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Heuristic routing methods in multiple-block warehouses with ultra-narrow aisles and access restriction

This paper focuses on multiple-block warehouses with ultra-narrow aisles and access restriction. These new features observed from one of the largest online retailers in China allow order pickers enter pick aisles from specific entrances but prohibit them from traversing the aisles. This impedes the application of traditional heuristic order picking methods. To address the order picking problem in such warehouses, we propose six heuristic routing methods by extending the basic Return, Largest Gap and Mid-point methods for the single-block warehouse. These six heuristic methods are named RNA, LNA, MNA, RNAP, LNAP and MNAP, respectively. The major improvements are achieved through setting rules with respect to determining the access mode of aisles as well as changing working aisles. Using real order information, a comprehensive simulation for comparison is conducted to evaluate the effectiveness of our improved routing methods under 12 warehouse layouts. The simulation results demonstrate that LNAP achieves the shortest average picking routes in most scenarios. The impacts of warehouse layout on performance measurements are analysed as well. It is ascertained that setting more cross aisles and connect aisles helps mitigate the negative impacts.