The effective use of dual hoist tandem spreaders
In my personal opinion, container terminals are made up of primarily four elements: equipment, process, operation system and manpower. Normally these four elements are developed keeping pace with one another, but sometimes the balance between them can be broken. In particular, the case that confuses terminal operators is often because the process can not catch up with the advancement of equipment – a typical example is the advent of quay cranes equipped with tandem spreaders.
In spite of rapid propagation of tandem quay cranes worldwide, those aren’t fully utilized because the process hasn’t been developed to back up tandem operation. Therefore, I intend to present problems and compensations on tandem operation to set up the appropriate process on the basis of the results of tandem discharge operation that has been performed during May and June 2011 at Pusan Newport Co. Ltd.
I personally think we should settle down tandem operation by all means because we have already invested extra funds in quay construction and manufacturing tandem quay cranes to perform tandem operation. I am convinced that tandem spreaders can be well used, although restricted, in actual operation. This article concentrates only on dual hoist tandem spreaders as shown in below Figure 1.
The primary precondition to be considered for dual hoist tandem spreaders operation is that tandem spreaders don’t allow a height difference of more than 35cm between the two 40-foot containers to be handled at one time. Therefore, tandem operation should be targeted at bays in rows of the same tier. In addition, the difference in the number of high-cubic containers stacked in each row in a bay should be only one or zero, otherwise preparation must be done prior to tandem operation to adjust the height difference between rows.
The second precondition is that bays that are mixed with full and empty containers should be excluded from tandem operation. We cannot expect productivity to increase by tandem operation in this case, because hoist speeds are sharply reduced by the weight difference between containers handled by the tandem spreader at the same time.
The third one is to make an even number of rows in the target bay before tandem operation begins. If it is composed of odd number rows, the preparation should be conducted by the single spreader first, until the number of rows become even. This is because hoist speed is reduced sharply when a single operation is done with a tandem spreader.
The last precondition is the number of yard tractors to be deployed for tandem operation. Tandem operation requires at least eight yard tractors per tandem quay crane for smooth operation and in order to reduce the time the quay crane must wait for two yard tractors to be aligned with the containers lifted.
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