Performance analysis of redundant safety-instrumented systems subject to degradation and external demands

• Published in: Journal of loss prevention in the process industries Vol. 62; p. 103946
• Main Authors: Zhang, Aibo, Barros, Anne, Liu, Yiliu
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.11.2019
• Subjects: Degradation; Probability failure on demand; Redundant structure; Safety-instrumented system; Degradation; Probability failure on demand; Safety-instrumented system; Redundant structure
• ISSN: 0950-4230
• DOI: 10.1016/j.jlp.2019.103946

Safety-instrumented systems (SISs) play a vital role in preventing hazardous events in the offshore facilities. Many of existing performance analysis of SISs are based on the constant failure rate assumption, which is however doubtful when it is applied to actuator sub-systems or mechanical final elements of a SIS. These mechanical SIS components can become vulnerable with time and with upcoming demands given the past exposures to shocks/demands. In this paper, we analyze SIS reliability and unavailability by considering that a failure occurs when total degradation of a SIS component, including continuous degradation and increments caused by random demands, exceeds to a predefined critical threshold. The dependency of two components in a redundant structure of mechanical actuators caused by random demands is also taken into account in the analysis. Approximation formulas for reliability and unavailability of the redundant SIS sub-system under a degradation process are developed. Finally, a numerical example is conducted to illustrate effects of degradation parameters on SIS performance. •Identification of combined effects of continuous degradation and random demands on a SIS with hidden failures.•New algorithms for calculating time-dependent PFDavg in different test intervals.•Guidance on updating proof tests of SISs based on the degradation situation to ensure compliance and cost-effectiveness.