Page 184 - Offshore Electrical Engineering Manual

Emergency Shutdown System 171

only do the various standards derive SILs differently, but none provides a clear and
detailed explanation of how they are derived and applied. The result is that SILs are
not well understood’ (Redmill, F., 2000). There are also doubts about the veracity
and accuracy of the calculation of the probability of failure on demand of the safety
system, as this figure is derived by operating on historical data by statistical means
in order to try and predict the future. Often a probability of failure on demand is
taken as a mathematically accurate figure, but it should only be used as a guide in the
comparison of different protective circuits, always bearing in mind that the demand
could result in the failure to act.
The standard referred to at the beginning of the chapter has recently been upgraded;
the eighth edition of API RP 14C Analysis, Design, Installation, and Testing of Safety
Systems for Offshore Production Facilities was published in February 2017, ‘On the face
of it, it seems to be a well overdue update with an element of catch up. But it still follows
the prescriptive line i.e. no allowance for risk based approaches’ (Wild, G. 2017).