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Inaugural lecture by Prof Stephan Visagie
Author: Ronel Beukes
Published: 08/05/2018

Prof Stephan Visagie, Professor in Operations Research and departmental chair of the Department of Logistics, presented his inaugural lecture on 3 May.

He was introduced by Prof Ronel du Preez, Vice-dean: Teaching and Learning of the Faculty of Economic and Management Sciences, who listed his research interest as non-linear knapsack problems and the application of optimisation techniques such as integer programming and metaheuristics in such diverse fields as agriculture, production, logistics, scheduling and voting theory.

In his lecture entitled Optimisation in distribution centres, Prof Visagie explained the use of distribution centres in the supply chains of retailers to buffer stock and to repack the bulk stock that arrive from suppliers to smaller shipments destined for stores.

“The retailer's central planning office releases batches of stock-keeping units that differ only in size. These batches are called distributions and are continuously released to the distribution centre to pick and ship to stores. A distribution thus contains one or more stock-keeping units together with instructions about the quantity to be shipped to individual stores. Each distribution also has a deadline, by which date it should have left the distribution centre.

“The system for the repacking of stock – or order picking – consists of parallel unidirectional picking lines. Order picking occurs in waves, when a set of distributions is assigned (and physically taken) to a specific picking line. A team of pickers then pick all the stock-keeping units on this particular picking line for all the stores. Once all the stock-keeping units are picked, the picking line is populated with a new set of distributions to start a new wave of picking."

According to Prof Visagie this methodology resulted in three layers of optimisation decisions, while the objectives considered during these decisions were to minimise the following: walking distance of the pickers, the volume of stock on the picking line, the number of packages of which the volume fail to exceed roughly the size of a shoebox, and the total penalties incurred to distributions for not being assigned to a picking line before a specified date.

“All proposed solution methods were tested on real historical data received from PEP Stores Limited. The preferred combination of algorithms – genetic algorithm for first tier, user-specified in the second tier and greedy-ending heuristic in the third tier – showed an average decrease of 41% in the walking distance of all the pickers and a 34% decrease in the volume of stock on the picking line with the largest volume of stock. The percentage of small packages was maintained while on average a 20% decrease in the average number of days that distributions are assigned later than their out-of-distribution centre-date was achieved."

Prof Visagie has received numerous awards including three Rector's Awards for outstanding achievement, and several awards from the Operations Society of South Africa including one for “an outstanding contribution towards the application of Operations Research on a national level" for the work he did in cooperation with Pep Stores. The Society further honoured him by making him a fellow of the Operations Research Society of South Africa, the most prestigious of its awards.

  • Photo: Prof Stephan Visagie with Prof Eugene Cloete, Vice-Rector: Research, Innovation and Postgraduate Studies and Prof Ingrid Woolard, Dean of the Faculty of Economic and Management Sciences.