Priority rules for twin automated stacking cranes that collaborate

A recent trend in container ports is to operate twin (i.e., identical non-passing) Automated Storage Cranes (ASCs) that collaborate to serve storage and retrieval requests from opposite ends of a storage block. Since the ASCs are unable to pass each other, there is an exchange point that serves as a temporary storage location so that one crane can start a request and leave it to the other crane to complete it. In this paper fourteen twin ASC priority rules are proposed and evaluated via simulation with respect to the makespan. The priority rules are then compared to the optimal priority, which is obtained using a Branch and Bound algorithm. Results from an exhaustive simulation study suggests that the best priority rules are LonRem and RemReq, which gives priority to the ASC with the longest remaining estimated workload or to the ASC with the most number of remaining requests, respectively, yielding solutions that are on average 1.3% from the optimal makespan. It is also found that best two-rule combination is LonRem and PriC2 (the latter gives priority to the seaside ASC). Besides their immediate practical use, the results of this paper are also useful when evaluating candidate solutions for the problem of assigning and scheduling requests in the twin collaborating ASCs system. This study is the first to compare priority rules for collaborating twin ASCs and quantify the impact of the priority rule selected. (C) 2015 Elsevier Ltd. All rights reserved.