Customized Ultrasonic Systems for Gas Pipeline Girth Weld...

International Pipeline Conference — Volume IIASME 1998

CUSTOMIZED ULTRASONIC SYSTEMS FOR GAS PIPEUNE GIRTH WELD INSPECTIONS

Michael D. C. MolesR/D Tech

Mississauga, Ontario, Canada

Noël DubéR/D Tech

Quebec City, Quebec, Canada

Ed A. GinzelMaterials Research Institute Waterloo, Ontario, Canada

AbstractGas pipeline ultrasonic inspection girth weld

systems have been well established for typical Canadian conditions, primarily through the efforts of TCPL and NOVA. However, the specific weld profiles, wall thickness, environment and operating conditions found in Canada do not necessarily represent girth weld inspections elsewhere. This paper describes some different inspection conditions, and some of the possible solutions, which may or may not have been implemented to date. For example, the extreme Canadian weather may require heating transducer wedges, or using dummy wedges to measure temperature. Sandy or salty conditions may require different designs and materials. Different weld profiles will require different ultrasonic set-ups, as will different inspection specifications. For flexibility, it is possible to design expandable inspection heads with replaceable probe modules, and to include extra

ultrasonic transducers for TOFD or additional scans. Tight time constraints can lead to quick' release levers for speedy head changes, as well as modular, replaceable components. Since the operator is critical in the inspection, a number of software additions can be added to speed inspection, interpretation and set'Up. For example, defect locations can be automatically displayed on the weld profile to aid interpretation. A table of defects can be shown so the operator can click on a selected defect to automatically display the appropriate scan region. Defects can be analyzed automatically by length and amplitude to determine accept/reject. Calibration set-up for a particular transducer can be recalled simply by clicking on the appropriate zone in the weld display. Automated recording and acceptance of calibration scans can be performed.

IPC1998-2074

Copyright © 1998 by ASME

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CURRENT MECHANIZED GIRTH WELD ULTRASONIC INSPECTIONS

Ultrasonic “Zone Discrimination"Gas pipelines have traditionally been inspected by radiography, but this technique has limitations. Over the last two decades, Canadian pipeline companies have developed an ultrasonic inspection technique with excellent results. The inspection approach which has evolved uses “zone discrimination” (see Fig. 1), whereby the weld is divided into several distinct “zones” (TCPL 1996). Each zone, typically 1-3 mm thick in Canadian pipes, is interrogated by a single focused transducer. Multiple transducers in pulse-echo mode are used to cover the different zones, plus extra scans as required. The transducers are mounted in a large probe pan in contact mode, with pumped water as couplant. The probe pan is mounted on a mechanized welding track for speed and convenience. The inspection head travels round the pipe on the welding track, with no axial scanning movement, to minimize inspection time. The complete scan cycle, including mounting, removal, moving and data analysis, can be performed in four minutes for a 36” pipe (de Raad et al. 1997).

Figure 1 Schematic of typical weld zones and defects (from TCPL 1996)

For simple zone discrimination, half a dozen transducers on each side of the weld centre line are required to scan a 10-mm thick weld. In practice, some zones of the weld cannot reasonably be inspected in pulse- echo, so a two-transducer tandem technique is required. Time-Of-Flight Diffraction and other scans are sometimes specified, requiring more transducers. Lasdy, welds require inspection from both sides, so the number of transducers is effectively doubled. Overall, up to twenty transducers are required to inspect a 10-mm thick mechanized weld with a bevel configuration as shown in Figure 1 using zone discrimination.

CALIBRATION\l

Figure 2 Calibration block containing flat- bottom holes and other reference reflectors (ASTM 1997)

On-shore, calibration is normally performed every ten welds or every two hours, depending on production rates. The calibration piece is mounted on a holder on the side or back of the truck. Only limited variations between calibration amplitudes from scan to scan are permitted. Also, to make the zone discrimination approach functional, only limited ultrasonic “beam overlay” from one zone to another is permitted. In this way, signals observed in one zone only can be safely assumed to be no deeper than that particular zone. This allows accurate sizing, which in turn permits larger defects using ECA calculations.

The Probe PanThe probe pan uses spring-loaded contact transducers on wear-resistant shoes or pins, mounted symmetrically on the upstream and


downstream sides of the weld (see Figure 3). Typically, the transducers are mounted in modules with facing zone pairs. Modules can be removed and replaced with just a few screws. Besides convenience, this permits coupling checks using pitch-catch. The pan is usually made of aluminum for low weight, and is connected to a standard mechanized welding drive apparatus. The ultrasonic instrumentation can be mounted on the pan, or in the truck with cabling. There is a considerable amount of cabling possible: ultrasonic connectors, one per transducer; water coupling lines; heating lines if required; temperature monitoring, if required; lifting cables or pneumatics, if desired. A probe pan typically weighs about 20 kg, and can be lifted on and off rapidly by one operator. Quick-release handles are included, for speed. Lifting the transducers permits easier probe positioning during setups and allows a lower coefficient of friction to permit a rapid return of the head without damaging the probes.

Figure 3 Sample mechanized ultrasonic probe pan

Output DisplayLastly, and perhaps most important, is operator interpretation. The scanning process, including mounting and removing the probe pan, takes a couple of minutes. To

keep the cycle down to four minutes per weld, the operator must be able to rapidly interpret the results. Experience has shown that a series of strip charts with signal amplitudes is essential, but insufficient. Each strip chart also includes the time of the signal (see Calibration above), displayed in colour for easy interpretation (see Fig. 4). In this way, operators can readily determine which signals come from what zone(s), and where in the zone(s). The standard display also shows B-scans (stacked A-scans, or waveforms) of the root and cap regions to identify porosity. If specified, a TOFD B- scan display is presented as well. The display shows coupling checks as a “go- no go” series of strip charts on one side, as well as the circumferential position of the probe pan.

Figure 4: Output display from calibration block

Advantages and Limitations of Mechanized UTIn summary, a combination of ultrasonic zone discrimination, precision positioning, and rapid data analysis permits reliable and repeatable inspection of girth welds using mechanized ultrasonics. The advantages of ultrasonic inspection over radiography are: no radiation hazard; rapid inspection allowing some process control; more accurate defect sizing, permitting larger defects in practice; hard copy output; faster inspection and construction cycles. The disadvantages of mechanized ultrasonic inspections include heavy and bulky probe


pans; limited flexibility in changing pipe dimensions; cost; precision set-up; sensitivity to temperature; and the lack of a suitable code. Both the heavy probe pans and limited flexibility are being addressed by phased arrays; for the code, a draft ASTM code is under review (ASTM 1997).CUSTOMIZING FOR SPECIFIC APPLICATIONSIn practice, most users want some

modifications to the “Canadian standard”, either by necessity or by personal choice. These preferences can be categorized as: Weather and Environment; Weld Profiles; Regulatory; and Output Display requirements.

Weather and EnvironmentCanadian girth weld inspection systems need to operate during the winter months, with temperatures as low as -40°C. To achieve this target, the couplant is changed to a methanol-water mix. The transducers, or more specifically the wedges, need to be kept within +. 10°C of the pipe temperature (TCPL 1996). To satisfy this criterion, the pipe can be inspected as soon as it cools to the appropriate temperature, and/or the wedges can be heated by using a propane heater on the calibration block, or by individual electrical heaters. As an aid, one transducer slot can be filled with a dummy wedge containing a thermocouple for temperature monitoring. Temperature can be displayed on the CRT, like a coupling check.Most gas pipelines are built in warmer climates than Canada. Mechanized ultrasonic systems have already been used in desert environments, where high temperatures are a potential problem. As long as the wedge remains within jilO°C of the calibration temperature, the scans are acceptable. It may be necessary to cool the pipe to achieve the correct control, especially for off-shore applications.Another potential problem with desert

environments is the presence of sand. Mechanized girth weld scanners have been used on the Magreb pipeline with few

problems. The CRC Evans drive mechanism for the probe pan couples directly to the welding band using a friction drive, which is effectively sand-proof. The other components, including the transducer wedges, have proved effectively sand-proof. For tropical, humid climates, temperature differences would be minimal, though corrosion could present problems.For offshore inspections, metal temperatures are typically higher, but can be controlled by cooling. However, the environment is corrosive due to salt, and care must be taken to ensure that only salt-resistant materials are used, particularly for components that move, e.g. transducer springs and mountings. Besides selecting appropriate materials, it would be necessary to minimize dissimilar metal combinations. Another major feature of offshore inspections is the tighter time frame, typically two minutes per weld versus the four or more for on-shore applications. Such inspections will require more mechanical fixturing to ensure rapid placement and removal of the probe pan. Normal on-shore calibration procedures may be inadequate for offshore and some other applications. Offshore, it is not practical to inspect ten welds before calibration, because rescanning these welds due to calibration failure would be a production disaster. A system for calibrating between every weld inspection is required and may use automatic calibration and data analysis.Weld ProfilesThe weld profile shown in Figure 1 is a commonly used bevel in mechanized welding and is well suited to the “zone discrimination” approach, though other weld profiles, such as J-shaped welds, can be inspected using mechanized ultrasonics. Each different weld profile will require defining a new set of ultrasonic zones and defect orientations, plus manufacture of the appropriate calibration block with suitable FBH reflectors. Then the ultrasonics must be manually set up to pass the acceptance procedures.


Mechanized ultrasonic has been successfully used to inspect manual welds (Heckhauser et al, 1995, Ginzel et al. 1997). There are inherently greater variations in weld profile and finished product with manual welding, though in principle the zone discrimination approach works.Regulatory and Selected Scan RequirementsThere are a number of special inspection requirements that might arise, either due to specific regulations, or due to a preference for specific inspection conditions or scans. For example, Time-Of-Flight Diffraction (TOFD) is commonly used as an additional inspection technique, both for accurate sizing and for detecting poorly oriented reflectors (Verkooijen 1995).Another possible requirement is detection of transverse defects, using transducer pairs in pitch-catch oriented at an angle to the weld. A second transverse pair can be mounted facing the opposite direction, if required. Using replaceable transducer holders, it is quite possible to insert and remove specific scans, depending on the circumstances. Transverse defects are very rare in the mechanized welds in Canada, so Canadian specifications do not call for these inspections. Other countries do require the capability to detect transverse defects. Creeping waves, used to detect and characterize surface-breaking cracks, can also be added.To date, none of the girth weld inspections has specified the use of a normal beam transducer, which has potential advantages for characterizing defects, particularly porosity. Due to the rough weld profile, it would be necessary to switch from a contact transducer to some form of bubbler or water jet. This normal beam transducer would be mounted in a removable module with a skirt to minimize water loss. Depending on the requirements, a B-scan could be displayed on the screen to identify porosity. Interface triggering might be required to compensate for variations in weld cap height.

In principle, the use of easily replaceable transducer modules should permit essentially any scan, though pulse-echo, tandem, pitch- catch, transverse, creeping waves and TOFD cover most normal possibilities.Output DisplayAs mentioned above, the output display is a key feature of the mechanized ultrasonics girth weld inspections (see Fig. 4). The operator must be able to rapidly detect, size and characterize any signal that occurs. While the strip chart/B-scan display now “standard” for Canadian inspections has worked well, there are other factors for many scans.First, if additional (or reduced) scans are specified, then the output must be enlarged (or shrunk). The current output display with multiple strip charts, B-scans for root, cap and TOFD, plus couplant checks, possibly temperature, and circumferential distance effectively fill a 21” monitor screen. The addition of extra channels might require removal of some other information.Second, different operators may prefer different screen lay-outs and color schemes. Besides personal preference, some colors are more suitable for specific applications than others. For example, if the operator intends to fax some of the hardcopy output, then black and white is preferable to color. The optimum color scheme for the CRT may print poorly, so a different palette may be better for printed output. TOFD B-scans are best viewed in black and white, while above- threshold signals are best viewed in a different color from the surrounding grass- level signals. As the transducers are physically at different positions around the circumference, the various signals are inherently offset circumferentially on the scan. This offset can be compensated for on the display.Third, for different weld profiles and wall thickness, the operator needs flexibility to accommodate varying numbers of transducers and conditions. Some form of set-up wizard might be appropriate, though


this issue has not arisen much yet, probably due to the hard fixturing required for the probe pan, with inherent loss of flexibility. Fourth, in addition to the “mandatory” displays above, it is possible to display defects in a user-friendlier manner for analysis. When a recordable signal is detected, its position can be automatically displayed on the weld profile and on a cross- section to show both depth and circumferential extent. A list of above- threshold signals can be displayed: when the operator clicks on one of these recorded signals, the screen automatically jumps to show that part of the output display containing this defect. In principle, it is possible to analyze defects using the ASME code on-line, to automatically determine which defects are acceptable and which

rejectable, based on the standard ASME criteria of length, depth and proximity.SUMMARYMechanized ultrasonic inspection of gas pipeline girth welds is well established in Canada, and several other countries around the world. Several unique features have been developed, such as zone discrimination, specialized calibration blocks, unique output displays. However, other users in other countries will have different specifications and needs, due to different weather, environment, weld profiles, regulations and scan requirements. Furthermore, many users will have preferred output displays and other specific display needs. This paper outlines what customization is currently or soon available to satisfy different conditions and regulations.


REFERENCESASTM 1997, Draft Standard

“Standard Practice for Mechanized Ultrasonic Examination of Girth Welds”, ASTM Committee Draft No. 1, April 1997.

de Raad, J. A., and Dijkstra, F. H., August 1997, “Mechanized Ultrasonic Testing on Girth Welds During Pipeline Construction”, Materials Evaluation.

den Boer Ginzel, E. P., and Hoff, M., July 1997, “Application of Mechanized Ultrasonic Inspection to Manually Welded Pipeline Girth Welds”, On-Line Ultrasonic.

httx>://www.ultrasonic.de/article/wsho0597/ginzel3/ginzel3.htm.

Hechhauser, H., and Schultz, S., June 1995, “Advanced Technology in Automated Ultrasonic Weld Inspection of Pipeline Girth Welds”, Insight, Vol. 37, No. 6.

TCPL 1996, TransCanada Pipelines Ltd., “Welder Inspector’s Manual 1996, Published by TransCanada Pipelines, Calgary, Alberta, Canada.

Verkooijen, J., June 1995, “TOFD Used to Replace Radiography”, Insight Vol. 37, No. 6.


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