Reliability assessment of safety instrumented systems operating in high demand mode
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Safety instrumented systems (SISs) play an important role in protecting human life, the environment, and material and financial assets. In order to achieve the expected protection function, high reliability of such systems is desired. The SIS reliability quantication is therefore an important topic. IEC 61508 is the most important standard for SIS. In IEC 61508, two SIS operation modes are differentiated. When the frequency of demands for SIS activation is greater than once per year or greater than twice of the functional test interval, the SIS is said to be operating in the high-demand mode, otherwise it is operating in the low-demand mode. In the same standard, different reliability measures and quantification methods are used to assess SIS operating in high-demand mode and low-demand mode. Extensive study has be done on SIS operating in the low-demand mode,however much less effort has been directed to SIS operating in the high-demand mode. One issue is the criterion for classication of low-demand mode and high-demand mode. Another issue is the reliability measure and the quantication method for high-demand SIS. This master thesis focuses on SIS operating in high-demand mode. The objectives are to provide a better understanding of high-demand SIS, propose a new reliability measure and quantication method, and identify and assess some key factors that influence high-demand SIS reliability performance. The thesis is written in the form of two scientific papers. The first paper focuses on the presentation of a new Markov model and its applicability, while the other paper focuses on the new reliability measure and the high-demand SIS reliability influencing factors. In addition to the demand rate, there are other factors which are important to the reliability of SIS. In the first paper, a discussion of these factors is given. A general Markov model, that incorporates these factors (including demand rate), is presented. The model is applied to a single componentsystem. The result from the proposed model is compared with results from other methods. It concludes that the proposed model is applicable to SIS operating in both low- and high-demand modes. In the second paper, a new reliability measure is proposed to quantify the risk reducing value of a SIS. Based on the new measure and the Markov model, an approach for determination of functional test interval for SIS operating in high-demand mode is presented. The effect of the demand rate and the functional test interval is assessed. The result shows risk reducing value of a SIS is dependent on the demand rate. Due to the time limit, many other important issues with high-demand SIS have not been investigated. Further research on this topic is needed.