In API 581 two methods of calculating POF are used: the generic failure frequency (GFF) method and a two-parameter Weibull distribution method. The GFF method is used to predict the loss of containment probability of failure from pressure boundary equipment. While the Weibull distribution method is used to predict the probability of failure for PRDs and heat exchanger bundles. Also, Inspection effectiveness is introduced into the POF calculation using Bayesian Analysis, which updates the POF when additional data is gathered through inspection.


  • API-581, 2016, Risk-based Inspection Methodology.

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