Utmattingsberegninger for stålkonstruksjoner iht gjeldende regelverk og Eurokode 3, del 1-9. Weld improvement and life extension.

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1 Utmattingsberegninger for stålkonstruksjoner iht gjeldende regelverk og Eurokode 3, del 1-9 Weld improvement and life extension 8-9 November 2010 Prof. P J Haagensen NTNU Institutt for konstruksjonsteknikk Trondheim per.haagensen@ntnu.no 1 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Topics S-N curves steel materials Good design practice Improvement during fabrication Post weld improvement methods 1. Geometry improvement 2. Residual stresses metods Improvement methods in design rules Upgrading and life extension projects: 1. Veslefrikk B platform 2. Troll C platform 2 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

2 The fatigue strength of welded joints is independent on steel strength because life consists manily of crack growth. Crack growth rate is independent of steel strength No effect of UTS on fatigue strength of as-welded joints 3 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Strategies for Achieving High Fatigue Strength 1. Good design practice Minimize fatigue loads, e.g. by avoiding resonance & vibration Use low SCF joints Avoid corrosion Place welds in areas of low stress 2. High quality fabrication ( good workmanship ) Correct choice of plate and weld materials Correct choice of welding process Correct weld preparation, e.g. groove geometry, cleaning Correct choice of welding process Highly qualified welders 3. Improvement Applied during fabrication Post fabrication treatment High fatigue strength 4 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

3 Good design practice 1. Keep welds outside highly stressed areas! 2. Reduce stress concentrations 5 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Good design practice Improve stiffener ends Improved detail design and finish gives FAT 90 instead of FAT 63, i.e. a strength increase of 43 %, corresponding to a factor of almost 3 on life 6 6 Weld Utmatting improvement flytende and lifekonstruksjoner extension Oslo, Oslo, april Nov P J P J Haagensen Haagensen

4 Influence of overall weld geomtry Improve shape of weld during welding to reduce local stress (API/AWS method) Works well only when weld toe is burr ground 7 7 Weld Utmatting improvement flytende and lifekonstruksjoner extension Oslo, Oslo, april Nov P J P J Haagensen Haagensen Influence of weld quality Improved welding electrodes with respect to better wetting and flow characteristics have been developed in Japan and USA to give large weld toe radii and low stress concentration factors. Fatigue strength improvements are of the order of 50 to 85 percent for high strength steels. However, problems related to positional welding have limited the use of these electrodes 8 8 Weld Utmatting improvement flytende and lifekonstruksjoner extension Oslo, Oslo, april Nov P J P J Haagensen Haagensen

5 Post weld improvement methods The three most important factors reducing the fatigue strength of welded joint are: Weld geometry the transition of the cross section gives a local stress peak Weld defects undercuts and slag intrusions at the weld toe provides early crack initiation Residual stresses high tensile stresses arising from weld metal shrinkage and from fit up loads reduce fatigue life 9 9 Weld Utmatting improvement flytende and lifekonstruksjoner extension Oslo, Oslo, april Nov P J P J Haagensen Haagensen Two groups of weld improvement methods: 1. Improve weld geometry and remove defects 2. Residual stresses remove tensile stresses and introduce compressive residual stresses Weld Utmatting improvement flytende and life konstruksjoner extension Oslo, Oslo, april Nov P J P J Haagensen Haagensen

6 Weld geometry modification methods BURR GRINDING MACHINING METHODS DISC GRINDING WATER JET GOUGING IIW rec WELD GEOMETRY IMPROVEMENT METHODS REMELTING METHODS SPECIAL WELDING TECHNIQUES TIG DRESSING PLASMA DRESSING LASER DRESSING WELD PROFILE CONTROL (AWS) SPECIAL ELECTRODES 11 Utmatting flytende 11 Weld improvement and life extensionkonstruksjoner Oslo, 8-9 Oslo, Nov april P J Haagensen 2008 P J Haagensen IIW rec 12 Utmatting flytende 12 Weld improvement and life extensionkonstruksjoner Oslo, 8-9 Oslo, Nov april P J Haagensen 2008 P J Haagensen

7 WELD IMPROVEMENT METHODS AND THEIR EFFECT ON FATIGUE STRENGTH Background data: IIW and other publications Guidance: IIW Verification 1: IIW round robin testing program Emphasis on reproducibility and stringent specifications Verification 2: IIW round robin testing program Emphasis on high strength steels, YS 700 to 1100 MPa 13 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Burr grinding Important to grind below surface 14 Utmatting flytende 14 Weld improvement and life extensionkonstruksjoner Oslo, 8-9 Oslo, Nov april P J Haagensen 2008 P J Haagensen

8 Quality control of burr grinding 1. Check depth of groove with special gauge 2. No visible defect in groove Weld toe inadequately ground OK 15 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Increase in fatigue strength from burr grinding steel weldments: Factor 1.3 on stress Benefit of weld toe grinding for steel can be claimed only for details in FAT 90 Class or lower 16 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

9 TIG (GTAW) dressing Positioning of torch important Use standard TIG equipment 17 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Quality control of TIG dressing 1. Shape of weld if it looks good, it is good 2. No visible defects undercut or pores Cleaning and removal of millscal is essential for a good result, to avoid pores and other defects 18 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

10 Q: Do we get larger improvements in fatigue strength for higher strength steel than for lower grades? A: Yes, to some extent Example 1: TIG dressing applied to 350, 590, 700 and 900 MPa YS steels TIG dressing Stress range at N = 2 mill cycles As welded Yield strength 19 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Increase in fatigue strength from TIG dressing steel weldments: Factor 1.3 on stress Benefit of TIG dressing for steel can be claimed only for details in FAT 90 Class or lower 20 Utmatting flytende 20 Weld improvement and life extensionkonstruksjoner Oslo, 8-9 Oslo, Nov april P J Haagensen 2008 P J Haagensen

11 Hammer peening Use standard equipment - e.g. riveting guns OK Unacceptable, with individual indentations visible due to insufficient coverage 21 Utmatting flytende 21 Weld improvement and life extensionkonstruksjoner Oslo, 8-9 Oslo, Nov april P J Haagensen 2008 P J Haagensen Needle peening When large areas - rather than the weld toe alone has to be treated, needle peening is chosen for the improvement. Needle gun with modified mouth piece 22 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

12 Increase in fatigue strength from hammer peening steel weldments Factor 1.5 on stress Benefit of hammer peening for steel can be claimed only for details in FAT 90 Class or lower 23 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Limitations in fatigue strength from hammer peening (1) It is recommended that for structures with plate thickness larger than 25 mm the benefit for hammer peening is assumed to be the same as for grinding and TIG dressing. i.e. a factor of 1.3 on allowable stress range limited to an increase to FAT 100 for steel and FAT 45 for aluminium weldments. It is not recommended to use hammer peening on thin plate structures; t min = 4 mm for steel, 8 mm for aluminium (use needle peening or UIT) 24 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

13 Limitations in fatigue strength from hammer peening (2) Due to the sensitivity of hammer peened welded joints to applied mean stress, the higher S-N curves can only be used under the following circumstances: When the maximum nominal compressive stress in the load spectrum is lower than 0.25YS When the applied stress ratio R < 0, the S-N curve is used in conjunction with the full stress range When the applied stress ratio R > 0 (all stresses in tension) the S-N curve is used in conjunction with the maximum stress instead of the full stress range. 25 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Hammer peening - benefits Hammer and needle peened peened joints: Upgrade S-N curve by 60%, max. IIW FAT 125 with fatigue limit at 2x10 6 cycles. Use full stress range when R 0 Use stress range = max. stress when R > 0 Applied stress ratio R 0 Applied stress ratio R > Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

14 Ultrasonic Impact Treatment UIT Ultrasonic Peening UP - two new high frequency peening techniques 27 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen UIT equipment Generator frequency 27 khz, tool pins: Hz 28 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

15 Typical UIT - results, 700 YS steel Failure away from weld UIT generally gives as high or higher increases in fatigue strength as e.g. hammer peening Approximately. 100% increase in fatigue strength 29 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Combination of methods more effective than single method Example TIG +UIT Combination of methods: Two methods, one from the geometry modification group, followed by a residual stress method tend to give larger improvements than a single method 30 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

16 Ultrasonic Impact Treatment is equally effective for aluminium as for steel 6082-T6 aluminium specimens As-welded UIT 31 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Fatigue test results for UIT improved aluminium weld 6082-T6 aluminium specimens P = plate failure P P Approximately. 100% increase in fatigue strength Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

17 Ultrasonic impact treatment - UIT Advantages As effective as hammer peening As effective as hammer peening Much less noise & vibration, easier to use Disadvantages Proprietary technique - not generally available Expensive equipment Less industrial experience than e.g. hammer peening 33 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen New improvement method: Low Temperature Transformation (LTT) weld metal Introduces compressive residual stress (instead of tensile stress) during welding Principle: Use weld metal with martensitic transformation that takes place at a relatively low temperature, C, compared with ~500 C for conventional electrodes. Restraint from the colder material compresses the weld metal during expansion. 34 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

18 Low Temperature (LTT) Transformation weld metal LTT consumable LTT consumable Low temperature martensitic transformation in weld metal gives volume expansion and large compressive strains 35 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Low Temperature Transformation (LTT) weld metal Improvements in fatigue strength similar to those obtained by peening methods. Results even better after PWHT 36 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

19 Low Temperature Transformation (LTT) weld metal 37 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Improvement methods - recent developments Nickelbearing weld metals used normally for duplex steel welding have shown promise for improving the fatigue strength. Verified in full scale tests on riser pipes (ExxonMobil & NTNU) 38 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

20 Film of test rig operating at 25 Hz Normal speed 39 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Film of test rig operating at 25 Hz (cont d) Slow motion 40 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

21 Improvement methods in design codes IIW Recommendations: New S-N curves (nominal stress and hot spot stress curves) for improved welds Higher design stresses for improved higher strength steel welds IIW new round robin testing program on high strength steels 41 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Improvement methods in design codes: DNV RP-C-203 Methods: Grinding, TIG dressing and hammer peening Application: Upgrading and repair of existing, not in design of new structures Improvement: Grinding, TIG dressing: Factor 1.3 on stress (factor 2.2 on life for m = 3) Peening: Factor 1.5 on stress (factor 3.4 on life for m = 3) 42 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

22 Examples of applications of improvement methods 1. Repair of 14 year old floating platform Veslefrikk B (VFB), Upgrading of new platform Troll C, Weld Utmatting improvement flytende and life konstruksjoner extension Oslo, Oslo, april Nov P J P J Haagensen Haagensen Case study 1 Veslefrikk B Repair & Upgrading Project Platform design and service history Platform was designed for exploration and drilling. Design fatigue life was 20 years, commissioned in Converted to production platform and placed in service in Leaks due to fatigue cracking detected in 1992 in pontoon at gusset plates at corners of columns. Cost of project: Approx NOK 900 mill 44 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

23 Case study 1 Veslefrikk B Repair & Upgrading Project 45 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Case study 1 Veslefrikk B Repair & Upgrading Project Objectives Add more production risers and processing equipment increased weight & loads Repair fatigue damage Extend fatigue life by 20 years Time available - weather window One month, August / September Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

24 Case study 1 Veslefrikk B Repair & Upgrading Project Structural modifications to extend fatigue life Columns/pontoons Use cast steel inserts Horizontal braces Extend longitudinal stiffeners Add new longitudinal stiffeners Enlarge scallops Remove all anode cover plates and interior attachments 47 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Case study 1 Veslefrikk B Repair & Upgrading Project Study of various cast steel inserts to reduce SCF 48 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

25 Welded details requiring upgrading 1. Columns and pontoons Insert-to-plate welds in column-pontoon corners Selected inside fillet welds in columns 2. Brace details Girth welds, inside & outside Ring stiffener fillet welds, at scallops and other highly stressed regions Ends of longitudinal stiffeners Attachments, e.g for cable ladders Anode cover plate welds 49 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Case study 1 Veslefrikk B Repair & Upgrading Project Additional structural modifications Add auxillary columns and sponsons for buoyancy and stability Add lifting devices and processing equipment on deck 50 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

26 Platform design & structural layout Auxilliary column Main column Sponsons Brace Pontoon Starboard Forward 51 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Case study 1 Veslefrikk B Repair & Upgrading Project Horizontal brace structure 52 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

27 Inside view of horizontal brace Scallop 53 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Finite element stress analysis 54 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

28 Stress distribution Castings used here 55 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Case study 1 Veslefrikk B Repair & Upgrading Project Improvement methods Choice of method depends on: 1. Type of detail (e.g. butt or fillet weld) 2. Degree of improvement required 3. Length of weld to be treated 4. Access 5. Work position (downhand, overhead etc.) 56 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

29 Examples of methods used (1) Brace girth welds & plate-to-cast infill welds: 1. Toe grinding by burr grinder 2. Remove excessive overfill by disk grinding 3. Sand blasting 57 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Case study 1 Veslefrikk B Examples of methods used (2) Fillet welds at rat holes: Toe burr grinding Needle peening Repair & Upgrading Project 58 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

30 Case study 1 Veslefrikk B Repair & Upgrading Project Fatigue tests on butt welds, cast inserts to existing plate structure Objectives Verify effectiveness of improvement methods Verify that butt welds perform to Class D S-N curve Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Conclusions VFB repair & upgrading 1. Weld improvement techniques were successfully implemented in a tight time schedule with operators and inspectors who were initially unfamiliar with the procedures. 2. The success of the program could in part be attributed to a comprehensive training course that motivated the participants and increased their knowledge. 3. The fatigue test results indicate that peening and toe burr grinding can increase the fatigue strength of butt welds tested in bending to almost the same level as that of unwelded plates. 60 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

31 How sucessful was the repair & upgrading work? All critical areas of the Veslefrikk B platform were inspected in the summers of 2004 & 2006 No cracks were found in repaired/upgraded areas Many new cracks were detected in regions near the improved welds, where stresses had been judged to be too low to justfy upgrading 61 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Case Study 2, : Troll C upgrading Activities: Needle peening of bellmouths Hammer peening of pipe attachments Major concerns over verification and quality control/assurance 62 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

32 South Korea, winter 1999 Norway, spring Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Upgrading of bellmouths Problem: Inadequate fatigue strength due to increased weight and loads 64 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

33 Bell mouths openings for pulling in pipes The problem: Following refined FE analyses low fatigue lives were predicted for areas of the bell mouths. Hull construction was almost completed. Highly stressed areas 65 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen The solution: Since large areas - rather than the weld toe alone had to be treated, needle peening was chosen for the improvement. Needle gun with modified mouth piece 66 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

34 Inadequate peening of ground area. Re-peened 67 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Acceptable peening Repaired weld defects Re-peened area. 68 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

35 Dubious peening. Re-peened Blue sickle-shaped areas indicate inadequate peening 69 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Score marks from disk grinder, area to be re-peened Extended peening area to cover damage from disk grinder. 70 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

36 Upgrading of fillet welds in pipe supports Problem: Attachments were welded to deck bulb beam to support pipes. Beams were downgraded from highest fatigue class to next lowest, and predicted fatigue lives were too low. Solution: Hammer peening was used since only weld toe needed to be treated. Peening tool 71 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Two types of attachments were used: 1. Flat plate 2. Angle profile Flat plate pipe support Test beams, Type 1 and 2 attachments Weld Utmatting improvement flytende and life konstruksjoner extension Oslo, Oslo, april Nov P J P J Haagensen Haagensen

37 Fatigue test were required to verify improvement Fatigue tests on full scale deck beam Failure away from weld, hammer peened plate attachment 73 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Fatigue test results confirmed that adequate fatigue life was obtained by hammer peening G design, Norsok ~150 % increase 74 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

38 Result Procedures were approved by DNV for a life extension by a factor of 4 Procedures have become informative guidance in Norsok Fabrication Standard M Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen Improvement methods - recent developments Nickelbearing weld metals used normally for duplex steel welding have shown promise for improving the fatigue strength. 76 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen

39 CONCLUSIONS Higher fatigue strength can be obtained for welded structures by: Improved detail design High quality welding Use of castings Use of improvement methods, particularly residual stress methods. The largest improvement is obtained by a combination of a shape modification method and a residual stress method, for example grinding plus peening IIW recommendations give solid basis for use of improvement methods Possible future changes to IIW recommendations include: 1. Higher design stresses for improved high strength steels 2. Guidance titanium alloys 3. Guidance for new methods, e.g. ultrasonic peening, LTT and Ni filler materials 77 Weld improvement and life extension Oslo, 8-9 Nov 2010 P J Haagensen