Containers are large boxes used to transport goods from one destination
to another. With containers a bulk unit can be created out of the individual
pieces of freight. As a result, containerisation can be defined,
according to The Containerization Institute, as the utilisation, grouping
or consolidating of multiple units into a larger container for more efficient
movement. Compared to conventional bulk, the use of containers has several
advantages, namely less product packaging, less damaging and higher
productivity Agerschou (1983). The
dimensions of containers have been standarised. The term TEU
(twenty-feet-equivalent-unit) is used to refer to one container with a
length of twenty feet. A container of 40 feet is expressed by 2 TEU.
Several transportation systems can be used to transport containers from
one destination to another. Transport over sea is carried out by ships.
On the other hand, trucks or trains can be used to transport containers
over land. To transship containers from one mode of transportation to
another, ports and terminals can be used. For example, at a container
terminal, a container can be taken off a train and placed on a ship.
Containers were used for the first time around 1950. Through the years, the proportion of cargo handled with containers has steadily increased. As a result of the enormous growth, the capacity of ships has been doubled since 1987. Large ships have a capacity up to 12,000 TEU. An extensive overview of the history of containers is given in Rath (1973). Furthermore, the importance of ports and terminals has grown. With the introduction of larger ships, small terminals have changed into large terminals. To ensure a fast transshipment process at large terminals information technology and automated control technology can be used. A detailed description of the use of these technologies in container terminals can be found in Johansen (1999). To use these kinds of technologies large investments have to be made and ongoing database management is required. Wan et al. (1992) show that the application of information technology in the port of Singapore results in more efficiency and a higher performance. In Leeper (1988) it is concluded that, in order to achieve an improvement of productivity and reduction in investment costs, an advanced automated control technology is a necessary condition.
The process of unloading and loading a ship at a container terminal may be described as follows: when a ship arrives at the port, the containers have to be taken off the ship. This is done by manned Quay Cranes (QCs), which take the containers from the ship's hold and the deck. Next, the QCs put the containers on vehicles, like automated guided vehicles (AGVs). After receiving a container, the AGV moves to the stack. This stack consists of a number of lanes where containers can be stored for a certain period. These lanes are served by, for example, automatically controlled Automated Stacking Cranes (ASCs). When an AGV arrives at a lane, the ASC takes the container off the AGV and stores it in the stack. After a certain period the containers are retrieved from the stack by the ASCs and transported by the AGVs to transportation modes such as barges, deep-sea ships, trucks or trains. This process is also be executed in reverse order, to load containers on a ship.