An essential part of any effective marine terminal is the tracking of goods that move through the facility. At low volumes, container tracking can be accomplished by a few people and an effective paper filing system.

Over the years these systems have become quite efficient and sophisticated. The inventory tracking parts of these systems are generally divided into two portions:

• Yard-side
• Vessel-side

The yard-side tracking has evolved into a large wall-mounted board onto which representations of the container are placed [see Figure 1]. This board not only provides a complete schematic representation of the container yard, but also allows color coding for easy visual identification.

On the vessel-side, there are three types of operations:

• Vessel Discharge
• Vessel Loading
• Transhipments

For vessel discharge, the yard board is consulted for empty locations in the yard and the ship's stow plan is marked to indicate where in the yard the container should be discharged.

For vessel loading, small pieces of paper are used to represent the containers. The paper squares have information on them about the container, including the container number and its current location in the container yard. These squares have an adhesive backing and can be placed on plastic sheets which contain an outline of the vessel.

A transhipment may be treated as both a vessel discharge and a vessel load. However it is not unusual for the planning operation to ignore transhipments and relegate these moves to the vessel operations personnel.

In any case, either a work list or the marked-up stow plan is sent to the supervisor for each gang working the vessel. The supervisor then directs the gang to perform the work required.

These two representations are conceptually identical. In fact, both systems are models Ñ a model is a representation of a real situation and how it functions. In this case, one representation is a model of the container yard; the other is a model of the ship. The vessel inventory model is physically smaller than the yard model to allow the stow plan required for the departing vessel to be created on a xerox machine.

The advantages of these models are:

• The inventory representation is highly visual.
• Fairly large amounts of information on the model are organized and reorganized by simply moving the representations of the container.

In fact, at low handling volumes, these models are an accurate enough presentation of the container terminal to allow planning activity without an actual view of the container yard.

The disadvantages are:

• The planning lacks short-term flexibility. Several hours of work must be planned and assigned to the work gangs. When a disruption occurs, this inflexibility stalls the entire operation. The problem is most noticeable during a vessel operation when a container cannot be loaded. When this happens, a change to the vessel load plan is needed. This change triggers a major effort to rewrite the work lists and then send them out to the now idle gangs.
• The information recorded in the model is not synchronized with the actual container terminal. This is due to the time delay inherent in using a paper work list. Consequently, a look at the yard board model must be tempered with some judgment about what is changing in the facility.
• The model is prone to inventory errors. The basic cause of the errors is unreported container moves. These moves occur when the equipment operator shifts other containers while searching for a particular container to satisfy a gate or vessel move. If the additional moves required to get the correct container are not on the work list, the operator may not report the other containers moved.

Throw People at the Problem

At low volumes the inaccuracies in the model can usually be ignored. Eventually, as the volume of containers increases, it is not at all unusual for an inventory system of this type to be as much as 20% inaccurate. Before the inaccuracies become intolerable, the terminal operator has several methods to slow the progress of the increasing problem:

• Perform manual inventories, routinely have yard personnel inventory either the entire facility or areas of the facility, and correct the information on the yard board model.
• Add mobile yard personnel, to locate those lost containers required for immediate delivery to either a truck or a vessel.

Both of these solutions continue to increase the amount of nonproductive labor in the terminal. However, these measures are expedient and carry the existing inventory-tracking system with its problems to the next bottleneck in the office.