Dependability & safety modeling and calculation: Petri nets

Since its birth, about fifty years ago, the reliability field develops methods and tools to satisfy its different needs and this is more and more difficult as industrial systems becomes more and more complex and required parameters more and more sophisticated. Then the simplistic reliability and availability calculations of the origins are no longer sufficient to fulfill the present time needs encountered when dealing with RAM (Reliability, Availability, and Maintainability) or SIL (Safety Integrity Level) analysis. The dynamical behavioral aspects and the combinatory explosion of the number of states of industrial size system imply to proceed to a qualitative jump to be able to manage them properly. This may be done simply by moving from the traditional analytical approach to the Monte Carlo simulation. Nevertheless to be implemented efficiently this approach needs a powerful model behaving a close as possible as the physical system which is modeled. Finite states automata are generally chosen for this purpose and, among them the stochastic Petri nets (SPN) have proven to be very effective. This is the aim of this paper to show how SPN may be used for RAM (i.e. dependability) and SIL (i.e. functional safety) objectives.