P.B. de Jong
Container storage at container terminals
Report 2004.TL.6916, Transport Engineering and Logistics.
Worldwide maritime container transport executed by different modalities has
increased enormously the last five decades. Standardisation of maritime
containers and transport equipment arrange this international accepted way
of transporting cargo. However, also the development of special purpose
containers like refrigerated containers, which can be handled by the same
handling equipment, affects this intercontinental transport. Due to these
large and increasing intercontinental cargo streams, the deep-sea container
vessels even as their sailing patterns also undergo a transformation.
In this container transport the container terminal functions in principle
as a rapid transit facility at the interface between land and sea transport.
The definition of a terminal can be described as: an organisation offering a
total package of activities and services to handle, store and control cargo
to and from transportation modes with a balance in handling and services to
the transportation modes against minimised costs (J.C. Rijsenbrij,
Terminal Design Delft University of Technology, Faculty Mechanical
Engineering and Marine Technology (1998)). A terminal consists of systems for
unloading and (re)loading, internal transport, storage and optionally
transformation. Most terminals are built up from the same components and have
a commonly used layout suited to transfer the goods properly.
Operations at container terminals are: the arrival of the ship,
(un)loading of the ship, the transport of containers from ship to stack
and vice versa, stacking of the containers and finally inter-terminal
transport and transport to other modes of transportation. These modes are
road and railway transportation, inland shipping (via barges) and feeder
line transport. Transhipment numbers, cargo flow direction and
service-orientation can classify terminals. Storage of goods is for
uncoupling time and place in the transportation chain because most of the
times it will not be possible to tranship directly two modes transport.
Storage systems are designed with three main criteria to improve:
container handling productivity, access to the stacked containers and land
To handle and transport containers, different types of equipment are used.
Most equipment has standardised handling tools to sufficient handle the
containers. Container terminal equipment can be subdivided in cranes for
(un)loading container vessels (ship to shore cranes), equipment for
internal horizontal transport, equipment that can handle as well transport
containers and equipment for storage of containers. Stacking of containers
is mostly done by rubber tired cranes (RTG) or rail mounted gantry cranes
(RMG), straddle carriers or overhead bridge cranes. Rail mounted gantries
are easy to automate. Such an automated stacking crane (ASC) can be found
at terminals with a high throughput. RTGs can switch between stacks and
have higher wheel loads compared to RMGs. Overhead bridge cranes (OBC) run
on elevated girders supported by concrete columns and the can stack
higher, wider and faster, are easy to automate and less energy consuming
than gantry cranes. For straddle carriers, the stack consists of (not too
lengthy) rows of containers, separated by lanes wide enough for the legs
and tires of the straddle carrier to drive over.
Containers can be stored on chassis or on the ground. Block stacking is
the most common stacking method. No storage yard is identical but general
configurations can be distinguished. The transfer from/to the stack can be
executed parallel to the stacking block if the transport vehicle travels
on a parallel traffic lane or on end at a special transfer point. Stacking
blocks can be separated for import- and export containers but can also be
nonseparated. The orientation can be parallel or perpendicular to the quay
wall. There are much variations and adoptions possible with these standard
configurations. The access to stacked containers is tied to the
dimensional characteristics of container handing equipment in the yard and
the number of stacking tiers. Estimation of the accessibility and the
number of rehandles can be done in different ways. In general the more
accurate methods use more computational time.
A lot of interrelated issues influence stacking performance. Ship sizes,
berth productivity and call sizes determine the container
arrival/departure, which preferably is uniform. Transhipment percentages,
dwell time, TEU-factor and sub-stack utilisation have impact on the number
of ground slots. Another relation is that of the stacking length, number
of substacks and the handling capacity.
Planning is done on different levels with different planning horizons.
This horizon is the time span in which the operations are planned. The
stack-planning operator stays in connection with seaside and landside
operators. Important issue in stack planning is the space assignment of
containers. A container terminal control system guards and controls all
terminal operations. Performance is measured with help of indices and
ratios for reason of analysis and evaluation.
A storage assignment policy is a set of rules, which determines where in
the storage system arriving containers be stored. Dedicated storage is a
policy where a set of locations is reserved for the items of a single
product during the entire planning period. With shared storage, a location
can be used successively for the storage of items or different products.
Block stacking is the most popular way of container storage.
Import containers arrive predictably at the yard, in large batches, but
depart one by one in an unpredictable order when the road carriers come to
collect them. Export containers depart predictably but arrive in random
order. Export containers are stacked higher than import containers because
more information about export containers is known compared to import
containers. Import and export containers share the same stacking blocks.
Within each block the bays are dedicated to export containers or import
containers. These bays are mixed in the stacking block, but import
containers are, if possible, stacked at the landside, while export and
transhipment containers are, if possible, stacked at the waterside of the
stacking block. In a stacking block, the 20 ft and 40 ft containers are
dedicated to different bays. There are different rules for storage of
import and export containers according to their characteristics.
Because export containers must be loaded according to a rigid ship stowage
plan, they must be rationally stored according loading vessel, weight and
discharge port to avoid re-handling if the stacking sequence not matches
the loading plan sequence. Export containers can directly be assigned to
their storage location with a static allocation strategy. To avoid
rehandling at the loading operation, the containers waiting in the yard
can be re-shuffled in advance according the 'sort & store'
strategy. Another policy is to store export containers temporary in a separated
area. When the ship's loading plan has been send by the shipping lines,
the containers are transferred to an assigned ground slot in the
marshalling rows. The 'sort & store' strategy and dynamic
assignment strategy ask for re-handling moves but save space. Which system is
implemented depends on the available storage space, number of rehandles,
and the allowed complexity of the processes.
Import containers arrive in large quantities together. They can be stacked
on containers currently in the yard with a non-segregating strategy. With
the segregation strategy the yard cranes empty space in advance to avoid
re-handling at the retrieval. Terminal operators must find an optimum
between re-handling and storage space.
Per stacking block one or two RMGs can be deployed sharing the same rail.
There are configurations with two RMGs on separated tracks of which one
can pass under the other. OBCs have the best land utilisation through high
stacking and more ground slots per hectare. High throughput can be
achieved and more OBCs per track are deployed. RTGs are not bound to
tracks and can be deployed on each stacking block. Algorithms help with
optimisation of the assignment of RTGs to stacking blocks.
Transfer cranes are for transferring container from and to the storage
location, remarshalling the storage yard and for re-locate containers to
other storage blocks. Task sequencing and scheduling is done to minimise
transport vehicles waiting times and maximise transfer crane capacity.
Housekeeping operations are executed to speeding up loading operations,
minimise travelling distances and equalise stacking heights.
Automation becomes an attractive approach in the design of handling
systems to control the increased scales at reduced costs. Automated guided
vehicles, automated lifting vehicles, automated stacking cranes and
automated straddle carriers are successfully deployed today. To increase
transfer capacity quay cranes and transport systems are adapted.
Concepts for storage are overhead rail systems. Individual vehicles with
spreaders can lift containers from the vessel, travel in the horizontal
plane and store/retrieve containers. Speedport and grid-on-rail are two
examples of these systems. Another concept is an automated storage and
retrieval system. An advantage is the accessibility of the stored
containers but construction costs are high and the space utilisation is
The used equipment and yard layout must fit the demands of customers and
terminal operations. Availability of land, transfer capacity and
information determines the storage policy for import and export
Good storage is applied when:
'Integrated optimisation' and simulation should be a field of increased
investigation. Recognition of transportation patterns and duration of stay
principles should be examined by simulation to decrease re-handling. A
further co-operation between large trucking companies and terminals will
allow for a better exchange of information and the announcement of an
estimated time of arrival in advance.
- Clients of all modalities are served according to the
service-contracts and internal appointments about service performance
- Dwell times are minimal (maximum 5 days)
- Re-handling during peak hours is minimised and containers are properly
and orderly stacked.
- Operations between storage yard and landside/seaside are harmonised
with each other and the total transport chain is optimised.
- Information is available when it is needed
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