Fault Tree Approach Research Articles

Fault-Tree Model of Bridge Element Deterioration Due to Interaction

Bridge engineers and bridge inspectors recognize that deterioration of one element influences the deterioration of another. The effect of this phenomenon may be crititcal if it increases the deterioration rate of the affected elements. The phenomenon is known as element interaction. The consequences of element interaction can help to explain why elements of the same type have varying deterioration rates. No practical model exists for modeling element interaction. This paper presents the use of a fault tree for qualitative and quantitative evaluation of element interaction phenomena. The fault-tree results can be used to better assess the deterioration rate and improve deterioration prediction of an element. An example of the fault-tree model for accelerated deterioration of concrete bridge decks is presented. The fault-tree model is not a deterioration model, but can be used to focus on mitigating accelerated element deterioration by certain maintenance or repair actions. Systematic implementation of the fault-tree approach presented herein must be preceded by field verification.

Comparison of Markov model and fault tree approach in determining initiating event frequency for systems with two train configurations

This study explores the use of Markov models in some areas of systems analysis in which time evolution of the system may be a significant factor in influencing the system reliability or availability. Comparisons are made between the Markov models and the time-averaged fault tree models for determining support system failure initiating event frequency in a nuclear power plant, for both power and shutdown conditions. Factors affecting consistency between the fault tree approach and the Markov model approach are studied for systems with common two train configurations. A correlation is developed to estimate the ratio between initiator frequencies through both approaches for a two parallel component system. Insights are developed as to when time averaged and simplified fault tree models support a good approximation to the more rigorous time-dependent Markov models.

The DYLAM approach for the dynamic reliability analysis of systems

In many real systems, failures occurring to the components, control failures and human interventions often interact with the physical system evolution in such a way that a simple reliability analysis, de-coupled from process dynamics, is very difficult or even impossible. In the last ten years many dynamic reliability approaches have been proposed to properly assess the reliability of these systems characterized by dynamic interactions. The DYLAM methodology, now implemented in its latest version, DYLAM-3, offers a powerful tool for integrating deterministic and failure events. This paper describes the main features of the DYLAM-3 code with reference to the classic fault-tree and event-tree techniques. Some aspects connected to the practical problems underlying dynamic event-trees are also discussed. A simple system, already analyzed with other dynamic methods is used as a reference for the numerical applications. The same system is also studied with a time-dependent fault-tree approach in order to show some features of dynamic methods vs classical techniques. Examples including stochastic failures, without and with repair, failures on demand and time dependent failure rates give an extensive overview of DYLAM-3 capabilities.

A data base oriented dynamic methodology for the failure analysis of closed loop control systems in process plant

Abstract Dynamic methodologies are defined as those which explicitly account for the time element in system operation for failure modeling. A dynamic methodology is presented for the accurate failure analysis of closed loop control systems (CLCS) in process plants, using: (a) a data base to describe the CLCS process physics, and (b) a Markov model to describe the probabilistic evolution of the controlled variables in discrete time and discretized controlled variable state space. Both the preparation of the data base and construction of the Markov model are mechanized. The methodology is advantageous if: (a) there is an existing computer code for the fast simulation of CLCS behavior under both normal and failed component operation, (b) the failure characteristics of the CLCS for given initial conditions and over a specified time interval are of interest, and (c) several analyses need to be performed in view of uncertainties in the component failure data. The methodology is illustrated on a CLCS for maintaining the reactor vessel pressure and water levels in Boiling Water Reactors within allowed ranges following a small-break loss of coolant accident (LOCA). The results for the probability of occurrence of the Top Events (e.g. high pressure, low level) during a 60 min interval following the LOCA are compared to those predicted by the fault-tree approach using digraphs.

Impact of PRA workstations on PRA applications

Abstract This paper briefly discusses the history of software tools for PRA and their development from the first commercial PRAs to the present. The characteristics of a usable (‘living’) PRA are then described (i.e., accessibility, maintainability and quality). A description of the small event tree/large fault tree approach to PRA is discussed, with special attention paid to the interaction of the various PRA codes. Finally, four different applications of PRA are listed, showing the functions and interactions of SAIC's PRA workstation codes.

Fault Tree Approach for the Reliability Analysis of Gear Trains

The application of fault tree techniques to the analysis of gear trains is considered. The techniques involve the decomposition of the system into a logic diagram or fault tree in which certain basic or primary events lead to a specified top event which signifies the total failure of the system. The fault trees are used to obtain minimal cut sets from which the modes of system failure and, hence, the reliability measure for the top event can be calculated. The method of constructing fault trees and the subsequent estimation of reliability of the system is illustrated through a nine speed compound gear train and a five speed epicyclic gear train.

Reliability Study On The Lawrence Livermore National Laboratory Water-supply System

We conducted a reliability analysis of the Mocho water-supply system for the Lawrence Livermore National Laboratory (LLNL) to determine if an adequate supply of water would be available in the event of a major fire. We used the digraph fault-tree approach for logic model generation. The initiating-enabling event interval reliability approach was used for a probabilistic evaluation of the Mocho system fault tree. In the event of a major fire, the Mocho system demand unavailability was calculated to be 3.6 x .0001. We identified 16 single-component failures that would cause failure of the control loop which monitors storage-tank level. These failures would go undetected by monitoring personnel at LLNL. Our recommended changes would provide a redundant measurement of the tank level, resulting in a decrease of the predicted system unavailability by about a factor of 50.

Limitations of Minimal Cut-Set Approach in Evaluating Reliability of Systems with Repairable Components

This work shows that, when system components are repairable, system reliability cannot be evaluated exactly by means of the fault tree approach; only a system reliability lower bound can be obtained.

Reliability considerations in the design of solar photovoltaic power systems

This paper presents a systems level approach to reliability analyses for solar photovoltaic systems and illustrates how these analyses can be useful in photovoltaic systems design even though detailed reliability data are not yet available. Traditional reliability techniques and trade-offs are discussed and relationships between reliability, maintainability and availability are reviewed using minimum life cycle cost as the parameter to be optimized. A list of available techniques which are useful today is presented, as well as a list which provides a series of techniques that will be useful when more field and test reliability data on photovoltaic systems are available. Three typical solar photovoltaic systems are illustrated in block diagram form and discussed. These are (1) a stand-alone photovoltaic system, (2) a stand-alone system with battery energy storage and (3) a system with a utility back-up. Each system is analyzed to the subsystem level using two reliability analysis techniques: the failure modes and effects analyses (FMEA) and the fault tree approach. The FMEA is an approach which starts at the lowest level of the system being analyzed and considers each subsystem failure one at a time and two at a time, and traces upward through the block diagram to identify the effect these failures have on the total system performance. A summary of these results gives the designer an indication of the critical subsystems. The fault tree analysis is a logic diagram approach which begins with a failure of the system at its highest level to be the primary output of the diagram. Inputs to the analysis are made from those levels which can cause failure at the highest level. Thus, only subsystems which can cause system failure are included in the diagram. If probabilities of failure are available for the various subsystems, fault trees may be used to construct a probability analysis of the system to predict the probability of success or reliability of the photovoltaic system involved. In this paper, this analysis is performed using assumed values of probability of success for the subsystems considered. This discussion presents the view that taking reliability and availability into consideration early in the design at the system level will prevent some major field problems. It will also favorably influence the subsystem design goals, especially the control subsystems. In addition, this approach should encourage the early development of complete photovoltaic systems that satisfy the demanding availability precedent set by the electric utilities.

A fault tree approach to quality control by variables: L. F. Paul and G. G. Weber. IEEE Trans. Reliab., R-28, (5) 344 (December 1979)

The stochastic behaviour of a single-server n-unit system has been investigated subject to different service strategies. The analysis is carried out under the supposition that the failure-time of a unit is exponentially distributed while all the other time distributions involved are arbitrary. At any instant t, the system is characterized by the probabilities of its being in the “up” or “down” state. Integral equations for these probabilities have been obtained by identifying the system at suitable regeneration epochs. Various system parameters of significance, namely, 1.(1) point-wise availability of the system at instant t,2.(2) steady-state availability of the system,3.(3) s-expected up-time of the system in [0, t],4.(4) s-expected inspection time of the server in [0, t] and5.(5) s-expected repair time of different types in [0, t], have been obtained. A cost-benefit analysis of the system has been developed by using the above obtained system characteristics.

A Fault Tree Approach to Quality Control by Variables

Based on the analogy between the events ``component failed'' (in reliability) and ``measurement out of tolerances'' (in quality control) acceptance sampling by variables is investigated when using the measurements made at several check-points for each device to be tested. The joint use of the fault-tree approach (especially in terms of min-cuts and min-paths) and of finite-sample hypothesis testing, leads to estimates of consumer and producer risks of a quality control procedure taking into account the internal design (functional diagram) of each device. As an example, a tuneable frequency multiplier is fully treated.

A laboratory model for system reliability analyzer

A laboratory model of a reliability analyzer is described. The idea behind the development of such an analyzer is to provide an analogue model for determining all pathsets or cutsets of a system and thereby determining the system reliability when the system configuration is known along with the reliability of its constituent elements. Any system with unreliable branches and nodes can be simulated on the reliability analyzer using the logic gates. The analyzer provides tremendous flexibility in changing of the system configuration for reliability studies. The users require manually plugging in or out of a particular link to simulate addition or removal of a branch. The reliability parameters such as terminal reliability and global reliability in the case of a communication network can also be studied. Block diagram or fault tree approaches can be used to analyze a system on the analyzer described in this paper.

Common mode failure analysis

Standard Reliability analysis techniques have demonstrated the capability to evaluate system vulnerability to random component failures. Recently, in the nuclear power industry, interest has been developing in methods to consider systematic, nonrandom, so-called Common-mode failures which are not tractable in quantitative terms using the usual fault tree or reliability block diagram approaches. This paper summarizes the state of the art as reliability engineers seek to extend the techniques to include multiple failures from a common cause (or in a common mode). Qualitative extensions of both the fault tree and the reliability block diagrams have been used to evaluate the adequacy of reactor protection circuitry for commercial power reactors.