To Wave or Not to Wave? Order Release Policies for Warehouses with an Automated Sorter

Abstract

Wave-based release policies are prevalent in warehouses with an automated sorter, and take different forms depending on how much waves overlap and whether the sorter is split for operating purposes. Waveless release is emerging as an alternative policy adopted by an increasing number of firms. While that new policy presents several advantages relative to waves, it also involves the possibility of gridlock at the sorter. In collaboration with a large US online retailer and using an extensive dataset of detailed flow information, we first develop a model with validated predictive accuracy for its warehouses operating under a waveless release policy. We then use that model to compute operational guidelines for dynamically controlling the main parameter of its waveless policy, with the goal of maximizing throughput while keeping the risk of gridlock under a specified threshold. Secondly, we leverage that model and dataset to perform through simulation a performance comparison of wave-based and waveless policies in this context. Our waveless policy yields larger or equal throughput than the best performing wave-based policy with a lower gridlock probability in all scenarios considered. Waveless release policies thus appear to merit very serious consideration by practitioners. Facilities using a non-overlapping wave policy should also consider overlapping waves or a split sorter policy.

The forgotten sons: Warehousing systems for brick-and-mortar retail chains

“…Note that if the SKUs arrive from a parts-to-picker system, then the human order picker typically has already retrieved the requested number of pieces, so that the complete bin is inducted. Further note that for some warehouses also the application of fully-automated induction technology is reported ( Gallien & Weber, 2010 ). Once all pieces of the current SKU are retrieved and placed onto the conveyor, the unit load is returned into the storage system or the empty bin is removed.…”

“…Here, aimless pieces can recirculate in the loop until some store order is completed and another packing station for a subsequent order is available again. However, if more and more orders have to recirculate until the complete loop is fully occupied, no further pieces can enter the conveyor to complete not yet 13:31 ] finished orders, no packing stations can be freed, and a gridlock occurs (see also, Gallien & Weber, 2010 ). The recovery after such a gridlock can be very laborious and is error-prone.…”

Route optimization for warehouse order picking operations via vehicle routing and simulation

“…The wave picking policy involves processing and combining orders to be transported in one shipment all at once regardless of zone or location, instead of sending pickers as soon as an customer demand arrives. It aims to take advantage of economies of scale in picking operations, and the topics such as optimal frequency, timing, and size of wave release have been studied [5,13,14]. Note that these picking policies can be implemented in various combinations based on requirements.…”

“…Hence, order picking has to be not only efficient but also robust enough to provide adequate and acceptable outcomes. In this regard, the order picking policy called wave picking can be considered where the frequency, timing, and size of order picking are to be optimized [5,13,14]. In order to ensure efficient resource utilization and timely fulfillment of customer demand, the order picking problem in general and the routing optimization in particular are a point of focus in this paper.…”

Integrated Zone Picking and Vehicle Routing Operations with Restricted Intermediate Storage

“…3) to ensure enough capacity for remaining suborders such that a scheduled delivery tour can be executed as planned. This concept generally corresponds to the assumptions of a waveless picking system that involves the continuous transfer of individual orders, based on a priority ranking of store orders that respects target delivery time windows and tour planning, for example (Gallien and Weber 2010). However, the release strategy within the modeling approach proposed prevents the system from being blocked when a missing suborder cannot be moved from the picking area to the intermediate storage area due to lack of space.…”