Low Demand Safety Instrumented System: Update of Function Test Intervals with Layer of Protection Analysis in the Operational Phase

In the oil & gas industry, the estimation and updating of function test intervals of critical safety instrumented systems (SIS) are critical for safe production. The definition of the new test interval of a SIS is closely related to its failure rate and average probability of failure on demand (PFD). In current practice, the evaluation process is employed on each individual SIS, and the overall safety level is usually not justified. When such analyses are performed on multiple safety instrumented protection layers, there is a tendency for the overall safety level of the protection system to be increased to a very high level, i.e. overprotection. This paper aims to suggest an alternative approach to estimating test intervals, with a focus on overall safety level. The results show the potential to extend function test intervals in a more ambitious way, without degrading the safety level of the overall protection system or each specific SIS.