Intelligent stacking as way out of congested yards? Part 1


Pursuit for intensifying yard operations

As container volumes keep increasing, new container terminals are being built and other terminals – in particular those that cannot easily extend their area – aim at increasing the density of the stacking yard. One way is to go for yard handling systems that can put through more containers per square meter, however this is a costly approach for existing facilities; equipment has to be sold or written off, during the transition the terminal has to work halfspeed,  and cannot handle the regular volume. Therefore, we suggest another approach: higher density stacking without changing the yard handling system in a physical way. We propose to apply stacking rules that can cope with higher densities without a performance decline. Typically, terminals experience problems with stacking containers at operational occupancy rates of 50 per cent or more. The question is what rules should be applied to accommodate more volume through the same facility, without loosing performance. An important issue here is the availability and quality of information about the container flow. Typically, information is late, lacking, or of bad quality. Besides, the information changes over time. This puts one requirement on the set of rules: they have to cope with changing or missing information.

Furthermore, nothing comes for free. It may be that the new set of rules, requires more equipment to be as effective. However, as volumes are going up, costs may also go up in absolute sense, as long as the cost per move stay at the same level, it should be acceptable to the terminal, which forms our second requirement. Finally, the service level at waterside (serving the vessel) and landside (serving road trucks and possibly trains) should remain at least at the same level.

Subject of our exercise

As most container terminals worldwide are using rubber tyred gantries in combination with terminal trucks, we have chosen to apply are stacking strategies to these type of terminals. Typically, the layout of such terminals looks like shown in Figure 1. Behind the berth, there are a number of blocks going into the depth of the terminal. Along the quay wall, these blocks form so called lanes. In principle there are two types of terminals: transshipment terminals and import-export terminals. The first type mainly moves containers from seagoing vessel to seagoing vessel, whereas import-export terminals move containers from seagoing vessels  to hinterland transportation modes, such as barge, trains and trucks. The issues are only partly comparable between the two types of terminals, however, no terminal is strictly the one type nor the other, although there are more and more terminals thatare almost 100 per cent transshipment. Examples are Tanjung Pelepas, Singapore, Salalah, Port Said, Gioia Tauro, Malta Freeport,  and Algeciras. All of these ports have transshipment rates over 90 per cent. Then, there are many terminals that hardly perform transshipment (typically below 15 per cent), especially in the United States, in England, and at the European continent. However, some of them (for instance Rotterdam and Antwerp) have a high barge share as part of the hinterland transportation modal split. These ports have excellent inland waterway connections deep into the continent. The container flow in those terminals show more  characteristics of transshipment terminals than ‘real’ import-exportterminals, because the volume over the quay wall is much higher than handled through gate or rail terminal.

In the reflection, we will discuss what the consequences of these differences are for our conclusions with regard to stacking rules. In the remainder, we will focus on transshipment terminals, as they provide (relatively speaking) the best information about where a container is going. The latter is caused by the extent to which the transportation is organized: hinterland transportation is a poorly organised, scattered processes, with many different suppliers, often one man companies, whereas the sea transportation is more  and more in hands of a couple of large shipping lines. Although not perfect at all, the information about seagoing containers is acceptable in terms of availability and quality.

Methodology of assessment of the stacking rules

In this paper, we try to assess the impact of yard operating rules, as well as we try to define a set of rules that will increase stacking capacity without increase in costs per move, and without a performance decline.

As containers are staying between a couple of hours until a couple of month on the terminal, the consequences of yard operating rules affect the process over a long period of time. Furthermore, the situation at any given moment is subject to dynamic effects. Vessels being delayed, trucks arriving without pre-notice, information coming in late, volumes that vary from vessel to vessel, etc.

These circumstances made us choose for applying simulation as way to assess the qualities of the yard operating rules. This approach has proven to be well suited for the analysis of these kinds of complex systems. Therefore, we have developed a terminal simulation model, containing all operations between the waterfront, and the gate. The waterside transportation is by means of tractor trailers, the landside operation is direct, which means that the RTGs handle the road trucks directly.

Outline of the paper

The outline of this paper is as follows. First, we will review literature with regard to yard operating rules and high density stacking. Secondly, we will elaborate on the yard operating rules in the reference case and in the alternative case, as suggested by us. Then, we will discuss the used simulation model and the experimental set-up. Subsequently, we will discuss the results of the simulation experiments. Finally, we will discuss the other relevant aspects and come to a conclusion.

Literature review: high density stacking

A broad range of research has been done on container terminal stack yard operations, in order to improve efficiency. Though, little has been published in scientific literature on stacking problems. A main reason may be that the practical problems are quite complex and do not easily allow for analytical results which are relevant for practice.

Stacking problems can be dealt with in two ways: simplified analytical calculations or detailed simulation studies. The first gives insight into the relationships between the various parameters on a more abstract level. The second can go in much more detail, with the negative by effect that it is time consuming and only few people really understand its ins and outs. No comprehensive stacking theory exist today, and a good stack design not only depends on local space conditions but also on the information characteristics of the ingoing an outgoing flow of containers which may vary from place to place. Examples of both approaches are given below.

Yvo A. Saanen, TBA b.v., Delft, & Rommert Dekker, Econometric Institute, Erasmus University Rotterdam, The Netherlands
Edition: Edition 31

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