Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering
Transport Technology

P.B. de Jong Container storage at container terminals
Literature survey, 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 availability.

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 relatively low.

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 containers.

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.

Reports on Transport Engineering and Logistics (in Dutch)
Modified: 2005.06.20; , TU Delft / 3mE / TT / LT.