Table of Contents
International Journal of Quality, Statistics, and Reliability
Volume 2008 (2008), Article ID 263895, 12 pages
Research Article

Fuzzy Risk Graph Model for Determining Safety Integrity Level

1LARPI Laboratory, Safety Department, Institute of Health and Occupational Safety, University of Batna, Road Med El-Hadi Boukhlouf, Batna 05000, Algeria
2LSPIE Laboratory, Electrical Engineering Department, Faculty of Engineering, University of Batna, Road Med El-Hadi Boukhlouf, Batna 05000, Algeria

Received 15 August 2007; Revised 15 November 2007; Accepted 14 January 2008

Academic Editor: Nagi Gebraeel

Copyright © 2008 R. Nait-Said et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


The risk graph is one of the most popular methods used to determine the safety integrity level for safety instrumented functions. However, conventional risk graph as described in the IEC 61508 standard is subjective and suffers from an interpretation problem of risk parameters. Thus, it can lead to inconsistent outcomes that may result in conservative SIL's. To overcome this difficulty, a modified risk graph using fuzzy rule-based system is proposed. This novel version of risk graph uses fuzzy scales to assess risk parameters, and calibration may be made by varying risk parameter values. Furthermore, the outcomes which are numerical values of risk reduction factor (the inverse of the probability of failure on demand) can be compared directly with those given by quantitative and semiquantitative methods such as fault tree analysis (FTA), quantitative risk assessment (QRA), and layers of protection analysis (LOPA).