Impact of gas detection coverage on SIF SIL Rating

Conclusion
It is obvious from this simple illustration that the percentage of detection coverage has a significant impact on the eventual SIL rating of a gas detection related SIF.

A field study report released by the Offshore Safety Division of HSE UK (OTO 2000 112, Dec 2000), suggests that only about 60% of hydrocarbon gas releases (out of 1801 reported gas releases cases between October 1992 and March 2000) on offshore facilities were detected by the equipment that were put in place for gas detection purpose. Reasons for the 40% non-detection were not detailed in the report, so arguably these failures could have been caused or influenced by other factors besides poor detection coverage. Nevertheless, the statistical evidence presented is enough to implicitly suggest that the likelihood of a lower than 95-99% detection coverage is a highly probable scenario (due to various causes), certainly in the case for offshore applications and likely to be similar for onshore applications.

If, as the HSE study suggests, a greater than 99% detection coverage is improbable, and if there are no other considerations that warrants its implementation, then it may not make a lot of sense to implement a costlier SIL3 or SIL2 SIF hardware solution for most gas detection systems, since the effective risk reduction is at best no greater than SIL1.

SIL Level	Risk Reduction
3	> 1,000 to ≤ 10,000
2	> 100 to ≤ 1,000
1	> 10 to ≤ 100

Finally, regardless of what SIF hardware is selected, good detection coverage remains an important design element for gas detection systems, as this determines whether or not the system would be effective in providing protection against dangerous gas leaks.

Some methods to improve detection coverage that can be considered are:
Usage of computational fluid dynamics (CFD) modelling techniques to approximate gas cloud dispersion scenarios under different environmental conditions. Knowing where the probable gas dispersion path would be will greatly increase the ability of the engineer to place gas detectors more precisely.

Where possible, apply detection technology that is more impervious to wind direction/speed and other environmental influences. An example of this technology would be the ultrasonic gas leak detector, which detects ultrasonic frequencies created by escaping pressurized gas at the leakage point, and is therefore not affected by onsite wind conditions or other common environmental factors.

Where possible, apply detection technology that supports post installation detection coverage verification. A technology that features this is the ultrasonic gas leak detector, where the effective detection coverage can be verified through N2 leak tests after the detector is installed with all surrounding process equipment, piping, vessels etc. fully constructed. If necessary, the gas leak detector could be re-positioned to improve detection coverage.

For large area monitoring of outdoor uncongested locations that are exposed to wind, apply a detection technology that is able to detect low concentration hydrocarbon gas clouds to account for dilution effects caused by the wind. An example of this technology would be the optical open path IR beam hydrocarbon gas detector that is sensitive to parts-per-million (ppm) levels of hydrocarbon gases.

Understand the process equipment construction and mechanical layout so that point type gas detectors can be placed as close as possible to potential gas leak points to increase the likelihood of detection.

Understand the process well so that the type of gas release (e.g. low/high fraction release, condensate release, liquid spills, gas jet release, flashing liquid release etc.) can be anticipated and the gas detection.