We have developed a methodology and tools, combining stochastic models to calculate the Probability of Failure on Demand (PFD) and Spurious Trip Rate (STR) with Genetic Algorithms (GA) and Whole Life Costing (WLC) to assess and optimise the testing strategies for low-demand Safety Instrumented Systems (SIS).

Our tool can assess Safety Instrumented Systems (SIS) with various Safety Instrumented Functions (SIF), taking into account the specified Safety Integrity Levels (SIL), the system architecture and voting logic, the failure rate of each component, and the ability of the final elements to perform Partial Stroke Tests (PSTs).

The costs associated with the execution of tests and business interruption (due to a spurious activation or downtime for a full test) are considered to produce a set of possible testing strategies that minimise the cost and at the same time allow the required SIL levels to be achieved.