TWI Industrial Member Report Summary 911/2008
By R Sanderson, P Catton and D Liaptsis
Over the past two decades, interest in guided waves has increased significantly and the potential for using them for the non-destructive testing of engineering structures has been recognised. The technology is most readily applicable to any structure of regular cross section whose axial dimension is typically long compared to other dimensions, such as a pipe. One significant advantage of guided waves is the ability to screen tens of metres of the structure from a single axial location.
TWI has researched and developed guided wave technology from the outset. More recently it has used modelling as a tool to help make new advances such as focusing the energy into one region of the pipe (Sanderson, 2003a and b, Sanderson, 2007a and b). Currently, the most commercially successful application has been the corrosion screening of straight industrial pipeline. However, there are two main challenges to overcome.
Firstly, whilst the technology is proven on straight pipe, inspecting beyond features such as bends can be problematic (Demma et al, 2005, Sanderson, 2007a and b) since the waves are distorted as they propagate round the bend and mode conversions occur. If these effects could be quantified, then optimised frequency selection or algorithms for a phased array style excitation could be used to overcome the effects of the bend.
Secondly, whilst the signal received gives the user an indication of the loss of total cross section, it cannot distinguish between uniform circumferential thinning and a severe patch of localised corrosion at one circumferential position. Thus, when corrosion is detected, a pipe usually has to be excavated and replaced (Mudge and Catton 2006).
It may be the case that a corroded section of pipe could be allowed to remain in place without any significant risk of failure for an extended period of time. This would have the advantage that the pipe could remain in service and the repair or replacement could be carried out at a convenient time.
If the extent of corrosion could be more accurately quantified, an improved assessment could be made.
In this project, work has been carried out using a combination of theoretical modelling and experiments to address these issues.
- Quantify the effects of pipe bends on guided waves.
- Find a technique for sizing locally thinned area defects in straight pipes using guided waves.