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

2-24 API RECOMMENDED PRACTICE 581
Equation (2.15) with t rdi from STEP 3, t min or t c from STEP 5, S and E from STEP 5, and flow stress,
FS Thin , from STEP 7.
Thin S⋅
E max( t min ,tc
)
SRP
= ⋅ (2.14)
Thin
FS trdi
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 should be used when appropriate.
Using Equation (2.16) with t rdi from STEP 3 and FS Thin from STEP 7.
Thin
SRP
=
α
P⋅
D
Thin
⋅FS
⋅trdi
(2.15)
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 (2.15) should be used.
i) STEP 9—Determine the number of inspections for each of the corresponding inspection effectiveness,
Thin Thin Thin Thin
N A , N B , N C , and N D , using Section 4.5.6 for past inspections performed during the inservice
time.
Thin Thin Thin
j) STEP 10—Calculate the inspection effectiveness factors, I1 , I 2 , and I 3 , using Equation (2.16),
prior probabilities,
Thin Thin
Prp
1 , p2 ,
Thin
p
Pr and Pr 3 , from Table 4.5, the conditional probabilities (for each
Thin
p
Thin
p
inspection effectiveness level), Co 1 , Co 2 , and Co 3 , from Table 4.6, and the number of
inspections,
Thin Thin
N A , B ,
Thin
C
Thin
D
Thin
p
N N , and N , in each effectiveness level from STEP 9.
= p ( p ) ( p ) ( p ) ( p )
= p ( p ) ( p ) ( p ) ( p )
=
Thin
Thin
Thin
NA
NB
N
hinC C ThinD
p ( p ) ( p ) ( p ) ( Co p 3 )
Thin Thin Thin Thin
NA NB NC N
Thin Thin ThinA ThinB ThinC ThinD D
I1 Pr 1 Co 1 Co 1 Co 1 Co 1
Thin Thin Thin Thin
NA NB NC N
Thin Thin ThinA ThinB ThinC ThinD D
I 2 Pr 2 Co 2 Co 2 Co 2 Co 2
Thin Thin ThinA ThinB T
I3 Pr 3 Co 3 Co 3 Co 3
Thin
ND
(2.16)
See Section 4.5.3 for guidance on selection of the prior probabilities. Conservatively, the low confidence
data could be chosen from Table 4.5.
k) STEP 11—Calculate the posterior probabilities,
Thin
Po p 1 ,
Thin Thin
I I 2 , and I 3 in STEP 10.
Thin
1 ,
Thin
Po p 2 , and
Thin
Po p 3 , using Equation (2.17) with