One of the typical major accident scenarios considered when building and operating an offshore drilling or production rig, is a gas leak that is ignited, leading to a jet fire, or even worse, an explosion. For this scenario to happen we need three things (from the fire triangle):
- Flammable material (the gas)
- Oxygen (air)
- An ignition source
The primary protection against such accidents is containment of flammable materials; avoiding leaks is the top safety priority offshore. As a lot of this equipment exists outdoors (or in “naturally ventilated areas” as standards tend to call it), it is not an option to remove “air”. Removing the ignition source hence becomes very important, in the event that you have a gas leak. The technical system used to achieve this consists of a large number of distributed gas detectors on the installation, sending message of detected gas to a controller, which then sends a signal to shut down all potential ignition sources (ie non-EX certified equipment, see the ATEX directive for details).
This being the barrier between “not much happening” and “a major disaster”, the reliability of this ignition source control is very important. Ignition sources are normally electrical systems not designed specifically to avoid ignition (so-called EX certified equipment). In order to have sufficient reliability of this set-up the number of ignition sources should be kept at a minimum; this means that the non-EX equipment should be grouped in distribution boards such that an incomer breaker can be used to isolate the whole group, instead of doing it at the individual consumer level. This is much more reliable, as the probability of a failure on demand (PFD) will contain an additive term for each of the breakers included:
PFD = PFD(Detector) + PFD(Logic) + Sum of PFD of each breaker
Consider a situation where you have 100 consumers, and the dangerous undetected failure rate for the breakers used is 10-7 failures per hour of operation, with testing every 24 months, the contribution from a single breaker is
PFD(Breaker) = 10-7 x (8760 x 2) / 2 = 0.000876
If we then have 6 breakers that need to open for full isolation, we have a breaker PFD contribution of 0.005 from the breakers (which means that with reliable gas detectors and logic solver, a full loop can satisfy a SIL 2 requirement). If we have 100 breakers the contribution to PFD is 0.08 – and the best we can hope for is SIL 1.