In the oil and gas and other process industries, there is an increasing need for the inspection of sections of pipes and vessels rendered inaccessible by components such as supports, clamps and saddles. Current ultrasonic techniques used for this in-service application include CHIME (R)((1)) and LORUS. These techniques do, however, have some limitations when quantitative measurements of wall loss are needed. In contrast, conventional TOFD can provide accurate through-wall information, but on the longer supports and saddles it is not effective because of the required large probe separation. This paper describes an extension of the TOFD technique to multi-skip inspection which allows significantly increased probe separations, and hence provides potential for the quantitative measurement of wall loss in the inspection of inaccessible components. Results from this new technique, referred to as M-skip, are described from a number of components, with probe separations up to approximately I m. The methods needed for the analysis of signal arrival times differ from conventional TOFD, but the accuracy of the resulting wall loss measurements can be as high as +/- 1 mm. Preliminary results suggest that the advantages and limitations of this technique are to some extent complementary to those of CHIME.
