Browsing by Author "Pascual, R."
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- ItemA methodology for integrated critical spare parts and insurance management(2016) Martínez, A.; Pascual, R.; Maturana Valderrama, Sergio
- ItemA practical procedure for the selection of time-to-failure models based on the assessment of trends in maintenance data(ELSEVIER SCI LTD, 2009) Louit, D. M.; Pascual, R.; Jardine, A. K. S.Many times, reliability studies rely on false premises such as independent and identically distributed time between failures assumption (renewal process). This can lead to erroneous model selection for the time to failure of a particular component or system, which can in turn lead to wrong conclusions and decisions. A strong statistical focus, a lack of a systematic approach and sometimes inadequate theoretical background seem to have made it difficult for maintenance analysts to adopt the necessary stage of data testing before the selection of a suitable model. In this paper, a framework for model selection to represent the failure process for a component or system is presented, based on a review of available trend tests. The paper focuses only on single-time-variable models and is primarily directed to analysts responsible for reliability analyses in an industrial maintenance environment. The model selection framework is directed towards the discrimination between the use of statistical distributions to represent the time to failure ("renewal approach"); and the use of stochastic point processes ("repairable systems approach"), when there may be the presence of system ageing or reliability growth. An illustrative example based on failure data from a fleet of backhoes is included. (c) 2009 Elsevier Ltd. All rights reserved.
- ItemA structured methodology to optimise throughput of production lines(2016) Pascual, R.; Madariaga, R.; Santelices, G.; Godoy Ramos, David R.; Droguett, E.
- ItemAn asset-management oriented methodology for mine haul-fleet usage scheduling(2018) Nakousi, C.; Pascual, R.; Anani, Angelina; Kristjanpoller, F.; Lillo, P.
- ItemBusiness-oriented prioritization: A novel graphical technique(ELSEVIER SCI LTD, 2009) Pascual, R.; Del Castillo, G.; Louit, D.; Knights, P.Traditionally, Pareto analysis has been used to select the most critical components and failure modes of a system. A clear disadvantage of this technique is that it requires preselecting a single criterion to establish priorities. More recently, a graphical log-scatter diagram technique has been proposed. It considers three key performance indicators simultaneously: reliability (MTBF), maintainability (MTTR), and unavailability (D). This technique considers only times and does not include economical effects explicitly. This article extends both techniques to explicitly consider both direct and indirect costs to prioritize from the point of view of an asset manager or from a maintenance decision-maker, as required. Due to the economic-based approach of this article, cost discounting is also considered inside financial costs such as-but not limited to-reliability-related investments. Also, the results are displayed on simple and accessible graphs which make them particularly useful for conveying results to non-technical managers. The methodology is illustrated by analyzing a shovel from the copper mine industry, and it clearly shows how the proposed technique facilitates business oriented decisions and how they should change under different market conditions. (C) 2009 Elsevier Ltd. All rights reserved.
- ItemIntegrated Model for Optimizing Strategic Overhaul Planning of Distributed Pump Stations(ASCE-AMER SOC CIVIL ENGINEERS, 2011) Pascual, R.; Rey, P. A.; Hodkiewicz, M.; Cruz, C.An important part of the lifecycle costs for pump stations are the support costs associated with energy and preventive maintenance of the pumps. Both cost terms are interrelated because a trade-off exists between maintaining pumps frequently (with increased pump efficiency, reduced energy costs and increasing maintenance costs, and pump unavailability) and extending the intervals between the overhauls (increased loss of efficiency, increased energy costs, and decreased maintenance costs). The planner needs to forecast pump overhaul demands to determine budget levels because economic resources are limited. This process is assisted by using mathematical programming methods to prioritize funding for equipment requiring renewal in consideration of limited resources in a strategic time horizon (several years). This work proposes an efficient model to define an overhaul maintenance program for a pump network with a large number of pumps (a few hundred in the case study). The model minimizes the discounted total (energy+overhaul) cost by selecting and scheduling pumps for overhaul subject to budget constraints. The formulation uses 0-1 integer programming. DOI: 10.1061/(ASCE)CP.1943-5487.0000085. (C) 2011 American Society of Civil Engineers.
- ItemIntegrating mining loading and hauling equipment selection and replacement decisions using stochastic linear programming(2017) Santelices, G.; Pascual, R.; Lüer Villagra, A.; Mac Cawley Vergara, Alejandro Francisco; Galar, D.
- ItemOptimal inspection intervals for safety systems with partial inspections(TAYLOR & FRANCIS LTD, 2011) Pascual, R.; Louit, D.; Jardine, A. K. S.The introduction of International Standard IEC 61508 and its industry-specific derivatives sets demanding requirements for the definition and implementation of life-cycle strategies for safety systems. Compliance with the Standard is important for human safety and environmental perspectives as well as for potential adverse economic effects (eg, damage to critical downstream equipment or a clause for an insurance or warranty contract). This situation encourages the use of reliability models to attain the recommended safety integrity levels using credible assumptions. During the operation phase of the safety system life cycle, a key decision is the definition of an inspection programme, namely its frequency and the maintenance activities to be performed. These may vary from minimal checks to complete renewals. This work presents a model (which we called rho beta model) to find optimal inspection intervals for a safety system, considering that it degrades in time, even when it is inspected at regular intervals. Such situation occurs because most inspections are partial, that is, not all potential failure modes are observable through inspections. Possible reasons for this are the nature and the extent of the inspection, or potential risks generated by the inspection itself. The optimization criterion considered here is the mean overall availability A(o), but also taking into account the requirements for the safety availability A(s). We consider several conditions that ensure coherent modelling for these systems: sub-systems decomposition, k-out-of-n architectures, diagnostics coverage (observable/total amount of failure modes), dependent and independent failures, and non-negligible inspection times. The model requires an estimation for the coverage and dependent-failure ratios for each component, global failure rates, and inspection times. We illustrate its use through case studies and compare results with those obtained by applying previously published methodologies.
- ItemOptimization models for critical spare parts inventories-a reliability approach(TAYLOR & FRANCIS LTD, 2011) Louit, D.; Pascual, R.; Banjevic, D.; Jardine, A. K. S.In industries characterized by heavy utilization of equipment and machinery, such as mining, oil & gas, utilities, transportation, adequate stockholding of critical spare parts becomes essential. Insufficient stocks affect overall performance of physical assets, as lack of spares may result in gross penalties, lower availability or increased operational risks. On the other hand, oversized inventories lead to inefficient use of capital and may imply severe expenditures. This paper presents various approaches for the determination of the optimal stock size, when the stock is composed of (i) non-repairable or (ii) repairable parts. The paper is focused on spares for relatively expensive, highly reliable components, rather than on fast-moving spare parts. Optimization criteria considered are minimization of costs, maximization of equipment availability, and the achievement of a desired stock reliability (probability that a spare part request will not be rejected because of the lack of spares in stock). For stock reliability, instantaneous and interval reliability calculations are considered. In addition, models directed to the estimation of the remaining life of a given stock of spare parts (at a certain stock reliability level) are introduced. The paper describes several models subject to practical industrial application, and presents case studies from utilities and mining to illustrate their use. Journal of the Operational Research Society (2011) 62, 992-1004. doi: 10.1057/jors.2010.49 Published online 2 June 2010
- ItemThroughput centered prioritization of machines in transfer lines(ELSEVIER SCI LTD, 2011) Pascual, R.; Godoy, D.; Louit, D. M.In an environment of scarce resources and complex production systems, prioritizing is key to confront the challenge of managing physical assets. In the literature, there exist a number of techniques to prioritize maintenance decisions that consider safety, technical and business perspectives. However, the effect of risk mitigating elements-such as intermediate buffers in production lines-on prioritization has not yet been investigated in depth. In this line, the work proposes a user-friendly graphical technique called the system efficiency influence diagram (SEID). Asset managers may use SEID to identify machines that have a greater impact on the system throughput, and thus set prioritized maintenance policies and/or redesign of buffers capacities. The tool provides insight to the analyst as it decomposes the influence of a given machine on the system throughput as a product of two elements: (1) system influence efficiency factor and (2) machine unavailability factor. We illustrate its applicability using three case studies: a four-machine transfer line, a vehicle assembly line, and an open-pit mining conveyor system. The results confirm that the machines with greater unavailability factors are not necessarily the most important for the efficiency of the production line, as it is the case when no intermediate buffers exist. As a decision aid tool, SEID emphasizes the need to move from a maintenance vision focused on machine availability, to a systems engineering perspective. (C) 2011 Elsevier Ltd. All rights reserved.