Improving the Reliability of Safety Instrumented Systems Under Degradation with an Alternating Testing Strategy

This paper presents an alternating testing strategy to improve the reliability of multi-state Safety Instrumented Systems (SIS) under degradation conditions. A Dynamic Bayesian Network (DBN) model is developed to assess SIS unavailability, integrating proof testing parameters and capturing multi-state component behavior. Applied initially to the actuator layer of a SIS with a 1oo3 (one-out-of-three), redundancy structure, the study examines the impact of extended test durations, showing that the alternating strategy reduces non-zero test durations compared to the simultaneous test strategy. The approach is then extended to a complete SIS, with a case study demonstrating its potential to enhance system reliability and optimize maintenance management by considering degradation and redundancy factors.