API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology
2-100 API RECOMMENDED PRACTICE 581βextcorr1=extcorr1−D ⋅A 1rt −SRSp22 2 2 2 extcorr 2 2⋅1rt ⋅ t ( 11rt )( )S ∆ + − ⋅ ⋅ +S S fp ⋅ PD A COV D A COV SR COV,βextcorr2=extcorr1−D ⋅A 2rt −SRSp22 2 2 2 extcorr 2 2⋅2rt ⋅ t ( 12rt )( )S ∆ + − ⋅ ⋅ +S S fp ⋅ PD A COV D A COV SR COV,(2.48)βextcorr3=extcorr1−D ⋅A S3rt −SRp22 2 2 2⋅3rt ⋅ 1 (S ∆t + − ⋅3rt ⋅ +S S fextcorr 2p ) ⋅2P( )D A COV D A COV SR COV.where D S1 = 1, D S2 = 2, and D S3 = 4. These are the corrosion rate factors for damage states 1, 2, and 3as discussed in Section 4.5.3 [17] . Note that the DF calculation is very sensitive to the value used for thecoefficient of variance for thickness, COV ∆t . The COV ∆t is in the range 0.10 ≤ COV ∆t ≤ 0.20, with arecommended conservative value of COV ∆t = 0.20.r) STEP 18—CalculateextcorrD fusing Equation (2.45).Dextcorrf( ( )) ( )( ) ( ( ))⎡ extcorr extcorr extcorr extcorr extcorr extcorrPo p1 Φ − β1 + Po p2 Φ − β2 + Po p3 Φ −β⎤⎢3 ⎥= ⎢1.56E-04⎥⎢⎥⎣⎦(2.45)where Φ is the standard normal cumulative distribution function (NORMSDIST in Excel).15.7 Nomenclatureageage coatage tkA rtC ageCoat adjC rC rBis the in-service time that damage is applied, yearsis the in-service time since the coating installation, yearsis the component in-service time since the last inspection thickness measurement with respectto wall loss associated with external corrosion or service start date, yearsis the expected metal loss fraction since last inspectionis the total anticipated coating life from the time of installationis the coating adjustment, yearsis the corrosion rate, mm/year (inch/year)is the base value of the corrosion rate, mm/year (inch/year)extcorp1Co is the conditional probability of inspection history inspection effectiveness for damage state 1extcorp2Co is the conditional probability of inspection history inspection effectiveness for damage state 2extcorp3Co is the conditional probability of inspection history inspection effectiveness for damage state 3
RISK-BASED INSPECTION METHODOLOGY, PART 2—PROBABILITY OF FAILURE METHODOLOGY 2-101COV PCOV S fCOV ∆tDis the pressure varianceis the flow stress varianceis the thinning varianceis the component inside diameter, mm (inch)D S1 is the corrosion rate factor for damage state 1D S2 is the corrosion rate factor for damage state 2D S3 is the corrosion rate factor for damage state 3extcorrD fis the DF for external corrosionextcorrDF pis the DF for external corrosionEF EQF IFFS extcorris the weld joint efficiency or quality code from the original construction codeis the adjustment factor for equipment design/fabrication detailis the corrosion rate adjustment factor for interface for soil and wateris the flow stressI 1 extcorr is the first order inspection effectiveness factorI 2 extcorr is the second order inspection effectiveness factorI 3 extcorr is the third order inspection effectiveness factorL eis the measured wall loss from external corrosion, mm (inch)extcorrN A is the number of A level inspectionsextcorrN B is the number of B level inspectionsextcorrN C is the number of C level inspectionsextcorrN D is the number of D level inspectionsPextcorrPo p 1extcorrPo p 2is the pressure (operating, design, PRD overpressure, etc.), MPa (psi)is the posterior probability for damage state 1is the posterior probability for damage state 2
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2-100 API RECOMMENDED PRACTICE 581
β
extcorr
1
=
extcorr
1−D ⋅A 1
rt −SR
S
p
2
2 2 2 2 extcorr 2 2
⋅
1
rt ⋅ t ( 1
1
rt )
( )
S ∆ + − ⋅ ⋅ +
S S f
p ⋅ P
D A COV D A COV SR COV
,
β
extcorr
2
=
extcorr
1−D ⋅A 2
rt −SR
S
p
2
2 2 2 2 extcorr 2 2
⋅
2
rt ⋅ t ( 1
2
rt )
( )
S ∆ + − ⋅ ⋅ +
S S f
p ⋅ P
D A COV D A COV SR COV
,
(2.48)
β
extcorr
3
=
extcorr
1−D ⋅A S3
rt −SR
p
2
2 2 2 2
⋅
3
rt ⋅ 1 (
S ∆t + − ⋅
3
rt ⋅ +
S S f
extcorr 2
p ) ⋅
2
P
( )
D A COV D A COV SR COV
.
where D S1 = 1, D S2 = 2, and D S3 = 4. These are the corrosion rate factors for damage states 1, 2, and 3
as discussed in Section 4.5.3 [17] . Note that the DF calculation is very sensitive to the value used for the
coefficient of variance for thickness, COV ∆t . The COV ∆t is in the range 0.10 ≤ COV ∆t ≤ 0.20, with a
recommended conservative value of COV ∆t = 0.20.
r) STEP 18—Calculate
extcorr
D f
using Equation (2.45).
D
extcorr
f
( ( )) ( )
( ) ( ( ))
⎡ extcorr extcorr extcorr extcorr extcorr extcorr
Po p1 Φ − β1 + Po p2 Φ − β2 + Po p3 Φ −β
⎤
⎢
3 ⎥
= ⎢
1.56E-04
⎥
⎢
⎥
⎣
⎦
(2.45)
where Φ is the standard normal cumulative distribution function (NORMSDIST in Excel).
15.7 Nomenclature
age
age coat
age tk
A rt
C age
Coat adj
C r
C rB
is the in-service time that damage is applied, years
is the in-service time since the coating installation, years
is the component in-service time since the last inspection thickness measurement with respect
to wall loss associated with external corrosion or service start date, years
is the expected metal loss fraction since last inspection
is the total anticipated coating life from the time of installation
is the coating adjustment, years
is the corrosion rate, mm/year (inch/year)
is the base value of the corrosion rate, mm/year (inch/year)
extcor
p1
Co is the conditional probability of inspection history inspection effectiveness for damage state 1
extcor
p2
Co is the conditional probability of inspection history inspection effectiveness for damage state 2
extcor
p3
Co is the conditional probability of inspection history inspection effectiveness for damage state 3
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