Modelling and simulation for the joint maintenance-inventory optimisation of production systems

Simulation methodologies are developed to model the joint optimization of preventive maintenance and spare parts inventory for a specific industrial plant under different production configurations. First, spare parts provisioning for a single-line system is considered, with the assumption that the demand is driven by maintenance requirements. The results indicate that a periodic review policy with replenishment as frequent as inspection is cost-optimal. Second, the joint optimization model for a multi-line (parallel) system is developed. It is found that a just-in-time review policy with inspection as frequent as replenishment produces the lowest cost policy. In this latter case, an implication of the proposed methodology is that, where mathematical modelling is intractable, or the use of certain assumptions make them impractical, simulation modelling is an appropriate solution tool. Under both production settings, the long-run average cost per unit time is used as the optimality criterion for the comparison of several policies

Joint optimisation of inspection maintenance and spare parts provisioning: a comparative study of inventory policies using simulation and survey data

The demand for industrial plant spare parts is driven, at least in part, by maintenance requirements. It is therefore important to jointly optimise planned maintenance and the associated spare parts inventory using the most appropriate maintenance and replenishment policies. In this simulation-based study, we address this challenge in the context of the random failure of parts in service and the replacement of defective parts at inspections of period T. Inspections are modelled using the delay-time concept. A number of simultaneous periodic review and continuous review replenishment policies are compared. A paper making plant provides a real context for the presentation of our ideas. We survey practitioners working with such plant to collect real data that inform the values of parameters in the models. Our simulation results indicate that a periodic review policy with ordering that is twice as frequent as inspection is cost optimal in the context of the plant that we study. For the purpose of comparison, we also present and discuss the characteristics of the various policies considered

Optimisation of inspection policy for multi-line production systems

This paper develops a simulation model to determine the cost-optimum inspection policy for a multi-line production system taking account of simultaneous downtime. The machines in the multi-line system are subject to a two stage failure process that is modelled using the delay-time concept. Our study indicates that: consecutive inspection of lines with priority for failure repair is cost-optimal, with a cost reduction of 61% compared to a ‘run-to-failure’ policy; and maintainers need to be responsive to operational requirements. Our ideas are developed in the context of a case study of a plant with three parallel lines, one of which is on cold-standby. Keywords: maintenance; delay-time model; simulation; production; parallel lines; manufacturing; preventive maintenance

Modelling and simulation for the joint optimisation of inspection maintenance and spare parts inventory in multi-line production settings

A simulation methodology is developed to model the joint optimisation of preventive maintenance and spare parts inventory in multi-line settings. The multi-line machines are subject to failure, based on the delay-time concept, and a selection of policies are used for the replenishment of the machines’ critical component. Production lines of varied configurations are modelled and described in three principal chapters. Firstly, the optimisation of preventive maintenance for a multi-line production system is developed in the context of a case study. The policy proposed indicates that consecutive inspection with priority for failure repair is cost-optimal, which suggests a substantial maintenance cost reduction of 61% compared to the run-to-failure policy. The contribution of this study is first and foremost in narrowing the gap between the theory and practice of managing multi-line systems, and in particular, that the scenarios and policies considered have important economic and engineering implications. In a second study, spare parts provisioning for a single-line system is considered, given that the demand for industrial plant spare parts should be driven, at least in part, by maintenance requirements. A paper-making plant provides a real context, for which simulation models are developed to jointly optimise the planned maintenance and the associated spare part inventory. This challenge is addressed in the context of the failure of parts in service and the replacement of defective parts at inspections of period T, using the delay-time concept, and a selection of replenishment policies. The results indicate that a periodic review policy with replenishment twice as frequent as inspection is cost-optimal. Further discussions and sensitivity analysis give insights into the characteristics and features of the policies considered. Finally, in the third study, the joint optimisation of preventive maintenance and the associated spare parts inventory for a multi-line system is developed using an idealised context. It is found that a review policy with inspection as frequent as replenishment using just-in-time (JIT) ordering is cost-optimal, and also the lowest risk policy; it is associated with the lowest simultaneous machine downtime and low stock-out cost-rates. This is a significant contribution to the literature. An implication of the proposed methodology is that, where mathematical modelling is intractable, or the use of certain assumptions make them less practical, simulation modelling is an appropriate solution tool. Throughout this thesis, the long-run average cost per unit time or cost-rate is used as the optimality criterion. In other contexts, one may wish to use availability or reliability instead. To do so would not change the methodology that is presented here

Link between single nucleotide polymorphism of rs266729 and rs2241766 in the ADIPOQ gene and gestational diabetes in an Iranian population

Background: The manuscript investigates the association between adiponectin gene (ADIPOQ) polymorphisms and gestational diabetes mellitus (GDM) in an Iranian population. Methods: We designed a case-control study involving 265 normal glucose tolerant (NGT) women and 297 GDM patients. Three adiponectin single-nucleotide polymorphisms (rs1501299, rs266729 and rs2241766) were genotyped by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP), and the protein level was measured by the enzyme-linked immunosorbent assay. Other biochemical parameters were assessed by conventional laboratory methods. Results: We found that rs266729 and rs2241766 were significantly associated with GDM. The genotypes analysis indicated that for rs266729, 51(61.4) of the 83 GDM patients had GG genotype, and 32(38.6) GC/CC genotype and for rs2241766, 75(58.6) of the 128 GDM patients had TT genotype, and 53(41.4) GG/GT genotype. The relation between other single-nucleotide polymorphisms (rs1501299) and GDM was not observed. Conclusion: The ADIPOQ genetic polymorphisms were associated with GDM in our studied population. © 201

Fit between humanitarian professionals and project requirements: hybrid group decision procedure to reduce uncertainty in decision-making

Choosing the right professional that has to meet indeterminate requirements is a critical aspect in humanitarian development and implementation projects. This paper proposes a hybrid evaluation methodology for some non-governmental organizations enabling them to select the most competent expert who can properly and adequately develop and implement humanitarian projects. This methodology accommodates various stakeholders’ perspectives in satisfying the unique requirements of humanitarian projects that are capable of handling a range of uncertain issues from both stakeholders and project requirements. The criteria weights are calculated using a two-step multi-criteria decision-making method: (1) Fuzzy Analytical Hierarchy Process for the evaluation of the decision maker weights coupled with (2) Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) to rank the alternatives which provide the ability to take into account both quantitative and qualitative evaluations. Sensitivity analysis have been developed and discussed by means of a real case of expert selection problem for a non-profit organisation. The results show that the approach allows a decrease in the uncertainty associated with decision-making, which proves that the approach provides robust solutions in terms of sensitivity analysis

Serum selenium level in patients with gastric non-cardia cancer and functional dyspepsia

BACKGROUND: Gastric cancer (GC) is the most common gastrointestinal cancer in Iran. Helicobacter pylori (H. pylori) accounts as one of the main risk factors for gastric non-cardia cancer (GNCC). It is suggested that high serum selenium level may have a protective role in GNCC. In this cross-sectional study, we determined the serum Se level and the status of H. pylori infection in two populations with GC and functional dyspepsia (FD). METHODS: The enrolled patients were 85 (27 women, 58 men) with recent pathologically proven GNCC (adenocarcinoma) and 85 (34 women, 51 men) FD patients. Serum Se was measured by atomic absorption spectrophotometry. H. pylori IgG antibody was detected by quantitative enzyme immunoassay. RESULTS: The mean age in the GNCC and FD patients were 62.85±14.6 and 58.9±14.7 years, respectively (P=0.08). The serum selenium levels were 111.6±27.7 and 129.9±32.1 μg/L (mean±SD) in GNCC and FD patients, respectively (P<0.001). The frequency of H. pylori infection was 49.4% (n=42) and 68.2% (n=58) in GNCC and FD patients (P=0.013). The crude and adjusted odds ratio (OR) between GNCC and the linear effect of serum selenium level were 0.98 and 0.982, respectively (P=0.002). This means that each unit increase in serum selenium level decreases the odds of cancer by 2%. CONCLUSION: Serum selenium level was significantly lower in GNCC cases. It suggests that lower serum selenium might have some association with the risk of GNCC. H. pylori infection does not play a significant impact on this association