Repair Policy Research Articles

Model for Comparing Performance of Various Transit Maintenance Repair Policies

This paper presents a simulation model suitable for comparing a bus maintenance system's performance under various repair policies. The scope of the model is restricted to measuring only the performance of a central, heavy maintenance facility, typically found at transit systems with multiple garages. Different activities within a transit maintenance facility are simulated in this effort and a superior repair policy is determined. The numbers of active and spare buses, failure distributions of bus components, and available repair resources are included as parameters in this model because they are directly related to the repair process. The main model consists of three sub‐models: (1) Service sub‐model; (2) repair sub‐model; and (3) status sub‐model. This paper includes a detailed description of each model along with its inputs and outputs. The model is tested with real world data. First‐come‐first‐served repair rule bounded by a waiting time limit is found to be most effective in improving system performa...

Workstage repair policies for sequential manufacturing systems

Dynamic stage repair policies where the stage repair probabilities are determined by the number of items in the buffer are examined through a model of a manufacturing system with two unreliable workstages connected by a minimal intermediate storage buffer. Definitions for workstage availability and line balance are extended for sequential manufacturing systems with dynamic repair policies. A measure for stage repair inputs and a general form of dynamic stage repair policies are presented. The steady-state probabilities of the resultant state-inhomogeneous Markov chain are computed by an efficient solution procedure. Insights obtained from extensive numerical studies are illustrated and discussed.

Current Technique of the Arterial Switch Procedure for Transposition of the Great Arteries

The arterial switch operation has recently become the preferred method of repair for patients with transposition of the great arteries in the majority of congenital heart centers. Concern regarding the late results in those patients undergoing atrial repairs has stimulated this major change in operative treatment. Although initially high, the current operative mortality has been reduced substantially to equal that of atrial repair in experienced institutions. The precise methods of coronary artery transfer, as well as aortic and pulmonary artery reconstruction have been refined to allow the routine performance of direct arterial repair within the first few weeks of life with low risk and excellent late survival. At the University of Michigan, a policy of arterial repair for all suitable infants with transposition of the great arteries with or without ventricular septal defect was adopted in July 1985. Although modifications in surgical technique have been made, the basic procedure has remained constant. This article reviews the current operative approach used in a consecutive series of 81 patients undergoing the arterial switch procedure.

Optimisation of crew size and repair policy in flexible manufacturing systems

This paper presents a simulation-based scheduling algorithm for evaluating the performance of different repair policies and computating the optimum size of repair-crews in flexible manufacturing systems. The simulation experiments were conducted for a system with numbers of machines ranging from six to twelve, processing 15 different types of parts. The experiments were planned according to the rules of central composite rotatable experiment design, and the system performance was optimised on the basis of minimum unit production cost. It has been concluded that for the particular system configuration studied, a repair policy based on shortest repair time yields the best results. It has been shown further that the optimum size of repair crews is much more sensitive to labour costs than to other production-associated costs.

Calculation of the Availability of a Two-Unit Parallel System With Cold Standby

A two-unit parallel system with cold standby, Markovian degradation of the working unit and one repair facility is considered. The calculation of the availability of this system under a control-limit repair policy is discussed. Iterative schemes for computing the availability in case of Erlangian repair-time distributions are presented along with numerical results.

Optimal quadratic control of jump linear systems with separately controlled transition probabilities

Abstract The optimal control of discrete-time jump systems that would be linear but for randomly jumping parameters are considered. These parameter jumps, which are indexed by a finite state form process, can be used to model systems subject to component failures or structural changes. The control of such systems when the jump probabilities can be controlled by choice of a finite-valued input is considered. This input (form control) can be thought of as corresponding to a maintenance or repair policy. The optimal control problem with quadratic costs (whose weighting matrices depend on the jumping parameters and form control) is formulated for such systems. The optimal solution for finite and infinite time horizon problems is developed. In addition, sufficient conditions for the existence of a steady-state, stabilizing solution to the infinite time problem are given in terms of appropriately defined controllability and observability properties.

Optimal quadratic control of jump linear systems with separately controlled transition probabilities

Abstract The optimal control of discrete-time jump systems that would be linear but for randomly jumping parameters are considered. These parameter jumps, which are indexed by a finite state form process, can be used to model systems subject to component failures or structural changes. The control of such systems when the jump probabilities can be controlled by choice of a finite-valued input is considered. This input (form control) can be thought of as corresponding to a maintenance or repair policy. The optimal control problem with quadratic costs (whose weighting matrices depend on the jumping parameters and form control) is formulated for such systems. The optimal solution for finite and infinite time horizon problems is developed. In addition, sufficient conditions for the existence of a steady-state, stabilizing solution to the infinite time problem are given in terms of appropriately defined controllability and observability properties.

A method for transformation of the system fault tree and repair policies into equivalent state-space diagram

In this paper is presented the algorithm which automatically forms the state space diagram from the fault tree. Proposed algorithm is designed to minimize the number of states. This is achieved by grouping identical modules in one base event, by merging identical system states and without expanding those branches in the space diagram which can not eventually lead to the system failure. Second algorithm described adds transitions which represent repairs to the state-space diagram formed by the first algorithm. Diagram, which is generated by the above algorithms can be solved by any numerical method, giving reliability or availability of the system.

A clinical comparison of mitral valve repair versus valve replacement in ischemic mitral regurgitation

Severe mitral regurgitation caused by acute myocardial infarction has been a particularly difficult management problem with disappointing clinical results. Over a 75-month period, ending March 31, 1987, 611 patients underwent mitral valve operations at Duke University Medical Center. Within this group, 55 patients had clearly defined ischemic mitral regurgitation, and 37 of these required emergency operations. Thirty-one of the 55 patients had isolated posterior papillary muscle dysfunction, nine had papillary muscle rupture, and 15 had severe ventricular dysfunction and generalized annular dilatation. Thirty-two patients were treated with primary mitral valve replacement, and 23 had mitral valve repair. In 18, repair was accomplished by a transventricular approach, combining the techniques of commissural annuloplasty, papillary muscle shortening or reimplantation, and infarct exclusion. Transventricular mitral valve repair proved to be safe, expeditious, and effective in restoring valve competence. Although the repair and replacement groups were similar with respect to all relevant baseline characteristics, improved operative survival was observed after valve repair, as compared to replacement, both for the overall group (p = 0.03) and for acute papillary muscle dysfunction (p = 0.05). These data suggest that a policy of predominant mitral valve repair, when appropriately applied in patients with ischemic mitral regurgitation, offers the potential for improving therapeutic results.

Probabilistic analysis of K-out-of-N:F three state-unit redundant system with common-cause failure and replacements

In this paper we carry out the analysis of K-out-of-N: F three-state unit redundant system with common-cause failure and replacements. The assumptions introduced are those of Chung [2], but with a main difference in the repair policy. Laplace transforms of the state probabilities, availability, mean up and mean down times are derived. The steady-state probabilities and the mean time to system failure (MTSF) are also obtained. Some results in literature are obtained as special cases. A numerical example is introduced to illustrate our results.

Actuarial survival, freedom from reoperation, and other events after repair of atrioventricular septal defects

Repair of an atrioventricular septal defect was performed in 134 patients from 1973 to 1985. Interventricular communication was present in 54 (40%) and none was present in 80 (60%). There were 12 hospital deaths (9%; 70% confidence limits 6%-12%). Earlier date of operation (p = 0.03) and younger age at repair (p = 0.005) were incremental risk factors for hospital death after repair of partial atrioventricular septal defect. Smaller size (p = 0.003) and longer cross-clamp time (p = 0.002) were risk factors for hospital death in those patients with complete atrioventricular septal defect. Higher preoperative pulmonary vascular resistance was a risk factor for death in both groups (p = 0.007). The 10 year actuarial survival rate was 82% overall. Increasing preoperative severity of left atrioventricular valve incompetence was not a risk factor for early (p = 0.001) or late (p = 0.002) death. The 10 year actuarial freedom from reoperation after repair of partial atrioventricular defects was 85%, and it was 68% for complete atrioventricular defects (p = 0.06). We conclude that the intermediate term results after repair of atrioventricular septal defects are good. Incompetence of the left atrioventricular valve has apparently been neutralized as a risk factor for hospital death in this series. However, the durability of the repair has been disappointing. A policy of earlier repair has been adopted and should improve hospital mortality and permit better preservation of the atrioventricular valve structures with resultant improved late term functional status.

Optimal Maintenance Decisions for Pavement Management

The object of this investigation was to develop a procedure for making optimal maintenance decisions for a deteriorating system. The particular system chosen for study is a pavement, and a methodology is developed to ensure that pavements meet certain performance criteria while minimizing the expected maintenance cost. A cumulative damage model based upon a Markov process was developed to model pavement deterioration. The optimal repair action for each possible pavement state in the planning horizon was found by means of probabilistic dynamic programming. Sample sequences of repair actions were generated during a simulation in which the optimal repair policy was applied to sample pavement condition histories. Several sensitivity studies were performed to study the variation in expected cost, including the effect of delaying the optimal program.

A multistate system under an inspection and repair policy : Janusz Karpinski. IEEE Trans. Reliab.R-35(1), 76 (1986)

A multistate system under an inspection and repair policy : Janusz Karpinski. IEEE Trans. Reliab.R-35(1), 76 (1986)

Availability analysis of a repairable system with common-cause failure and one standby unit

This paper investigates the mathematical model of a system consisting of two non-identical parallel redundant active units, with common-cause failure, and a cold standby unit. The failed units are repaired one at a time or are repaired together, if they fail due to common cause failure. All repair time distributions are arbitrary and different. The analysis is carried out under the assumption of having a single service facility for repair and replacement. Applying the supplementary variable technique, Laplace transforms of the various state probabilities are developed. Explicit expressions for the steady state probabilities and the steady state availability are derived. Some well known results are obtained as special cases. A numerical example is given to illustrate the effect of the repair policy on the steady state probabilities and the availability of the system.

Factors affecting the cost of fissure sealants: a dental insurer's perspective.

The slow adoption of sealants into dental practice is attributed partly to the fact that few prepayment plans pay for sealants. An important concern of insurers is how a sealants benefit will affect premiums. In this analysis, caries rates for various surfaces and teeth, failure rates for both sealants and amalgam restorations, fees and copayments, interest rates, reimbursement policies for replacement and repair, and the passage of time were studied to assess the cost to an insurer of amalgam restorations versus sealants. The results indicate: Sealants as a covered benefit for molars may not cause premiums to increase if fees and copayment levels are properly balanced. If caries in children continues to decline, however, higher premiums may be required if a sealant benefit is provided. Sealants as a covered benefit for bicuspids are likely to require higher premiums. The level of dental caries in the population should be monitored closely, because a further substantial decline in caries prevalence could diminish the economic argument for sealants.

Social function of adults with isolated ventricular septal defects. Possible negative effects of surgical repair?

Possible long-term deterioration of social function has been studied in adults over 30 years of age with congenital, isolated ventricular septal defects (VSD). This deterioration may possibly have been caused by brain damage connected with open heart surgery performed after age 10 years. Thirty-five patients with a mean age of 39 (range 31-61) years have been followed up for an average of 15 (3-21) years when restudied after surgical repair of VSD performed at a mean age of 23 (10-51) years (Group 1). Their social status was compared with 61 non-operated subjects with basically smaller defects, mean age 43 (31-73) years, who had been followed up for an average of 14 (3-21) years. Group 1 had a higher educational level, were less stressed at work, had a higher gross income (NS) and were less physically disabled (p less than 0.01). Both groups had a higher educational level than normal 40-year-old Norwegians. The percentages receiving disablement pension were 12% in group 1 and 13% in group 2. Despite a higher abortion rate (33% vs. 18%), group 1 females had a higher number of liveborn babies than those in group 2 (NS). Regular medical check-ups were attended by 45% and 60% in groups 1 and 2 respectively, and about half took antibiotics prophylactically. Unlike when initially investigated, group 1 had a better cardiac condition than group 2. Compared with the non-operated group, the operated patients had suffered no detrimental effects on their social function. We therefore advocate a liberal policy for surgical repair of VSD after age 10 years.

A Multistate System under an Inspection and Repair Policy

This paper presents a multistate system under a certain inspection and repair policy over an infinite time span. A complete failure state of the system is detected at once, but other states can be identified only by inspection; so the system is periodically inspected. Two types of repairs are considered; the repair type depends upon the detected state of the system. These repairs are imperfect, ie, after any repair the system goes to the normal state with a certain probability. In order to analyse this system, state probabilities are determined, and then the inspection time interval is evaluated which maximizes average profit rate of the system. The problem is solved by using Laplace transforms.

N-unit parallel redundant system with multiple correlated failures

This paper deals with mathematical models of an N-unit parallel redundant system with one repair facility and multiple correlated failures. These models are generalized on the basis of the results obtained previously and related to a system subject to a process of single failures and double and N-unit correlated failures. The method of the supplementary variable in two models differing by the repair policy is used. We obtain the following results: for both models the stationary availability, and for the second model the Laplace transform (L.T.) of the system reliability, L.T. of the pointwise a vailability, and the mean time to system failure (MTSF).

RAM analysis of a navigational system by Markov technique: A case study

An analysis is made for a typical repairable Dual-VHF Omni Range (Dual-VOR), for prediction of its reliability, availability and maintainability by adopting Markov technique. Dual-VOR is a ground based azimuth navigational system for aircraft. The failure rates of various functional blocks for the Dual-VOR have been derived from the MIL-HDBK. The repair rates have been based on maintenance practices and repair policies adopted for the system. Markov modelling for reliability involves solving a set of differential equations. Markov modelling for availability involves solving a set of linear simultaneous equations. A simple technique which involves only the availability model has been adopted for determining the mean time to repair (MTTR). RAM analysis of “hot-standby” configuration of Dual-VOR has been discussed.

A Note On “Determination of Coordinated Maintenance Scheduling Frequencies For A Group Of Machines”

Abstract In this note we present a simpler total cost model when the repair work on a group of machines is coordinated. We demonstrate that coordination of repair work may not be economically beneficial under the generally accepted conditions. We suggest an improvement in the solution procedure in order to consider coordinated as well as uncoordinated repair policies for a group of machines.