Availability and Reliability Studies
Since Verification Schemes require that the reliability of SCEs (Safety Critical Elements) be determined it might be considered logical to carry out formal studies into this requirement. Availability studies determine whether a system or component will operate when required to do so. This is often expressed as "Available on Demand".
No system can be always available, but it can be available 99.9 percent of the time. This is known to the reliability experts as “three nines”. The process is more complex than one might expect, and an example that is often used is the fire detection system. The system will detect fires, but of course one does not know whether it is going to work until it actually detects a fire, and when demanded to do so, it might not work.
The availability of the system must therefore be calculated, and this is done using failure rate data which is available in the public domain, often OREDA (Offshore Reliability Data), or which is provided by the manufacturer of the equipment, and the test intervals which are probably part of the planned maintenance system. Today of course there are many systems that are self-testing, often testing every detector head several times a second. Since such a system will identify a failure as soon as it occurs it will only be if there are frequent failures that the system will be less reliable than “three nines”.
Marex uses RELEX reliability software, which is effective in operation and which provides graphics which are easy for people other than safety engineers to understand.