The alternating current field measurement testing technique was developed in the 1980s to estimate the depth of penetration of fatigue cracks underwater at the welded tubular intersections of offshore oil platforms in the North Sea. Such defects were generally detected using fluorescent magnetic particle inspection (MT), which also provided information on the surface length of the defect. However, the severity of the defect, and hence the remaining life of the structure, depends on how far the defect penetrates through the wall. Conventional eddy current systems at the time were not well developed for underwater use, or for inspection of welds in ferritic steel, and in any case were not able to accurately measure penetration depths greater than about 5 mm (0.2 in.). The AC potential drop technique (ACPD) was the one technique being used for this purpose at the time, but it was very difficult and slow to use underwater because of the need to maintain very good electrical contact between the voltage probe and the surface of the steel. To solve the problem, a noncontact equivalent to ACPD was required. At that time, the mechanical engineering department at University College London, UK (UCL) had many years’ experience in growing fatigue cracks on tubular welded joints in the laboratory and was making both theoretical and practical developments to improve the accuracy of ACPD for measuring crack growth rates. A group of UK oil companies therefore approached them to develop a new noncontact technique, which became known as the alternating current field measurement technique (Lugg, 2010).
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