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

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).

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.

Sectorial (Azimuthal) Scans

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 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, with a sizing error of + 0.3 mm. 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 are normal, with up to 50 mm 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 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

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.

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 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, instead of the 4-6 mm 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, 100 mm, 120 mm and infinite (the bottom image is the standard unfocused flat probe).

Small diameter pipes

Clad pipe

Short Cutback

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.