Robust storage assignment in stack- and queue-based storage systems

In stack- and queue-based storage systems, items stored behind the foremost position are only accessible once all blocking items have been removed. A prominent example are deep-lane storage systems, which are often applied in the refrigerated warehouses of the food industry. The price for the high space utilization of these compact storage systems is, thus, a larger retrieval effort whenever items are not properly stored according to increasing due dates. Due dates, however, are often bound to uncertainties, e.g., due to untimely arrivals of the outbound vehicles picking up the stored items. This paper introduces a new way to derive robust storage assignments, such that excessive retrieval effort is avoided in spite of due date uncertainty. Specifically, we aim to maximize the minimum time difference between due dates of items dedicated to different vehicles and stored in the same stack or queue. The resulting optimization problem is defined, computational complexity is proven, and suited solution procedures are derived. Furthermore, a simulation study investigates whether our novel storage assignments are indeed more robust against unforeseen delays than previous approaches.