Reliability importance analysis is usually used to detect design weakness, to identify which components contribute the most to the failure of a system, and thus to help find the most cost-effective way for upgrading the system. In this paper, we consider the reliability importance analysis of components in a generalized phased-mission system using the Birnbaum's measure. Our method is based on multi-state binary decision diagrams. In addition, it accounts for two-level modular imperfect coverage that arises from the combination of traditional imperfect coverage and more general than OR-ed combinatorial phase requirements. An example three-phased mission system is analyzed to illustrate the applications and advantages of the proposed approach.
Received on June 8, 2006
References: 22

The cost of maintenance is too high in mining industry to ignore. This is mainly due to poor understanding of the maintenance process which is greatly influenced by the reliability characteristics of the operating system. The paper introduces an approach for maintenance scheduling of a mining system based on reliability analysis which is divided into two parts. The first part introduces a methodology for optimal maintenance scheduling based on analysis of maintenance data in the form of time-between-failure and time-to-repair distributions. In the second part, we present a case study from Jajarm Bauxite Mine in Iran to illustrate the applicability of the maintenance scheduling model.
Received on April 18, 2006
References: 15

A digital system is multi-functional and it may cause a risk concentration if it is installed without any redundancy. In order to reduce this risk of a concentration, multiple redundancies are applied for the safety-critical systems. Simplified common cause failure (CCF) modeling technique is commonly used for a practical reason when the target system consists of many redundant components. This technique enables us to avoid an impractically large number of CCF events for a system fault tree. One of its demerits is that it may hide important information regarding a system's status. Condition-based human reliability assessment (CBHRA) method which is a practical method to develop a more realistic fault-tree model with a consideration of the various conditions endured by a human operator uses the system status information for post-processing of minimum cut sets. Since the CBHRA post-processing should be performed based on the status information of the sensors and the signal-processing equipment and noting that the simplified CCF modeling technique may cause the loss of some information, in order to perform a more realistic analysis, we have to overcome this possible information loss. In this study, we also present a case study of a fault tree modeling which adopted both the CBHRA method and the simplified alpha factor method.
Received on July 30, 2006
References: 08

The results of the internal fire events analysis in a Level 1, power operation probabilistic safety assessment (PSA) for a pressurized water reactor (PWR) plant in Taiwan was updated in the late 1990's. This fire analysis adopted a scenario-based PSA approach to systematically evaluate the fire and smoke hazards and their associated risk impacts on the PWR plant. The analysis was divided into two phases: spatial interaction analysis phase (also known as the screening phase) and detailed analysis phase. In the screening phase, it was determined that the contributions to core damage frequency (CDF) from such fire risk-insignificant zones as administration building, radwaste building, etc. were relatively low. The remaining fire zones were further evaluated quantitatively to identify the risk-significant zones using a risk criterion of "greater than 0.1 percent of the internal events CDF". The main control room and the identified risk-significant zones were analyzed in detail in the second phase. The analysis of the main control room adopted a more refined methodology based on a panel-oriented scenario analysis. The extensive fire growth modelling was performed using the COMPBRN-IIIe code for the risk important fire zones. The plant model used in this analysis, which was developed and quantified using the WinNUPRA software to estimate the CCDP, was based on the living PSA models of the PWR plant completed in December 1995. The plant models were modified to reflect the impact of fire scenarios on human events and recovery actions. EXCEL? spreadsheets were used for the evaluation and integration of the fire risks. For the detailed analysis of fire sub-scenarios, the CDF of a fire sub-scenario was derived by multiplying the fire initiating event frequency, the conditional core damage probability, and the fire hazard factors associated with the sub-scenario. Five fire hazard factors were used to account for some of the effects of fire and smoke in the fire PSA model. This paper presents the results of the fire PSA updates and discusses the risk impacts associated with the fire hazard factors. These fire analysis results are the bases for the advanced risk-informed fire analyses performed for the cable tray wrapping at the PWR plant in Taiwan.
Received on October 10, 2006
References: 06

The ALARP principle expresses that risk should be reduced to a level that is low as reasonably practicable, and risk reducing measures should be implemented provided the costs are not grossly disproportionate to the benefits gained. To verify ALARP, different tools are being used, including cost-benefit analyses and cost-effectiveness analyses. In this paper we discuss the role of such analyses in the ALARP process; in particular we address the common procedures for increasing the value of a statistical life and adjusting the discount rate to check against the gross disproportion criterion. We conclude that standard cost-benefit analyses should be used with care as the expected net present value calculations to large extent ignore uncertainties. The adjustment procedures to account for the uncertainties are considered ad hoc with a strong element of arbitrariness.
Received on July 3, 2006
References: 26

The confined space inside a tunnel poses a peculiar set of risks to the users, especially in the event of a fire. There is limited escape facilities and restricted intervention access by emergency personnel in these structures. A major fire in a tunnel can result in costly damage to the infrastructure in terms of reparation and economic loss from downtime. This paper looks at the fire risks of metro tunnels, where a large number of users are potentially exposed to these risks. A brief literature review of tunnel incidents in the past is conducted to present the factors that have led to the occurrence of various disasters in these tunnels. A risk assessment technique for a limited analysis of fire risks in metro tunnels is presented. The method is based on a simplified model of estimating the consequences from these risks by calculating the effects of fires in a tunnel configuration and the impact on people that may be exposed to these effects. The factors contributing to the occurrence of fires in metro tunnels and their consequences are assessed and the effectiveness of various means of mitigating these risks is presented. The key factors that have a significant impact on the level of risks from both the contributing factors and the mitigation measures are identified. Means by which the contributing factors may reduce the level of risks in order to achieve a cost-effective design solution are discussed.
Received on October 10, 2006
References: 09

This paper describes a software risk assessment approach based on the software conditional risk concept recommended in the PRA Procedure Guide for NASA Managers and Practitioners. This concept is used with the Dynamic Flowgraph Methodology analytical tool and risk-based software test strategies. The input space of the software is subdivided into a set of "contexts" corresponding to operating environments and functional conditions for the space system controlled by the software. The formulation of a "conditional risk index" that expresses critical software risk in terms of "condition coverage" is then enabled with a combination of DFM modeling/analysis and software defect rate estimation techniques.
Received on October 10, 2006
References: 06

In this paper, we investigate an important and interesting problem in a dependable secure computing system. The problem is to determine an optimal time that the secret shares should be updated on a (k, n) threshold-based secret sharing system with the proactive secret sharing (PSS) capability. In an earlier survivability study for a reconfigurable system, we developed a new definition for the survivability assessment. We extend this new definition for the survivability of the dependable secure computing system. From the survivability assessment perspective, we can easily determine an appropriate updating time for safeguarding secret information on the dependable secure computing system.
Received on April 12, 2007
References: 04

Mean Time Between Failures (MTBF) is a commonly used metric to indicate the reliability of a reparable item. For items with an increasing failure rate (wear-out failure), periodic maintenance is often performed to improve their operational reliability or increase the operational MTBF. This paper develops a very simple but highly accurate approximation of MTBF for items subjected to periodic maintenance, upon which engineers can easily do a quick calculation and perform design-for-reliability analyses.
Received on April 17, 2007
References: 04

We propose a hierarchical availability analysis methodology for multi-tiered Web applications. The methodology partitions the analysis into three levels, namely, server, request and session, and considers only the relevant factors at each level. The levels are connected using a hierarchical approach; the results obtained from one level are propagated for use in the analysis at the next one. The methodology thus decouples the different factors that influence availability and yet provides an integrated framework to consider them simultaneously.
Received on May 8, 2007
References: 07