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

RISK-BASED INSPECTION METHODOLOGY, PART 2—PROBABILITY OF FAILURE METHODOLOGY 2-119
If multiple inspections of a lower effectiveness have been conducted during the designated time period, they
can be equated to an equivalent higher effectiveness inspection in accordance with Section 3.4.3.
17.6.3 Calculation of the DF
The following procedure may be used to determine the DF for ExtClSCC; see Figure 17.1.
a) STEP 1—Determine the susceptibility using Table 17.2 based on the driver and the operating
temperature. Note that a High susceptibility should be used if cracking is confirmed to be present.
b) STEP 2—Determine the Severity Index, S VI , using Table 17.3 based on the susceptibility from STEP 1.
c) STEP 3—Determine the in-service time, age crack , since the last Level A, B, or C inspection was
performed with no cracking detected or cracking was repaired. Cracking detected but not repaired
should be evaluated and future inspection recommendations based upon FFS evaluation.
d) STEP 4—Determine the in-service time, age coat , since the coating has been installed using
Equation (2.60).
agecoat
= Calculation Date − Coating Installation Date
(2.60)
e) STEP 5—Determine the expected coating age, C age , based on coating type, quality of application and
service conditions. C age should be 0 years for no coating or poorly applied coating. Lower quality
coatings will typically have a C age of 5 years or less. High quality coatings or coatings in less harsh
external environments may have a C age of 15 or more years. C age may be adjusted based on an
evaluation of the coating condition during a high-quality inspection.
f) STEP 6—Determine the coating adjustment, Coat adj , using Equations (2.61) and (2.62).
If agecrack
≥ agecoat:
( )
Coat = min C ,age
adj age coat
(2.61)
If agecrack
< agecoat:
1. If the coating has failed at the time of inspection when age tke was established, then
Coat adj = 0
2. If the coating has not failed at the time of inspection when age tke was established, use Equation
(2.63) to calculate Coat adj
( ) ( )
Coat min C ,age min C ,age age
adj = age coat − age coat − tke
(2.63)
g) STEP 7—Determine the in-service time, age, over which ExtClSCC may have occurred using Equation
(2.75).
age age Coat
= crack − adj
(2.75)
h) STEP 8—Determine the number of inspections performed with no cracking detected or cracking was
repaired and the corresponding inspection effectiveness category using Section 17.6.2 for past