Automated Ultrasound Testing
Automated ultrasonic testing (AUT) is a non-destructive testing method that combines an ultrasound testing technology, whether conventional or phased-array, with a motorized scanner and an encoder. Even though this term covers a wide variety of applications, it is mostly used in reference to pipeline girth weld inspection because of the early acceptance of this inspection technique as well as its widely spread use.

AUT in the pipeline industry usually makes use of either phased-array or conventional ultrasound in a way to target specific zones of the weld. Phased-array UT is gaining more popularity because it is a lot easier and convenient to use than conventional UT. As a matter of fact, the only way to achieve the same results with conventional UT would be to use a large number of probes which would make it cumbersome to install, and adjusting the beams’ positions would take a significant amount of time due to the impressive quantity of scanner parts necessary to hold each and every probe. And because the use of ultrasound offers no radiation problem, it is a lot easier and safer than radiography.

Most scanners are adaptable for onshore or offshore projects and can be used on a wide variety of weld profiles and wall thicknesses.

Zone Discrimination

The ultrasound array probes are installed on the scanner so that the sit on the pipe on both sides of the weld. Sound beam are emitted at different angles to cover separate areas of the weld. The number of areas depends on the bevel type and on the pipe thickness. For example, a CRC-bevel weld for a pipe thickness of 9.8 mm will be divided into 6 zones:


The ultrasound beams are focused in order to optimize the inspection of each zone. When setting up for inspection, the spreading of a beam to a neighboring zone will be kept between -6 dB and -14 dB, in order to ensure that the zone is well covered. The refracted angles of the beams are selected according to the angle of the fusion face. The expected defect along the fusion line of a weld, made with automated equipment, is usually lack of fusion. To obtain the highest probability of detection (POD), the beams should arrive at the expected defect at an angle of 90 degrees. This is accomplished by using a mix of pulse-echo and pitch catch configurations.

 

The detection of other typical defects is done by focusing other beams in the area where they usually occur. These defects include:
- Lack of penetration
- Cracks
- Inclusion
- Porosity
- Burn through

For indications appearing in the volume of the weld, the use of time of flight diffraction (TODF) is often added to the phased-array ultrasound inspection. TOFD probes are added onto the scanner and focused at a certain depth to ensure full coverage of the weld. TOFD is an excellent technique to evaluate depth and size of an indication to qualify it as a defect or not. TOFD is also very helpful for detection and sizing of misoriented defects.

Data Processing and Analysis
The encoded data collected from the inspection allows very accurate location of defects. It takes powerful data processing equipment to collect, process, and display data on-site in order to quickly analyze the inspection results. Rapidity is important when one wants to monitor the welding equipment at the same time and bring corrective actions before too many welds have to be repaired.

Data analysis has greatly improved with phased-array ultrasound and TOFD techniques. Because more data is collected and processed, and many display types are available, data analysis is easier and more precise than with conventional ultrasound. It is this level of precision that brought the use of engineering critical assessment (ECA) to the pipeline girth-weld inspection industry. ECA uses the defect location, its vertical extent, and its interaction with other defects to establish the acceptance or reject of a weld. It is a well documented fact that some flaws in welds do not affect their fitness for purpose. Therefore, a precise characterization of an indication lowers the number of unnecessary costly repairs.

Most AUT inspection systems offer a reporting feature and, since data is digitized, it is saved in digital format and can be recalled for comparison and/or ulterior analysis.

Standards and Codes
Automated UT is mentioned in many codes. The 1998 ASTM E-1961 includes some general rules for the use of AUT. E-1961 specifies zone discrimination, output displays, and customized calibration blocks. E-1961 also mentions the use of ECA. The 1999 API 1104 includes AUT for girth weld inspection. This code is less specific than ASTM E-1961, but both can be used in jointly.