Alternative techniques for detection of inaccessible pipe corrosion

Abstract

Testing for corrosion in the petrochemical industry has always been a significant challenge which takes up a large portion of the operating expenditure. Whereas major advancements have been made for the detection of general corrosion, inspection at inaccessible locations, such as at pipe supports, remains a demanding prospect; this signifies the need for an alternative technique, capable of dealing with various surface conditions encountered when testing at such locations including weld patches, T-joints, surface roughness and coatings. Long range guided waves are commonly used to detect relatively severe defects in plain sections of pipe but are less suited to inspection at supports because the support itself gives significant reflection. The reflection coefficient at the support reduces with frequency so it would be beneficial to test at higher frequencies, which can also improve the sensitivity of the test to smaller, pitting-type defects. Following the attractive properties of the Higher Order Mode Cluster (HOMC) proposed by Balasubramaniam et al. (IIT Madras), this research starts by investigating the nature of the mode cluster and shows that the features of this method are essentially those of the A1 mode in the high frequency-thickness regime. The study then goes on to investigate the possibility of exciting a single mode Lamb wave with low dispersion at a frequency-thickness of around 20 MHz-mm. Excitation of the A1 mode was considered because of its relationship with HOMC and due to its non-dispersive nature and low surface motion at such frequency-thickness products; this makes it attractive for inspection at supports since it will be unaffected by the support itself and also by surface roughness and attenuative coatings. The thesis then explores the relative ability of different transducer types for single mode excitation in the medium and high frequency-thickness regimes; here the practical feasibility of exciting the A1 mode at around 20 MHz-mm, in spite of its low surface motion, is investigated. Next, a systematic performance analysis of the A1 mode compared to the existing inspection techniques is carried out and, finally the sensitivity of this technique to realistic 3-D pitting-type holes is established. The thesis shows that the A1 mode is an attractive tool for the detection of localized, sharp, severe defects that will be missed by standard, lower frequency guided wave testing.