Doctoral Degrees (Logistics)

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Browsing Doctoral Degrees (Logistics) by Author "Matthews, Jason"

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    SKU assignment in a multiple picking line order picking system.
    (Stellenbosch : Stellenbosch University, 2015-12) Matthews, Jason; Visagie, Stephan; Stellenbosch University. Faculty of Economic and Management Sciences. Dept. of Logistics.
    ENGLISH ABSTRACT: An order picking system in a distribution center (DC) owned by Pep Stores Ltd. (PEP) is investigated. Twelve unidirectional picking lines situated in the center of the DC are used to process all piece picking. Each picking line consists of a number of locations situated in a cyclical formation around a central conveyor belt. Pickers walk in a clockwise direction around a conveyor belt picking stock for stores. The picking lines are managed in waves due to PEPs policy to push stock to stores. For each wave of picking a subset of released stock keeping units (SKUs) is selected and assigned to an available picking line. The physical stock is then brought to the assigned picking line before multiple pickers pick all the store requirements (or orders) de ned by the SKUs within that wave. Once all of the orders have been picked a new mutually exclusive set of SKUs, de ning a new wave, is brought to the picking line for picking. In this way picking lines function in parallel to and independently of each other. The order picking system is deconstructed into three decision tiers. Firstly at the start of each day SKUs are assigned to available picking lines which de nes the Picking Line Assignment Problem (PLAP). Once a set of SKUs has been assigned to a picking line each SKU is assigned a speci c location within the picking line which de nes the SKU Location Problem (SLP). Finally once pickers are brought to the picking line the individual orders are sequenced for each picker. This de nes the Order Sequencing Problem (OSP). The focus of this dissertation is on the rst two subproblems namely, the SLP and PLAP as the OSP has already been solved in a previous study. This picking line setup considered here has many similarities to carousel systems. Several heuristic approaches for arranging SKUs within carousel systems are adapted for use in this picking line environment. These heuristics are compared to two novel lower bound formulations as well as trivial lower bound to evaluate their performance. Both historical as well as generated problem instances are used to compare the relative performances of each heuristic. An average saving of 2% for large and 6.5% for medium sized problem instances is achieved if the best solution form the four heuristics is selected. Three goals are used when assigning SKUs to picking lines in the PLAP. Firstly walking distance should be reduced, secondly the number of small cartons produced should be minimal and nally the number of pallet movements required to populate any one picking line for a wave of picking should be manageable. The concept of a maximal cut is used as an estimate for total walking distance and it is shown that by minimising the maximal cut within each picking line the total walking distance is reduced. A greedy phased insertion heuristic is introduced which minimised the maximal cut and therefore walking distance. Although the total walking distance was reduced by on average 22% compared to historical assignments the number of small cartons produced and the number of pallet movements required to populate some picking lines is undesirable. Four measures using SKU correlations are introduced and used within a phased greedy insertion framework. These measures reduce the number of small cartons produced with a marginal increase in total walking distance compared to approaches which minimized the maximal cut only. The total walking distance is reduced by on average 20% compared to historical assignments with the number of small cartons produced within an acceptable range. However, the number of pallet movements required to populate some of the picking lines remains at an undesirable level. A nal picking line segmentation approach is introduced using a sequence of integer programming formulations. These formulations include capacity constraints which limit the total volume of stock (and therefore the number of pallet movements) assigned to any one picking line. This approach delivers individual picking lines that have a manageable number of pallet movements to populate all picking lines with stock. A nal hybrid approach is also introduced which switches between this segmentation approach and a correlations approached when appropriate. This results in a 15% reduction in walking distance compared to historical assignments while maintaining a good number of small cartons produced and improving on the historical assignments in terms of the number of pallet movements required to populate any one picking line with stock. The managers within the DC are responsible for doing both the SKU to picking line assignments as well as the SKU arrangements within each picking line. A new warehouse management system (WMS) is in the process of design and implementation. A proof of concept interface which illustrated how the approaches to both the SLP and PLAP can be implemented in the new WMS while still allowing for managerial exibility is therefore proposed.