The put-away problem in parallel-aisle warehouses

In warehouses, put-away operations involve the replenishment of capacitated item slots in forward storage areas from reserve storage. These items are later picked from these slots as their demand arises. While order picking constitutes the majority of warehouse operating costs, put-away operations might be as costly in warehouses where pick lists generally consist of only a few lines (e.g., order fulfillment warehouses).

In this study, we consider the put-away problem in a parallel-aisle warehouse, where put-away and order picking operations are carried out in successive waves with time limits. The aim is to determine the item slots that will be replenished and the route of the put-away worker in each put-away wave, so as to minimize the total labor and travel costs, and ensure the availability of items at the start of the wave they will be picked. The problem is analogous to the inventory routing problem due to the inherent trade-off between labor and travel costs.

We present complexity results on different variants of the put-away problem and show that the problem is NP-hard in general. Consequently, we develop a number of heuristic approaches. We test and compare the performances of these heuristics on randomly generated warehouse instances, and further analyze the effect of different storage policies (random, turnover-based, or class-based) and demand patterns (e.g., highly skewed or uniform).