External corrosion of uninsulated carbon steel pipework is a major problem in the oil and gas and other industries. As the corrosion progresses, corrosion product or scale builds up on the pipework. Removal of this corrosion product for fabric maintenance purposes on live process pipework can have significant risks and hence there is often a requirement to assess the remaining steel wall thickness under the corroded area, without removal of the corrosion product. Within the HOIS JIP, the performances of a number of NDT methods for sizing the remaining ligament under corrosion scabs on uninsulated components have been assessed by means of blind trials. One method that has shown promise for thicker walled components (greater than about 15mm) is the ultrasonic pitch-catch technique referred to as M-skip, which was originally developed for inspection of inaccessible components such as corrosion under pipe supports (CUPS). Provided the extent of the corroded area in the circumferential or axial direction is less than about 11 times the wall thickness, trial results have shown very good agreement between the remaining ligament under the external corrosion and the corresponding benchmark values. This paper outlines the principle of M-skip for this application and gives details of the components inspected and trial results obtained.
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