This article focuses on the configuration of the container storage and retrieval (S&R) system of the next generation of automated container terminals. For this discussion, the definition of an automated terminal is: 1. Waterside (WS) and landside (LS) (over-the-road) traffic must be separated. 2. Both waterside horizontal transport and container storage and retrieval must be automated. We have selected two S&R systems; end-loaded twin (ELT) automated stacking cranes (ASCs), which has become somewhat of a standard, and side-loaded (SL) ASCs, a new, at least to fully automated terminals, concept. This SL concept would require landside transfer cranes in order to comply with the definition of automated above. Another S&R option is end-loaded pass-over ASCs which has benefits in some situations, but is not included here. We will look at a three-berth terminal,or component of a larger terminal, with a throughput capacity of three million annual twenty-foot equivalent units (TEU), with varying proportions of import-export (I-E) and transhipment (TS). The berth is 1,320 metres long to accommodate three 400-metre long vessels. We will compare throughput capacity, waterside and landside handling capacity, and capital cost of equipment. We will assume that large vessels call with significant interchange per call and require in excess of 200 net moves per hour (mph). So, our three-berth system must provide waterside handling capacity of at least 600 net mph.
Why compare these two systems?
For terminals with high transhipment, end-loaded is not a good functional fit simply because all the handling demand is on the waterside end of the stacks. The landside ASC has little to do, other than support the WS crane, but there is reluctance to have only one crane per storage block due to the possibility of a breakdown. Even in terminals with high import-export, the end-loaded twin design has problems with the unbalanced workload and productivity of the WS and LS ASCs. Typical WS productivity is 17-20 mph while LS is 10-13 mph. Compounding this problem, where there is an import-export loaded box imbalance, such as in the US, the LS actually requires more total moves than the WS due to empty depot activity. Add the fact that, in an ELT import-export terminal, the gantry movement of the ASC accomplishes necessary box movement from WS to LS and vice versa, while in a TS environment, the ASC gantry motion is an unnecessary container movement. The side-loaded design, on the other hand, is very well suited for transhipment since the ASCs only store and retrieve and all ASCs can support the waterside operation. Necessary horizontal movement of boxes is accomplished by much smaller vehicles. SL ASCs are up to 40 percent more productive than EL ASCs, averaging 22-24 mph WS and LS. Finally, we have found that the proportions of some sites simply will not support one or the other of the two alternatives. So which is better, or which makes more sense? As always, it depends.
Throughput capacity and handling demand
Figure 1 shows comparable numbers …
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