Safety instrumented systems (SIS) are widely used in the process industry, like oil and gas, to prevent the initiation of dangerous events and mitigate their consequences if hazardous events are developed into an accident. International standards like IEC – 61508 and 61511 sets this requirement interms of Safety Integrity Level or SIL to be maintained during the entire safetylife cycle of the SIS. The standard developed by the PSAN – NOG 070 also has reliability requirements for SIS that align with those set by IEC standards.

A SIL is given a number between 1 and 4 which indicates SIF’s reliability in the increasing order. A single SIS can be designed to perform one or more Safety Instrumented Function (SIF). The SIL of the SIF defines a limit of themaximum tolerable average probability of failure on demand (average PFD) which is usually determined during the design phases and must be complied with during the operational phases as well.

In this thesis, the SIS in its operational life is reviewed by implementing a casestudy of the SIF called PSD PAHH (preassure alarm high high) by the closing of safety valves. The SIS and the SIF are implemented on Equinor’s Johan Castberg FPSO on the Northern Continental Shelf. Experienced failure rates of the components from the operational phase form important inputparameters that are required to carry out the reliability follow-up analyses. The guidelines recommend optimising the proof test interval in case the experienced failure rates, and consequently the average PFD, differs significantly from the initial values determined at design. The proof test optimisation may be done by extending the test interval which indicates that the PFD has improved (decreased) or by shortening the interval which shows that PFD has increased meaning the reliability of the SIF has degraded.

Reducing the proof test interval is not seen as an ideal situation by the end users of the SIS as it is a costly and resource-consuming affair that usually stops the production and bring additional safety related issues for the duration of thetest. Therefore, proof test alternatives are suggested and the benefits of using the partial proof test are discussed qualitatively and quantitatively.