Order routing in sequential zone picking systems

Abstract Sequential zone picking is a strategy to pick stock keeping units in a warehouse. It is preferred for its flexibility and scalability. Among others, it is applied in use cases in which the overall number of stock keeping units is low, and the volume of orders is high. If there are only very few different stock keeping units compared to the order volume, the same stock keeping units are placed in multiple zones to improve performance. This gives rise to the problem of order routing, which determines where the required stock keeping units for every order should be picked. Although there is a large body of literature on picker-to-parts order picking, this paper is the first to consider order routing for sequential zone picking. From a theoretical perspective, we analyze problem complexity and provide insights into the mechanics of the problem. Based on these insights, we develop three different model-based solution heuristics. To evaluate the routing approaches, we collaborated with HelloFresh, one of the largest cook-and-eat meal-kit providers worldwide. An extensive case study based on real-world data shows that the proposed routing approaches are applicable in practice and outperform heuristics currently in use with an up to 30% higher throughput.