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Driving inspection around the bend

Teletest long range
Fig 1. Teletest long range guided wave equipment being used to inspect insulated and cased oil production pipes for corrosion

Guided wave inspection using Teletest® has the advantage of providing full volumetric coverage of tens of metres of pipe from a single test location. The technique is already widely used for pipeline inspection and is particularly valuable for unpiggable sections of pipeline such as at cased road crossings. TWI has recently been developing techniques to derive more quantitative information from the inspections including an estimate of the through wall extent of flaws. However, there are often bends in the pipe, and these are known to distort the guided wave signals, making quantitative analysis problematic. In order to address this issue, TWI has recently completed research into methods for inspecting beyond pipe bends. A study has been carried out using a combination of finite element analysis and practical trials to understand the behaviour of guided waves in a relatively tight pipe bend example.

guided waves distorting
Fig 2. Finite element simulation of guided waves distorting as they propagate around a pipe bend.

Initially, finite element analysis was used to investigate the behaviour of guided waves propagating around a pipe bend. A relatively small diameter pipe with a tight bend radius was taken as a case study. Guided waves propagate in several different vibrational modes, so both the mode conversion that occurs through the bend and the orientation of the wave modes after the bend were investigated. Experimental validation was then carried out to confirm the findings of the finite element analyses. Building on the understanding of the guided wave behaviour gained in the first stage of the project, analytical methods were then developed both to predict the behaviour and to be used as a tool to 'undo' the signal distortion. The analytical distortion correction methods were then verified against finite element solutions. The aim of the analytical method is to provide computationally fast techniques that can be used to give quantitative rather than qualitative measurements of flaws beyond bends in the field.

Teletest Focus
Figure 3 Teletest equipment

The finite element modelling showed that although the incident wave mode can be made to pass through a pipe bend, mode conversion occurs and the orientation of the wave modes are often altered. The orientation of these modes can be used to determine the location of the flaw so this knowledge is an important step forward in ensuring inspection results from beyond a pipe bend are valid.

Next, an analytical distortion correction method was developed and tested against experimentally validated finite element modelling results. It was shown to work well; however, the distortion correction method works for one journey through the pipe bend whereas guided wave inspection is normally used in a pulse-echo configuration which means the waves will travel twice through the pipe bend. Therefore, a method was developed to alter the excitation algorithm to generate a non-standard mixture of wave modes. The mixture is selected so that, after propagating around the bend, the guided wave is the same as a conventional excitation propagating in a straight pipe. This was successfully demonstrated in a validated finite element model.

This combination of pre- and post-processing techniques has therefore opened up the potential for more refined inspection beyond pipe bends. Compensating for the distortion of guided waves will also allow flaw sizing techniques to be used beyond pipe bends.

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