Abstract
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.
Highlights
Safety instrumented systems (SISs), which generally consist of sensor, logic solver- and actuator-subsystems, are widely used to prevent the occurrences of hazardous events or mitigate their consequences (Rausand, 2014)
Common cause failures (CCFs) in such a 1oo2 configuration are excluded, with the purpose to illustrate the effects of degradation on a redundant architecture apparently
If after testing we discovered an anomaly, we can schedule intervention, such as lubrication; On the contrary, if the valves is functional after each proof test, theoretically, we will not act on the valves
Summary
Safety instrumented systems (SISs), which generally consist of sensor-, logic solver- and actuator-subsystems, are widely used to prevent the occurrences of hazardous events or mitigate their consequences (Rausand, 2014). Almost all reliability assessments of SISs are based on an assumption that the failure rates of the components within the systems are constant, such as (Guo and Yang, 2008; Liu and Rausand, 2011; Catelani et al, 2011; Jin and Rausand, 2014), even in (IEC 61511, 2010) and (IEC 61511, 2003). It means that all components or SIS channels are as-good-as-new when they are functioning, and their failures follow the exponential distribution.
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