Container ships are nowadays unloaded and loaded at large container terminals. Figure 1 illustrates the unloading and loading process at a typical container terminal. This loading and unloading process can be divided into different subprocesses, described below. When a ship arrives at the port, quay cranes (QCs) take the import containers off the ship's hold or off the deck. Next, the containers are transferred from the QCs to vehicles that travel between the ship and the stack. This stack consists of a number of lanes, where containers can be stored for a certain period. Equipments, like cranes or straddle carriers (SCs), serve the lanes. A straddle carrier can both transport containers and store them in the stack. It is also possible to use dedicated vehicles to transport containers. If a vehicle arrives at the stack, it puts the load down or the stack crane takes the container off the vehicle and stores it in the stack. After a certain period the containers are retrieved from the stack by cranes and transported by vehicles to transportation modes like barges, deep sea ships, trucks or trains. To load export containers onto a ship, these processes are also executed in reverse order. Most of the terminals make use of manned equipments, like straddle carriers, cranes and multi-trailer-systems. However, a few terminals, like some terminals in Rotterdam, are semi-automated. At such terminals Automated Guided Vehicles (AGVs) are used for the transport of containers. Furthermore, the stacking process can also be done automatically by Automated Stacking Cranes (ASCs). This page describes the different subprocesses and their corresponding types of material handling equipments in more detail.
Automated and manned terminals both use quay cranes. QCs are manned
because automation of this process encounters practical problems, like
exact positioning of containers. Figure 2 illustrates a QC.
The QCs have trolleys that can move along the crane arm to transport
the container from the ship to the transport vehicle and vice versa.
A spreader, a pick up device attached to the trolley, picks the
containers. The QCs move on rails to the different holds to take/put
containers off/on the deck and holds. It can occur that at the same
moment one QC is unloading containers while another QC is loading
containers.
The number of import containers that has to be unloaded at the terminal
is in practice usually only known shortly before the arrival of the
ship. At the operational level an unloading and loading plan have to
be made. The unloading plan indicates which containers should be unloaded
and in which hold they are situated in the ship. Successively, these
containers are unloaded. In a hold the crane driver is almost free to
determine the order in which the containers are unloaded. The unloading
time of a container depends on its place in the ship.
In contrast with the unloading process, there is hardly flexibility in
the loading process. A good distribution of containers over the ship is
necessary. At the operational level a stowage planning is made. A
stowage plan indicates for each container the exact place in the ship.
Containers with the same destination, category, weight, size, contents
and so on, belong to the same category. Sometimes, only for each category
the positions in the ship are given. Locations of containers belonging
to the same category can be exchanged between containers of this category.
In making the stowage planning attention should be paid to the order in
which containers need to be unloaded. Unnecessary moves should be avoided
by placing containers designated for a terminal visited later during the
journey on top of containers designated for the earlier visited terminals.
For the transport of a container at a manned terminal, vehicles like forklift trucks, reackstackers, yard trucks or straddle carriers can be used. Straddle carriers (see figure 3), reackstackers (see figure 4) and forklift trucks (see figure 5) can pick up containers from the ground.
A crane is needed to put the container on the yard truck (see figure 6). For the transport of multiple containers, multi-trailer systems can be used.
At an automated container terminal Automated Guided Vehicles (AGVs) are
used for the internal transport. AGVs are robotic vehicles which travel
along a predefined path. The road system consists of electrical wires
in the ground, or a grid of transponders, that control accurately the
position of the AGV.
Usually, an AGV has a capacity of 60 tonnes and can carry either two 20 feet containers, one 40 feet or one 45 feet container.
The concept of transporting two loads at the same time by a single AGV are, however, in practice, hardly being applied due to a lack of efficient control policies.
AGVs are only practical in ports with high labour costs
because of the high initial capital costs. In ports with low labour costs,
the system of manned vehicles is preferable. A new type of automated vehicle has been introduced, namely an Automated
Lifting Vehicle (ALV). Some terminals already use this new type of vehicle.
Two ways of storing containers can be distinguished: storing on a chassis
and stacking on the ground. With a chassis system each container is individually
accessible. With stacking on the ground containers can be piled up,
which means that not every container is directly accessible. As a
consequence of limited storage space, nowadays stacking on the ground
is most common.
The stack (see Figure 7) is the place where import and export containers
can be stored for a certain period. The stack is divided into multiple
blocks/lanes, each consisting of a number of rows. The height of stacking
varies per terminal between two and eight containers high. At the end of
each lane a transfer point might be situated. At this point the crane takes/places
the container off/on the vehicle that transports the container. Empty
containers are usually stored separately.
A decision that has to be made, is choosing the type of material handling equipment that will take care of the storage and retrieval of containers in and from the stack. Systems like forklift trucks, reach stackers, yard cranes and straddle carriers can be chosen. Yard cranes (see Figure 8) move on rubber tires or on rails over the containers. They can provide high density storage and can be automated. These automated cranes are called Automated Stacking Cranes (ASCs). ASCs move on rails and are controlled by the central operating system. The ASC takes/places the container with a spreader from/on the AGV. At the port of Rotterdam, the containers can be stacked six wide and two or three levels high per ASC.
Most of the described terminal operations have their origin and destination
at the stack, for example the transport of containers from the stack to
the ship and vice versa. The process of storing and retrieving containers
should be executed such that the remaining operations in the terminal can
be carried out effectively. The efficiency of stacking depends among
other things on the stack height and strategies for storage and retrieval
planning of import and export containers.
Consequences of higher stacking are a higher number of reshuffles/rehandles.
To reach a specific container it can be necessary to rehandle containers
that are placed on top of the demanded container. To minimise delay by
removing containers, reshuffling of the stack can be done in advance.
On the other hand, the higher the stacking the less ground space is
needed for the same number of containers.
Containers have to be transported from the stack to other modes of
transportation, like barges, rail and road. It is expected that, with
the growth of terminals in the future, this inter terminal transport
becomes more and more important. According to
Van Horssen (1996)
new concepts and technologies have to be developed to handle the large
numbers of inter terminal container transports expected in the future.
Furthermore, research has to be done to the various transport systems
by which containers can be transported between the terminals.
Multi-trailer systems and automated guided vehicles can carry out this
inter terminal transport. In certain terminals it is possible that containers
are put directly on, for example, trains without using transport vehicles.
One way of transporting containers to other destinations is by rail
(see Figure 9).
Another way of transporting containers to other destinations is on the road by trucks.
