Bi-objective workforce-constrained maintenance scheduling: a case study

In this paper, a real maintenance workforce-constrained scheduling problem is formulated as a bi-objective mixed-integer programming model with the aim of simultaneously minimizing the workforce requirements and maximizing the equipment availability. The skilled workforce is provided by internal and external resources using regular time, overtime and contracting. The equipment availability is measured by the downtime required for preventive maintenance (scheduled) and failure repair (unscheduled) jobs. We also encounter imminent or potential failures whose priorities depend on the severity of the failure on the system (secondary failure). The total weighted flow time is used as a scheduling criterion to measure the equipment availability; the weight of each job directly depends on the expected downtime resulting from the associated failure. The proposed model is verified using two comprehensive numerical examples and some sensitivity analyses. We conclude by discussing the results.     

Measurement in the workplace: the case of process improvement in manufacturing industry

In the context of education, measurement is often defined as the process of assigning a numerical value to an attribute of an object or event. Using three case studies of ??process improvement??, the purpose of this article is to show how measurement takes place in manufacturing industry. We ask: What are the key issues involved in measurement that should inform the design of education and training for measurement? First, our research suggests that the definition of measurement should be enhanced so as to include the quantification of processes and multivariate constructs that aim to capture key performance indicators of production processes. Secondly, technology plays a complex role because it can lead not only to automation and increased invisibility of data, but also to the availability of information otherwise not accessible. Thirdly, we suggest that the use of the inferentialist concept of ??web of reasons?? in addition to more commonly used concepts such as ??practice?? or ??activity?? can help to focus not only on the what and how, but also on the why of measurement.     

Balancing stocks, flexible recipe costs and high service level requirements in a batch process industry: A study of a small scale model

Process industries often obtain their raw materials from mining or agricultural industries. These raw materials usually have variations in quality which often lead to variations in the recipes used for manufacturing a product. Another reason for varying the recipe is to minimize production costs by using the cheapest materials that still lead to a satisfactory quality in the product. A third reason for using recipe flexibility is that it may occur that not all materials for the standard recipe are available. If variations in supply and demand are large, keeping sufficient safety stock to cope with these variations may incur prohibitive high costs. This means that the costs of keeping safety stock should be balanced with the costs of sometimes using more expensive recipes. The question now is for what situations and to what extent the use of recipe flexibility is justified. In this paper we study this question by means of a small scale model. For this simple situation we derive a decision procedure to balance safety stock costs and flexibility costs. This procedure is applied to a range of different situations, that are characterized by a set of parameter values, in order to determine for which situations recipe flexibility should be used.     

Modelling in the absence of data: a case study of fleet maintenance in a developing country

Adequate and relevant objective data for modelling maintenance decision problems are often incomplete or not readily accessible. This is particularly true in developing countries. In this paper the experience gained between 1991–95 in conducting a maintenance study of an inter-city express bus fleet in a developing country is presented. The lack of available maintenance records and operating data rendered the study the most data-starved maintenance modelling exercise the authors have met before or since. The study required the use of subjective methods to both define the problem and to estimate parameters, and the application of recently developed concepts in maintenance modelling along with snapshot analysis and delay time modelling. This imposed a structured approach to problem recognition and problem solution. The study contributed both directly and indirectly to a change in work culture and to a reduction in bus breakdown rate. The company was re-visited 5?years later specifically to seek evidence of lasting impact. Some evidence existed and is reported in the paper.     

Benchmarking of maintenance performance: a case study in two manufacturers of furniture

When profit margins of a plant are decreasing, the need forreliable and efficient maintenance policy becomes more important.Measuring maintenance performance is important for companiesto recognize whether their planned goals are achieved or not.Also, such measurements can be utilized for benchmarking, whichis one of the tools for never-ending improvement. But, theseobjectives cannot be achieved without well-documented data ofthe relevant variables. Better data coverage and quality isnecessary for following maintenance performance developmentand it would, in many cases, clarify the ambiguity concerningthe main problem in the context, namely that neither the productionnor maintenance department can show what effect maintenancehas on profitability. A more effective maintenance policy indirectly implies improvementsin product quality and manufacturing process effectiveness.Elongation of the production time, i.e. reducing the downtimedue to failures, planned replacements and repair, in additionto the improvement in the total maintenance activities, arealso some of the results that can be expected when an efficientmaintenance policy is used. Measuring and monitoring maintenanceperformance measures is required partly for detecting, and eventuallytreating as soon as possible, undesirable changes and partlyto make benchmarking with the best in the branch, which savesappreciable economic losses for companies. In this paper, a model for how to identify the measurable variables,which are needed to develop measures for monitoring maintenanceperformance behaviour systematically, is developed. Five maintenanceperformance measures are proposed and applied. An additionalmodel for systematically analysing the trend of maintenanceperformance measures, for an overall assessment of the company'ssituation, is presented. Two case studies in manufacturers offurniture are conducted to verify these models.     

Supply chain performance evaluation using fuzzy network data envelopment analysis: a case study in automotive industry

Supply chain performance evaluation problems are evaluated using data envelopment analysis. This paper proposes a fuzzy network epsilon-based data envelopment analysis for supply chain performance evaluation. In the common data envelopment analysis models which are used for evaluation of decision-maker units efficiency, there are several inputs and outputs. One of the bugs of such models is that the intermediate products and linking activities are overlooked. Considering these intermediate activities and products, the current study evaluates the performance of decision-maker units in an automotive supply chain. There are ten decision-maker units in the supply chain in which there are three suppliers, two manufacturers, two distributors, and four customers. Moreover, the overall efficiency of input-oriented (input-based) model and input-oriented divisional efficiency are calculated. In order to improve the efficiencies, the projections onto the frontiers are obtained by using the outputs of the solved model and Lingo software. In order to show the applicability of the proposed model, it is applied on automotive industry, as a case study, to evaluate supply chain performance. Then, the overall efficiencies of DMUs and each sections (divisions) of DMUs were calculated separately. Therefore, every organization can apply this evaluation method for improving the performance of alternative factors.

     

Workforce-constrained maintenance scheduling for military aircraft fleet: a case study

The problem is related to a fleet of military aircraft with a certain flying program in which the availability of the aircraft sufficient to meet the flying program is a challenging issue. During the pre- or after-flight inspections, some component failures of the aircraft may be found. In such cases, the aircraft are sent to the repair shop to be scheduled for maintenance jobs, consisting of failure repairs or preventive maintenance tasks. The objective is to schedule the jobs in such a way that sufficient number of aircrafts is available for the next flight programs. The main resource, as well as the main constraint, in the shop is skilled-workforce. The problem is formulated as a mixed-integer mathematical programming model in which the network flow structure is used to simulate the flow of aircraft between missions, hanger and repair shop. The proposed model is solved using the classical Branch-and-Bound method and its performance is verified and analyzed in terms of a number of test problems adopted from the real data. The results empirically supported practical utility of the proposed model.     

Modeling safety culture as a socially emergent phenomenon: a case study in aircraft maintenance

Safety culture is often understood as encompassing organizational members’ shared attitudes, beliefs, perceptions and values associated with safety. Safety culture theory development is fraught with inconsistencies and superficiality of measurement methods, because the dynamic and political nature of culture is often ignored. Traditionally, safety culture is analyzed by survey-based approaches. In this paper we propose a novel, systemic, interdisciplinary approach for investigating safety culture that combines multi-agent system modeling with organizational ethnography. By using this approach, mechanisms of emergence of safety culture from daily practices, operations and interactions of organizational actors can be modeled and analyzed. The approach is illustrated by a case study from the aircraft maintenance domain, based on existing ethnographic data. Using the proposed approach we were able to reproduce and explain emergent characteristic patterns of commitment to safety in the maintenance organization from this study. The model can be used for theory development and as a management tool to evaluate non-linear impacts of organizational arrangements on workers’ commitment to safety.     

Reducing depot-related costs of large bus operators a case study in Bangkok

Despite high passenger levels, many bus operators in large cities operate at a loss because of low fares and inefficient operations management. Methods to reduce the operating costs need to be adapted to the operational environment. This paper addresses one such “unproductive” cost: the idle running of buses between depots and routes. Models are developed to tackle the bus allocation problem in which many different brands of buses exist for which maintenance is organised in the depots. Based on an assessment using 1992 data for the city of Bangkok, a hierarchical modelling approach is retained. It is shown that important cost savings can be made.     

Delay-time-based preventive maintenance modelling for a production plant: a case study in a steel mill

This paper presents a case study of delay-time-based preventive maintenance (PM) modelling for a production plant system. Since production stoppages caused by waiting for raw materials provide windows to inspect and maintain the system, these production stoppages can be incorporated into the PM model. Considering the nature of different defects that can cause failures, two types of defects are modelled: small and large defects. Small defects are normally dealt with during production stoppages, but both small and large defects can be dealt with over a longer duration during PM. The parameters of the model are estimated using the maximum-likelihood method based on the real data. The model aims to find the optimal PM interval by minimizing the expected total downtime within an overhaul cycle. Management suggestions are also recommended.     

Budget-constrained maintenance in the building industry

Given records of building component renewals which occurredduring the year and also theage profile of the building stock,the theory of alternating renewal processes in discrete timeis used to deduce the underlying probability mass functionsof the natural lifespan of building components and of delaydue to limited resource. In an investigation into the economicsof renewal delay, the expected number of component renewalsin each year of the life of a building is deduced from the convolutionsof the probability mass function of natural lifespan: theseexpected numbers depend on the chosen delay. The nett presentvalue of the expected cost associated with each specified delayis calculated, so that a judgement can be made as to the optimumdelay (if any). These calculations are conveniently performedusing a spreadsheet macro.     

An approach for analysing transportation costs and a case study

One of the important parameters in the determination of optimal transportation system is economy. Therefore, a realistic method based on the technical, economical and operational parameters of various transportation modes, namely, road, railway, and sea routes is required in the analysis of costs. This method will take into consideration the probable price escalations during the lifetime of a certain transportation system. The cost of a unit of cargo or passenger per route length should be considered since it is an indicator of economics. In this paper, an approach for transportation cost analysis based on the economic analysis of the alternative modes of cargo or passenger transportation, is presented.     

Rationalisation of building maintenance by Markov decision models: a pilot case study

In this paper we develop a Markov decision model for rationalising building maintenance at a strategic level. Through a global modelling of deterioration we are able to determine the maintenance policy that ensures a specified average quality level at minimal cost. In this way we are able to produce a trade-off curve between overall quality level vs the minimum required level of maintenance costs. The model is applied in a pilot case for four main building elements, viz. masonry, pointing, window frames and painting of buildings owned by a building society. This demonstrates the value of the model as management instrument to determine and allocate budgets.     

An optimization-based decision support system for strategic planning in a process industry: the case of aluminium company in India

We describe how a generic multi-period optimization-based decision support system (DSS) can be used for strategic planning in process industries. The DSS is built on five fundamental elements—materials, facilities, activities, storage areas, and time periods. It requires little direct knowledge of optimization techniques to be used effectively. Results based on real data from an aluminium company in India demonstrate significant potential for improvement in profits. We conclude with a comparison of similar studies in two other process industries.     

Using soft OR principles for collaborative simulation: a case study in the Dutch airline industry

Collaborative simulation is a method to handle problem situations that are technologically and socially complex. It combines discrete event simulation with methods, techniques and tools designed to help groups progress with their problems. Discrete event simulation and group support both appear to have clear promises in embedding soft OR principles, yet also contain limitations. Furthermore, simulation and group support appear to be complementary, balancing each other's limitations. Therefore, the combination of group support and discrete event simulation seems to be a fruitful vehicle for the application of soft OR principles. Through experiences in a case study at the cargo department of a major airline, we examine how collaborative simulation can be applied in practice as problem structuring method. We conclude that collaborative simulation has definite potential as a soft OR technique, but further research is required to speed up different steps of simulation to fully use simulation in interactive decision making sessions with multiple stakeholders.     

Efficiency analysis of chain stores: a case study

An efficiency analysis problem of a hairdressing company is addressed in this paper. The service system has the characteristics of different trade area types, multiple workers with different levels, crisp facilities and expenses and customer relationship orientations. A specialized imprecise data envelopment analysis (IDEA) model is proposed so that the efficiencies of stores from different trade areas can be compared and all workers in different ranks are included. The analysis results exhibit that the inefficiency in resource utilization is largely caused by inefficient operation. Based on the measurement results, eight stores and two stores are aimed with high priority for improving the efficiencies in operation and scale, respectively. The regional manager agrees that IDEA is helpful in evaluating relative efficiency and detecting the reasons caused inefficiency.     

Forecasting demand for single-period products: A case study in the apparel industry

The problem considered is that of forecasting demand for single-period products before the period starts. We study this problem for the case of a mail order apparel company that needs to order its products pre-season. The lack of historical demand data implies that other sources of data are needed. Advance order data can be obtained by allowing a selected group of customers to pre-order at a discount from a preview catalogue. Judgments can be obtained from purchase managers or other company experts. In this paper, we compare several existing and new forecasting methods for both sources of data. The methods are generic and can be used in any single-period problem in the apparel or fashion industries. Among the pre-order based methods, a novel ‘top-flop’ approach provides promising results. For a small group of products from the case company, expert judgment methods perform better than the methods based on advance demand information. The comparative results are obviously restricted to the specific case study, and additional testing is required to determine whether they are valid in general.     

Stochastic games on a product state space: the periodic case

We examine so-called product-games. These are n-player stochastic games played on a product state space S ¹ × ... × S ⁿ , in which player i controls the transitions on S ⁱ . For the general n-player case, we establish the existence of 0-equilibria. In addition, for the case of two-player zero-sum games of this type, we show that both players have stationary 0-optimal strategies. In the analysis of product-games, interestingly, a central role is played by the periodic features of the transition structure. Flesch et al. (Math Oper Res 33, 403–420, 2008) showed the existence of 0-equilibria under the assumption that, for every player i, the transition structure on S ⁱ is aperiodic. In this article, we examine product-games with periodic transition structures. Even though a large part of the approach in Flesch et al. (Math Oper Res 33, 403–420, 2008) remains applicable, we encounter a number of tricky problems that we have to address. We provide illustrative examples to clarify the essence of the difference between the aperiodic and periodic cases.     

Stochastic modeling of the growth process

In this paper we estimate the upper boundary of the number of trees in the selection stand determined for harvesting in a future. At the same time we showed that the present resource of the number of trees in selection stand is sustained. This is achieved by stochastic modeling of the number of trees and the number of felled trees and by solving the partial differential equation. The same problem is solved in the papers, Mitrović (Stochastic modeling of the number of trees and the number of felled trees in selection stands, YUJOR, vol 14(1), pp 57–64, 2004; Stochastic modeling of the number of felled trees in selection stands, Computational and Applied Mathematics, vol 24(2), pp 285–292, 2005). In this paper the modified mathematical model is represented.      

Developing geographical indicators of mileage-related costs: a case study exploring travelling public services in English local areas

The context of this study is the public sector provision of services involving travelling in local authority areas in England. Such travelling services are costly and the relative levels of these costs across different local areas have raised a number of policy issues, particularly how performance assessments of local authorities and capitation-based funding by central government take into account (or fail to take into account) the differential travel costs faced in geographical areas that differ in population dispersion (sparsity) characteristics. The research presented here is concerned with identifying and evaluating practical indicators of mileage-related costs faced in local areas and a range of indicators have been explored for three services: domiciliary care, refuse collection and home-to-school transport. The findings suggest that currently used population dispersion indicators could be improved and that the current sparsity allowances in England underestimate the relative cost effects by a considerable amount.