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

2-110 API RECOMMENDED PRACTICE 581
NOTE Use flow stress (FS Thin ) at design temperature for conservative results, using the appropriate Equation
(2.54) or Equation (2.55).
m) STEP 13—Calculate strength ratio parameter,
Thin
SR
P , using Equation (2.54) or Equation (2.55).
1) Use Equation (2.54) with t rde from STEP 5, S, E, and t min or t c (Section 4.5.6, Step 5), from STEP 10,
and flow stress FS CUIF from STEP 11.
CUIF S⋅
E min( t min ,tc
)
SRP
= ⋅ (2.54)
CUIF
FS trde
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) Use Equation (2.55) with t rde from STEP 4 and flow stress FS CUIF from STEP 12.
CUIF
SRP
=
α
P⋅
D
CUIF
⋅FS
⋅trde
(2.55)
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 (2.54) should be used.
CUIF CUIF CUIF
n) STEP 14—Determine the number of inspections, N A , N B , N C , and N D
CUIF , and the
corresponding inspection effectiveness category using Section 16.6.2 for all past inspections.
o) STEP 15—Determine the inspection effectiveness factors, 1 CUIF I
, I 2 CUIF , and I 3 CUIF , using Equation
Pr CUIF
1
(2.56), prior probabilities, p Pr CUIF
2
, p Pr CUIF
3
, and p
, from Table 4.5, conditional probabilities (for
each inspection effectiveness level),
CUIF
Co p1
,
CUIF
Co p2
and
CUIF
Co p3
, from Table 4.6, and the number of
inspections,
CUIF
A
CUIF
B
N , N , N
CUIF
C
, and
CUIF
N D , in each effectiveness level obtained from STEP 13.
= p ( p ) ( p ) ( p ) ( p )
= p ( p ) ( p ) ( p ) ( p )
=
CUIF
CUIF
CUIF
NA
NB
N
UIFC C CUIFD
p ( p ) ( p ) ( p ) ( Co p 3 )
CUIF CUIF CUIF CUIF
NA NB NC N
CUIF CUIF CUIFA CUIFB CUIFC CUIFD D
I1 Pr 1 Co 1 Co 1 Co 1 Co 1
CUIF CUIF CUIF CUIF
NA NB NC N
CUIF CUIF CUIFA CUIFB CUIFC CUIFD D
I 2 Pr 2 Co 2 Co 2 Co 2 Co 2
CUIF CUIF CUIFA CUIFB C
I3 Pr 3 Co 3 Co 3 Co 3
CUIF
ND
(2.56)