A safety instrumented system can comprise single or multiple safety functions.
Establishing the Safety Integrity Level (SIL) for each function involves establishing the combinations of failure of equipment and its controls which lead to Hazards.
It is then possible to analyse these hazardous failures in such a way that the risk targets can be met.
This in turn leads to the establishment of an IEC61508 SIL Target for each function. The two most common methods of SIL targeting are the Risk Graph (RG) and the Layers of Protection Analysis (LOPA).
RG provide a qualitative (quicker) method of establishing a SIL Target whilst LOPA is a semi quantitative method which provides a less conservative result.
Both methods require a review by a multidiscipline group with the actual offshore operators being key to the activity success. Data must be provided by the end user in the form of corporate information on acceptable risk criteria.