Porvoo, Finland
Porvoo, Finland

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The time-dependent unavailability and the failure and repair intensities of periodically tested aging standby system components are solved with recursive equations under three categories of testing and repair policies. In these policies, tests or repairs or both can be minimal or perfect renewals. Arbitrary distributions are allowed to times to failure as well as to repair and renewal durations. Major preventive maintenance is done periodically or at random times, e.g. when a true demand occurs. In the third option process renewal is done if a true demand occurs or when a certain mission time has expired since the previous maintenance, whichever occurs first. A practical feature is that even if a repair can renew the unit, it does not generally renew the alternating process. The formalism updates and extends earlier results by using a special backward-renewal equation method, by allowing scheduled tests not limited to equal intervals and accepting arbitrary distributions and multiple failure types and causes, including failures caused by tests, human errors and true demands. Explicit solutions are produced to integral equations associated with an age-renewal maintenance policy. © 2015 Elsevier Ltd. All rights reserved.


Vaurio J.K.,Prometh Solutions
Reliability Engineering and System Safety | Year: 2010

This paper shows an efficient order for calculating importance measures and develops several new measures related to fault diagnostics, system failure intensity, system failure count, and configuration control. Definitions and relationships are extended to certain non-coherent systems and to models with mutually explicit events. Useful interpretations and applications are pointed out, and many roles of the Birnbaum importance are highlighted. Another important topic is the accuracy of various alternative methods used for quantification of accident sequence probabilities when negations or success branches of event trees are involved. Finally, the role of truncation errors is described, and criteria are developed for selecting truncation limits and cut-off errors so that importance measures can be estimated reliably and risk-informed decision making is robust, without unreasonable conservatism and without unwarranted optimism. © 2009.


Vaurio J.K.,Prometh Solutions
Reliability Engineering and System Safety | Year: 2016

Component importance measures have been defined and applied so far mostly for coherent systems. This paper develops and compares possible extensions of the traditional measures to non-coherent systems. The focus is on Birnbaum- and Criticality-type importances, both with respect to system unavailability and system failure intensity. Several versions are suggested for both measure types, each with different interpretation and potential applications. The measures are presented in terms of Boolean system logic functions so that they can be quantified with usual fault tree techniques even for large systems without manually solving and derivation of lengthy analytical functions. Examples demonstrate the method and discover some potential problems in system design if a component can initiate an accident while it is also part of a safety function to prevent an accident. Results are compared to earlier published results obtained with different algorithms. © 2015 Elsevier Ltd. All rights reserved.


Vaurio J.K.,Prometh Solutions
Reliability Engineering and System Safety | Year: 2011

This paper describes roles, extensions and applications of importance measures of components and configurations for making risk-informed decisions relevant to system operations, maintenance and safety. Basic importance measures and their relationships are described for independent and mutually exclusive events and for groups of events associated with common cause failures. The roles of importances are described mainly in two groups of activities: (a) ranking safety significance of systems, structures, components and human actions for preventive safety assurance activities, and (b) making decisions about permissible permanent and temporary configurations and allowed configuration times for regulation, technical specifications and for on-line risk monitoring. Criticality importance and sums of criticalities turn out to be appropriate measures for ranking and optimization. Several advantages are pointed out and consistent ranking of pipe segments for in-service inspection is provided as an example. Risk increase factor and its generalization risk gain are most appropriately used to assess corrective priorities and acceptability of a situation when components are already failed or when planning to take one or more components out of service for maintenance. Precise definitions are introduced for multi-failure configurations and it is shown how they can be assessed under uncertainties, in particular when common cause failures or success states may be involved. A general weighted average method is compared to other candidate methods in benchmark cases. It is the preferable method for prediction when a momentary configuration is known or only partially known. Potential applications and optimization of allowed outage times are described. The results show how to generalize and apply various importance measures to ranking and optimization and how to manage configurations in uncertain multi-failure situations. © 2011 Elsevier Ltd. All rights reserved.


Vaurio J.K.,Prometh Solutions
Reliability Engineering and System Safety | Year: 2011

This paper updates and extends earlier published analytical results for the unavailabilities (probabilities of failure on random demand) of redundant standby systems with k-out-of-n logic. Such systems are mostly used in safety and protection systems and are subject to latent and detectable failures. Latent failures are detected by periodic tests and repaired immediately after discovery. Many potential failure and error modes are included in the formalism. Interesting relations between two approaches are pointed out. Both consecutive and staggered testing schemes are evaluated, and methods for including certain common cause failures in the analyses are pointed out. © 2010 Elsevier Ltd.


Vaurio J.K.,Prometh Solutions
Reliability Engineering and System Safety | Year: 2011

Phased missions consist of consecutive operational phases where the system logic and failure parameters can change between phases. A component can have different roles in different phases and the reliability function may have discontinuities at phase boundaries. An earlier method required NOT-gates and negations of events when calculating importance measures for such missions with non-repairable components. This paper suggests an exact method that uses standard fault tree techniques and Boolean algebra without any NOT-gates or negations. The criticalities and other importance measures can be obtained for events and components relevant to a single phase or to a transition between phases or over the whole mission. The method and importance measures are extended to phased missions with repairable components. Quantification of the reliability, the availability, the failure intensity and the total number of failures are described. New importance indicators defined for repairable systems measure component contributions to the total integrated unavailability, to the mission failure intensity and to the total number of mission failures. © 2010 Elsevier Ltd.

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