Comparison of Predicted and Measured Residual Stresses in a Pipeline Girth Weld

Transcription

1 Industry Sector RTD Thematic Area Date Power & 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 Summary The 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. 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

2 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. 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. 2

3 Axial Stress Profiles in Pipeline Girth Weld as Recommended by BS7910 and R6 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. Normalized Axial Residual Stress 1.2 E/B < 50 J/mm < E/B < 120 J/mm Low Heat Input -0.4 Medium Heat Input E/B > 120 J/mm High Heat Input (z/b) SMAW = shielded metal arc welding Case 1 Assumed As Welded Residual Stresses (Hoop Direction) Weld elements at tensile yield (475MPa) in hoop direction. 3

4 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. Case 1 Post Hydrotest Residual Stresses (Axial) Modified Axial Stress Distribution after application and then removal of hydrotest loading.. Tensile axial stress at inner surface of pipe close to weld has reduced to approx 31MPa. 4

5 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. 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. 5

6 Case 2 Post Hydrotest Residual Stresses (Axial) Modified Axial Stress Distribution after application and then removal of hydrotest loading.. Tensile axial stress at inner surface of pipe close to weld has marginally increased to approx 40MPa. 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 BS

7 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: %C, %Mn, %Si 3m long 600 Pipe A - API X65 36-in OD (914.4mm) 500 Wall thickness: 5/8 and ½ in (15.9mm and 12.7mm) Stress-strain curves were measured for both base metal and weld metal True Stress, N/mm Parent M01-04 Parent M01-05 Parent M True Strain 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) E E E E E (cap) E o

8 Typical Welds 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 8

9 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 Surface Residual Stress Measurements Air abrasive centre hole drilling method. 9

10 Finite Element Analysis (Engineering Mechanics Corporation of Columbus) 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 Welding Process & Parameters Steel & Weld Metal Compositions Weld Heat Flow Model Microstructure & Property Model Thermal History Weldment Microstructure & Mechanical Properties Mechanical Model Experiment Validation Residual Stress Distribution An Integrated Thermal-Mechanical-Metallurgical Weld Stress Model 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 10

11 Temperature Dependent Material Properties Used Yield and flow stress functions of both temperature and microstructure. Other temperature dependent properties included: Elastic Modulus 500 HAZ Poisson s Ratio Specific Heat Conductivity Stress (MPa) Base Metal Temp (K) FE Model Results Predicted Microstructures consistent with those in Actual Weld. Ferrite Fraction Pearlite Fraction Bainite & Acicular Ferrite Fraction 11

12 FE Model Results Predicted Hardness consistent with those in Actual Weld Weld Metal Base Metal Hardness (VHZ) X 50 O X (mm) FE Model Results As Welded Residual Stresses Axial Stresses Max value 392MPa Hoop Stresses 12

13 FE Model Results Residual Stresses After Hydrotest Axial Stresses Max value 196MPa Hoop Stresses Comparison of Predicted Through Wall & Experimental Surface Residual Stresses As Welded Results Inner Surface Max Axial Stress Point Outer Surface B Location 2 Location 1 Weld Centerline z Normalised Axial Stress FE simulation - Weld Centreline FE simulation - Location 1 FE simulation - Location 2 BS7910 High Heat Input Distribution Inner Surface Measurements Outer Surface Measurements z/b 13

14 Comparison of Predicted Through Wall & Experimental Surface Residual Stresses Results Following Hydrotest Normalised axial stress FE simulation - Weld Centreline FE simulation - 7.2mm from Weld Centreline -0.6 FE simulation mm from Weld Centreline BS7910 High Heat Input Distribution -0.8 Inner Surface Measurements Outer Surface Measurements z/b 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? 14

15 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. 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, 15

16 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. 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