An optimization approach for safety instrumented system design

An optimization approach for Safety Instrumented System (SI S) design minimizing structure-dependant Life-Cycle Costs (LCC) is presented in this paper. It provides a systematic procedure to determine the most cost efficient SIS configuration for an equal degree of functional safety. The approach determines the optimal SIS architectures for sensor and final element subsystems, the optimal component sets (including heterogeneous redundancies), and the optimal distribution of Probability of Failure on Demand (PFD) among the subsystems. Downtime costs caused by proof tests and spurious trips of SIS are considered. The introduced optimization approach is applied to a practical case study. SIS design optimization is performed for a given PFD to a number of SIS components with different parameters. With regard to incurred LCC a large difference between analyzed SIS configurations is observed. Additionally, it is demonstrated that low total purchase costs of SIS components can lead to high LCC. The presented method is effective to reduce LCC by optimal SIS design.