Guided waves for nondestructive testing of pipelines

Mona Lieb, winter semester 2023/2024

The Guided Wave Testing uses ultrasonic waves that are guided along the length of pipelines to identify defects ^[1,2].

1. Motivation

Pipeline systems are the preferred choice for transporting large quantities of liquid, as this method is the safest and most cost-effective. Therefore, pipeline systems are essential for the transportation of water, wastewater, gas and oil, as well as in industry, where pipeline networks are used to transport oil, gas, and chemical products. It is necessary to inspect pipeline systems regularly to detect defects at an early stage and to obtain information on wall thickness to ensure reliable operation. Certain areas of the pipelines are difficult to access, especially large sections of pipelines that run under populated or environmentally protected areas. Interfaces, such as at the pipeline supports, are also difficult to reach. In addition, operation cannot be interrupted to inspect the pipeline systems. Guided wave ultrasonic testing (GWT) is an efficient method for this, as it is a non-destructive testing technique that can be used to inspect large areas of pipeline structures in a non-destructive manner ^[3,1].

2. Functional principle and measurement setup of GWT

Guided wave testing uses the Ultrasonic Pulse-Echo Method to transmit and receive ultrasonic signals ^[3]. In the conventional ultrasonic method, the wave propagates through the structure, as shown in Figure 1 in the upper graph. In comparison, the GWT is shown schematically in the lower graphic in Figure 1. Here, the ultrasonic waves propagate along the structure (e.g. along a pipe). Along a pipe, guided ultrasonic waves can form standing wave modes and propagate over long distances with a low energy loss, whereby the amplitude remains almost unchanged ^[4]. As with the conventional method, defects are detected based on the reflection of the transmitted ultrasonic waves ^[3,5].

Figure 2 shows the conceptual design of a test with guided waves. The ring of transducer is attached to the pipeline at one point, which arranges the piezoelectric elements evenly around the circumference and transmits and receives the signals. The signals are processed using the inspection system and can then be displayed on a laptop or computer ^[3,5]. The trend for these measurement setups is increasingly shifting towards inspection with permanently installed sensors so that the ring of transducers no longer needs to be removed. This enables reliable and continuous monitoring of the pipeline condition and a direct comparison with previous measurements ^[1].

3. Properties of GW in pipelines

3.1. Wave modes and dispersion curves

Compared to the conventional ultrasonic method, the GWT supports an infinite number of wave modes. The modes for cylindrical structures can be divided into longitudinal modes L(0,m), torsional modes T(0,m) and bending modes F(n,m). The variable n indicates the order of the wave modes and m is the mode designation. Longitudinal and torsional modes are axisymmetric. The distinction between these modes is shown in Figure 3 ^[5,3,2,6].

The prediction of wave mode characteristics is based on detailed mathematical models, usually represented in dispersion diagrams. Dispersion curves show the phase velocities for each mode as a function of frequency. The characteristics of the wave modes are predominantly dependent on the frequency and the wall thickness of the tube, which is referred to as dispersion ^[5]. The dispersion curves are determined by calculating the eigenvalues over the frequency range ^[4]. This results in the following equations for the eigenvalues of an unbounded harmonic hollow cylinder ^[3]:

$//<![CDATA[ \begin{array}{l}|C_{ij}|_{(8 \times 8)} = 0, \text{ of which } K_1 = \frac{\omega}{C_1} \text{ and } K_2 = \frac{\omega}{C_2}\end{array} //]]>$

with:

K: Wavenumber function

ω: angular frequency [rad/s]

C_ij: Diameter [m]

C₁: shear velocity [m/s]

C₂: longitudinal volume wave velocity [m/s]

The eigenvalues determined from this allow the dispersion curves to be displayed ^[3]. Figure 4 shows an example of a dispersion diagram.

3.2. Selection of suitable wave modes

The selection and monitoring of the wave mode is of crucial importance, as the use of a suitable mode enables more precise detection of defects ^[4]. The mode should be as non-dispersive as possible, otherwise the signal amplitude will decrease, and noise will drown out the signal. In addition, many different modes can occur simultaneously at one frequency, which makes evaluation more difficult ^[3]. In practical applications, axisymmetric modes, such as longitudinal and torsional modes, prove to be particularly advantageous ^[2]. In contrast, bending modes are not axisymmetric, which makes their propagation characteristics more complex. Therefore, bending modes should be avoided when inspecting pipelines ^[6]. In practice, for example, the L(0,2) mode is suitable for identifying defects that show up as changes in depth and circumference ^[2].

It has a high propagation speed and therefore requires the shortest time to receive the reflected waves. In addition, the mode shows no dispersion in a wide frequency range. The torsion mode shows no dispersion over the entire frequency range. The T(0,1) mode is therefore also suitable for this application ^[4,2]. Large propagation distances of the signal in the pipelines to be tested require low frequencies. However, it should be noted that the sensitivity of defect detection decreases with lower frequency. For this reason, a screening mode is often used with the GWT. This means that potentially affected areas of the pipeline are identified first and then a more localized inspection is carried out to assess the damage in detail ^[4].

3.3. Damping of GW in pipelines

Attenuation refers to the decrease in amplitude with increasing distance ^[2]. Various factors such as surrounding materials, ambient temperature, coatings, and surface roughness have an attenuating effect on the guided waves in a pipeline ^[3,2]. This can occur, for example, in pipelines that are laid in water or underground. Despite this attenuation, it is possible to inspect the pipes ^[1]. However, it is crucial to select the excitation frequency and wave mode in such a way that the attenuation remains low and a large propagation distance is achieved ^[4].

The attenuating influence can be reduced by using a lower excitation frequency. The T(0,1) mode, for example, is particularly suitable for use in coated pipelines and those surrounded by liquid or through which liquid flows. The reason for this is the mode's insensitivity to insulating materials and liquids ^[2].

4. Interaction of guided waves with defects

The pipeline wall limits the propagation of GWs so that the waves propagate along the structure ^[3,5]. If the cross-section of the structure changes, the wave is reflected [5]. This partial reflection of the signal occurs when the waves encounter a change in geometry, such as weld seams, flanges, corrosion, or cracks ^[3]. The process is shown in Figure 5.

By analysing the reflections, the position and magnitude of changes in pipelines can be determined ^[3]. The location of defects is determined from the propagation speed of the GWs generated and the arrival time of the echo ^[3,5]. The amplitude magnitude increases with the echo and thus enables an estimation of the defect sizes ^[5]. The measurement of bent pipes is also possible. However, problems can occur when detecting defects on the outer side of the bend. Guided waves tend to propagate via the shortest path and are therefore more likely to be located on the inner side of the bend. Corrosion or erosion on the outside of the bend is particularly difficult to identify ^[4].

5. Advantages and disadvantages of the procedure

The advantages and disadvantages of the GWT compared to conventional non-destructive testing methods are shown below.

Advantages:

• Possibility to test large and inaccessible structures ^[4]
• Defects can be detected at a distance of up to 100 m ^[4]

Disadvantages:

• Generation of a specific wave mode is challenging ^[3]
• Weld seams and flanges are also detected as defects ^[3,5]

6. Literature

1. Jacques, Ricardo C., Oliveira, Henrique H. de, dos Santos, Rafael W. F., Clarke, Thomas G. R.: Design and In Situ Validation of a Guided Wave System for Corrosion Monitoring in Coated Buried Steel Pipes. J Nondestruct Eval (2019) 65:38, p. 1-12.
2. Guan, Ruiqi, Lu, Ye, Duan, Wenhui, Wang, Xiaoming: Guided waves for damage identification in pipeline structures: A review. Structural Control and Health Monitoring (2017) 24:11, p. 1-17.
3. Ghavamian, Aidin, Mustapha, Faizal, Baharudin, B. T. Hang Tuah, Yidris, Noorfaizal: Detection, Localisation and Assessment of Defects in Pipes Using Guided Wave Techniques: A Review. Sensors (2018) 18:12, p. 1-48.
4. Fromme, Paul: Guided Wave Testing. In: Nathan Ida und Norbert Meyendorf: Handbook of Advanced Non-Destructive Evaluation. Cham: Springer (2019), p. 1-30.
5. Diogo, Ana Rita, Moreira, Bruno, Gouveia, Carlos A. J., Tavares, João Manuel R. S.: A Review of Signal Processing Techniques for Ultrasonic Guided Wave Testing. Computer Methods in Metallic Materials (2022) 12:6, p. 1-20.
6. Ling, En Hong, Abdul Rahim, Ruzairi Hj.: A review on ultrasonic guided wave technology. Australian Journal of Mechanical Engineering (2020) 18:1, p. 32–44.