Testing the system
To validate the PIGWaves system, a section of pipe was deliberately damaged. Initially, a crack was simulated with a slot increased in depth in seven equal increments until the entire wall thickness was penetrated (Figure 2, measurements 1–7). A secondary defect simulating wall thickness loss was introduced on the outside wall of the pipe in six steps (Figure 2, measurements 8–13).
The results presented on an A-scan (Figure 2a) an end‑of-pipe reflection, which reduced in amplitude in the presence of cracking and wall thinning, and as the width of the simulated corrosion was increased. In other regions of the filtered A‑scan, where anomalies are present (Figure 2a), there are several significant increases in signal amplitude due to the loss of wall thickness. The A-scan amplitude changes, following an increasing trend deviation from the defect-free signal.
Looking at the frequency domain of the signals with and without loss of wall thickness, there is a greater loss of energy over a wider range of frequencies around 30kHz compared to the results from the inserted cuts and thinning.
Changes on the A-scan’s amplitude reveal the presence of wall thickness loss. These different areas are evaluated by analysing the similarity of the signals at a given location.
Similarity is equal to one minus the error function of the rate of change of the A-scan’s amplitude:
*Equation 1.
Figure 2b shows the correlation between each of the signals acquired in comparison with the flawless pipe reference signal. The similarity is measured from one to zero, with one being equivalent to 100% equal signals. The error is calculated in one dimension; thus the similarity between two points on the real line is the absolute value of their numerical rate of change difference.
The Euclidean distances were analysed for one dimension, comparing the obtained signals on each measurement against the defect-free pipe signature. The distance between two points in one dimension is simply the absolute value of the difference between their coordinates. Mathematically, this is shown as follows:
*Equation 2.
Where p1 is the first coordinate of the first signal and p2 is the first coordinate of the second signal.
In Figure 2c, results show an increment on the distance between each signal measured and the reference signal when there is defect growth. The most important aspect considered is that the distance on amplitude change follows a trend with increasing deviation from the defect-free signal.