Performance Assessment of Safety-instrumented Systems Subject to Cascading Failures in High-demand Mode

Abstract

Safety-instrumented systems are designed to act upon hazardous events and reinforce safety. IEC 61508 specifies two possible reliability measures of safety-instrumented systems: the average probability of failure on demand for low-demand mode systems, and average frequency of dangerous failures for high/continuous-demand mode systems. Redundancy is applied to ensure the reliability of safety-instrumented systems so that they are commonly constructed as K-out-of-N systems. The potential effects of dependency must therefore be included in the reliability analysis. So far, both standards and literature focus primarily on common cause failures as the source of dependencies. With the technology trends (e.g. cyber-physical and programmable electronic technologies), cascading failures caused by functional dependencies and shared resources may be issues in the implementation of safety-instrumented systems. Few attempts have been made to investigate the effects of cascading failures in the reliability of safety-instrumented systems. This paper aims to propose approximation formulas for average frequency of dangerous failures for high/continuous-demand mode systems that are subject to cascading failures. This research is an extension of previous research where the focus was directed to low-demand mode systems