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Yayın Analysis of different maintenance policies on a multi-component system using dynamic bayesian networks(Işık Üniversitesi, 2019-01-15) Karacaörenli, Ayşe; Özgür Ünlüakın, Demet; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Endüstri Mühendisliği - Yöneylem Araştırması Yüksek Lisans ProgramıRecently, system components and interactions between them have become more complex and this situation has made it di?cult to provide maintenance decisions. Herewith, determining e?ective decisions has played an important role. In multicomponent systems, many methodologies and strategies can be applied when a component or a system has already broken down or when it is desired to identify and avoid pro-actively defects that could lead to future failure. In dynamic systems, it is important for proactive maintenance to increase system reliability by performing early diagnosis-based maintenance activities without waiting for a problem. In this study, we focus on proactive maintenance of a complex multi-component dynamic system. Components are hidden although there exists partial observability to the decision maker. Components deteriorate in time. It is possible to replace or repair components with a given cost. We want to ?nd a policy that minimizes the total maintenance cost in a prede?ned time horizon. We propose several maintenance policies and compare the performance of these by simulating them via Dynamic Bayesian Networks on an empirical model. Furthermore, a dynamic Bayesian network is constructed for the maintenance of an endo generator system to show how the proposed methods can be implemented in real life.Yayın Medical device maintenance strategy for post pandemic: case on ventilators(IEOM Society International, 2022-06) Aksezer, Sezgin ÇağlarEquipment-demand of healthcare providing institutions increased drastically during the Covid-19 pandemic. While majority of the demand (also the most publicized) has been occurring for disposable products such as masks, gloves, gowns, sterilizers, and test kits; the demand for electronic devices also increased during the initial waves of the pandemic but dropped afterwards as it was satisfied by the manufacturers. Since devices like vital monitors, dialysis systems, and ventilators are not single use products, handling of such systems after acquisition has become an issue of operations management for healthcare professionals. Utilization of such devices seem to have a cyclic behavior correlated with pandemic waves (as the hospitalizations peak so the usage and vice versa) and tend to have a longterm decreasing trend as the Covid-19 pandemic transforms to epidemic. Ventilators have found extensive usage in the treatment of Covid-19 related patients for ICU and emergency admissions. Medical ventilator device provides mechanically simulated air into the lungs of patients who are completely unable to breathe by themselves or breathes but insufficient to carry the necessary oxygen. So, the total functionality is crucial and service readiness is imminent. Number of active ventilators in Turkey increased to 22.000 units by the first year of the pandemic. However, utilization ratio of ventilators dropped to 29.6% (as of March 17, 2022) from the heights of 55% observed in the winter of 2021. Because of such decline, devices have become idle or operational times have dropped significantly. Health institution under investigation is a full-service government hospital operating in the city of Istanbul, Turkey. There are currently 142 ventilators in the hospital with less than 35% utilization. Maintenance task is only carried out by manufacturer approved service providers and parts are supplied from abroad. Since the operator of devices is a non-profit government hospital, maintenance outsourcing is done by bidding with strict budget and usually under the pressure of currency exchange rates even after the maintenance contract is signed. Failure analyses indicate mean time to failure of devices has not changed significantly with respect to pre-pandemic operation. Periodic maintenance strategy that has long been adopted for such devices has been modified to consider cyclic operation, extended standby durations, and lead time of spare parts. Further proposals are under consideration for group maintenance involving multiple healthcare institutions.Yayın Analyzing the performance of different costBased methods for the corrective maintenance of a system in thermal power plants(World Academy of Science, Engineering and Technology (WASET), 2019-08-16) Özgür Ünlüakın, Demet; Türkali Özbek, Busenur; Aksezer, Sezgin ÇağlarSince the age of industrialization, maintenance has always been a very crucial element for all kinds of factories and plants. With today’s increasingly developing technology, the system structure of such facilities has become more complicated, and even a small operational disruption may return huge losses in profits for the companies. In order to reduce these costs, effective maintenance planning is crucial, but at the same time, it is a difficult task because of the complexity of systems. The most important aspect of correct maintenance planning is to understand the structure of the system, not to ignore the dependencies among the components and as a result, to model the system correctly. In this way, it will be better to understand which component improves the system more when it is maintained. Undoubtedly, proactive maintenance at a scheduled time reduces costs because the scheduled maintenance prohibits high losses in profits. But the necessity of corrective maintenance, which directly affects the situation of the system and provides direct intervention when the system fails, should not be ignored. When a fault occurs in the system, if the problem is not solved immediately and proactive maintenance time is awaited, this may result in increased costs. This study proposes various maintenance methods with different efficiency measures under corrective maintenance strategy on a subsystem of a thermal power plant. To model the dependencies between the components, dynamic Bayesian Network approach is employed. The proposed maintenance methods aim to minimize the total maintenance cost in a planning horizon, as well as to find the most appropriate component to be attacked on, which improves the system reliability utmost. Performances of the methods are compared under corrective maintenance strategy. Furthermore, sensitivity analysis is also applied under different cost values. Results show that all fault effect methods perform better than the replacement effect methods and this conclusion is also valid under different downtime cost values.
