Defect Assessment of a Pressure Vessel Nozzle - NAFEMS · PDF fileFENET THEMATIC NETWORK COMPETITIVE AND SUSTAINABLE GROWTH (GROWTH) PROGRAMME Advantica Technology • A leading provider

FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME

Industry Sector RTD Thematic Area DatePower & Pressure Systems Durability and Life Extension Jun-02

Defect Assessment of a Pressure Vessel Nozzle

Keith Wright - Structural Integrity Assessments Ltd, Melbourne, Derby, United Kingdom&

Janak Patel – Robust Solutions Ltd, Oadby, Leicestershire, United Kingdom

SummaryThe use of FEA in the defect assessment of a pressure vessel nozzle is described along with a summary of some possible future workshop activities for the Durability and Life Extension technology areas of FENet.


Advantica Technology

• A leading provider of technology and engineering services to customers in gas, pipelines and associated industries internationally.

• Heritage going back over 30 years.• Formerly “BG Technology Ltd“ with origins in the research, technology and

engineering arms of “British Gas“.• An £80 million turnover with operating experience in over 30 countries.• Part of the Lattice Group Organisation that was demerged from BG Group in 2000.

• Lattice Group has annual turnover of over £3 billion• Lattice Group has operating profit of over £1 billion


High Pressure Gas Storage Vessels

–Used to smooth out diurnal pressure fluctuations.



–Typically 3m Diameter

– Length of 55m to 75m

– Manufactured between 1968 to 1972


Flaw Assessment Procedures

• BSI PD6493 (First published 1980) “Guidance on methods for assessing the acceptability of flaws in fusion welded structures“

• BSI PD6493 (Second edition 1991)

• BS7910 (1999) “Guide on methods for assessing the acceptability of flaws in metallic structures“

Also:

• R6• SINTAP• API 579



–Inlet/Outlet nozzle.


HPSV Nozzle FE Model

• Inlet/Outlet nozzle

• Protruding design


HPSV Nozzle FE Model

• Inlet/Outlet nozzle

• Max principal stress distribution, normalised to unit hoop stress in vessel shell, due to internal pressure.


Nozzle Defect Assessment

• The uncracked body through wall stress distribution are input to:• CRACKWISE3 or• R6


Nozzle Defects


CRACKED BODY FE ANALYSIS ?• Concerns over COST and TIME• 80 vessels x 6 nozzles x ? Defects per nozzle

Also Technical difficulties:• Revised defect dimensions for crack growth analyses• Modelling of ‘SECONDARY‘ welding residual stresses• Sensitivity studies for material property variations

Aim is to develop a PRAGMATIC assessment method• Use Flat Plate Solutions• Use Mode 1 radial crack opening stress distributions• JUSTIFY BY UNDERTAKING SOME CRACKED BODY FE ASSESSMENTS


HPSV Nozzle Cracked Body FE Model

• Embedded Defect.• Defect length fully

circumferential.• Defect height

13.5mm.• 3mm ligament.• Displaced shape

shown due to pressure loading.


HPSV Nozzle Cracked Body FE Model Displaced Shape

• Midside nodes to ¼ point position.

• ABAQUS v6.2.• C3D20R

elements.• Internal pressure

loading only.


Elastic Stress Intensity Factors Calculated By:• ABAQUS• Type=K Factors on *CONTOUR INTEGRAL

Effective Stress Intensity Factor (Keff) Calculated By:

}1

{2

22

υ−++= III

IIIeffKKKK


Keff & KI, KII and KIII

• Near Surface Crack Front.

• 3mm ligament• Keff dominated

by mode 1.

Shallow Point KI, KII and KIII.(13.5mm defect, 3mm ligament)

-15.0-10.0-5.00.05.0

10.015.020.025.0

0 10 20 30 40 50 60 70 80 90

Crack Front Location (degrees)

Stre

ss In

tens

ity F

acto

rs, K

I, K

II, K

III

KI Shallow Front

KII Shallow Front

KIII Shallow Front

Keff Shalow Front


Keff & KI, KII and KIII

• Deepest Crack Front.

• 3mm ligament to near surface.

• Keff still dominated by mode 1 at deepest point.

Deepest Point KI, KII and KIII.(13.5mm defect, 3mm ligament)

-5.0

0.0

5.0

10.0

15.0

20.0

0 20 40 60 80

Crack Front Location (degrees)

Stre

ss In

tens

ity F

acto

rs, K

I, K

II, K

III

KI Deepest Point

KII Deepest Point

KIII Deepest Point

Keff Deepest Point


Keff derived from J

• Good Agreement with Keff derived from KI, KII and KIII.

• But an Elastic analysis.

K Effective along Crack Front.(Gillingham 6" nozzle, 13.5mm defect, 3mm ligament)

0.0

5.0

10.0

15.0

20.0

25.0

0 20 40 60 80 100Crack Front Location (degrees)

Stre

ss In

tens

ity F

acto

r (M

Pam

)

Shallow PointDeepest Point

)1( 2υ−= JEKeff


Elastic-Plastic J Integrals Calculated By:• Crack tip nodes allowed to move apart to model blunting.• ABAQUS Default option on *CONTOUR INTEGRAL

Stress-Strain Curve• Assumed to be Bi-linear• Negligible (1.5%) work hardening increase of yield stress after 1% plastic strain.

Equivalent Effective Stress Intensity Factor (Keff) Calculated By:

)1( 2υ−= JEKeff


Keff Comparison From Elastic/Plastic & Elastic Analyses

• Higher Keff at near surface crack front from elastic/plastic analysis.

• Good agreement at the deepest crack front.

K Effective along Crack Front.(Gillingham 6" nozzle, 13.5mm defect, 3mm ligament, Elastic/Plastic)

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80 90Crack Front Location (degrees)

Stre

ss In

tens

ity F

acto

r (M

Pa√m

)Elastic/PlasticShallow Front

Elastic/PlasticDeepest FrontElastic ShallowFront

ElasticDeepest Front


HPSV Nozzle Cracked Body FE Model

• Embedded Defect.• Defect length fully

circumferential.• Defect height

13.5mm.• 1mm ligament.


Keff Comparison From Elastic/Plastic & Elastic Analyses 1mm Ligament

• Much Higher Keff at near surface crack front from elastic/plastic analysis.

• Differences at the deepest crack front too, unlike 3mm case.

K Effective distribution along crack front.(Gillingham 6" nozzle, 13.5mm defect, 1mm ligament, Elastic/Plastic)

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90Crack Front Location (degrees)

Stre

ss In

tens

ity F

acto

r (M

Pa√m

)

Elastic/PlasticShallow Front

Elastic/PlasticDeepest Front

Elastic ShallowCrack Front

Elastic DeepestCrack Front


How to Use The Cracked Body FE Nozzle Defect Assessments?

• Comparisons made with Flat Plate Solution for Embedded Defect.• Linearised stress distribution over defect location obtained from uncracked FE

model.

Results of Comparison Between Elastic/Plastic FE and Flat Plate Solution

1.0125.1 MPa.m^0.525.4 MPa.m^0.53mm

1.1541.4 MPa.m^0.547.8 MPa.m^0.51mm

Mixed Mode / Elastic-Plastic Factor

Max KI from CRACKWISE3 Flat

Plate Solution

Max Keff from elastic/plastic Cracked

Body FE AnalysisLigament


What Lessons Have Been Learnt?

• Flat plate solutions predict highest KI value to occur at 0 degree position. However cracked body FE work shows highest Keff occurs at 45 degree position. This is due to the mixed mode loading effects.

• MORE SIGNIFICANTLY, for embedded defects with small (<3mm) ligament dimensions the effects of plasticity on Keff for the near surface crack front must be considered in an analysis.

• HENCE, when using pragmatic assessment methods such as flat plate solutions to assess defects in a nozzle geometry then allowances for mixed mode and plasticity effects need to be made.


What Next?

• Example discussed was an Inlet/Outlet nozzle (Internal Diameter approx 150mm) of a set through or protruding design. There are other nozzle designs such as set on or flush with significantly different dimensions. Eg Manways with compensating pads.

• Also, nozzles with multiple defects and possibilities of defect interaction.


Example of Multiple Defects in a Nozzle Weld – Defect Interaction?


What Next?

• Example discussed was an Inlet/Outlet nozzle (Internal Diameter approx 150mm) of a set through or protruding design. There are other nozzle designs such as set on or flush with significantly different dimensions. Eg Manways with compensating pads.

• Also, nozzles with multiple defects and possibilities of defect interaction.

• T-stresses (those that act parallel to the crack flank) obtained from FE analysis can be used to modify the Failure Assessment Diagram (FAD) if they can be shown to be compressive. (Can be considered as an increase in the apparent fracture toughness).

• Provide a pointer to existing Guidance, extend and improve it.• NAFEMS - “How to Undertake Fracture Mechanics Analysis“• API 579, Section B.6.4.3• R6 Revision 4, Section III.2


Suggestions For Future FENet Activities

• Benchmarks for verification and validation• Particularly for Defect assessment of pressure vessel nozzles• Guidelines for application of 3D cracked body FE analysis

• Provide guidance to, and liaise with, other Thematic Networks such as FITNET on the use of FE methods in defect assessments.

• Defect Interaction Studies Using FEA• Accuracy, identify potential pitfalls.

Documents

Defect Assessment of a Pressure Vessel Nozzle - NAFEMS · PDF fileFENET THEMATIC NETWORK COMPETITIVE AND SUSTAINABLE GROWTH (GROWTH) PROGRAMME Advantica Technology • A leading provider