Browsing by Author "Hofmann, Flora"

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    Choosing optimal reverse channel structures for the collection of used products
    (Stellenbosch : Stellenbosch University, 2017-03) Hofmann, Flora; Visagie, S. E.; Stellenbosch University. Faculty of Economic and Management Sciences. Dept. of Logistics. Logistics.
    ENGLISH SUMMARY : The re-manufacturing of used products has become more important in literature and practice. Governmental legislation forces manufacturers to take care of their end-of-life products. Additionally, re-manufacturing may increase a companies' revenue through direct savings in production costs by the recovery of valuable material. With these external and internal developments, there is a growing interest of manufacturers in determining an optimal channel for the collection of used products. The overall objective of this optimisation lies in the maximisation of the companies' profit. Therefore, the problem of increasing waste streams of end-of-life products need to be addressed by identifying the most protable reverse channel structure to collect and re-manufacture used products. Three different collection channel options are modelled as decentralised decision-making systems. Therefore, a game theory approach is applied. The first channel is the manufacturer carrying out the collection. The retailer making use of the retail store network to collect from customers and sell back to the manufacturer describes the second channel. A third-party logistics service provider acts as a third channel for collecting and selling returns. The thesis focuses on the detailed cost of collection that each potential collecting agent accommodates. A non-cooperative game between the three collecting agents is modelled first, followed by the extension to a cooperative game. The cooperation can be caused by external in influences like legislative regulations or by a change in perspective. The stability of both versions of the game is evaluated by changing single parameters. Additionally, by changing the market scenario, the influence of the market environment on the channel choice is investigated in particular. The benchmark scenario of the non-cooperative and cooperative version of the game is stable in its parameters. In general, changes in single parameters influence the level of the highest payoff achievable by each player. In the non-cooperative version of the game the manufacturer gains the highest payoffs followed by the retailer, as both benet from the sales of new products. If the market scenario is changed, this ranking only shifts with a change in the market area size. Therefore, the retailer obtains a prot higher than the manufacturer. The third-party is able to work with different clients, turning the collection of returns into a successful business. The results of the cooperative version of the game are consistent with the observations in the non-cooperative game. Forming the grand coalition is the best option to obtain the highest payoffs if collection rate fees are imposed externally. With a change of perception to the manufacturer, the same customer density identifies the retailer as an optimal collection channel. However, subcontracting the third-party obtains the highest payoff in the benchmark scenario as well as in the larger market areas. In conclusion, a cooperation between different options should be taken into account while designing optimal reverse channel structures for every scenario. Additionally, the point of view is crucial in choosing the partner to obtain the highest payoff.
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    Order picking optimisation on a unidirectional cyclical picking line
    (Stellenbosch : Stellenbosch University, 2020-12) Hofmann, Flora; Visagie, Stephan E.; Stellenbosch University. Faculty of Economic and Management Sciences. Dept. of Logistics. Logistics.
    ENGLISH SUMMARY : The order picking system in a company's distribution centre is the biggest contributor to the operational cost within the DC. Optimisation should thus aim at running this activity as effciently as possible. The order picking process consists of three main activities, namely walking to the stock, picking stock in fullment of a customer order and handling the picked stock for further processing. While the total amount of work for the picking and handling activities remain constant, the minimisation of walking distance becomes the main objective when minimising the total picking effort. The minimisation of walking distance can be translated into a reduced overall picking time which can lead to a decrease in the total cost of operating the picking system. The main objective of this dissertation is to optimise the order picking system on a unidirectional cyclical picking line. Order batching is introduced to the picking system, since it is an effective methodology that minimises walking distance in operations research literature. Order batching has been introduced to the standard single block parallel-aisle warehouse layout, but not to the specic layout of a unidirectional cyclical picking line. Additionally, the unidirectional cyclical picking line can offer two conguration options that change the physical set up and thereby inffuence the way in which pickers walk during the order picking process. Order batching is introduced to the unidirectional cyclical picking line through picking location based order-to-route closeness metrics. These metrics are further extended by taking the characteristics of the layout into account. The distribution centre of a prominent South African retailer provides real life test instances. Introducing the layout specic stops non-identical spans metric in combination with the greedy smallest entry heuristic results in a reduction of 48:3% in walking distance. Order batching increases the pick density which may lead to higher levels in picker congestion. In a discrete event simulation, the reduction of the overall picking time through a decrease in walking distance is thus conrmed. On tested sample picking waves, the overall picking time can be reduced by up to 21% per wave. A good number of pickers in the picking system is dependent on the pick density. The pick density, amongst other explanatory variables, can also be used to predict the reduction in picking time. The effects of different structural options of the unidirectional cyclical picking line, namely the U- and Z-conguration, are investigated. This results in four decision tiers that have to be addressed while optimising the order picking system. The rst decision tier assigns stock to picking lines, the second arranges stock around a picking line, the third chooses the conguration and the last sequences the orders to be picked. Order batching is added as an additional layer. An increase in pick density benets the reduction of walking distance throughout the decision tiers and supports the choice of the U-conguration after evaluating different test instances. The total completion time of a picking wave can thus be reduced by up to 28% when compared to benchmark instances. The dissertation is concluded by suggesting further research directions.