some issues with the operation of such a trailer, not least that it is much wider than the tractor, making it difficult for the driver to see down either side of the vehicle. Reversing therefore becomes very difficult and the chances of damage during tight manoeuvring will go up. The latter problem could be much reduced if the trailer had a steerable rear bogey set-up so that the rear of the trailer exactly follows the front. With such a fitment, there is no cutin by the trailer and the driver knows that if the front of the trailer has got through a gap, the rest of the vehicle will get through.

The more serious difficulty with a double-width trailer is that it needs a six metre wide roadway, effectively taking over an extra lane under an RTG. With a conventional six or seven wide RTG stack, taking out an extra traffic lane substantially reduces the stack capacity. It is perfectly possible to build RTGs which span up to nine containers and a conventional vehicle lane so in a new yard layout, the double-width trailer could be used with an eight-wide stack.

FOUR TIMES THE COST The other solution being considered is to operate the stack with RMGs which can lift under side cantilevers with the full distance between the rails being occupied by containers. This gets around the loss of stack space but the side cantilever RMGs are approximately four times the cost of an RTG with a similar work rate. In compensation it is easier to build an RMG that can itself do tandemlift, possibly gaining some speed by reducing the number of lifts. This system would however require a specially developed yard planning system if productivity is not to be lost to increased shuffling later in the operation.

As has happened before, the technology available on the latest quay cranes has outstripped the ability of the yard system to handle the resulting flow of containers. This article has just addressed the issues with containers in the import part of the handling cycle. The export handling system has the same challenges but with the added complication of requiring the containers to arrive in exactly the right sequence as well as in the right pairings.

Several organisations are working hard on yard solutions to match multi-lift crane technology but there is still some way to go before a preferred methodology emerges. This methodology will have to integrate the quay crane, internal movement vehicles, yard cranes, the ship planning and the yard planning in a much closer system than that used at present with single lift handling. Only when an efficient operating system has been demonstrated will the use of multi-lift systems become a common part of the industry.