API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

Risk-Based Inspection Methodology
Part 2—Probability of Failure Methodology
Annex 2.B—Determination of Corrosion Rates
2.B.1
Overview
2.B.1.1 Determination of Corrosion Rate
The corrosion rate should be calculated from measured thickness data available from equipment
inspection(s). However, if a calculated corrosion rate is not available, estimated corrosion rates may be
determined for each potential thinning mechanism using this document or as estimated by a corrosion
specialist.
Screening questions are used to determine which of the thinning mechanism sections apply. These
applicable sections will be entered to determine conservative estimated corrosion rates for possible thinning
mechanisms. The screening questions listed in Table 2.B.1.1 are used to select the applicable thinning
mechanism.
2.B.1.2 Determination of Thinning Type
The results of effective inspections that have been performed on the equipment or piping should be used to
designate the type of thinning (i.e. general vs localized). If this information is not known, then Table 2.B.1.2
lists the type of thinning (general or localized) expected for various thinning mechanisms. If both general and
localized thinning mechanisms are possible, then the type of thinning should be designated as localized. The
type of thinning designated will be used to determine the effectiveness of inspection performed.
2.B-1