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Picker routing in scattered storage warehouses: an evaluation of solution methods based on TSP transformations

New publication of the Chair for Management Science/Operations Research

2024/08/08 by

One major challenge in e-commerce warehouses is to pick a vast number of small orders from a large assortment under high time-pressure. Scattered storage, a warehousing strategy in which items of the same product are located at different positions within the warehouse, meets these requirements and is successfully implemented in many e-commerce warehouses worldwide. Still, reducing the travel times of pickers is a challenging task (even more so since products can be picked from multiple storage locations in scattered storage warehouses), such that a lot of research has been conducted on the so-called picker routing problem, aiming to find a shortest picking tour through a (scattered storage) warehouse.

To optimize their order fulfillment processes, many e-commerce warehouses employ a storage assignment strategy known as scattered or mixed-shelves storage. Under this approach, unit loads of homogeneous products are divided, and individual pieces are stored in various shelves throughout the warehouse. This arrangement ensures that products that appear together on unpredictable pick lists are stored in close proximity somewhere in the huge warehouses, reducing the travel distance for pickers. Despite these advancements, efficiently guiding pickers through the warehouse remains a significant planning challenge. Since the same products can be found in multiple storage positions, the traditional picker routing problem becomes more complex, as an additional selection task arises regarding which shelf to retrieve each requested product from. While previous research has developed several tailor-made solution algorithms, it is demonstrated that known transformation schemes used for different variants of the well-known Traveling Salesman Problem (TSP) can be utilized to convert the single picker routing problem with scattered storage (SPRP-SS) into a classical TSP. This approach makes it possible to leverage the extensive array of state-of-the-art TSP solvers. The purpose of this paper is to explore the performance of these solvers when applied to solving the SPRP-SS. Through a computational study, it is found that existing TSP solvers exhibit good performance, allowing near-optimal solutions to be obtained in less than a second for real-world scale SPRP-SS instances. Moreover, the efficiency of these TSP solvers remains unaffected by the number of cross aisles in the warehouse. Consequently, this flexibility is exploited to investigate the impact of cross aisles on picking performance in scattered storage warehouses.

Wildt, C., Weidinger, F. & Boysen, N. Picker routing in scattered storage warehouses: an evaluation of solution methods based on TSP transformations. OR Spectrum (2024). https://doi.org/10.1007/s00291-024-00780-0