• Journal of Applied Reliability
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• v.13 no.2
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• pp.77-86
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• 2013

Recently, an extended warranty of a system following the expiration of the basic warranty is becoming increasingly popular to the user. In this respect, we suggest a replacement model following the expiration of extended warranty with minimal repair warranty from the user's point of view in this paper. Under extended warranty, the failed system is minimally repaired by the manufacturer at no cost to the user during the original extended warranty period. As a criterion of the optimality, we utilize the expected cost rate per unit time during the life cycle from the user's perspective and suggest the optimal replacement period after extended warranty. Finally, a few numerical examples are given for illustrative purpose.

• Communications for Statistical Applications and Methods
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• v.12 no.3
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• pp.697-704
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• 2005

Computer software has gradually become an indispensable elements in many aspects of our daily lives and an important factor in numerous systems. In recent years, it is not unusual that the software cost is more than the hardware cost in many situations. In addition to the costs of developing software, the repair cost resulting from the software failures are even more significant. In this paper, a cost model with warranty cost, time to remove each fault detected in the software system, and delivery delay cost is developed. We use a software reliability model based on non-homogeneous Poisson process (NHPP). We discuss the optimal release policies to minimize the expected total software cost. Numerical examples are provided to illustrate the results.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.28 no.4
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• pp.191-201
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• 2016

A stochastic process has been used to develop a condition-based model for preventive maintenance of armor units of rubble-mound breakwaters that can make a decision the optimal interval at which some repair actions should be performed under the perfect maintenance. The proposed cost model in this paper based on renewal reward process can take account of the interest rate, also consider the unplanned maintenance cost which has been treated like a constant in the previous studies to be a time-dependent random variable. A function for the unplanned maintenance cost has been mathematically proposed so that the cumulative damage, serviceability limit and importance of structure can be taken into account, by which a age-based maintenance can be extended to a condition-based maintenance straightforwardly. The coefficients involved in the function can also be properly estimated using a method expressed in this paper. Two stochastic processes, Wiener process and gamma process have been applied to armor stones of rubble-mound breakwaters. By evaluating the expected total cost rate as a function of time for various serviceability limits, interest rates and importances of structure, the optimal period of preventive maintenance can easily determined through the minimization of the expected total cost rate. For a fixed serviceability limit, it shows that the optimal period has been delayed while the interest rate increases, so that the expected total cost rate has become lower. In addition, the gamma process tends to estimate the optimal period more conservatively than the Wiener process. Finally, it is found that the more crucial the level of importance of structure becomes, the more often preventive maintenances should be carried out.

• Journal of Korean Society of Industrial and Systems Engineering
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• v.41 no.3
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• pp.145-153
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• 2018

Taguchi regarded the concept of quality as 'total loss to society due to fluctuations in quality characteristics from the time of supplied to the customer.' The loss function is a representative tool that can quantitatively convert the loss that occurs due to the deviation of the quality characteristic value from the target value. This has been utilized in various studies with the advantage that it can change the social loss caused by fluctuation of quality characteristics to economic cost. The loss function has also been used extensively in the study of producer specification limits. However, in previous studies, only the second order loss function of Taguchi is used. Therefore, various types of losses that can occur in the process can't be considered. In this study, we divide the types of losses that can occur in the process considering the first and second loss functions and the Spiring's reflected normal loss function, and perform total inspection before delivering the customer to determine the optimal producer specification limit that minimizes the total cost. Also, we will divide the quality policy for the products beyond the specification limits into two. In addition, we will show the illustration of expected loss cost change of each model according to the change of major condition such as customer specifications and maximum loss cost.

• Journal of Integrative Natural Science
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• v.9 no.3
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• pp.206-214
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• 2016

The basic goal of software development is to produce high quality software at low cost. Therefore, when to stop software testing and release the software product is a significant point in the software development. The software cost model is an effective tool used to help software developers control costs and determine the release time. In this paper, we discuss the cost model to apply all 6 models with consideration of time to remove errors, cost of removing each error and risk cost due to software failure. We show the impact of cost coefficients and parameter values on the expected total cost by changing the values and comparing the optimal release times.

• Journal of the Korea Society of Computer and Information
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• v.11 no.1 s.39
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• pp.203-222
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• 2006

This study proposes the methodology of analyzing the expected performance of the Integrated Computing Environment(ICE) on the assumption that the ICE of the agencies and the affiliated organizations under Ministry of Culture and Tourism is constructed and operated. The main objectives of the study are as follows : 1) deriving the performance criteria divided into the operational expected value and the strategic expected value of constructing and operating the ICE for Culture and Tourism informatization, 2) proposing the ways of extracting the detailed criteria for four performance areas(the degree of jnformatization efficiency, the degree of public service, the innovation and development of IS organization, the culture and tourism value creation) to evaluate the operational expected value based on the BSC(Balanced Scorecard) Perspective by using the CSF(Critical Success Factors) methodology, and 3) developing the detailed performance criteria and measures for the economic evaluation of the informatization efficiency from the TCO(Total Cost of Ownership) perspective and suggesting the practical evaluation method by applying them to the case of Culture and Tourism informatization.

• Journal of Korean Society of Industrial and Systems Engineering
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• v.6 no.8
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• pp.93-103
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• 1983

In this paper, we study an optimal service policy of the M/M/2/K queueing system with two types of customers. The incurred costs consist of waiting cost, service cost and incurred costs consist of waiting cost, service cut and changeover cost. The changeover cost occurs when a server who assigned to serve a particular type of customers reassigned to the other types of customers. Two servers serve two types of customers who arrive to the two separate queues. The two types of customers differ in respect of their arrival rate, service rate, waiting cost, and service cost. The servers require a policy, for determining when they should change their service type, which minimizes the long run expected total cost. The policy is obtained by a Markov decision process model that consists of a finite number of states and actions. In order to find the optimal service policy, we define states and actions of the system, compute onestep transition probabilities, and apply to the successive approximations algorithm.

• Proceedings of the Korean Reliability Society Conference
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• 2005.06a
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• pp.251-257
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• 2005

Software system which is essential in operating the computer has gradually become an indispensable element in many aspects of our daily lives and an important factor in numerous systems. In recent years, software cost sometimes exceeds the cost of maintaining the hardware system. In addition to the cost necessary to develop the new software system and to maintain the system, the penalty costs incurred due to software failures are even more significant. In this paper, a cost model incorporating the warranty cost, debugging costto remove each fault detected in the software system, and delivery delay cost is developed. A software reliability model based on non-homogeneous Poisson process(NHPP) is established and the optimal software release policies to minimize the expected total software cost are discussed. Numerical examples are provided to illustrate the results.

• Journal of Korean Society of Steel Construction
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• v.13 no.4
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• pp.337-349
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• 2001

This study present an optimum deck and girder system design for minimizing the life-cycle cost (LCC) of orthotropic steel deck bridges. The problem of optimum LCC design of orthotropic steel deck bridges is formulated as that of minimization of the expected total LCC that consists of initial cost, maintenance cost, expected retrofit costs for strength, deflection, and fatigue. To demonstrate the effect of LCC optimum design of orthotropic steel deck bridges, the proposed optimum LCC design is compared with the conventional method for orthotropic steel deck bridges design. From the numerical investigations, it may be positively stated that the proposed optimum design procedure for orthotropic steel deck bridges based on the LCC will lead to more rational, economical and safer design.

• Bulletin of the Society of Naval Architects of Korea
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• v.27 no.4
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• pp.67-71
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• 1990

This study is concerned with the optimum design of stiffened cylindrical members frequently found in floating offshore platforms with constraints on reliability. Minimised is the expected total cost which is composed of the structural cost and the expected failure cost. Some design requirements drawn from variotus design codes are also considered as constraints. Reliability of critical component in a structure only is considered in this paper and the system failure is discarded since the probability of system failure is in general much smaller than the probability of component failure and it is very difficult to evaluate the cost due to system failure. Ultimate strength only is considered and not the fatigue strength. Several parametric studies are illustrated and the optimum solutions for different strength models which are now in use for the design of stiffened cylinders are derived to show the optimum designs against different strength models for the same type of structural component. The present results lead to the important conclusions relating to the posibility of more cost saving in the design of such structure through the reliability-based optimisation process.