A snapshot of methods and tools to assess safety integrity levels of high-integrity protection systems

摘要

In the oil industry, high-integrity protection systems tend to replace traditional safety systems. In this paper, a snapshot is provided of the methods and tools designed by the authors to evaluate efficiently safety integrity levels of high-integrity protection systems, as required by IEC 61508 and 61511 standards. These standards provide rigorous processes to build the safety of safety-instrumented systems. They are very efficient from an organizational point of view. However, difficulties arise with definitions and probabilistic calculations. To overcome these difficulties, the existing methods and tools were extended. These extensions are presented by means of simple examples. The focus is on the treatment of high-integrity protection systems working in low-demand mode (that is, with less than one demand per year according to the standards), e.g. systems such as high-integrity pressure protection systems. Some indications for safety systems working in continuous mode are given. Some practical conclusions are drawn from the various experiments performed. First, fault trees, when properly used, are very efficient for low-demand safety systems. Second, multi-phase Markov processes provide accurate results, even if only very small systems are tractable with this approach. Finally, behavioural modelling coupled with Monte Carlo simulation on Petri nets is both efficient and accurate. From the authors’ point of view, these approaches are simpler to handle than the informative formulae proposed in the present issue of IEC 61508 and 61511 standards.