Section 9 - Assessment of Locally Thinned Areas

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Copyright 2005, TWI Ltd World Centre for Materials Joining TechnologyWorld Centre for Materials Joining Technology

Assessment of LocallyThinned Areas

Assessment of LocallyThinned Areas


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Locally Thinned AreasLocally Thinned Areas

Many procedures:Many procedures:

API 579 Sections 4 and 5API 579 Sections 4 and 5

BS 7910 Appendix GBS 7910 Appendix G

ASME B31GASME B31G Numerous papersNumerous papers


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Apart from API 579 Section 4 (thicknessApart from API 579 Section 4 (thickness

averaging), the theory behind theaveraging), the theory behind the

different methodology is usually based ondifferent methodology is usually based onan effective area method.an effective area method.

Where metal loss is treated as a partWhere metal loss is treated as a part

through flaw and evaluated using athrough flaw and evaluated using aFoliasFolias bulging stress magnification bulging stress magnification

factorfactor

Methodologies are validated against aMethodologies are validated against alarge burst test databaselarge burst test database


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Most procedures require theMost procedures require thedetermination of critical thickness profilesdetermination of critical thickness profiles

(CTP)(CTP) CTP based on river bottom of theCTP based on river bottom of theminimum thickness readings in bothminimum thickness readings in both

circumferential and longitudinalcircumferential and longitudinaldirectionsdirections

Average and minimum thickness can beAverage and minimum thickness can bedetermineddetermined

Recommendations in API RP 579 toRecommendations in API RP 579 toderive inspection grid spacingderive inspection grid spacing


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Line M critical thickness profile from inspectionLine M critical thickness profile from inspection


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Locally Thinned Areas section 4Locally Thinned Areas section 4

Section 4 basic procedureSection 4 basic procedure

Compute remainingCompute remainingthickness ratio (thickness ratio (RRtt))

Determines the lengthDetermines the lengthof thickness averagingof thickness averaging

(L) from equation given(L) from equation givenin standardin standard

Determine s and c , theDetermine s and c , thedimensions of metaldimensions of metal

loss in the longitudinalloss in the longitudinaland circumferentialand circumferentialdirectionsdirections

ttmmmm = minimum measured thickness= minimum measured thickness

D = Inside diameter of the cylinder or otherD = Inside diameter of the cylinder or othersection (see API 579)section (see API 579)

Q = Factor from Table 4.4 based on anQ = Factor from Table 4.4 based on an RSFRSFaaand the remaining thickness ratio,and the remaining thickness ratio, RRtt


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S

C

S

C

Example of s and c determinationExample of s and c determination

tmin

Bold line (x) circumferential CTPBold line (x) circumferential CTP

Dashed line (o) longitudinal CTPDashed line (o) longitudinal CTP


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If s < L then metal loss is acceptableIf s < L then metal loss is acceptable

If s > L then:If s > L then: Assume average thickness equal toAssume average thickness equal to

measured minimum thicknessmeasured minimum thickness

Direct averaging of the thickness readingsDirect averaging of the thickness readingsthat reside within length L.that reside within length L.

Area averaging method of the thicknessArea averaging method of the thicknessreadings that reside within length L.readings that reside within length L.

If minimum required thickness (If minimum required thickness (ttminmin) to) to

code calculation is less than or equal tocode calculation is less than or equal toaverage thickness (taverage thickness (tamam) minus the FCA) minus the FCAthen the metal loss is acceptablethen the metal loss is acceptable

Refinem

ent Si

mplicity


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Critical thickness profile from inspectionCritical thickness profile from inspection

Assuming the minimum measuredAssuming the minimum measuredthicknessthickness


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Average thickness in a set lengthAverage thickness in a set length

ttminmin== thickness points within Lthickness points within L


Set length fromSet length from

procedure (L)procedure (L)


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A1

A2

Area averaging method in LArea averaging method in L

ttminmin= (A= (A11 + A+ A22) / L) / L


Set length fromSet length from

procedure (L)procedure (L)


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Section 4 metal loss - exampleSection 4 metal loss - example

Corrosion on a cylindrical section of pressureCorrosion on a cylindrical section of pressure

vessel has been found during an inspection.vessel has been found during an inspection.

The vessel was designed and constructed toThe vessel was designed and constructed tothe ASME B&P VIII division I. Evaluate if thethe ASME B&P VIII division I. Evaluate if the

pipe vessel is fitpipe vessel is fit--forfor--service.service.

Design Conditions = 300 psig @ 350Design Conditions = 300 psig @ 350ooFF

Inside Diameter = 48 inchesInside Diameter = 48 inches

Nominal Thickness = 0.75 inchesNominal Thickness = 0.75 inches

Future Corrosion Allowance = 0.10 inchesFuture Corrosion Allowance = 0.10 inches

Uniform metal loss = 0 inchesUniform metal loss = 0 inches

Material = SA 516 Grade 70 (S = 17500 psi @Material = SA 516 Grade 70 (S = 17500 psi @

temperature)temperature)

Weld Joint Efficiency = 0.85Weld Joint Efficiency = 0.85


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API 579 Section 5 MethodAPI 579 Section 5 Method

Level 1Level 1 Screening curves longitudinalScreening curves longitudinal

extent of a flawextent of a flaw


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API 579 Section 5 MethodAPI 579 Section 5 Method

Level 1Level 1 Screening curve circumferentialScreening curve circumferential

extent of a flawextent of a flaw

c = Circumferential dimension ofc = Circumferential dimension of

the metal lossthe metal loss

D = Inside diameter of theD = Inside diameter of the

cylinder or other section (see APIcylinder or other section (see API

579)579)


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Assessment includes groove like flawsAssessment includes groove like flaws

(not shown refer to API 579)(not shown refer to API 579)

Screening curves for bothScreening curves for both

circumferential and longitudinalcircumferential and longitudinal

directionsdirections

Level 2 similar to level 1 but provide aLevel 2 similar to level 1 but provide a

better estimate of the RSFbetter estimate of the RSF

Includes variation for thickness profileIncludes variation for thickness profile

to find RSF (based on the RSTRENGto find RSF (based on the RSTRENGslicing methodology)slicing methodology)


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AAii = area of metal loss for increment length= area of metal loss for increment length

AAiioo = Original area for increment length (= Original area for increment length (ttminminxx SSii))

RSF calculated for each incremental lengthRSF calculated for each incremental length

Variation for thickness profile to find RSFVariation for thickness profile to find RSF


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ttmin

min

SS11

R

SF

R

SF

incrementincrement

SS11


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ttmin

min

SS22

R

SF

R

SF

incrementincrement

SS11

SS22


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ttmin

min

SSii+n+n

R

SF

R

SF

incrementincrement

SS11

SSii+n+n

Minimum RSFMinimum RSF

for assessmentfor assessment

SS22


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Section 5 Level 3Section 5 Level 3

Finite element analysis either elasticFinite element analysis either elasticor elasticor elastic--plasticplastic

No limitations e.g. can includeNo limitations e.g. can include

fatiguefatigue


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A region of localized corrosion has beenA region of localized corrosion has been

found on a pressure vessel during afound on a pressure vessel during a

scheduled turnaround. The vessel wasscheduled turnaround. The vessel wasdesigned and constructed to the ASMEdesigned and constructed to the ASME

B&PV Code, Section VIII, Division 1.B&PV Code, Section VIII, Division 1.

Determine if the vessel is acceptable forDetermine if the vessel is acceptable forthe current MAWP using a Section 5the current MAWP using a Section 5

Level 1 Assessment.Level 1 Assessment.


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Vessel DataVessel Data

Design Conditions = 300 psi @ 650Design Conditions = 300 psi @ 650ooFF

Inside Diameter = 96 inchesInside Diameter = 96 inches

Fabricated Thickness = 1.25 inchesFabricated Thickness = 1.25 inches

Uniform Metal Loss = 0.10 inchesUniform Metal Loss = 0.10 inches

FCA = 0.125 inchesFCA = 0.125 inches Material = SA 516 Grade 70 (S= 17500 psi)Material = SA 516 Grade 70 (S= 17500 psi)

Weld Joint Efficiency = 0.85Weld Joint Efficiency = 0.85


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Documents

Section 9 - Assessment of Locally Thinned Areas