RETOUR D EXPERIENCE SUR LE CONTROLE ULTRASONORE MULTIELEMENT (PAUT) DE SOUDURES HOMOGENES/ INHOMOGENES EN ACIER AUSTENITIQUES / AUSTENO- FERRITIQUES.

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1 JOURNEES COFREND 2017 RETOUR D EXPERIENCE SUR LE CONTROLE ULTRASONORE MULTIELEMENT (PAUT) DE SOUDURES HOMOGENES/ INHOMOGENES EN ACIER AUSTENITIQUES / AUSTENO- FERRITIQUES. S.Demonte (Institut de Soudure Industrie-90 rue des Vanesses Villepinte s.demonte@isgroupe.com) La technique des ultrasons multiéléments (PAUT) a été mise en œuvre pour le contrôle de soudures austénitiques, austéno-ferritiques, homogène ou inhomogène, lors de la fabrication de tuyauteries et d appareils à pression pour un important projet pétrolier et gazier. Plus de 350 soudures d atelier ou de montage ont été contrôlées. En considérant les difficultés inhérentes au contrôle ultrasonore appliqué aux soudures non ferritiques, les exigences des codes, et parfois les restrictions associées à la configuration des composants, le développement des procédures de contrôle et les examens sur sites requièrent davantage de précautions en comparaison de cas «standard», tels que le contrôle ultrasonore de soudures bout à bout en acier carbone. Cet article décrit le retour d expérience relié à trois cas associés à trois types de soudures et stratégies de contrôles. Certains éléments des procédures, les avantages et limitations de la technique en regard d autres méthodes ou techniques, ou lorsque l examen encodé est comparé à l examen manuel ainsi que des résultats obtenus sur des soudures de production, en termes d acceptation, de rejet ou de productivité, sont présentés. FEEDBACK ON PHASED ARRAY ULTRASONIC TESTING (PAUT) EXAMINATIONS OF HOMOGENEOUS / INHOMOGENEOUS AUSTENITIC, AUSTENO-FERRITIC STEEL WELDS. Phased Array Ultrasonic Technique (PAUT) has been implemented for the testing of austenitic, austeno-ferritic, homogenous or inhomogeneous welds, during the fabrication of piping and pressure vessels for a major Oil &Gas project. More than 350 shop or erection welds have been tested. Considering the inherent difficulties of the ultrasonic method applying to non-ferritic welds, the code requirements, and sometimes the restrictions associated to the components configurations, the testing procedure development and the on-site examinations require much more precautions compared to standard cases, such as carbon steel butt welds ultrasonic testing. This paper describes the feedback related to three cases associated to three types of welds and testing strategies. Some elements of the procedures, the advantages and limitations of the technique regarding other methods or techniques, or when comparing encoded to manual testing as well as some results obtained on production welds in terms of acceptance, rejection or productivity are presented. 1

2 GLOSSARY, LIST OF ABBREVIATIONS: o NDT/NDE: Non Destructive Testing/Non Destructive Examination. o RT : Radiography Testing o MUT: Manual Ultrasonic Testing Standard volumetric NDT using probes generating ultrasonic beams with fixed features (angle, exit point, beam size, wave mode). Probes are manually moved. o PAUT : Phased Array Ultrasonic Testing More advanced NDT compared to MUT where ultrasonic beams features are electronically controlled (angle, beam size, exit points, wave mode). Probes are manually moved (MPAUT) or using a scanner: (semi)-encoded PAUT, data may be recorded. o Manual raster scanning: basic probe displacement commonly used with MUT or MPAUT. o Linear scanning at fixed distance: the probe is moved at a fixed distance from the weld axis. Commonly used with MPAUT o MUT TRL or 2D matrix Array (DMA PAUT probe): Compared to standard MUT or PAUT probes, elements working in transmission and reception separately, with specific focusing features, giving advantages for coarse grain material testing. o SDH: Side Drilled Hole. Standard reference reflector, used for the MUT or PAUT instrument setting and calibration. SDH sizes are defined by codes. o Shear Waves/compression waves: type of mechanical vibrations defined by the way the particles oscillate. Shear wave mode is usually preferred in UT testing, as it offers better resolution and is a more flexible way to interrogate a weld due to its capability to rebound on component surfaces. o Creeping Waves: Waves traveling parallel and close to the surface, aiming at detecting surface defects. o DAC: Distance Amplitude Correction. It provides a means of establishing a graphic reference level sensitivity as a function of UT sound path in material. o TCG: Extracted from DAC. Signal amplitude of reflectors is electronically amplified according to their sound path location, in order to reach a fixed value. Shear wave mode is usually preferred in UT testing, as it offers better resolution and is a more flexible way to interrogate a weld due to its capability to rebound on component surfaces. o CS: Carbon Steel. SS: Stainless Steel. DSS: Duplex Stainless Steel. o SNR: Signal to Noise Ratio. Parameter associated to the comparison of a discontinuity indication (relevant) and the noise level (structural and or electronic, non-relevant) Figure 1: Raster scanning pattern Figure 2: Shear/compression waves oscillations 2

3 INTRODUCTION PAUT is commonly used since more than 15 years for the weld testing of carbon steel components. It is a volumetric NDT which can be implemented in lieu of RT, MUT, or TOFD. The physical ultrasonic phenomena involved, especially those related to defect/ultrasonic beam interaction are considered as almost identical to conventional ultrasonic technique. Consequently, the PAUT strategies follow the same fundamental rules as those of MUT. PAUT is usually carried out, and known, in automated (or semi-automated) mode: probes are fixed to encoding devices: for every probe position (considering a given increment) a raw ultrasonic signal is collected and recorded during the scanning process, therefore traceability of the examination is obtained. PAUT can also be performed in manual mode: the process may be equivalent to MUT (raster scanning), or similar to encoded scan: probe is moved manually at a fixed distance from the weld. Data may be recorded but without the accurate information of the probe position along the weld length. In this case, full traceability is lost. When employing encoded PAUT, the resulting data set is displayed through various types of views that are interpreted for the detection, sizing and acceptance assessment of defect s indications according to codes and standards. This is easily achieved when testing carbon steel butt welds of simple geometry, with sufficient access from both sides of the welds. The type of ultrasonic wave mode (shear waves) combined to the carbon steel or low alloyed steel metallurgic structure allow obtaining beams/defect interactions that can be interpreted without too much difficulties, therefore clear and accurate assessment of the defect can be achieved. Difficulty increases depending on weld geometry, scanning restrictions due to component configuration, on site constraints such as surface condition, actual weld preparation, interferences or obstructions with accessories, etc MUT or PAUT applied to austenitic or austeno-ferritic welds, or more generally to coarse grain materials, is of a much higher difficulty : the metallurgical structure of the weld itself, sometimes of the base material, and at the base material/welded volume interface does not usually allow using ultrasonic beams with the same parameters as those employed for carbon steel welds : wave mode (compression instead of or additionally to shear waves), frequency, beam size, acoustic pressure, sensitivity settings, are to be selected and adjusted on a case by case basis, depending on base and filler material, welding procedure, weld thickness and bevel preparation. Combined with the acoustic response associated to the anisotropic metallurgic structure, optimizing defect/beam interactions and interpreting the ultrasonic indications require more precautions and efforts compared to carbon steel weld examinations. This is what is observed when performing trials on mock-up in lab conditions, and on simple geometry welds. 3

4 When PAUT testing is implemented on production components, and considering real on-site constraints, the level of difficulty also dramatically increases and drives to the development of testing strategies that will produce acceptable results, in terms of detection and assessment, but which are far from the ideal encoded PAUT examination which provides clean top/side/end views of a weld where all relevant defects are clearly shown. Three cases are described in this paper. They correspond to the examinations of three types of welded components during an Oil and Gas project construction phase, from 2014 to Examinations were conducted by Institut de Soudure Industrie within regulatory inspection or crosschecking (verification of examinations performed by contractor) scope of work. From the testing of more than 350 welds is extracted a feedback showing what can be reasonably achieved with and what are the actual advantages and limitations of the PAUT technique applied to austenitic, austeno-ferritic, homogeneous or inhomogeneous welds in Oil &Gas projects. 4

5 CASE 1: DISSIMILAR METAL WELDS - CARBON STEEL + STAINLESS STEEL SS316 WELD OVERLAY+ INCONEL 625 Type of component: The welded components are high pressure vessel nozzle butt welds described as follows: - Diameter DN50 and DN75, - Nominal thickness 25mm + SS316 weld overlay. - Material: Base metal carbon Steel DIROS 500HT, filler material Inconel Construction code : PD 5500 Figure 3: Nozzle butt weld configuration. To be noted the significant width of the welded volume, actual production welds up to 45mm wide in the OD area. The NDE requirements, as per code and Client specification, were: - Welds to be tested by 2 volumetric methods : RT+UT, - NDE standards: PD5500, 2012 Edition, refers to EN1714, which is not applicable to austenitic welds and to PAUT. EN ISO and EN ISO were used as guidelines. - Ultrasonic acceptance criteria: provided by PD5500, same as those given for carbon steel welds. Maximum allowable flaws lengths are given, depending on type of indications (planar, threadlike, volumetric ) and their amplitude. It was initially expected that MUT would be implemented by the local NDT sub-contractor. Preliminary trials and investigations on reference blocks: Reference blocks, to be used for the setting and calibration of ultrasonic equipment, are welded blocks of the same configuration as the production weld, with SDH and surface notches. 5

6 Figure 4: Reference blocks DN50, thickness 25mm with 3mm SDH and 2.5mm deep notches. Welding procedure identical to production welds. Weld cap is removed. - MUT preliminary trials : Trials performed by the local subcontractor with his available MUT probes, corresponding to a basic set of probes dedicated to SS weld testing, did not produce any acceptable results. SDH could be detected with a very poor signal to noise ratio, or even not detected at all. To be noted that the performance of these probes was degraded by the small diameter (100mm OD) of the nozzle regarding the probe contact surface. - PAUT preliminary trials : Trials performed using standard linear 1D array probe, of various frequencies, aperture size, and fixed to standard wedges generating compression waves could produce slightly better results but were still not acceptable. Severe noise, beam distortion and deviation could be observed. To be noted also the performance of the 1D linear array, fixed to curved wedges which did not exactly fit to the small diameter (100mm OD) of the nozzle, was also degraded due to the mismatch between the contact surfaces. 6

7 Conclusions: MUT and standard PAUT technique failed to demonstrate testing feasibility of this type weld. MUT customized TRL and creeping waves probes would possibly provide better or even acceptable results, but the uncertainty of results and the delay for provision of customized probes (more than 2 months) was not in line with the project schedule. Also, several probes of various focusing spots, size, frequencies, and possibly nominal angles, would have to be made available for the conduction of trials aiming at selecting the suitable probe set, hence increasing their number (probably more than 12 probes for preliminary investigations only). The main parameters at the origin of the testing difficulty are associated to the metallurgic structure; the small diameter combined the thickness of the component. TRL DMA PAUT preliminary trials: Trials were conducted using one Dual Matrix Array PAUT probe and various wedges shaped to 100mm. A single DMA probe and 2 wedges allow, by focal laws computation, to produce a wide range of angle beams (from 40 to 90 ), and focusing spots. Therefore, it is supposed to replace a large number of MUT TRL/creeping waves probes to be used for preliminary investigations, and, later, for settings optimizations carried out on other mock-ups. Consequently, the process for search unit selection is made significantly faster compared to MUT, when dealing with this type of weld Results: all SDH and surface notches were correctly resolved with acceptable SNR (>18dB), whatever were the accesses (from base metal and weld bead). Figure 5: (left) TRL DMA 2.25 MHz Figure 6: Sectorial images of reference reflectors SDH and (19 wedge) Conclusions: Further to preliminary trials on the reference blocks, it is concluded that the TRL DMA was likely to produce acceptable results, and would be therefore selected for nozzle butt Inconel welds. 7

8 Settings and calibrations: Ultrasonic and focal laws settings (driving the way the ultrasonic beams are generated and received in phased array technique) are not established according to the normal process followed for carbon steel welds testing, where some parameters such as velocity and other calibration parameters are easily measured on standardized calibration blocks. Where it may take a few hours to set and calibrate a PAUT instrument for a carbon steel weld testing, 3 days were dedicated to setting and calibration for this application. As explained later in this paper, settings and calibrations are based on Manual PAUT raster scanning, which drives to a different approach than those associated to encoded PAUT. In this case, to overcome the problems related to various ultrasonic velocities (carbon/inconel) and beam distortion, the settings are adjusted according to iterative approach: several set ups are computed with various velocities and focusing values then checked on the reference block, until satisfying good parameters are identified in order to : - Optimize defect location errors, as the same setup is used when scanning from base metal (CS) and from weld bead (Inconel). - Optimize the reflector response, mainly in term of SNR and echo shape. Based on the implementation of manual raster scanning, therefore that indications may be interrogated with beams traveling through CS, or Inconel, or both, conventional DAC or TCG that are normally employed with MUT and PAUT cannot be established and are not relevant. The following approach was considered: indications reflectivities were compared to SDH response, at similar depth, with similar access and for specific beam angles. Figure 7: Extract of table to be used for indication s amplitude assessment against reference reflectors. 8

9 Testing strategy development and procedure demonstration: Testing strategy development, including «fine tuning» of settings, and finally the procedure performance demonstrations are carried out on one validation block with artificial reflectors simulating planar defects, which may be more difficult to be detected than volumetric reflectors, such as SDH, as their reflectivity depends on the targeting UT beam s angle. Figure 8: Validation block DN50, thickness 25mm, 3 x 10 x0.5 mm (height/length/width) 9

10 Figure 9: PAUT images, S Scans of some of the validation reflectors, including OD/ID connected notches of reference block. All reflectors are correctly detected (above evaluation, notch 14 and root centreline notch being the most difficult to highlight). The indications nature assessment Planar/Volumetric or threadlike is mainly based on variations of reflectivity through beam angles. To ensure detection and assessment of all validation reflectors, the weld cap has to be removed. Conclusion: the testing strategy was validated in double side access configuration. For defect detection and assessment, sufficient clearance shall be available on both sides of the weld. 10

11 Scanning for near OD surface defects detection (creeping waves and high angle beams) Figure 10: Extract of manual PAUT Scan plan 11

12 Testing strategy: why using manual raster scanning- no records- instead of encoded scanning at fixed distance-recordable data? - Due to anisotropy and defect orientation, obtaining the maximum amplitude of defect indication sometimes needs probe tilting (this can however be electronically achieved with dual matrix array) - Actual scanning surface on these components is not perfectly flat; this may generate coupling problems/beam alterations when performing scans at fixed distance. Also, due to the combination of small component diameter and relatively large foot print of the wedges fixed to the probes, coupling variations or loss of coupling are likely to occur. Such variations can be better compensated by manual scanning and the use of viscous coupling paste. - Accurate evaluation of the indication s nature and location needs probe s positioning at specific locations and probe s displacement perpendicularly to the weld axis. Unless many scans at fixed distance are implemented, manual investigations are more accurate and are less time consuming for this type of joint. Also, some indications produced by the structure are observed and may be considered as relevant through scan at fixed distance. Such indications are clearly identified as being non relevant when being manually investigating. - The various components configurations encountered on site (clearance on both sides of the welds) do not usually allow the use of «standards» scanners. Customized and complex scanners would have been to be developed for this application. - Encoded scan is not required by code, as manual PAUT is used in lieu of MUT. Also, traceability was given by RT. Except traceability, what would be the advantages of encoded scans for this application? - Investigations of root defects would have been easier through analysis of encoded data at fixed distance instead of time coded data. - Length sizing would have been more accurate. However, manual length sizing usually produces oversizing therefore is on the safe side. Results of the examinations on production welds 52 joints have been tested on 10 vessels. All joints were judged acceptable by RT. - 5 joints were judged rejectable by PAUT, with several unacceptable indications on some of the joints. - Defects were confirmed during excavation when using pencil grinder (defects may be closed when using normal grinder). Location, size as well as nature were confirmed: planar (inter run and side wall lack of fusions). One root defect was also detected (probable lack of root fusion). These defects were not shown on RT films. - Other joints found with acceptable indications: nature and size within tolerances of PD 5500 acceptance criteria. 12

13 - In terms of productivity, the time for examination of 1 joint with RT was approximately 5 hours shooting time (Ir-30Cu). With PAUT, 1 hour (no indication) to 2 hours (with indications to be assessed) was needed. PAUT results were given immediately after testing, defect location was directly marked on the weld, allowing immediate repair. To be noted that less than 20 minutes would be needed for carbon steel homogeneous butt welds of the same configuration. Figure 11: Extract of a report showing a defect indication. 13

14 CASE 2: DUPLEX WELDS AND DISSIMILAR METAL WELDS - CARBON STEEL + SS316 WELD OVERLAY+ INCONEL 625 TO DUPLEX PIPING Type of component to be examined, NDE requirements. The welded components are high pressure nozzle drain butt welds described as follows: - Diameter DN80 and DN250 for duplex to duplex (s31803) pipe and fitting, thickness 11mm to 25mm, - Diameter DN150: carbon steel boss + Weld overlay SS316 + Inconel 625 filler material to duplex tee or pipe, thickness 14.3 to 22mm. - Same NDE requirements as case 1. The examinations were performed after development and completion of case 1. Figure 12: Example of drain nozzle butt weld locations and configurations. All weld caps to be flushed ground. 14

15 Preliminary trials and investigations on reference blocks: Figure 13: Examples of reference block: CS boss + Inconel + DSS, associated PAUT Sectorial images with DMA and PAUT shear wave probes. - MUT preliminary trials : Trials performed by the local subcontractor using the same probes as those mentioned in case 1, results were slightly better, but still not acceptable. The welds could be partially inspected: areas of the fusion lines, as well as welded volume with one angle only (straight beam testing) could be interrogated. - PAUT preliminary trials, using similar strategy as case 1 : The TRL DMA probe, with specifically contoured wedges, could immediately provide good results. It was observed that the beam distortion was clearly lower compared to CS/Inconel component, when interrogating the Inconel welded volume from the duplex parent metal, obviously due to a lower structural/acoustic impedance mismatch between the 2 materials. On duplex to duplex components (homogenous weld), no obvious beam distortion or structural noise was observed when using the DMA probe. PAUT 1D linear array producing shear waves could also be used for DSS/DSS welds fusion lines and heat affected zones. However, strong attenuation and beam deviations were observed. The shear waves could not be used for the welded volume examination. 15

16 Conclusions: MUT could be implemented but could not ensure 100% weld examination Similar PAUT technique as described in case 1, with a complementary PAUT shear wave examination, was identified as suitable. Settings and calibrations: - For the PAUT DMA probes : - Same approach is used as for case 1 when testing CS/Inconel/DSS welds. - For DSS to DSS welds, standard setting and calibration process could be followed, similarly to carbon steel weld testing. - For the PA 1D probe generating shear waves and used when scanning from DSS parent metal: due to much stronger attenuation and beam deviation, time base settings were requiring to use fake information, differing from the physical one: shear waves velocity and wedges parameters, not corresponding to their real values, were used for focal law computation in order to partially compensate errors in location. The sensitivity calibration was established according the standard process of TCG correction with beam path through the parent metal only. Testing strategy development and procedure demonstration: - Compared to case 1, due to larger OD, thinner weld, probable lower material structure mismatch, lower gain, better SNR, lower beam distortion were obtained and observed on the reference block. - Also, the use of shear waves is an «additional» weld interrogation (fusion lines, including root area) which would improve the detection and evaluation performances. It was therefore considered that the case 1 testing strategy was also validated for case 2. - However, some part of the welds joining fittings (e.g. boss to Tee, intrado, specific area of pipe to reducer ) could not allow full interrogation from both sides using the DMA probe, therefore possibly reducing the probability of detection on specific areas. However, this could be partially balanced by the use of shear wave PAUT probes of a small footprint, interrogating some of these specific areas. These areas were identified and reported with probable limitations. 16

17 Figure14: Extract of scan plans given in the procedure (manual raster scanning) illustrating the testing strategy as well as areas with limited interrogation. 17

18 Testing strategy: why using manual raster scanning- no records- instead of encoded scanning at fixed distance-recordable data? - For the same reasons as those given in case 1. Except traceability, what would have been the advantages of encoded scans, related to this application? - Encoded PAUT would have increase probability of defect detection for root interrogation from single side (e.g. pipe to flange/reducer), which is more delicate compared to case 1 where the root could be interrogated from 2 sides. Elements of comparison between encoded and manual scans are also detailed in case 3. Results of the examinations on production welds - 85 joints on 15 vessels have been tested, all joints judged acceptable by RT. - 7 joints judged rejectable by PAUT, with several rejectable indications on some of the joints. 3 joints were DSS to DSS welds (total 70 joints of this type), 4 joints were CSS/Inconel/DSS (total 15 joints of this type) - Defects were usually associated to inter run and side wall lack of fusion. Two were associated to lack of root fusion. - Defects usually confirmed by MUT, mainly straight beam probe, but sometimes with lower amplitude or with difficulties to resolve them from geometry/noise response. - Other joints were found with acceptable indications (size or nature within tolerances of PD 5500 ultrasonic acceptance criteria). - In terms of productivity, the time for examination of 1 joint with RT was between 2 hours and 5 hours shooting time (Ir-30Cu). With PAUT, 0.5 hour (no indication) to 2 hours (with indications to be assessed) was needed. PAUT results were given immediately after testing, defect location was directly marked on the weld, allowing immediate repair. Figure 15: Extract of a report showing a defect indication. 18

19 CASE 3: DUPLEX AND STAINLESS STEEL PROCESS PIPING WELDS This case was treated after completion of case 1 and case 2. Type of components to be examined: The welded components are process piping welds described as follows: - DSS UNS Piping welds OD 6 to 36 pipe, thickness 15mm to 62mm. Girth, branch, nozzle and sweepolets/flange olet welds. For girth welds, Pipe to pipe, pipe to fitting, fitting to fitting girth welds. - SS 316L piping welds OD 6 to 16, thickness 15mm to 55mm. Pipe to pipe, pipe to fitting, fitting to fitting girth welds. - Construction code: ASME B All weld caps to be flushed ground, whatever is the component s configuration. The NDE requirements, as per code and Client specification, were: - Welds to be tested by 1 volumetric method: RT or UT. MUT for Erection welds and shop welds with thickness 40mm. MUT has been validated through procedure performance demonstration carried out on various mock-ups and was implemented by Contractor for regulatory examination. - NDE standards: ASME V art.4. Also, EN ISO was used as guidelines. - Ultrasonic acceptance criteria provided by ASME B31.3 are based on workmanship: planar defects not allowed, non-planar acceptance depending on amplitude and length. Encoded PAUT, for girth and branch welds, and manual PAUT, for nozzle/sweepolets or for some of the girth welds, were implemented for crosscheck purpose (verification of Contractor s examinations). Discrepancies between PAUT and MUT, if any, were finally judged by MUT investigations of PAUT indications. Preliminary trials and investigations on reference blocks: - Manual UT (carried out by local NDT subcontractors) A wide range of probes (more than 30 different probes) dedicated to SS weld testing, of various crystal sizes, focusing spots and frequencies, being contoured in order to fit the various pipe diameters, was available. All SDH were correctly detected with satisfactory SNR on SS and DSS welded reference blocks. The shear wave testing could not be used on the project DSS material, due to very strong attenuation affecting this wave mode. On SS 316L, they could be used complementarily to compression waves. - PAUT (carried out by Institut de Soudure, for crosschecking purpose) 19

20 PAUT probes/wedges were selected as follows: For DSS welds : 3,5MHz 1D linear Array (large aperture), compression waves, with 2 types of wedges. (and wedges contoured to various pipe OD). 5MHz 1D linear array (small active aperture/footprint), compression waves, MPAUT sweepolet testing for thickness<25mm and pipe diameter equal to or less than 14. DMA probe (with wedges for high angle beams/creeping waves) for branch welds. As for MUT on this particular DSS material, the PAUT probes could not produce acceptable results when generating shear waves. For SS welds : 1,5MHz 1D linear array (very large aperture, allowing beam focusing at a wide depth range), for SS weld thickness >30mm, DMA probe alone for weld thickness 30mm, combined to 1,5MHz for weld thickness >30mm. The 3.5MHz could not be used on SS welds, due to poor quality of the signal induced by the welded structure. On SS (as for MUT), shear waves could be used. However, only compression waves were considered for the PAUT weld interrogation.. Figure 16: Example of reference block, compliant to ASME V art.4, ISO EN and client specifications. Same material/wps as production welds A total 17 reference blocks needed to cover the OD/thickness range of SS and DSS pipe welds. 20

21 Settings and calibration: The time base and sensitivity settings follow a standard process, similar to those employed for CS weld testing with encoded PAUT. As for MUT, the reference reflectors were 2.5mm or 3mm diameter SDH, depending on weld thickness, located along the weld centerline. Therefore, sensitivity between MUT and PAUT was supposed to be the same (regardless of probe frequency or focusing spot) Validation: 7 validation blocks have been fabricated, in order to pronounce MUT validation in accordance with ASME V Article 14, Examination System Qualification Intermediate Rigour Level. i.e. qualifying the procedural document, the personnel and the inspection equipment capabilities. - 2 Girth welds blocks for SS (smallest diameter and thickest weld) - 2 Girth weld blocks for DSS (smallest diameter and thickest weld) - 1 Branch weld, and 2 sweepolet welds (corresponding to the smallest diameter and the thickest weld) The main reflectors were EDM notches with height of 3mm, length 10mm, and width of 0,5mm. 1,5mm side drilled holes along fusion lines and reference notches OD/ID connected could also be considered as validation reflectors. Location and orientation of notches were identified according to possible welding induced defects: e.g. vertical notch above the root to simulate hot cracking on pipe girth welds that could unlikely occur outside of this area. All reflectors were detected and correctly assessed in term of location, length size and nature from one side only, proving the feasibility of the testing for pipe to fitting configuration (single side access, other side with limited or no access at all), both by MUT and PAUT. 21

22 Figure 17: Example of validation block (branch weld, DSS thickness 31mm), and associated PA sectorial images of the validation reflectors. Below, applicable scan plan for this joint configuration 22

23 Testing strategy: - As far as possible, the root should be interrogated from both sides, to facilitate discrimination with geometry response such as root protrusion, counterbore, sharp tapering, high low, etc, and to maximize probability of detection in case of severe misalignment. However, thorough evaluation of specific signal patterns allows identifying root defects from one side only. Sometimes, additional investigations using manual PAUT or MUT may confirm or not relevant/not relevant nature of root indications. - When a weld was tested from one side only, mode conversion signal (LLT, selfroundtrip tandem) were also evaluated in order to detect possible mis-oriented planar defects regarding the UT beam s angles. Generally, 4 scans are sufficient to cover the weld for detection, and preliminary nature evaluation. To be noted that additional scans should be added to ensure complete evaluation of the indications (planar, non-planar), in lieu of Manual PAUT or MUT evaluation. Figure 18: example of scan plan for SS welds, thickness 42mm, case of pipe to fitting configuration. 23

24 Results of the examinations on production welds - More than 150 DSS and 12SS welds examined by semi-encoded PAUT. - In terms of productivity: from 3 to 6 welds / day (scanning only), 6-8 welds in 2 days for 1 team (1 technician and one assistant) interpretation and further MUT/MPAUT investigations. This is to be compared with 2-4 welds in 2 days with MUT for 1 technician. - More than 70 DSS welds crosschecked by manual PAUT (mainly nozzles and sweepolets welds) - More than 90% of the welds cross checked inside the offshore structure-erection welds. Other welds were tested in shop. - Discrepancies between MUT and PAUT on less than 20% approximately of the welds crosschecked. - Discrepancies between MUT and PAUT confirmed on less than 10% of the welds crosschecked. (Other 10% not confirmed by MUT). Figures 19: scanning of DSS and SS welds (above). MPAUT and MUT investigations of encoded PAUT indications. (Below) 24

25 Nature of discrepancies between MUT (contractor) and PAUT (3 rd party) Defects indications found and confirmed by MUT, not by PAUT (not detected or with amplitude < evaluation level): Mainly defects- inter run lack of fusion, parallel to surface- which were found by straight beams (0 ) from the weld bead. Additional PAUT scans should be carried out from the weld bead, interrogating the weld with low angles less than 35. Few mis-oriented defects (planar and not parallel to the weld axis), requiring probe tilting (detected with encoded scans but of very low amplitude). Few defects located in welds areas with surface condition not suitable encoded scan with the large footprint of the PAUT probe, but suitable for MUT. Remark: such type of discrepancies were not observed with manual PAUT Defects indications found-initial MUT assessment-, accepted by PAUT, then confirmed accept by MUT Rejectable weld areas were identified during the first MUT assessment. Scan data show multiple point reflectors without interference (small pores/slags) which could not be judged as single defect. More accurate MUT evaluation/mapping could confirm this evaluation. Root geometry instead of root defect. Too high sensitivity induced by improper setting of the MUT instrument. After MUT recalibration, it was confirmed that some indications were below evaluation level, therefore acceptable. Errors in location -initial MUT assessment-, identified by PAUT, then confirmed by MUT Wrong location along weld transverse axis, or multiple defects not correctly plotted during the first MUT assessment. PAUT location was then confirmed by MUT, therefore complete removal of defects was ensured during the weld repair process. Defects indications found by PAUT, not by initial MUT examination, then, confirmed by MUT. Defects not easily detected through MUT signal display (Ascan), partially hidden by geometry response (root) or other spurious echoes, or multiple point reflectors hiding an elongated rejectable defect. Defects «missed» or not correctly evaluated (amplitude not maximized, mix of acceptable and rejectable defects indications, or uneasy access for the MUT operator that made the MUT scanning process difficult) Defects indications found by PAUT, not confirmed by MUT (therefore classified as accept). Higher indication s amplitude obtained by PAUT. This is probably due to : o Higher PAUT probe s frequency, or location within the focusing spot area, 25

26 o Possible actual defect that do not properly interfere with MUT beams, but with PAUT beam: e.g. 53 PAUT beam produce optimized reflection whereas MUT 45 or 60 beam angles don t. o Also, PAUT beam wave front slightly differing from those of MUT, hence interaction is not identical. This was also sometimes being observed in the opposite direction: MUT probe produced higher amplitude, for the same angle, compared to PAUT. Question: who is right? No excavation could be performed to confirm who, MUT or PAUT, was right Spurious reflections due to coupling noise, structural noise or surface waves at high angles. However, such indications were considered as doubtful during interpretation and would need confirmation before final judgment. Examples of data-comparison Encoded PAUT/MUT Figure 20: PAUT scan of DSS weld, thickness 62mm. Area fully rejected by the MUT technician at initial assessment, considered as a single rejectable indication. PAUT scan data shows multiple point reflectors, except to individual acceptable indications with measurable length-see red circles. Further accurate MUT investigations confirmed the PAUT judgment: acceptable indications, not requiring repair. 26

27 Figure 21: PAUT scan of DSS weld thickness 62mm. Missed defects, confirmed after further MUT investigations and excavation (linear and rounded indications found down to 58mm depth). Figure 22: PAUT scan of SS316 L thickness 42mm with DMA probe. Non relevant indication further to manual investigation, associated to structural noise. The amplitude of the noise was similar, however dynamic MUT echo pattern could demonstrate the non-relevant nature of this indication. 27

28 GENERAL FEEDBACK FROM CASES 1 TO 3: - Similar ultrasonic difficulties as for MUT were encountered: beam distortion, attenuation, poorer resolution, high structural noise, etc. - For testing strategy development, the capability to electronically drive some UT beams features, such as angle or focusing spot, allows quickly identifying the optimized probe parameters, instead of proceeding to the evaluation of a high number of MUT probes. 4 PAUT probes and various wedges were needed, instead of more than 40 MUT probes, to cover the 3 cases. - Skilled operator s specific training (for a given application) required lots of trials on various mock-ups and blocks highly representative of welds to be tested, before starting the examination on production welds. This is also the case for MUT. - The limitations raised during the project of encoded PAUT scans (recordable UT) on SS/DSS/homogenous or inhomogeneous welds were: o Not always achievable (problems of clearance for some components) o Unless a high number of scans are carried out, compared to CS standard PAUT inspections- evaluation, and sometimes detection, of any expected defects frequently required additional manual testing. o For an equivalent probe aperture between MUT and PAUT compression waves probes (in order to generate beams with similar acoustic pressure and diameters), the footprint of PAUT probes is larger than those of MUT. Consequently, there are more sensitive to coupling problem on uneven surface, hence affecting the quality of the signal. This might be partially compensated when implementing MPAUT. o Actual on-site constraints (compared to shop conditions) such as suitability of scanning surface preparation, obstruction between scanner and accessories, etc greatly affect the performance of encoded scans. Especially, actual fit up conditions, for single Vee welds, where counterbore, internal tapering (slopes commonly <1/3 instead of 1/ 4), high lows, produce signals which make the weld roots investigation difficult. However, the difficulty is lower compared to MUT. - Main advantages (regarding MUT) o An overall better probability of detection, combined to an improved evaluation of relevant/non relevant nature of indications, especially in the root area, could be observed. As mentioned above, it may require sometimes further manual PAUT investigations. o For «dirty» but acceptable welds, it s easier, through the advanced PAUT signal display, to «accept» a weld than with MUT. Weld cap removal, in these cases, is considered as mandatory for SS/DSS welds. Also, to increase productivity, efficiency, to increase the probability of detection and to reduce false call rate (unnecessary repairs), sufficient access to probes on both sides of butt welds should be considered at the component design stage. 28