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

RISK-BASED INSPECTION METHODOLOGY, PART 2—PROBABILITY OF FAILURE METHODOLOGY 2-99
NOTE
The t min is based on a design calculation that includes evaluation for internal pressure hoop stress,
external pressure and/or structural considerations, as appropriate. The minimum required thickness calculation
is the design code t min . Consideration for internal pressure hoop stress alone may not be sufficient. t c as
defined in STEP 5 may be used when appropriate.
2) Using Equation (2.41) with t rde from STEP 5 and FS extcorr from STEP 12.
extcorr
SRP
=
α
P⋅
D
extcorr
⋅FS
⋅trde
(2.41)
where α is the shape factor for the component type. α = 2 for a cylinder, 4 for a sphere, 1.13 for a
head.
NOTE This strength ratio parameter is based on internal pressure hoop stress only. It is not appropriate
where external pressure and/or structural considerations dominate. When t c dominates or if the t min is
calculated using another method, Equation (2.40) should be used.
n) STEP 14—Determine the number of inspections,
extcorr
N A ,
extcorr
N B ,
extcorr
N C
, and
extcorr
N D
, and the
corresponding inspection effectiveness category using Section 15.6.2 for past inspections performed
during the in-service time (see Section 4.5.5).
o) STEP 15—Determine the inspection effectiveness factors, 1 extcorr I
, I 2 extcorr
, and I 3 extcorr , using Equation
Pr extcorr
1
(2.46), prior probabilities, p Pr extcorr
2
, p
Pr extcorr
3
, and p
, from Table 4.5, conditional probabilities
(for each inspection effectiveness level), Co extcorr
p1
,
extcorr
Co p2
, and
extcorr
Co p3
, from Table 4.6, and the
extcorr
number of inspections, N A ,
from STEP 12.
extcorr
N B ,
extcorr
N C
, and
extcorr
N D
, in each effectiveness level obtained
= p ( p ) ( p ) ( p ) ( p )
=
extcorr extcorr extcorr
NA NB NC
p ( p ) ( p ) ( p ) ( p
D
)
= p ( p ) ( p ) ( p ) ( p )
extcorr extcorr extcorr extcorr
NA NB NC N
extcorr extcorr extcorrA extcorrB extcorrC extcorrD D
I1 Pr 1 Co 1 Co 1 Co 1 Co 1
extcorr extcorr extcorrA extcorrB extcorrC extcorr
I 2 Pr 2 Co 2 Co 2 Co 2 Co 2
extcorr
N D
extcorr extcorr extcorr extcorr
NA NB NC N
extcorr extcorr extcorrA extcorrB extcorrC extcorrD D
I3 Pr 3 Co 3 Co 3 Co 3 Co 3
(2.42)
Po extcorr
1
p) STEP 16—Calculate the posterior probabilities, p
(2.43) with 1 extcorr I
, I 2 extcorr , and I 3 extcorr in STEP 14.
extcorr
Po 2
, p
extcorr
Po 3
, and p
, using Equation
extcorr
extcorr
I1
Po p1
=
extcorr extcorr extcorr
I1 + I2 + I3
extcorr
extcorr
I 2
Po p2
=
extcorr extcorr extcorr
I1 + I2 + I3
extcorr
extcorr
I 3
Po p3
=
extcorr extcorr extcorr
I1 + I2 + I3
(2.43)
extcorr
q) STEP 17—Calculate the parameters, β 1 , β 2 extcorr
extcorr
, and β 3
assigning COV ∆t = 0.20, COV S f = 0.20, and COV P = 0.05.
, using Equation (2.48) and