|NDT.net - March 2000, Vol. 5 No. 03|
|TABLE OF CONTENTS|
Today we seem to take for granted the speed and accuracy associated with mechanised UT inspections. One of the main beneficiaries of mechanisation enhancements in ultrasonic testing is the pipeline industry. It has taken the best part of half a century but the traditional radiographic inspection of pipeline girth welds has now managed to be replaced, in many locations, by mechanised UT.
This article will attempt to identify the highlights of the progression of events that led from an idea to a major industry.
Fig 1: RTD Three Probe Rotoscan (single channel unit illustrated. circa 1959)
Simple applications of ultrasonic testing in pipe mills on long or helical seams was about the extent of pipeline UT during the 1960's. During the 1970's there again seemed to be an increased interest in a faster radiation-free option to inspect the girth welds.
One of the early efforts in the 1970's came out of Japan. This was described in a paper by M. Nakayama, Y. Kato, and E. Isono entitled "Investigation of the improvement of speed and reliability in the inspection of field welded pipelines". This paper was presented in the 1974 Quality Control and Non-Destructive Testing in Welding, International Conference, London, England. The authors worked in the Nippon Steel Product Research and Development Laboratories, Sagamihara City.
The system they described was a prototype with two probes and was calibrated on 3.2mm diameter through holes (similar to the calibration technique for the submerged arc long seam welds). Scanning could be done from 100mm/min. to 1000mm/min and coupling checks could be made using the opposing probe configuration which used a pair of 5MHz 10x 10mm 70° probes. Recorded outputs were made on a polar graph plot that indicated amplitude with angular position around the circumference.
Nakayama et al noted that detection of flaws was closely related to test sensitivity but if sensitivity was too great the echoes from the weld bead geometry lowered the signal-to-noise ratio.
|Fig 2: Polar Plot from the Nippon Steel Prototype system|
On the left is a polar-graph made if the eight 3.2mm diameter through holes used for the calibration. On the right is a weld scan showing ; "slag" (1 o'clock), "crack" " (3 o'clock), " lack of fusion (7 o'clock).
Around this time, work was being done to mechanise the welding process. Some early efforts had been made during the 1940's using an oxy-acetylene heated upset-butt welding process. In 1958 Esso Research and Engineering Company funded a project at Battelle in Columbus Ohio, USA. This used a then new process called Gas Metal Arc Welding (GMAW) with a CO2 cover gas. Problems with the process were mostly due to fit-up as it had used the standard 60° included bevel angle.
In the mid 1960's Crutcher-Rolf-Cummings merged with M.J. Crose Manufacturing and operated as CRC-Crose International until sometime in the 70's when they became Crutcher Resources Corp. In the mid 60s Jerry Nelson (who had been the engineer working on the Battelle project) approached CRC with an idea for a new and improved system. He purposed to make a system that would be composed of a bevel facing machine, an internal line-up clamp/welding machine to deposit the root pass and then external welders run on bands clamped to the pipe OD. This was the foundation of the CRC Automatic Welding System and it remains essentially unchanged today.
The CRC welding system had some growing pains. Welding parameters were then poorly understood, the operators were not welders and needed to have extensive training and like most new technology, it was not readily accepted in the industry. Finally some trial runs were made and welding parameters were improved and the operators became proficient. In about 1983 CRC Pipeline International broke away from Crutcher Resources Corp and became a privately held operation. In 1985, CRC Pipeline International merged with Evans Pipeline Equipment to form CRC-Evans Pipeline International
Mechanised GMAW welding is now the preferred method of producing large diameter pipeline girth welds. CRC Automatic Welding was one of the first systems developed but now others are also proving effective. Vermaat from the Netherlands, SERIMER from France, and RMS from Canada, are but a few of the systems used today. Some use internal clamp/welding machines while others use just the internal clamp with a copper backing-ring. When well tuned, all produce a narrow uniform cap and a smooth, nearly flat root profile.
Fig 3: Tony Richardson with the First Multiprobe scanner mounted on a CRC Welding Band
Fig 4: Tony Richardson's Scanning Head using 4 immersion probes (1972)
This system was ahead of its time. Attempts to get TransCanada Pipelines Ltd. interested failed as they saw no application for it in 1972. It was sent to the Vetco European office and had an evaluation by British Gas but nothing further developed with Vetco with that specific design.
However, in the same year NOVA (then Alberta Gas Trunk Ltd.) began development of the GMAW process using the CRC Automatic Welding system and by 1977 decided that UT was the best option for the 37.5° root bevel and 45° hot pass bevel angle. Not being aware of the Canadian development already established 5 years previous using the CRC equipment, NOVA opted to have RTD develop a system based on the CRC Automatic Welding apparatus. RTD adapted their existing yoke version Rotoscan used on offshore applications and put it on a CRC welding band. This land-based version was dubbed the Bandscan to differentiate it from the offshore version that was called Rotoscan. Soon the name Bandscan was dropped and reverted to Rotoscan.
In the late 1970's there were several other companies developing mechanised systems but when demonstrating their abilities on the girth welds, all wanted to show how much information could be collected and duplicate manual style scanning . To do this they used a raster scan. Scanning with a raster movement can provide useful information and the relationship of signal-to-position (echo-dynamics) using raster motion could be helpful in sorting out defects from geometries; but the scan is exceedingly slow. When demonstrated on typical 42" (1067mm) diameter welds the scanning time was on the order of 10-15minutes and interpretations would require even more time. The linear scan showed the best hope for production rates typically achieved by welding (80-100 welds per day at that time).
The Rotoscan used standard contact probes (just as the Nippon Steel system had) but used several probes on each side trying to optimise for the weld bevel (just as the Vetco scanner had). With the combined interest of a potential pipeline user (NOVA) and the manufacturer/service company (RTD) the development moved faster. However, standard contact probes and simple amplitude gating still resulted in many false calls. It became apparent that probe design had to be reconsidered.
Although the principles were well documented, the previous uses for this technology had not demanded the sort of precision required by this application.
Improved techniques for the relatively thin pipeline girth weld applications were possible only with focused probes. The first of these applied to the girth weld inspections used internal lensing and was developed by BAM in Germany. Later, others experimented with shaped elements. By the mid 1980's spot sizes were consistently held to around 2-4mm diameter at the area of interest and false calls that were caused by edges of the beam interacting with weld surface geometries were virtually eliminated.
This improved signal-to-noise ratio allowed a new philosophy to be considered; Engineering Critical Assessment (ECA). The ECA concept uses the principles of fracture mechanics to assess the severity of a defect based on its vertical extent. The small spot sizes now achieved allowed the weld to be divided into several zones. This linking of ultrasonic results to fracture mechanics was probably the single most important aspect in the development of mechanised UT on girth welds.
Although the concept of dividing the weld into zones was popularised in the pipeline industry by Glover et al[Mechanised Ultrasonic Inspection of Pipeline Girth Welds, part 4, Glover, A.G., Fingerhut, M.P., Dorling,D.V., Dept. of Supply & Services DSS File # 23SQ-23340-2-9027-4, Nov. 1988]. in the late 1980's, the concept was used in a much earlier report (work done in 1981/82) by Moles and Allen[Tandem Probe Ultrasonic Measurement of Cracks in Economizer Inlet Header Sections, Moles. M.D.C., Allen, A.L., Materials Evaluation, May, 1984] not only utilised the concept of zones to size flaws but also used a computer display to indicate the zones and also used the tandem pitch-catch arrangement of elements later "re-developed" by Canmet in the study reported by Glover as the recommended option for the inspection of the 5° vertical bevel of the GMAW weld. The application of the tandem probe arrangement has long been popular in Europe for heavy walled vessel inspections. Krautkramer references A. de Sterke[Some aspects of radiography and ultrasonic testing of welds in steel with thicknesses from 100-300mm, de Sterke, A., Br. Journal of NDT, No. 9, 1967] (of RTD) regarding this technique who described the practice which had been used in Europe for some time by then.
|Fig 7: SGS Scanner|
|Fig 8: SGS Chart Display|
|Fig 9: AIB Scanner|
|Fig 10: RD Tech Chart Display used by AIB circa 1992|
|Fig 11: WeldSonix scan head|
|Fig 12: RD Tech Display 1997|
Now the speed of development was accelerated even more. RTD developed a DOS based computerised system and developed a "mapping" display in 1993 to improve the discrimination between flaws and surface geometry and it had the serendipitous advantage of characterising porosity. "Mapping" was just the modern equivalent of the B-scan. They collected stacked A-scans and assigned colours to the various signal amplitudes thereby making porosity "LOOK" like porosity on the computer monitor.
In the early 1990's parallel development was being done by two other companies. SGS Gottfeld in Germany had designed its MIPA system and RD Tech in Canada had developed its initial system subsequently used by AIB Vinçotte in Belgium.
The SGS Gottfeld system seemed to be a flashback to the Vetco system of the 1970's but now modernised with computer technology. It too used immersion probes and a skirt to hold the water. The monitor display consisted of a series of bands for each channel with amplitude represented as a colour (a single line C-scan). Figures 5 and 6 are courtesy of Rolf Diederichs and www.NDT.net and can found at http://www.ndt.net/article/schulz/schulz.htm
Fig 5: Original RTD Strip-chart format printed on RMS recorder and light sensitive paper
Fig 6: SPSL Computerised Strip-chart format printed on laser printer
The RD Tech System used a probe array with contact probes similar to that used in Canadian projects but their system also had a single line colour per channel C-scan display as the hard copy.
Both the RD Tech and SGS systems were used on a project in North Africa but failed to get accepted in Canada. By then the ease with which the RTD stripchart format using time and amplitude information combined on the same chart could be read by the operators made it the preferred presentation method.
An interesting side note; in 1992 a Canadian company, Canspec, used a prototype version of the AIB system and experimented with the B-scan display available in the RD Tech software package. They found that root geometry and porosity was more easily identified using the B-scan information. This was over a year before RTD officially came out with their Roto-map which was designed for the same purpose (another example of parallel thinking). In 1996 WeldSonix introduced their system. This system came up with a smaller scanning head and full waveform data collection for all channels but also used the "stripchart" format.
In 1997 RD Tech decided to conform to the Data Presentation initiated by RTD (nearly 20 years ago) and developed a newer version of both hardware and software based on their Tomoscan technology. In 1998/99 RD Tech used this display for the data collected by the new Phased Array system they had developed.
Much of the acceptance of systems by the pipeline industry has been dictated by presentation. QA 9000 Ltd. introduced their Acuscan in the mid 1990's. Mechanically this looked very similar to the WeldSonix scanner but the report presentation had reverted to the "Top-Side-End View" presentation making it difficult to use with the ECA analyses now common with the other systems.
|Fig 13: Acuscan Scanner||Fig 14: Acuscan Display|
I would like to extend thanks to the following:
Anthony Richardson, of Inspectech Analgas Group, Scarborough, Ontario, Canada, Tel. (416) 757-1179, Fax: (416) 757-8096, E-mail: firstname.lastname@example.org. Tony provided me with his original photos and the US patent that he obtained for the system he put together using the immersion probes.
Robert van Agthoven, RTD Rotterdam, The Netherlands Tel.: +31 10 208 8262, Fax +31 10 415 8022 E-mail: email@example.com. Robert has provided me with the image of the earliest "Rotoscan" and commented extensively on its early development years B.C. (Before Computers).
Rolf Diederichs for his on-line collection of technical papers relating to the girth weld inspections, www.NDT.net
RD Tech for providing copies of their early brochures. RD Tech, Québec, Canada Tel. (418) 872-1155 Fax (418) 872-5431, www.RD-Tech.com
Henk van Dijk in WeldSonix International, Inc. for his open and frank discussions on ways to improve the technology. He has done a much for the industry in transferring his early experience with equipment in the filed as a technician to improve designs for the convenience of the operator. Web site www.weldsonix.com
Blaine Mitchell at CRC-Evans for providing information on the early developments of the GMAW process.
Jan A. de Raad, RTD Rotterdam, The Netherlands Tel.: +31 10 208 8262, Fax +31 10 415 8022 E-mail: firstname.lastname@example.org. Jan provided extensive proof reading, corrections and opinions on several aspects of the details of RTD's role in this history.
|© NDT.net - email@example.com|||Top||