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Pipe Stiffener Crack Detection using Alternating Current Field Measurement and Phased Array Ultrason

Advanced non-destructive testing (NDT) techniques like alternating current field measurement (ACFM) and phased array ultrasonic testing (PAUT) are being deployed on hydropower and water navigation structures more and more frequent as these structures age. This article presents a dual approach using ACFM and PAUT for detection and sizing of fatigue cracks in discharge tube fillet welds that connect a variety of different stiffener types to large diameter stiffener tubes used to transport water downstream from a dam to an upstream irrigation canal.

Fatigue Cracks in Discharge Tubes

The discharge tubes cited in this case study are large diameter steel or galvanized steel pipelines that transport water upward from the river downstream of a dam to a canal used for irrigation water. Several types of stiffeners are deployed to reinforce these discharge tubes including collar stiffeners, “T” type stiffeners and “H” type stiffeners. Each discharge pipe also uses periodically spaced supports which consist of two fillet welded collars connected via cross bracing which is also welded to the discharge tube.



Figure 1: Discharge tubes showing periodically spaced supports.

The potential zones for fatigue cracks to be detected with alternating current magnetic field measurement (ACFM) and phased array ultrasonic testing (PAUT) are shown in Figure 1 and 2: 1) the fatigue crack propagates inward through the fillet weld throat 2) the fatigue crack initiates at the fillet weld toe and propagates through thickness 3) the fatigue crack propagates along the fusion line of the fillet weld and discharge pipe / weld. An example fillet weld fatigue crack detected by ACFM is shown in Figure 2. The crack is approximately 3” long and was classified as a Type 2 fatigue crack per Figure 1 and follow-up testing with PAUT and magnetic particle (MT) was performed. The fatigue crack shown at the fillet weld toe was further confirmed with dry visual magnetic particle testing using an AC yoke and red powder.



Figure 2: Discharge pipe fillet weld fatigue crack locations