Picker routing and storage-assignment strategies for precedence-constrained order picking

Order picking describes the process of retrieving items from their storage locations to satisfy customer orders. Because order picking is considered the most labor-intensive process in warehousing, effectively routing order pickers through a warehouse can result in considerable time and cost savings. In practice, picker routing is often influenced by precedence constraints, i.e., the order-picking sequence is partially predetermined due to fragility restrictions, stackability, shape, size, and preferred unloading sequence. Although many warehouses face such precedence constraints for picking items (especially in the grocery sector), they are hardly considered in the scientific literature. This paper is inspired by a practical case observed in a warehouse of a German manufacturer of household products, where heavy items are not allowed to be stored on top of light items to prevent damage to the light items. Currently, the sequence for retrieving the items from their storage locations is determined by applying a picker-routing strategy that neglects this precedence constraint, and the order pickers pack the items respecting their weights after picking. To avoid this sorting effort at the end of the order-picking process, we propose a picker-routing strategy that incorporates the precedence constraint by picking heavy items before light items. We develop an exact solution method to evaluate this strategy. Furthermore, we examine the influence of different problem parameters on the proposed picker-routing strategy, and we derive managerial insights for dealing with precedence constraints in order picking.