We can distinguish between three planning and control levels in making
decisions to obtain an efficient terminal, namely the strategic level,
the tactical level and the operational level. At the strategic level it
is, for example, decided which layout, material handling equipment and
ways of operation are used. The time-horizon of decisions at this level
covers one to several years. These decisions lead to the definition of
a set of constraints under which the decisions at the tactical and
operational level have to be made.
This page gives a classification of the decision problems that arise at container terminals. Furthermore, some related literature is indicated.
When a ship arrives at the port, it has to moor at the quay. For this
purpose, a number of berths (i.e. place to moor) are available. The
number of berths that should be available at the quay is one of the
decisions that has to be made at the strategic level.
At the operational level the allocation of a berth to the ship has to
be decided on. Imai (1997) studies how to
allocate berths to ships while optimising the berth utilisation. On one
hand optimal berth allocation can be obtained by minimising the sum of
port staying times. This leads to ships mooring at the quay according to
the first come first served principle. On the other hand berths can be
allocated, without consideration of ship's arrival order, by allocating
ships at a berth closest by the area in the stack in which most containers
for this specific ship are located. As a result, the resulting terminal
utilisation will be maximal, but ship owners will be dissatisfied by the
long waiting times of the ships. Consequently, a trade-off exists between
the total staying time in the port and the dissatisfaction of ship owners
caused by the order in which ships are berthed. The berth allocation problem
could be considered as a machine scheduling problem.
One of the questions arising, is the determination of the type of material handling equipments which will be used for the unloading and the loading of containers from the ship. Automated and manned terminals both use quay cranes.
At the tactical level the number of QCs have to be determined that work
simultaneously on one ship. One of the objectives is to minimise the
staying time of ships at the terminal. The most general case of the
crane scheduling problem, is the case in which ships arrive at different
times in the port and queue for berthing space if the berths are full.
The objective in this case is to serve all the ships while minimising
the total delay of the ships.
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.
In designing a terminal, one of the decisions at the strategic level concerns the type of material handling equipment, that takes care of the transport of containers.
After the decision which system will be used has been made, one of the
problems at the tactical level that has to be solved is the determination
of the necessary number of transport vehicles to transport all containers in time.
At the operational level it should be decided which vehicle transports
which container and which route is chosen. Objectives are, for example,
to minimise empty-travel distances, to minimise the delay of the ship
or to minimise the total travel time of the vehicles.
A decision at the strategic level 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.
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. Obviously,
one of the problems at the strategic level is to determine a good
At the tactical level the number of transfer cranes has to be determined
necessary to ensure an efficient storage and retrieval process.
If straddle carriers take care of the storage and retrieval of containers
from the stack, it has to be decided at the operational level how to
route straddle carriers through the stack.
Another typical problem for a container terminal is that containers have
to be stored and retrieved at two sides of
the stack, namely seaside (to/from the ship) and landside (to/from
other modalities). This can be done by the same yard crane/ASC. Some of
the decisions that have to be made to ensure an efficient process are:
which side has the highest priority (commonly seaside) and how long can
containers wait before they are stored or retrieved.
The problem to decide which ASC carries out which job, can be examined in two ways. If every container is treated as an individual (QC asks for a specific container from the stack), then it is clear which ASC should carry out the job. However, one can also distinguish container categories in a stack. This holds especially for empty containers. Containers with, for example, the same destination, the same weight, contents and size belong to the same category.
Operational questions concerning storage planning are, for example, where is an incoming container stored, in which order are containers store, when is a container repositioned and in which way and which crane handles which container. For retrieval planning it has to be decided in which order containers are retrieved and which crane handles the request.