Stochastic Modelling and Analysis of Warehouse Operations

This thesis has studied stochastic models and analysis of warehouse operations. After an overview of stochastic research in warehouse operations, we explore the following topics. Firstly, we search optimal batch sizes in a parallel-aisle warehouse with online order arrivals. We employ a sample path optimization and perturbation analysis algorithm to search the optimal batch size for a warehousing service provider, and a central finite difference algorithm to search the optimal batch sizes from the perspectives of customers and total systems. Secondly, we research a polling-based dynamic order picking system for online retailers. We build models to describe and analyze such systems via stochastic polling theory, find closed-form expressions for the order line waiting times, and apply polling-based picking to online retailers. We then present closed-form analytic expressions for pick rates of order picking bucket brigades systems in differe

Behavioral Operations in Logistics

In the world of logistics, a considerable share of all work is automated and performed by machines or robots. An examination of the existing logistics r

Order picking optimisation on a unidirectional cyclical picking line

Thesis (PhD)--Stellenbosch University, 2020.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.AFRIKAANSE OPSOMMING : Die opmaak van bestellings op 'n uitsoeklyn in 'n onderneming se distribusiesentrum is die grootste bydraer tot die bedryfskoste van 'n distribusiesentrum. Dit is dus belangrik om hierdie aktiwiteit so doeltreffend moontlik te maak. Die proses om bestellings op te maak bestaan uit drie hoofaktiwiteite, naamlik stap na die voorraad, uitsoek (kies en bymekaarsit) van die voorraad vir 'n bestelling en die pak van die gekose voorraad in kartonne vir verdere verwerking en verspreiding. Omdat die totale hoeveelheid werk vir die uitsoek- en hanteringsaktiwiteite konstant bly, word die vermindering van loopafstand die hoofdoelwit om die totale koste van hierdie proses te minimeer. Die minimering van loopafstand lei tot 'n vermindering in totale tyd om bestellings op te maak, wat op sy beurt weer lei tot 'n afname in die totale koste van die stelsel om bestellings op te maak. Die hoofdoel van hierdie proefskrif is om die stelsel vir die uitsoek van bestellings op 'n eenrigting sikliese uitsoeklyn te optimeer. Metodes vir die samevoeging of groepering (Eng.: batching) van bestellings (om gelyktydig opgemaak te word) word ontwikkel vir hierdie uitsoekstelsel aangesien operasionelenavorsingsliteratuur aantoon dat groepering van bestellings 'n effektiewe metode is om loopafstand te verminder. Groepering van bestellings is reeds gedoen vir die standaard blokuitleg van distribusiesentra, maar nie vir hierdie spesieke uitleg van 'n eenrigting sikliese uitsoeklyn nie. Daarbenewens het die eenrigting sikliese uitsoeklyn twee kongurasie-opsies wat die siese opstelling verander en sodoende die manier beinvloed waarop werkers tydens die uitsoekproses loop. Die groepering van bestellings word ontwikkel vir 'n eenrigting sikliese uitsoeklyn deur middel van 'n plek-gebaseerde maatstaf wat die nabyheid van bestellings se roetes meet. Hierdie maatstaf word verder uitgebrei deur die eienskappe van die uitleg in ag te neem. Regte voorbeelde van die probleem uit 'n distribusiesentrum van 'n prominente Suid-Afrikaanse kleinhandelaar word gebruik vir toetsing. Die ontwikkeling en implementering van 'n uitlegspesieke stop-nie identiese-strek-maatstaf in kombinasie met die gulsige kleinste-invoegingsheuristiek lei tot 'n vermindering van 48:3% in stapafstand. Die groepering van bestellings verhoog die digtheid van plekke waar werkers stop vir voorraad, wat kan lei tot ho er vlakke van kongestie vir werkers. 'n Diskrete-gebeurtenis-simulasie bevestig dat 'n afname in loopafstand ook 'n vermindering van die totale voltooiingstyd tot gevolg het. Met behulp van werklike historiese data kon die totale tyd vir die uitsoek van bestellings met tot 21% per golf verminder word. 'n Goeie aantal werkers in die uitsoekstelsel is afhanklik van die uitsoekdigtheid. Die uitsoekdigtheid en andere verklarende veranderlikes, kan ook gebruik word om die vermindering in totale tyd om bestellings op te maak, te voorspel. Die invloed van verskillende strukturele opsies van die eenrigting sikliese uitsoeklyn, naamlik die U- en Z-kongurasie, word ook ondersoek. Dit het tot gevolg dat vier besluitnemingsvlakke aangespreek moet word om die uitsoekstelsel te optimeer. Die eerste besluitnemingsvlak ken voorraad aan die uitsoeklyne toe, die tweede rangskik voorraad binne die uitsoeklyn, die derde kies die kongurasie van die lyn en die laaste kies die volgorde waarin die bestellings uitgesoek word. Groepering van bestellings word bygevoeg as 'n addisionele vlak. 'n Toename in werksdigtheid bevoordeel die vermindering van loopafstand deur die besluitvlakke en bevoordeel die U-kongurasie na evaluering van verskillende toetsdata. Die totale voltooiingstyd van 'n uitsoekgolf kan dus verminder word met tot 28% in vergelyking met eweknie voorbeelde. Die studie word afgesluit deur verdere navorsingsmoontlikhede voor te stel.Doctora

Incorporating Worker-Specific Factors in Operations Management Models

To add value, manufacturing and service operations depend on workers to do the job. As a result, the performance of these operations is ultimately dependent on the performance of individual workers. Simultaneously, workers are major stakeholders of the firm. Workers spend a considerable amount of time in their lives at their job and depend on that job to sustain themselves and their families. As a result, firms wishing to satisfy their primary stakeholders should consider workers’ job satisfaction in the design of their operations. Especially given that job satisfaction can promote other positive outcomes for the firm, including lower personnel turnover and accident rates. This thesis addresses the key question of how common operations management decisions may have an impact on a worker’s individual performance and his job satisfaction. In particular, we first provide a literature survey of psychology and ergonomics linking operations decision variables with performance and job satisfaction. Next, we study the effects of assigning goals on performance and work pace regulation. We identified steady work pace regulation patterns associated with challenging goals. Finally, we studied the problem of where to store items in a warehouse such that efficiency (cycl

A POST-NORMAL SCIENCE APPROACH TO UNDERSTANDING THE REAL ISSUES, CHALLENGES AND CONTEXTS OF MUNICIPAL SOLID WASTE (MSW) MANAGEMENT IN DEVELOPING COUNTRIES – A CASE STUDY OF ABA-URBAN IN ABIA STATE, NIGERIA.

Effective management of waste is a complex task requiring appropriate technical solutions, sufficient organisational capacity and the collaboration of a wide range of stakeholders. The more advanced, high-income economies and developed nations of the world have evolved their current systems in a series of steps. It is now widely recognised that it is counterproductive for developing countries to use strategies and policies developed for high-income economies. There are no quick fixes. Therefore, it is unrealistic for a developing country to expect to go from uncontrolled dumping of waste to a ‘modern’ state of the art waste management system in one great leap. Rather, the process should be locally sensitive, critical and creative and owned by the community of concern. By adopting this approach, many cities and small towns in other developing countries have recorded considerable progress while the same cannot be said of cities in Nigeria, where there appears to be a lack of understanding and appreciation of the enormity of the challenges posed by MSW. The main aim of this study therefore, is to understand the real issues, challenges and contexts of MSW management in developing countries, using the Nigerian city of Aba as a case study. The study adopted a purely qualitative methodology, and by utilising the approach of Post Normal Science (PNS) and Adaptive Methodology for Ecosystem Sustainability and Health AMESH), particular attention was given to the oral testimonies and lived experiences of the participants drawn from the wider peer community of stakeholders of MSW management in the city. The results provide the first historical review of MSW management in Aba and show that, over the period reviewed; the MSW management processes remained rudimentary, often involving the evacuation of refuse from one point to another. It also shows that currently, indiscriminate dumping, littering and illegal dumping of refuse is common in the city. Further analysis of the data revealed the inadequacies in the national sanitation policy and the current MSW management system implemented by ASEPA – the agency responsible for MSW management in the city. The level of planning and organisation of MSW management activities was found to be shambolic and there were shortages in manpower and availability of equipment needed to effectively collect and dispose waste. The common method of waste disposal was found to be open dumping in dumpsites that were unplanned and unsanitary. Despite these realities, the study found that contrary to the commonly held popular notion that residents of Aba prefer a dirty environment to a clean one, most participants in this study showed a good understanding of the implication of poor MSW management practices on public health, and expressed willingness to pay higher sanitation fees if it will guarantee a cleaner environment. To curb most of the conflicts that currently exist between ASEPA and other stakeholder groups and move towards sustainable MSW management as indicated in the sustainable development goals (SDGs) and millennium development goals (MDGs), the direction of travel of MSW management in the city must change form a modernisation approach of expending scarce public resources on imported sophisticated refuse collection and transportation vehicles, that are unsuitable and does not stand the test of time for various reasons; to adopting a local approach that encourages genuine participation of all relevant stakeholders in the policy decision making, design, implementation and evaluation of the MSW management system. Such approach will help improve the livelihood of informal waste workers who are currently maligned, intimidated and harassed by MSW management authorities

Modernity's garbage: Struggles over municipal solid waste in urban India

The challenges associated with waste are worsening with increasing urbanization and economic growth in contemporary India. A range of actors--public and private, formal and informal--currently manage urban waste. Challenged with managing growing quantities of waste, municipalities across the country have recently been outsourcing waste management services to private firms, and in the process privatizing what used to function as the urban commons of waste for the informal sector. This new form of privatization has created a struggle over access to waste. Using a combination of qualitative and quantitative methods based on fifteen months of fieldwork in Delhi, this dissertation examines waste as a contested object of urban planning in contemporary India, through four inter-related arguments. First, privatization of waste management services reveals a contradiction between neoliberal theory and practice. While free markets are privileged in neoliberal theory, the reforms disrupt existing well-functioning markets by displacing (or threatening to displace) countless independent informal entrepreneurs and set the stage for national oligopolies where a few firms dominate the landscape. Second, in response to such displacement or threats thereof, informal sector workers are organizing into formal entities that can enter into contracts to provide certain waste management services, through a process called formalization. This process requires disciplining informal workers and an expenditure of free labor on their part. Under a persistent threat of displacement and dispossession, formalization is both strategic and necessary for the informal sector. Third, even though the composition of Indian waste has been deemed unsuitable for incineration, such technologies are increasingly being promoted. The emergence of such a technological fix can be explained in practical terms but also ideologically in the promise of modernity that such technologies embody. If implemented, it will necessitate that informal workers no longer extract recyclable materials and a livelihood, from the waste stream. Fourth, behavior change has recently emerged as the focus of how to make cities in India cleaner. However, a discourse of behavior change privileges aesthetic over infrastructural concerns. It also makes visible the voluntary labor of the privileged while erasing from memory the economically necessary labors of the marginalized

Advanced Automation for Space Missions

The feasibility of using machine intelligence, including automation and robotics, in future space missions was studied