• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2021년도 학술발표회
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• pp.145-145
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• 2021

Water distribution networks (WDNs) are designed to satisfy water requirement of an urban community. One of the central issues in human history is providing sufficient quality and quantity of water through WDNs. A WDN consists of a great number of pipelines with different ages, lengths, materials, and sizes in varying degrees of deterioration. The available annual budget for rehabilitation of these infrastructures only covers part of the network; thus it is important to manage the limited budget in the most cost-effective manner. In this study, a novel pipe replacement scheduling approach is proposed in order to smooth the annual investment time series based on a life cycle cost assessment. The proposed approach is applied to a real WDN currently operating in South Korea. The proposed scheduling plan considers both the annual budget limitation and the optimum investment on pipes' useful life. A non-dominated sorting genetic algorithm is used to solve a multi-objective optimization problem. Three decision-making objectives, including the minimum imposed LCC of the network, the minimum standard deviation of annual cost, and the minimum average age of the network, are considered to find optimal pipe replacement planning over long-term time period. The results indicate that the proposed scheduling structure provides efficient and cost-effective rehabilitation management of water network with consistent annual budget.

• 물과 미래
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• 제29권3호
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• pp.197-205
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• 1996

상수도 관망시설에서 각 관로는 시간이 경과함에 따라 각종 부식이 일어나 통수능이 저하된다. 그러므로 적절한 시기에 갱생이나 교체 등의 처리를 해주어야 한다. 본는 연구에서는 각 수요절점에서 요구수량, 요구압력 등 수리학적 조건을 만족시켜주며 동시에 최소의 비용으로 노구된 관로를 처리해주는 의사결정모형을 개발하였다. 교체비용, 관파열보수비용, 양수비용, 관 파열율, 연이율 등은 각 관로의 교체시기를 경제적으로 결정하는데 이용된다. 그리고 배수시설의 수리학적 만족여부는 관망계산 프로그램 KYPIPE에 의해 점검된다. 시스템이 수리학적 조건을 만족지 못할 경우 교체시기는 조건이 만족될 때까지 앞당겨진다. 본 모형은 기존 송배수시설인 수도권 광역상수도 제1단계에 적용되었다. 모형을 이용해 산출한 최적교체시키는 수리학적으로 모의해본 결과 문제가 없으며를 알 수 있었다.

• 한국국방경영분석학회지
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• 제4권1호
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• pp.113-123
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• 1978

Consider a series system of two units, named 1 and 2, respectively. Two units are observed at the beginning of discrete time periods t=0,1,2, $cdots$ and classified as being in one of a countable number of states. Let (i, r) be a state of the system at time t, when the state of unit 1 is i and state of unit 2 is r at time t, Under some conditions, the opportunistic replacement policy that minimizes the expected total discounted cost or the average cost of maintenance is shown to be characterized by the control limits $i^{*}(r)$ (a function of r) and $r^{*}(i)$ (a function of i) : (a) in observed state (i, r), the optimal policy for unit 1 is to replace if $i{\ge}i^{*}(r)$ and no action otherwise; (b) in observed state (i, r), the optimal policy for unit 2 is to replace if $r{\ge}r^{*}(i)$ and no action otherwise. In addition, this paper also develops optimal policy in the finite time horizon case, where time horizon is fixed or a finite integer valued r.v. with known pmf.

• 대한산업공학회지
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• 제21권4호
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• pp.555-570
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• 1995

We consider a preventive replacement policy for a cold-standby system with N components, in which only one component is in operation at a time. If the component in operation fails, a standby component is immediately switched into operation. If all components fail, the system fails. The system is inspected at random poins in time to determine whether it is to be replaced or not. If the number of failed components at the time of inspection exceeds a threshold value r, the system is replaced. Otherwise the decision is put off until the next inspection point arrives. Under the cost structure which includes a replacement cost, a system down-time cost and a holding cost of the components, we develop an efficient procedure to find the optimal control values N and r, which minimize the expected cost per unit time.

• 산업경영시스템학회지
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• 제17권32호
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• pp.209-220
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• 1994

In this paper, we study the timing and equipment of replacement. Those affect both available possibility and technological levels of new equipments in the future. These problems are classified according to technological levels and in consequence generated four circumstance. Models are formulated in this research. These are able to make us decision which we replace equipment at the first time. A method is suggested to find the optimal age for replacement at given each circumstance. A solution procedure and numerical example are given.

• Communications for Statistical Applications and Methods
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• 제15권6호
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• pp.909-923
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• 2008

본 논문에서는 수리가 가능한 시스템에 대하여 혼합보증이 종료된 이후의 교체모형과 예방보전모형을 고려하는데, 만약 보증기간이 종료된 이후에 시스템에 고장이 발생하면 최소수리를 수행한다. 최적의 교체정책과 최적의 예방보전정책을 결정하기 위한 기준으로는 기대비용과 기대비가동시간에 근거한 총밸류함수를 사용한다. 그리고 시스템의 고장시간이 와이블분포를 할 때 수치적 예를 통해서 제안된 최적의 교체정책 및 예방보전정책을 자세히 설명하고자 한다.

• 한국국방경영분석학회지
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• 제7권2호
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• pp.100-113
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• 1981

A replacement policy for a finite time span is proposed for the cumulative process where an item can fail only when the total amount of deterioration exceeds a prespecified failure level. The optimal deterioration limit level is determined to minimize the total cost expected per unit for a given time span. An illustrative example in case of periodically inspected reformer tubes in ammonia plant is also presented.

• 한국국방경영분석학회지
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• 제18권2호
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• pp.114-125
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• 1992

A policy of periodic replacement with minimal repair at failure is considered for a complex system. Under such a policy the system is replaced at periodic times. iT(i=1,2, $\ldots$), while minimal repair is performed at any intervening system failures. The cost of the j-th minimal repair to the component which fails at age t is g(C(t). $c_j$ (t)), where C(t) is the age-dependent random part, $c_j$(t) is the deterministic part which depends on the age and the number of the minimal repair to the component, and g is a positive nondecreasing continuous function. The cost of replacement is expensive when the number of failures occurring in (0. T) is greater than a threshold level. The problem of determining the optimal replacement period, $T^{\ast}$, which minimizes the total expected cost per unit time over an infinite time horizon is considered. Various special cases are considered.

• 한국신뢰성학회지:신뢰성응용연구
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• 제5권3호
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• pp.373-379
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• 2005

In many cases, it is more practical and economical to repair a system than to replace the whole system or to perform a complete overhaul when the system fails. Two basic replacement policies were proposed by Barlow and Hunter(1960) and Morimura (1970), in which the minimal repair times are identically distributed. But, as Lam(1988) pointed out, in many cases of deteriorating system, in view of ageing and cumulative wear, the repair time will tend to be longer and longer. In this note, the two basic replacement policies are considered for a repairable system with linearly increasing repair times. Optimal policies, which maximize the steady state availability of the system, are obtained for the Weibull failure rate case.

• Journal of Chest Surgery
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• 제24권10호
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• pp.967-972
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• 1991

Between Feb. 1982 and July 1990, 173 patients [male: 89, female: 84] Who underwent heart valve replacement for acquired valvular heart disease on the Department of Thoracic and Cardiovascular Surgery, School of Medicine, Pusan National University, were reviewed for return to work after heart valve replacement. The replaced valve were mitral [128, 74.0%], aortic[10, 5.8%], mitral & aortic[35, 20.2%]. Two tricuspid valve replacement were excluded. Several important factors influencing the return to work were age, the employment status before surgery, the number of replaced valve, the pre - op NYHA functional class and cardiac function [ejection fraction]. These factors were closely related to the optimal time of heart valve replacement. It can be concluded that the rate of return to work and the quality of life would be improved if valve replacement were performed at an earlier stage of valvular heart disease.