3 An Efficient Approach for the Reliability Analysis of Phased-Mission Systems with Dependent Failures (PSAM-0206)

We consider the reliability analysis of phased-mission systems with dependent common-cause failures in this paper. A phased-mission system (PMS) is a system supporting a mission characterized by multiple, consecutive, and non-overlapping phases of operation. System components may be subject to different stresses as well as different reliability requirements throughout the course of the mission. As a result, component behavior and relationships may need to be modeled differently from phase to phase when performing a system-level reliability analysis. This consideration poses unique challenges to existing analysis methods. The challenges increase when common-cause failures (CCF) are considered in the analysis. CCF are multiple dependent component failures within a system that are a direct result of a shared root cause, such as sabotage, flood, earthquake, power outage, or human errors. It has been shown by many reliability studies that CCF tend to increase a system's joint failure probabilities and thus contribute significantly to the overall unreliability of systems subject to CCF.

We propose a separable phase-modular approach for the solution of fault trees, our chosen formalism, as one way to meet the above challenges in an efficient and elegant manner. Our methodology is twofold: first, we separate the effects of CCF from the base PMS analysis using the Total Probability Theorem and the concept of a common-cause event space based on elementary common-causes; next, we apply an efficient phase-modular approach to determine the reliability of the PMS. The phase-modular approach employs both combinatorial and Markov-chain solution methods as appropriate, based on the characterization of a given phase module. We use the example of a hypothetical system subject to CCF to illustrate the basic steps of our proposed methodology.