Quantitative measurement of remnant thickness in corrosion under pipe supports

Abstract

There have been many attempts to quantify the wall loss in corrosion under pipe support (CUPS) applications without lifting the pipe to gain access to the corroded area, but none have yielded satisfactory quantitative results. A new method has been developed to enable quantitative estimation of wall loss in CUPS and other applications where direct access to the affected region is not possible. It uses a combination of the non-dispersive SH0 wave and the dispersive SH1 and potentially higher-order modes propagating around the pipe, both in transmission across the defect and reflection from it. The key feature is the rapid reduction in transmission and increase in reflection when the product of the frequency and the remnant wall thickness under the defect approaches the cut-off frequency of the SH1 or higher-order modes. Initial implementation of the method gives quantitative remnant wall thickness results when the wall loss is up to 50%, with qualitative indications of severity at greater defect depths. Blind trial results on a pipe with six defects show a maximum error in the estimated remaining wall thickness of 0.5 mm. The instrument currently covers pipes in the 6-24 inch diameter range and with 6-12 mm wall thickness. This will shortly be expanded and a similar tool to scan circumferentially around the pipe, transmitting and receiving SH waves in the axial direction, is also being developed. This will enable the same defect to be tested from two directions to further increase confidence in the results.