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Innal F.,University of Batna | Cacheux P.-J.,Total S.A. | Collas S.,Total S.A. | Dutuit Y.,Total S.A. | And 3 more authors.
Journal of Loss Prevention in the Process Industries | Year: 2014

This article casts a new glance over some methods dedicated to the calculation of the likelihood (probability or frequency) of failure of systems and, in particular, safety-related systems working alone or in association with other protection layers. It consists first in examining with a critical eye the relevancy of the aforementioned methods, which are still often used in spite of their restrictive limitations, and second in proposing an alternative approach for each of them. The correctness of the examinated methods is tested by applying them to very simple systems modeled by fault tree models, with intent to show why these methods are debatable and how they can be replaced by other ones, more appropriate. The particular case of several protection layers having to react on the demand resulting from the global failure of their associated control system is considered. That case leads to revisit the common assumption of the independence between the above protection layers and control system, by taking into account the order of their respective failures from a qualitative and quantitative point of view. © 2014 Elsevier Ltd.


Signoret J.-P.,Total S.A. | Dutuit Y.,Total S.A. | Cacheux P.-J.,Total S.A. | Folleau C.,SATODEV | And 2 more authors.
Reliability Engineering and System Safety | Year: 2013

For more than 30 years, the Petri nets (PNs) have proven to be very powerful for safety/dependability modeling and calculations. The number of publications is increasing and a standard is even going to be issued soon. Nevertheless, the dissemination is slow and the PNs are not yet commonly used by reliability engineers. As a matter of fact, the current PNs are often intricate and difficult to understand even if, in reality, they model simple systems. This is discouraging both for the PN designers and the PN readers and this is certainly one of the causes impeding the dissemination of this approach. Hopefully, some simple additional graphical rules can be easily implemented to undertake the PN modeling of large industrial systems while keeping the readability and the understandability all along the building process. The aim of this publication is to deal with the graphical aspects of Petri nets and it proposes first some very simple tricks and guidelines to structure and improve the drawing of standard PNs. It explains how the introduction of predicates and assertions allows developing modules (i.e., generic sub-PNs) in order to build the PNs in a modular way. Then it shows how reliability block diagrams (respectively flow diagrams) may be used as guidelines to build large PNs by using the above modules. It describes the RBD driven PNs which are very effective to model safety systems (e.g. safety instrumented systems) and explains how to extend them to the flow diagram (FD) driven PNs which allow to undertake simplified production availability modeling and calculations. © 2012 Elsevier Ltd.


Cacheux P.-J.,Total S.A. | Collas S.,Total S.A. | Dutuit Y.,Total S.A. | Folleau C.,SATODEV | And 2 more authors.
Reliability Engineering and System Safety | Year: 2013

The safety and reliability analysis of any system necessarily requires first to model its behavior or its failure logic, and second to assess its performance by using the previously chosen model. Among these performance indicators there is the failure frequency. In spite of its interest, it is not very often used, except in the standards devoted to the so-called functional safety, in which its genuine nature is not always made explicit. A sound method dedicated to the modeling and computing of the failure frequency of periodically tested systems is proposed in this paper. This type of systems comprises components with an availability curve which is discontinuous at deterministic instants. These discontinuities induce, in turn and at the same instants, frequency discontinuities. The impact of the latter on the overall value of the failure frequency is important, but largely unknown and therefore not taken into account in the safety and reliability area, in spite of the fact that it generally induces optimistic estimation, if no compensatory action is planned. The presented method is based on the notion of "critical state" and the use of advanced fault tree models (FT). The results obtained from these models are checked by using Monte Carlo simulation technique supported by equivalent Petri nets models (PN). © 2013 Elsevier Ltd.

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