Applications

Notes d’application

Inspecting pipeline girth welds, both for onshore and for offshore use

Ultrasonic phased arrays present major improvements over conventional multiprobe ultrasonics for inspecting pipeline girth welds, both for onshore and for offshore use. Probe pans are lighter and smaller, permitting less cutback; scans are quicker due to the smaller probe pan; phased arrays are considerably more flexible for changes in pipe dimensions or weld profiles, and for different scan patterns. More important, some of the potential advantages of phased arrays are now becoming commercially available. These include:
• Seamless pipe inspections;
• Premium inspections on thick section risers;
• 1.5 D arrays for improved sizing in risers-tendons;
• Focusing for thin-walled, large diameter pipes;
• Small diameter pipes;
• Clad pipes;
• Short cutback;
• Double jointing;
• Portable phased arrays.

The paper will introduce the concept of phased arrays, a standard phased array inspection system, and then describe the latest phased array UT results for special applications.

Introduction

Pipelines are typically constructed by joining sections of pipe together, using either manual or automated welding. Since pipelines operate at a high percentage of yield strength, these welds must be constructed and inspected to a high standard. In recent years, Engineering Critical Assessment (ECA -also called Fracture Mechanics, Fitness-For-Service or Structural Integrity) has been used to evaluate defects because ECA is considerably less conservative than traditional "workmanship" criteria. ECA is advantageous as it can reduce the reject rate significantly (1).
Compared with radiography, automated ultrasonic testing (AUT) offers the further advantage of process control, as welds can be inspected soon after completion, and feedback given rapidly to the welding crew. Overall, AUT can save construction costs by process control and the use of ECA to minimize the reject rate, often below 1% (2).
Ultrasonic phased arrays present major improvements over conventional multiprobe ultrasonics for inspecting pipeline girth welds, both for onshore and for offshore use. Probe pans are lighter and smaller, permitting less cutback; scans are quicker due to the smaller probe pan; phased arrays are considerably more flexible for changes in pipe dimensions or weld profiles, and for different scan patterns. More important, some of the potential advantages of phased arrays are now commercially available. These applications include:
•Seamless pipe inspections;
•Premium inspections on thick section risers;
•1.5 D arrays for improved sizing in risers-tendons;
•Focusing for thin-walled large diameter pipes;
•Small diameter pipes;
•Clad pipes;
•Short cutback;
•Double jointing;
•Portable phased arrays.

Ultrasonic Phased Arrays for Pipeline Girth Welds:
Phased arrays are fully described in reference (3).
Phased arrays use an array of elements, all individually wired, pulsed and time-shifted. These elements are typically pulsed in groups of ~16 elements at a time for pipeline welds. In order to make a user-friendly system, a typical set-up calculates the time-delays from operator-input, or uses a pre-defined file calculated for the inspection angle, focal distance, scan pattern etc (see Figure 1). The time delay values are back calculated using time-of-flight from the focal spot, and the scan assembled from individual "Focal Laws". Time delay circuits must be accurate to around 2 nanoseconds to provide the accuracy required. Due to the limited market, complexity, software requirements and manufacturing problems, industrial uses have been limited until the last few years (4).

From a practical viewpoint, ultrasonic phased arrays are merely a method of generating and receiving ultrasound. Consequently, many of the details of ultrasonic inspection remain unchanged; for example, if 7.5 MHz is the optimum inspection frequency with conventional ultrasonics, then phased arrays would typically use the same frequency, focal length, and incident angle.

Figure 1: Schematic showing generation of linear and sectorial scans using phased arrays.

While it can be time-consuming to prepare the first set-up, the information is recorded in a file and only takes seconds to re-load. Also, modifying a prepared set-up is quick in comparison with physically adjusting conventional transducers.
Using electronic pulsing and receiving provides significant opportunities for a variety of scan patterns.

Electronic Scans
Multiplexing along an array produces electronic scans (see Figure 2). Typical arrays have up to 128 elements, pulsed in groups of 8 to 16. Electronic scanning permits rapid coverage with a tight focal spot. If the array is flat and linear, then the scan pattern is a simple B-scan. If the array is curved, then the scan pattern will be curved. Linear scans are straightforward to program. For example, a phased array can be readily programmed to inspect a weld using both 45o and 60o shear waves, which mimic conventional manual inspections or automated raster scans.

Figure 2: Schematic illustration of electronic scanning.

Sectorial (Azimuthal) Scans

Sectorial scans use a fixed set of elements, but alter the time delays to sweep the beam through a series of angles (see Figure 3). Again, this is a straightforward scan to program. Applications for sectorial scanning typically involve a stationary array, sweeping across a relatively inaccessible component like a turbine blade root (5), to map out the features (and defects). Depending primarily on the array frequency and element spacing, the sweep angles can vary from + 20° up to + 80°.

Figure 3: Schematic showing sectorial scanning used on turbine rotor.

Combined Scans
Phased arrays permit combining electronic scanning, sectorial scanning and precision focusing to give a practical combination of displays. Optimum angles can be selected for welds and other components, while electronic scanning permits fast and functional inspections. For zone discrimination scans of pipeline welds, specific angliven welies, as shown in Figure 4.

Figure 4: Schematic showing zones on a CRC-Evans weld profile, and ultrasonic beamsfrom a phased array probe.

Pipeline AUT Inspections:
Pipeline AUT uses fully automated equipment travelling round the pipe on a welding band in a linear scan, with the array pulsing to cover all the weld zones as in Figure 4 above. Besides linear scanning, there are four specific features of pipeline AUT:
•zone discrimination,
•special calibration blocks,
•dual gate output display and
•rapid defect sizing. These features are described in detail elsewhere (1).

The special output display uses multiple strip charts, with colour-coded detection; the dual gate display shows both signal amplitude and time-in-the-gate for defect location in the weld; the calibration blocks use an angled notch or side-drilled hole to represent lack of fusion, one reflector for each zone. Rapid defect sizing is performed by counting the number of zones where above-threshold signals occur. These features are defined by ASTM E-1961-98 (6). R/D Tech has a commercial phased array system, PipeWIZARD (6), which can also meet or exceed any of the other codes -API 1104, DNV OS101, ISO 13847 (8-10).

Advanced Inspections:
One significant feature of phased arrays is their ability to perform "specials"; some active examples are shown below.
Compensating for variations in seamless pipe wall thickness
Offshore seamless pipe has significant variations in pipe wall, up to 10-15%. For a 20 mm (.787 in.) wall, these variations are sufficient for the zone discrimination beams to completely miss their targets. One phased array solution is to run multiple setups, typically the nominal, minimum and maximum walls (see Figure 5); the minimum and maximum set-ups can be performed electronically, based on a nominal calibration. The operator selects which "view" to watch based on wall thickness measurements (11).

Figure 5: Ray tracing showing beams for nominal, minimum and maximum wall thicknesses, and errors.

Premium inspections for risers, tendons and other components
Risers and tendons are nominally built to much higher quality than standard pipelines or other welds. For example, acceptable defect sizes on 35 mm walls may be only 0.3 mm (.011 in.), with a sizing error of + 0.3 mm (.011 in.). Phased arrays work better on such applications since they can use multiple beams at multiple angles to guarantee better coverage and defect detection.
Risers and tendons also tend to be thick-walled; thicknesses of 35-40 mm (1.377-1.74 in.) are normal, with up to 50 mm (1.968 in.) possible. This is another advantage for phased arrays, since PipeWIZARD can run an additional eight conventional transducers. This permits detailed inspections with highly focused transducers up to 50+ mm (1.968+ in.) walls.
Figure 6 shows a schematic ray tracing, showing enhanced coverage of the root, cap and volumetric areas using an increased number of beams and angles. In this application, the phased array system used 84 beams (not all shown), which would have been impractical with a multiprobe system.

Figure 6: Ray tracing showing partial coverage of premium weld.

1.5D Arrays for Improved Sizing in Risers and Tendons

Defect sizing requirements are becoming more stringent as new high strength pipes and new applications are being introduced, particularly to thick-walled risers and tendons offshore. A curved 1.5D matrix array was developed, primarily for improving focusing in the horizontal direction. Initially, the focusing was modeled for a variety of arrays, including the standard flat array, and for curved arrays. The optimum array turned out to be 120 elements long, with three rows, plus a slight curvature for initial focusing. Detailed focusing can be performed using the electronic capability of phased arrays. The results from this array show significantly improved horizontal focusing, with oversizing down from a few mm to ~1 mm (0.039 in.). Figure 7 shows an example of the modeling results, and Figure 8 a comparison of the focusing results.

Figure 7: PASS modeled beam profiles for focused and unfocused arrays. Standard flat array is at right, showing significant side lobes. Selected array is left center.

Fig. 8: Beam spread comparison for HP2. Target: 3 mm or .011 in., FBH. (Left) Scrolling views and B-scan for 1.5D array (length=3.25 mm or 127 in.). (Right) Strip chart for 1D array (length=5 mm or 0.196 in.).

Improved Focusing for Thin-walled Pipes
With the arrival of high strength pipe steels, improved defect sizing is becoming more critical. This is because repairs become more detrimental to the pipe material properties, and Engineering Critical Assessment requires better sizing as the pipes become thinner. Improved focusing is particularly relevant in the horizontal (circumferential) direction, to both minimize repairs and to more accurately characterize intermittent defects. Extensive computer modeling was performed to determine the optimum array for thin-walled pipes. Mechanical focusing (i.e. a curved array) was modeled with various curvatures and compared with an unfocused array. The modeling showed that a 60 element, 1 mm pitch array with a 100 mm (3.937 in.) curvature gave significant improvements over a standard array.
The experimental results showed that the curved 1D array had significantly better focusing than the standard (unfocused) array, as expected. The curved array oversized flat-bottomed hole reflectors by only ~ 1 mm (0.039 in.), instead of the 4-6 mm (0.157 in.) from the standard array. These curved arrays can be used on standard systems with no modifications to the general mechanics or software. Figure 9 shows a sample modeling result, with the flat array at the bottom.

Figure 9: Fill 1 channel. The red vertical cursors represent the theoretical focal positions. The probe is at the left. Curvatures (top to bottom) = 80 mm (3.149 in.), 100 mm (3.937 in.), 120 mm (4.724 in.) and infinite (the bottom image is the standard unfocused flat probe).

Small diameter pipes

Small diameter pipes are difficult to inspect well using conventional ultrasonics since there is a limit to the number of transducers that can be placed on the pipe. Phased arrays can generate an almost unlimited number of beams to provide coverage at different angles, locations and rastering. Figure 10 shows a small diameter pipe scanner which can be added to PipeWIZARD or operated independently. This scanner requires four rings to cover diameters from 60 mm (2.362 in.) to 400 mm (2.362 in.).

Clad pipe

Clad pipe is becoming more common for corrosion resistance. Normally cladding is austenitic stainless steel or nickel alloy-based. Both materials can be very difficult for conventional shear wave ultrasonics; large austenitic grains skew and attenuate shear wave beams. Longitudinal waves (L-waves) are significantly less affected, so standard practice in the nuclear industry (which uses a lot of austenitic piping) is to perform L-wave inspections. L-waves can be easily generated by phased arrays and PipeWIZARD, but the standard zone discrimination approach (6) will not work since it is not practical to bounce beams off the inside of the clad pipe. Developments are on-going in this area. Figure 11 shows an L-wave scan of a pipe, showing notch and notch tip using an S-scan.

Short Cutback

The current 60-element probe configuration requires a 100 mm (3.937 in.) cutback; however, in some cases, only shorter space is available, e.g. for risers with nearby flanges. In this situation, 32-element probes allow for an available space of only 50mm on one side. Some limitations may apply for heavy wall thickness. Figure 12 shows the approach.

This approach can adapt to complex weld profiles, restricted geometry and specific requirements.

Double Jointing
Figure 13 shows a typical double jointing operation. Using phased array AUT has the following advantages:
• No radiation hazard, or chemicals to dispose of.
• No work disruption
• Rapid feedback
• Rapid data interpretation
• Excellent defect detection capability
• Phased arrays offer custom scans for critical areas, e.g. root and cap
• Better process control reduces rejects and saves money.

Portable phased arrays for tie-ins and repairs
While large AUT phased array systems can be used for both tie-ins and repairs, economics and practical considerations favour smaller, portable systems. An encoded array, calibration block and appropriate set-up are sufficient for a rapid linear scan; C-scan and B-scan displays are generated in real-time. The OmniScan system (12) in Figure 14 can perform both electronic and S-scans; the resultant scan patterns are closer to ASME-type raster scans than to ASTM E1961 zone discrimination, but are suitable and acceptable for tie-ins and repairs.

Conclusions

1. Ultrasonic phased arrays offer considerable technical advantages over conventional multiprobe AUT systems, or radiography.
2. Using phased array AUT, operators can simply load a file to provide rapid scanning.
3. For pipelines, phased arrays offer some specific applications that are difficult or impossible for multiprobe systems to match:
a. Compensating for variations in seamless pipe wall thickness.
b. Premium inspections for risers, tendons and other components.
c. Improved focusing for both thin-and thick-walled pipe.
d. Small diameter pipes.
e. Clad and austenitic pipe.
f. Short cutback.
g. Double jointing.
h. Portable phased arrays for tie-ins and repairs.

References

1. Moles M.D.C, N. Dubé and M. Russell, "Ultrasonic Phased Arrays for Pipeline Girth Weld Inspections", 3rd International Pipeline Technology Conference, Brugges, Belgium, May 21-24, 2000.
2. Connelly T., "Update on the Alliance Pipeline", International Conference on Advances in Welding Technology, October 26-28, 1999, Galveston, Texas, sponsored by EWI and AWS.
3. R/D Tech, "Introduction to Phased Array Ultrasonic Technology Applications - R/D Tech Guideline", published by R/D Tech Inc., August 2004.
4. Lafontaine G. and F. Cancre, "Potential of Ultrasonic Phased Arrays for Faster, Better and Cheaper Inspections", NDT.net, vol. 5, no. 10, October 2000
5. Ciorau P., D. MacGillivray, T. Hazelton, L.Gilham, D. Craig and J.Poguet, "In-situ examination of ABB l-0 blade roots and rotor steeple of low-pressure steam turbine, using phased array technology", 15th World Conference on NDT, Rome, Italy, October 11-15, 2000.
6. ASTM, American Society for Testing and Materials, ASTM E-1961-98, "Standard Practice for Mechanized Ultrasonic Examination of Girth Welds Using Zonal Discrimination with Focused Search Units", September 1998.
7. Moles M.D.C., N. Dubé and E.A. Ginzel, "Pipeline Girth Weld Inspections using Ultrasonic Phased Arrays", International Pipeline Conference, Calgary, Alberta, Canada, Sept 29-Oct 3, 2002, ASME Paper number IPC02-27393.
8. API, American Petroleum Institute Standard 1104, "Welding of Pipelines and Related Facilities", Nineteenth Edition, September 1999.
9. DNV OS-F101, "Submarine Pipeline Systems, Appendix D, Non-Destructive Testing (NDT)", January 2000.
10. ISO 13847, "Petroleum and natural gas industries - Pipeline transportation systems - Welding of pipelines", First edition, 2000-09-15, Reference number ISO 13847:2000(E), © ISO 2000.
11. Moles M.D.C, Stewart D., Gray M., Godinot H., and Romazzotti H., "Inspecting Seamless Pipe Welds of Variable Wall Thickness using Ultrasonic Phased Arrays", 4th Pipeline Technology Conference, Oostende, Belgium, May 913, 2004.
12. R/D Tech, OmniScan, January 2004.

Products used for this application

OmniScan MX PA

L'OmniScan PA effectue des inspections multiéléments manuelles et automatisées. Il offre toutes les fonctionnalités d'affichage du A-scan, du B-scan, du S-scan et du C-scan, ainsi que le traitement de données en temps réel de pointe. Configurations d'éléments de 16:128 de base et possibilités de 16:16 M, M 16:64, 32:32 et 32:128.

OmniScan MX2NEW

L'appareil de recherche de défauts par ultrasons multiéléments OmniScan MX2 avec écran tactile permet des inspections de grande efficacité. Il inclut des fonctionnalités logicielles puissantes qui permettent une performance d'inspection manuelle et d'inspection automatisée supérieure et des configurations, des cycles d'essai et une production de rapports rapides. Il est compatible avec tous les modules multiéléments existants.

PipeWIZARD

Le PipeWIZARD est un système automatisé de contrôle des soudures circonférentielles par ultrasons multiéléments et ultrasons conventionnels (AUT). Spécialement conçu pour l'inspection soudure à soudure sur site dans des environnements extrêmes, sur terre et en mer.