Development of New Inspection Method for LSAW Pipes Using Matrix Phased Array UT

Abstract

A new method was developed for inspection of welds in LSAW (Longitudinal Submerged Arc Welded) steel pipes using ultrasonic testing. Defects called I.P. (Incomplete Penetration) may occur in the central part of the wall thickness in welds of LSAW steel pipes, and these defects have a flat shape in the thickness direction. In order to detect these defects with high sensitivity without influence of the welded part shape, transmission of the ultrasonic wave perpendicular to the defect at a high refraction angle is effective. On the other hand, there were also problems in use of the angle beam method with a high refraction angle, including a decrease the sensitivity of ultrasonic testing and the need to maintain the angle accurately. In the new method proposed here, a sector scan with a line focus beam using matrix phased array UT was adopted, securing high sensitivity and stability in inspections. It’s characterized by using a matrix array probe in which the small vibrators are arranged so that the width of small vibrators gradually decreases from the center of the matrix probe toward the outside. In the verification test using the actual steel pipes, good result was obtained with a S/N ratio of 24 dB or more and good repeatability for a flat bottom hole with 3 mm diameter that is simulated I.P.. The operability of probe setting is also better than tandem method because only one probe is used and the incident angle can be adjusted by the phased array technology.