Performance assessment of K-out-of-N safety instrumented systems subject to cascading failures

Abstract

Safety instrumented systems often employ redundancy to enhance the ability to detect and respond to hazardous events. The use of redundancy increases the fault tolerance to single failure but remains vulnerable in case of dependent failures, including common cause failures and cascading failures. Reliability analysis of safety instrumented systems therefore involves the impact of dependent failures. The used approaches have primarily focused on common cause failures. In this paper, it is argued the need to consider the efforts of cascading failures that are caused by functional dependencies, hazardous events, and shared resources. A recursive aggregation-based approach is proposed for performance analyzing of K-out-of-N safety instrumented systems with consideration of cascading failures. General approximation formulas are developed for estimating the average probability of failures on demand of different configurations of safety instrumented systems. These formulas are compared with those for common cause failures. Then a case of fire water pump is studied to illustrate the effects of cascading failures on safety instrumented systems.