Improved Lateral Focusing for Thin-Walled Gas Pipelines Girth Welds Using Phased Arrays

With the arrival of high strength, thinner walled pipe steels, improved defect sizing is becoming more critical. 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. This paper describes the development of an ultrasonic phased array specifically to improve focusing. Extensive computer modeling was performed to determine the optimum array for thin-walled pipes with mechanical focusing (i.e. a curved array) and various curvatures. The modeling showed that a 60 element, 1 mm pitch array with a 100 mm curvature in the passive aperture gave significant improvements over a standard array (60 elements, 1 mm pitch, unfocused). This curved array was manufactured and tested on a commercial calibration block with flat bottomed holes. 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-bottom hole reflectors by only ∼ 1 mm, instead of the 4–6 mm from the standard array. These curved arrays can be used on standard AUT systems with no modifications to the general mechanics or software.