On the scattering of elastic waves from a non-axisymmetric defect in a coated pipe

Wenbo Duan,Ray Kirby,Peter Mudge

doi:10.1016/j.ultras.2015.09.019

Abstract

Viscoelastic coatings are often used to protect pipelines in the oil and gas industry. However, over time defects and areas of corrosion often form in these pipelines and so it is desirable to monitor the structural integrity of these coated pipes using techniques similar to those used on uncoated pipelines. A common approach is to use ultrasonic guided waves that work on the pulse-echo principle; however, the energy in the guided waves can be heavily attenuated by the coating and so significantly reduce the effective range of these techniques. Accordingly, it is desirable to develop a better understanding of how these waves propagate in coated pipes with a view to optimising test methodologies, and so this article uses a hybrid SAFE-finite element approach to model scattering from non-axisymmetric defects in coated pipes. Predictions are generated in the time and frequency domain and it is shown that the longitudinal family of modes is likely to have a longer range in coated pipes when compared to torsional modes. Moreover, it is observed that the energy velocity of modes in a coated pipe is very similar to the group velocity of equivalent modes in uncoated pipes. It is also observed that the coating does not induce any additional mode conversion over and above that seen for an uncoated pipe when an incident wave is scattered by a defect. Accordingly, it is shown that when studying coated pipes one need account only for the attenuation imparted by the coating so that one may normally neglect the effect of coating on modal dispersion and scattering.

Highlights

Viscoelastic materials are often used as coatings on the outer surface of pipelines in order to protect the pipe from external damage and corrosion
The previous section investigated wave propagation in the frequency domain; long range ultrasonic testing (LRUT) is normally undertaken in the time domain because this is a convenient way to analyse scattering from a defect
This article has used a hybrid semi analytic finite element (SAFE)-finite element based method to analyse the scattering of guided waves from non-axisymmetric defects in coated pipes

Summary

Introduction

Viscoelastic materials are often used as coatings on the outer surface of pipelines in order to protect the pipe from external damage and corrosion. The application of LRUT to coated pipes involves sending a guided wave along the pipe wall, but this technique is less successful for coated pipes because the viscoelastic coating attenuates the ultrasonic wave as it travels along the pipe wall This has the effect of significantly reducing the range over which LRUT can be successfully used in the location of defects such as corrosion. This presents a significant problem because LRUT is an important tool for interrogating pipelines and the use of viscoelastic coatings is relatively widespread It is desirable, to try and develop a better understanding of the way in which a coating attenuates an ultrasonic wave, as well as how it affects the scattering of waves from defects. In doing so this model moves away from relying on dispersion curves in order to analyse a scattering problem that is more representative of problems found in the field

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