Verifying Diversity Defense Mechanisms Against Common Cause Failures for Safety Instrumented Systems Using a Stress-strength Model

Common cause failure (CCF) is a serious threat to Safety Instrumented Systems (SIS) reliability. It is therefore important to model and quantify CCF in reliability analysis. Diversity has long been used to protect redundant systems against CCF. Whilst there is clear evidence that diversity can bring benefits when compared with identical redundancy systems, these benefits can be difficult to quantify. Therefore, a novel CCF model which studies CCF from the viewpoint of Root Cause (RC) and Coupling Factor is proposed in this paper. The key parameters of the novel CCF model can be determined based on a stress-strength model by classifying variables of stressors and strengths according to RC categories. An application example is provided that illustrates how the proposed CCF model performs for a chemical reactor safety system (CRSS) for protection against high pressure and temperature. The results not only confirm that diversity techniques lower the CCF probability but also provide quantitative assessment of how large the improvements can be in different diversity techniques.