Picking Optimization in U-Shaped Corridors with a Movable Depot

We consider an order-picking system for a warehouse divided into corridors with two-layer shelves being arranged in the shape of a U in each corridor. Given an order in a corridor, the focus is on the optimization of the picking sequence and on locating the movable depot in the most convenient location. Two iterative algorithms based on constraint programming are proposed. Computational experiments position the new methods in the existing literature, showing that they are operatively effective. We also show how allowing the depot to be allocated away from the central axis of the corridor can lead to substantial time savings, especially for small orders. This strategic option had not been considered in the previous literature, but can be easily implemented in modern warehouses.