In large ports – and particularly those confronting high labour costs – end-loaded automatic stacking cranes (ASCs) are quickly becoming the 21st-century standard for container operations. These systems feature an appealing combination of high-density storage, clean electric power, and driverless crane operations that can work in combination with either automated guided vehicles (AGVs) or straddle carriers/shuttles for waterside transport. The consideration of ASC systems gained significant momentum with the highly successful development of such a system at CTA (Container Terminal Altenwerder) in Hamburg, illustrated in Figure 1.
ASC systems are not without their drawbacks, however. The primary limitation of these systems is the rigid nature of their layouts and the resulting ASC requirements. Most ASC terminals feature two ASCs of the same gauge sharing the same set of rails. This design mandates an identical ASC fleet size for waterside and landside service.
A further limitation of ASC systems is the need for rapid ASC acceleration and deceleration, and for a high top gantry speed in order to achieve high productivity. The energy required to accelerate an entire ASC weighing hundreds of tonnes is substantial, and so are the forces generated on the rail systems by these accelerations.
Nearly all of the ASC systems currently in place feature a container storage block length of approximately 40 twenty-foot ground slots (TGS). In terminals with less than perhaps 30 TGS of storage length, the ratio of ASCs per TGS of storage may become unacceptably high. Operators would be forced to choose between placing two ASCs (costing perhaps $2.5 million each) in a short row where they will be lightly utilised, or using only one ASC per row and facing the attendant risk of lack of access to any containers in that row if the single ASC is not operational.
Many of these limitations can be overcome with an alternative system: an automated container yard (CY) featuring two or more sets of large nested cantilever rail mounted gantries (RMGs) running parallel to the quay, with containers stored perpendicular to the quay. Figure 2 illustrates this concept, with shuttle carriers for waterside transport.
The waterside RMG will have sufficient cantilever to cover a buffer that is four TGS in length. The double cantilever crane shown on the landside row will have responsibility for horizontal rehandling of containers between the two rows. The length of the cantilever overlap between the two rows can be optimised to suit each terminal. As with traditional ASC layouts, the operating zone of automated RMGs is physically separated from the manual shuttle and truck operations.