Efficient Management of Compact Storage Systems

Warehouses are important nodes in supply chains. They decouple supply from demand in time, assortment, quantity, and space. By doing so, economies of scale can be achieved in transport, as warehouses allow to regroup transport flows leading to lower cost. They also allow postponement of value addition, increasing service levels at lower inventory levels. Warehouses are particularly needed in densely populated areas, close to where demand is generated and where the labor is available. Warehouses require much space to realize economies of scale. Warehouse buildings are often quite large, more than 10,000 m2 built space is common, and much infrastructure outside the building is needed. Unfortunately, in many of the urbanized areas, space for such large facilities has become short. In order to address this issue, enterprises are moving toward next generation storage systems, namely “three-dimensional (3D) compact storage systems”. These systems are designed in order to save floor space and labor costs, and increase the reliability of the order picking process. Several types of 3D compact storage systems exist with different handling systems (like S/R machines, conveyors, shuttles, or elevators) taking care of the horizontal, vertical and depth movements. This dissertation focuses on new types of 3D compact storage systems, in particular live-cube storage systems, where each unit load rests on a shuttle which can move in horizontal and depth directions, in cooperation with shuttles of other unit loads. In such a system, at least one empty storage slot per level is required. One of the key performance measures for compact storage systems, which is also the focus of this dissertation, is response time. We analyze these systems at two decision-making levels. At the tactical (design) decision level, we focus on the rack-dimensioning problem aiming at short response times. At the operational decision level, we investigate storage assignment and retrieval sequencing problems to shorten the response time.

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