# Deriving Situation-Adaptive Policy for Container Stacking in an Automated Container Terminal
## 2 Operations in the Storage Yard of an Automated Container Terminal  
The descriptions in this section are mostly based on the material given in [4; 6; 8]. As can be seen in Figure 1, the automated container terminal can be largely divided into four regions: quay, apron, storage yard, and hinterland. The quay is where the vessels borth and a number of quay cranes (QC) load/unload containers onto/from the vessels. The apron is the area for the automated guided vehicles (AGV) to deliver containers between the quay and the storage yard. The hinterland is where the external trucks (ET) bring in containers to the storage yard or bring out the containers picked up from the storage yard. The storage yard consists of dozens of rectangular blocks that are laid out in the perpendicular direction to the quay. Each block consists of hundreds of container stacks several tiers high, where the stacks are arranged in dozens of bays in the perpendicular direction and in several rows in the horizontal direction. Since a bay is of the length of a 20 ft container, a stack of 40 ft container spans two consecutive bays. For safety reasons, the containers cannot be stacked together if their sizes are not the same. Each block is equipped with two automated stacking cranes (ASC) to handle the containers. Since the two ASCs are of the same size, one cannot move across the other and thus may interfere with each other at close ranges. The container transfer to and from an AGV is made by the seaside ASC at a seaside handover point (HP) located at the seaside end of the block, while the transfer to and from an ET is made by the landslide ASC at a landslide HP located at the opposite end. The containers in the storage blocks are categorized into three groups by their directions of the flow of logistics: inbound, outbound, and _transshipment_ containers. The inbound and outbound containers are already mentioned in the previous section. The transshipment containers are those that are unloaded from certain vessels and stored in the yard, but unlike the inbound containers, they are to be loaded onto other vessels for further sea transportation. A container stored in a block is often relocated to some other place within the block. If container \(X\) is placed on top of container \(Y\) in a stack but \(Y\) has to be taken out earlier than \(X\), then \(X\) has to be moved to another location before \(Y\) can be retrieved. This relocation, or rehandling, is hard to avoid because the retrieval schedule of containers is usually unknown at the time the containers arrive at the yard and are piled up. Thus, we need a good policy that can select good stacking locations for the incoming containers so that both the container handling time and the possibility of rehandling are minimized. Note that a stacking policy should also be able to recommend good stacking locations for the rehandled containers to minimize further rehandling. Rehandling is considered the biggest cause of delay of container handling in the storage yard. There arises another difficulty with container handling when the two ASCs of a block cannot move across each other. To avoid collision, an ASC sometimes has to stop and wait until the other one finishes its job and backs up. This _interference_ deteriorates the throughput of the ASCs. Interferences are more likely to occur as the travel distance of an ASC becomes longer because it gets close to the other ASC with a higher probability. Unfortunately, the ASCs often cannot avoid long-distance travel in such a block layout as that shown in Figure 1. The outbound containers brought in by the ETs enter the block through the landslide HPs and are usually stored at locations near the landslide end. However, they eventually go out of the block through the seaside HPs when they are loaded onto their target vessels. On the other hand, the inbound containers unloaded from the vessels  
Figure 1: Layout of an automated container terminal. enter the block through the seaside HPs and then later exit through the landslide HPs.