Separate Analysis for SIL are NOT Needed; and Overuse of LOPA
Since I’m one of the two originators of LOPA in 1995 and one of the two primary authors of the CCPS books on LOPA (2001 and 2012), I (and many others) believe we are doing Too Many analysis. A well-structured and well led PHA team does 95% of the risk analysis and 99% of the risk identification. LOPA is only needed for 1 to 5% of the scenarios; for those that are too complex for the PHA team to complete in the team setting.
The same goes for SIL analysis. For the PHA teams we lead (and we have led many thousands) or that our graduates lead, there is no need for a separate SIL determination analysis. The PHA team decides if they have enough IPLs (qualitatively; no numbers), after first deciding that they understand the scenario well enough, and then one of the IPLs they consider are existing or needed SIF; the teams also qualitatively determine what SIL is needed for each SIF. On “some” of these, we will double-check using LOPA or other methods to get a different picture of the scenario; but note that LOPA and other quantification methods are Not “better” than PHA methods (such as HAZOP or What-if or FMEA) but they are certainly different views of the scenario.
Note that the SIS standards (like IEC ) and guidelines (such as from CCPS) do Not require a separate analysis for “determining” if an SIF is needed and what SIL is then needed; in fact, from the beginning of SIS standards development, it was anticipated that the SIF determination would occur in the PHA team. The industry is doing too many analysis of the same issues, in our opinion, and yet still missing a great many scenarios because as a rule (with exceptions) the industry is Not focusing on accidents during non-routine modes of operations. Ask if your company has done a PHA of startup mode or on-line maintenance modes of operations.
If you want to read some papers on these topics, including the two original papers on LOPA by me and Art Dowell III and including using PHA and LOPA, then visit PII’s website and download the related free papers. They can be found in the Free Resources section at: www.piii.com.