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FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Industry Sector RTD Thematic Area DatePower & Pressure Systems Durability and Life Extension Feb-03
Comparison of Predicted and Measured Residual Stresses in a Pipeline Girth Weld
Keith Wright - Structural Integrity Assessments Ltd, Melbourne, Derbyshire, United Kingdom&
Vinod Chauhan – Advantica Technologies Ltd, Loughborough, Leicestershire, United Kingdom
SummaryThe use of FEA in the prediction of pipeline girth weld residual stresses and a comparison with experimental measurements is described. The effects of hydrotesting on the weld residual stresses are also considered. A summary of some possible future workshop activities for the Durability and Life Extension technology areas of FENet are presented.
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Acknowledgement and Reference of Published Work:
• “Pipeline Girth Weld Residual Stresses and the Effects of Hydrotesting.“• Vinod Chauhan, Advantica Technologies Ltd, UK.• Zhilli Feng, Engineering Mechanics Corporation of Columbus, USA.
• ASME 4th International Pipeline Conference, October 2002, Calgary, Canada.
• Reference: Proceedings of IPC’02 - 27140
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Advantica
• A premier provider of advanced technology and systems solutions that help high performance energy and water delivery companies world-wide improve their operating performance.
• Origins in “British Gas“ in the UK and in “Stoner Associates“ in the US.
• Proven track record of over 30 years experience servicing more than 550 clients in over 50 countries.
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Weld Residual Stresses
• Pipeline girth welds are not Post Weld Heat Treated.
• Hence weld residual stress is an important factor in Fitness-For-Purpose Assessments.
• Many Codes (BS7910, R6 Revision 4, API RP579) recommend residual stress profiles in weld region that could be overly conservative.
• Evidence that welding residual stresses are reduced following hydrotesting.
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Axial Stress Profiles in Pipeline Girth Weld as Recommended by BS7910 and R6
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0 0.2 0.4 0.6 0.8 1
(z/B)
Nor
mal
ized
Axi
al R
esid
ual S
tress
Low Heat InputMedium Heat InputHigh Heat Input
50 < E/B < 120 J/mm2
E/B < 50 J/mm2
E/B > 120 J/mm2
• Residual stress is normalized to yield or 0.2% proof strength of weld metal.
• Derived from upper bound data – not necessarily self equilibrating.
• For girth weld made with manual SMAW process, codes suggest using the high heat input profile.
SMAW = shielded metal arc welding
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Case 1 – Assumed As Welded Residual Stresses (Hoop Direction)
• Weld elements at tensile yield (475MPa) in hoop direction.
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Case 1 - Assumed As Welded Residual Stresses (Axial Direction)
• Axial Stress Distribution at Equilibrium with Weld elements at tensile yield (475MPa) in hoop direction.
• Tensile axial stress at inner surface of pipe close to weld of approx 49MPa.
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Case 1 – Post Hydrotest Residual Stresses (Axial)
• Modified Axial Stress Distribution after application and then removal of hydrotestloading..
• Tensile axial stress at inner surface of pipe close to weld has reduced to approx 31MPa.
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Case 2 – Assumed As Welded Residual Stresses (Hoop Direction)
• Weld elements have a through thickness variation of hoop stress from tensile yield (475MPa) at outer surface to 100MPa tensile at inner surface.
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Case 2 - Assumed As Welded Residual Stresses (Axial Direction)
• Axial Stress Distribution at Equilibrium with Case 2 through thickness variation of hoop stress.
• Tensile axial stress at inner surface of pipe close to weld of approx 38MPa.
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Case 2 – Post Hydrotest Residual Stresses (Axial)
• Modified Axial Stress Distribution after application and then removal of hydrotestloading..
• Tensile axial stress at inner surface of pipe close to weld has marginally increased to approx 40MPa.
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Confidence in the Simplified Approach?
• NONE !!
• Experimental data required.
• Fortunately an experimental programme had been initiated.
Objectives of Work Programme:
• Conduct detailed experimental and finite element analysis to determine the residual stress fields in the vicinity of gas transmission pipeline girth welds.
• Determine the effects of hydrotesting on pipeline girth weld residual stress fields.
• Compare the results to those recommended in BS7910.
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Experimental Programme – (Advantica)
• Materials and pipe geometry selected are representative of most common pipeline within UK gas transmission systems.
• API 5L grades X60 and X65 pipes: 0.13-0.2%C, 1.3-1.6%Mn, 0.31-0.33%Si• 3m long• 36-in OD (914.4mm)• Wall thickness: 5/8 and ½ in(15.9mm and 12.7mm)
0
100
200
300
400
500
600
0 0.005 0.01 0.015 0.02 0.025
True Strain
True
Str
ess,
N/m
m2
Parent M01-04Parent M01-05Parent M01-06
Pipe A - API X65
• Stress-strain curves were measured for both base metal and weld metal
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Welding Details
• Manual SMAW girth weld. Six passes, 60 deg V-groove
• AWS E6061 electrode for root and second pass, and E8010 for other 4 passes.
• Two welders simultaneous vertical down progression.
Pass No.
Travel Speed
(cm/min)
Interpass
Temp. (ºC)
Electrode
Size (mm)
Electrode
Type AWS
Heat Input
(kJ/mm) 1 26 50 4 E6010 0.84
2 35 60 4 E6010 0.76 3 24 80 5 E8010 1.26
4 12 50 5 E8010 2.16
5 13 70 5 E8010 1.86 6 (cap) 13 90 5 E8010 1.59
30o
1.5+1 0
+5 0
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Typical Welds
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Full Scale Hydrotesting
• 6m long girth welded pipes with two end-caps • Hydro-test pressures
• High pressure case: 120 bar on two ½-in thick X60 pipes– correspond to the 105% SMYS requirement (UK pipeline design code
IGE/TD/1) (SMYS = Specified Minimum Yield Stress)• Low pressure case: 105 bar on one 5/8-in thick X65 pipe
– simulate above-ground installation (AGI) pipework (IGE/TD/13)
Girth Weld
6 meters
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Surface Residual Stress Measurements
• Air abrasive centre hole drilling method• Hole was about 2-mm diameter and depth• Estimated measurement accuracy:
– About 8% if stress is below 65% of SMYS– About 16% otherwise, due to plastic deformation in hole drilling
• Measurements both before and after hydro-test
• Measurements on both inside and outside surfaces • Initially, at one circumferential position at weld centreline and HAZ• Subsequent measurements on weld centreline at every 45 deg position
around the circumference, covering weld start and stop locations
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Surface Residual Stress Measurements
Air abrasivecentre hole drilling method.
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Finite Element Analysis – (Engineering Mechanics Corporation of Columbus)
Weld Heat Flow Model
MechanicalModel
Experiment Validation
Microstructure & Property Model
Welding Process & Parameters
ThermalHistory
Residual Stress Distribution
Weldment Microstructure & Mechanical Properties
Steel & Weld Metal Compositions
Weld Heat Flow Model
MechanicalModel
Experiment Validation
Microstructure & Property Model
Welding Process & Parameters
ThermalHistory
Residual Stress Distribution
Weldment Microstructure & Mechanical Properties
Steel & Weld Metal Compositions
• Sequentially coupled approach• Weld heat flow• Microstructure, mechanical
property• Stress
• ABAQUS, enhanced with set of proprietary user subroutines developed specifically for microstructure and welding computations
An Integrated Thermal-Mechanical-Metallurgical Weld Stress Model
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Finite Element Analysis – Axisymmetric Mesh
• Applicable for girth weld with the exception of the weld start/stop positions• Half model for perfectly aligned pipes, Full model for misaligned pipes• Four noded linear isoparametric quadrilateral element• Very fine mesh (element length about 0.1mm) in the weld and HAZ region for
microstructure analysis
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Temperature Dependent Material Properties Used• Yield and flow stress functions of both temperature and microstructure.
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100
200
300
400
500
600
700
200 400 600 800 1000 1200 1400 1600
Temp (K)
Stre
ss (M
Pa)
Base Metal
HAZ
Other temperature dependent properties included:
• Elastic Modulus
• Poisson’s Ratio
• Specific Heat
• Conductivity
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
FE Model Results – Predicted Microstructures consistent with those in Actual Weld.
Ferrite Fraction
Pearlite Fraction
Bainite & Acicular Ferrite Fraction
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
FE Model Results – Predicted Hardness consistent with those in Actual Weld.
200
250
0 5 10 15 20X (mm)
Weld MetalBase Metal
0
50
100
150
Har
dnes
s (V
HZ)
O
X
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
FE Model Results – As Welded Residual Stresses
Axial Stresses
• Max value 392MPa
Hoop Stresses
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
FE Model Results – Residual Stresses After Hydrotest
Axial Stresses
• Max value 196MPa
Hoop Stresses
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Comparison of Predicted Through Wall & Experimental Surface Residual Stresses
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z/B
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mal
ised
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al S
tres
s
FE simulation - Weld Centreline FE simulation - Location 1 FE simulation - Location 2 BS7910 High Heat Input Distribution Inner Surface Measurements Outer Surface Measurements
Weld Centerline
Location 1
Location 2
Outer Surface
z
B
ace
Max Axial Stress Point
As Welded Results
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Comparison of Predicted Through Wall & Experimental Surface Residual Stresses
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tres
s
FE simulation - Weld Centreline FE simulation - 7.2mm from Weld Centreline FE simulation - 13.5mm from Weld Centreline BS7910 High Heat Input Distribution Inner Surface Measurements Outer Surface Measurements
Results Following Hydrotest
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Some Observations
• The objectives of the experimental programme of work were achieved in that it did demonstrate that hydrotesting does significantly reduce girth weld residual stresses.
• The through wall axial residual stress distribution recommended by BS7910 was shown to be conservative at and near to the inner surface of the weld. At the outer surface of the weld this is not the case, for both as welded and after hydrotesting.
• The supporting analytical work using a sequentially coupled Thermal-Mechanical-Metallurgical Weld Stress Model was able to predict reasonably well the as-welded microstructure and the mechanical properties (Hardness) observed in actual welds. Hence the predicted residual streses from this Model would appear to have greater credibility than the simplified modelling approach described at the outset.
• But the experimentally measured residual stresses exhibit significant variability at both inner and outer surfaces. Is it real or measurement error?
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Observations Continued
• The scatter in the as welded measured axial stresses at the inner surface ranged from high tensile (244MPa) to compressive (-136MPa). The high tensile value was measured at the weld start/stop positions. The axisymmetric FE simulation is unable to predict stresses at the weld start/stop positions. If the weld start/stop locations are not considered then the comparison of the measured and FE stresses is much better.
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
So What About the Residual Stresses in the Practical Case?
• Confidence in the simplified approach simulation model was LOW as subsequently borne out by the experimental measurements.
• In light of the more advanced Thermal-Mechanical-Metallurgical simulation model would reliance be placed on predicted residual stresses now ?
• UNLIKELY – unless it was used with some form of safety factor.
• Therefore, an upper bound to the experimentally measured axial residual stress was used instead in the Fitness-For-Purpose assessments,
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
What Next ?
• It would be nice to have:-
• Simulation model extended to 3D and attempt to reproduce weld start/stops.
• Confidence limits associated with the through wall residual stress profiles. Hence a stress distribution to an appropriate confidence level could be used in defect assessments. Possibly by using Monte Carlo Simulation techniques with the Thermal-Mechanical-Metallurgical Weld Stress model.
FENET THEMATIC NETWORKCOMPETITIVE AND SUSTAINABLE GROWTH(GROWTH) PROGRAMME
Suggestions For Future FENet Activities
• Benchmarks for verification and validation
• Provide guidance on accuracy of simulations, identify potential pitfalls eg the simplified approach described.
Reference of Published Work:
• “Pipeline Girth Weld Residual Stresses and the Effects of Hydrotesting“, Vinod Chauhan & Zhili Feng.
• ASME 4th International Pipeline Conference, October 2002, Calgary, Canada, Proceedings of IPC‘’02 – 27140.