PSI - Issue 28

I. Shardakov et al. / Procedia Structural Integrity 28 (2020) 1795–1801 Author name / Structural Integrity Procedia 00 (2019) 000–000

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2. Experimental results 2.1. Vibration response of the pipe to impact on the wall

To study the response of the pipeline to shock impacts, a hammer was struck on the pipe wall in a section located on a ground surface. This section was located at a distance of 30 m from the S1 sensor. The S1 sensor recorded the passage of a shock wave with longitudinal and radial components. The wavelet spectra of vibration accelerations are shown in Figure 2. As can be seen from the figure, the dominant frequency of the longitudinal acceleration component was 100 Hz, and the radial one was 200 Hz. It was noted that the speed of propagation of the longitudinal and radial waves along the pipe was different. The propagation time of longitudinal and radial waves from the impact site to the S1 sensor differed by 0.1 s. The wave provoked by the impact on the pipe wall decayed quickly. The remote sensors did not register any response to this action.

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Fig. 2. Wavelet spectra of pipe wall accelerations at point S1 in longitudinal (a) and radial (b) direction

2.2. Vibration response of the pipe to impact on the wall To study the response of the pipeline to shock impacts on the surrounding soil, a series of impacts were made in the immediate vicinity of each accelerometer. The impact was produced by a striker weighing 16 kg, the impact energy was 320 J. Figure 3a shows the layout of the impact points. The impacts were made on the ground surface directly above the pipe at different distances from the sensor (points 1 and 6–9) and on the ground at different distances from the pipe (points 2–5). As a result, a series of vibrograms was obtained from each sensor placed directly on the pipe wall and the frequency ranges of vibrations were estimated using Fourier and wavelet transforms. An example of the signal received from the accelerometer located at point 2 and a tone image of its wavelet spectrum are shown in Figures 3b and 3c The analysis of these results showed that the impact on the soil in the vicinity of the pipeline generates oscillations of the pipe with radial component having a dominant frequency of ~ 50–100 Hz. The vibrations caused by impact on the ground are quickly attenuated. The damping effect can be estimated using the coefficient K = A I /A 0 , where A I is the amplitude of the acceleration recorded by the accelerometer mounted on the pipe wall when the impact on the ground is made at the remote point S I , and A I is the same amplitude obtained when the impact is made at the point S 0 directly above the sensor. Figure 3c shows the change in the attenuation coefficient K with increasing distance from the point of impact to the sensor in the direction along the pipe (curve 1) and in the transverse direction (curve 2). It can be seen from the figure that when the impact point is removed at a distance of 25 m, the vibration amplitude decreases by 50–100 times. For an impact with energy of 320 J, the radius of the sensor's sensitivity zone is about 60 m. At this distance, the signal amplitude decreases 1000 times. The analysis of the vibration response spectra shows that at the impact on the ground, the pipe vibrations are excited in the frequency range from 10 to 150 Hz. As the distance from the excitation source to the sensor increases, the upper boundary of the spectrum is shifted towards lower frequencies (60-70 Hz).