Monitoring pipe wall integrity using fiber Bragg grating-based sensing of low-frequency guided ultrasonic waves
Introduction
Transmission pipelines are widely used in oil, petrochemical and power plant industries where efficient structural health monitoring (SHM) techniques ensuring the reliability and early defect detection are essential [1]. Ultrasonic guided waves are attractive for SHM of pipes, due to their ability to propagate over long distances from a single transducer location [2], [3], and potential to inspect both internal and surface defects [4].
A common issue in such pipe structures is cross-sectional irregularity such as eccentricity or local wall thinning which can arise either due to manufacturing limitations or due to prolonged usage and hence is an essential parameter to be monitored continuously [5], [6]. One example of cross-sectional irregularity due to prolonged usage is flow-accelerated corrosion (FAC), which is a common chemical corrosion process in carbon or low-alloy steels leading to wall-thinning over an extended axial region [7]. Since wall thinning might ultimately lead to wall rupture, it is important to detect such irregularities early in pipes.
Recent research has shown that thinned areas of a pipe annulus can act as features that can confine and guide longitudinal L(0,2) ultrasonic guided wave modes, commonly known as ‘feature-guided waves’ (FGW) [8], [9], [10]. Such FGW modes can be potentially exploited for detecting pipe eccentricity. The feature-guiding effect (mode confinement) is strongly dependent on the extent of the eccentricity (wall thinning).
The key to observe FGW arising from pipe wall anomalies, is to detect accurately the amplitudes of a chosen guided-wave mode at various angular positions around the pipe circumference. Typically, laser Doppler vibrometry has been used to detect feature-guided waves [9], [11], [12], [13], [14], [15]. Unfortunately, measurement of mode amplitudes at various positions across the circumference of a pipe using LDV or even piezoelectric or Magnetostrictive (MsS) sensors is a challenging task on field.
Using LDVs for such applications is beyond the scope of most site conditions, as they may require custom-created robotized gantries and also full-field access to the pipe circumference. MsS Sensors, as they exist today, cannot provide individual amplitude at specific positions, and only yield a value averaged over all circumferential measurement nodes. On the other hand, with piezoelectric sensors, the major problem in measuring amplitudes accurately, is to ensure uniform contact with the pipe at all monitoring nodes, and this is very difficult to achieve in practice. Mistakes can be made in the prediction, if mode amplitude is not recorded accurately at one of the sensors due to poor contact – and this can be wrongly interpreted as ‘mode focusing’ or FGW in this approach. Moreover, using piezoelectric sensors would require creating customized rings that can hold them in place and special mechanisms or rings of different sizes would be needed to account for various pipe diameters. In the current article, a more practical approach to detect such FGW modes using FBG sensors is presented, and is the key aspect of this work.
Since FBG sensors are pasted directly on and can conform to the pipe surface, no special mechanisms are necessary to deploy them [16], [17]. Moreover, for pipe health monitoring, in addition to the ability to perform in harsh environmental conditions, the low-loss property of an optical fiber allows long distance health monitoring from a single end of the fiber [18]. Additionally, FBG sensors have a highly directional response due to their cylindrical geometry with high aspect ratio [19]. This property aids in preferential detection of guided wave modes propagating in the structure based on their orientation, which has been demonstrated recently by the authors [20], [21].
In this paper, a FBG-based field-deployable technique for monitoring pipe wall loss using changes to modal characteristics of low-frequency guided waves is presented. Our studies focuses on the second axisymmetric longitudinal pipe mode, L(0,2). Even though uniform eccentricity which causes wall-thickness loss over an extended axial region arises primarily due to manufacturing errors, this could also approximate those caused due to effects such as FAC. The experimental results based on pipe dimensions that are typically used in process industries confirm that with increase in pipe eccentricity, the L(0,2) mode loses its axisymmetry and gets focused in the thinner region of the pipe cross-section. Moreover, the L(0,2) mode also has a reduction in its velocity. Together, these two measurements provide a novel route to detect pipe wall anomalies such as extended wall loss.
Section snippets
Materials and methods
A key challenge in the structural health monitoring of metallic pipes is the excitation of a pure longitudinal wave. The particle motion in an L(0,2) mode is along the cylindrical axis and the strain is uniformly distributed through the pipe wall [9], thereby making it suitable to detect cross-sectional changes in pipes.
Fig. 1 shows the dispersion profile of a concentric seamless mild steel pipe of 60 mm outer diameter (OD) and 50 mm inner diameter (ID) obtained using DISPERSE software package
Results
Initial experiments were conducted on a seamless mild steel concentric pipe having OD of 60 mm and ID of 50 mm. The FBG sensor is placed 50 cm away from the acoustic source which provides longitudinal excitation at a frequency of 100 kHz. The waveform captured by the FBG sensor is compared with that captured using a Doppler vibrometer to verify that the measurements are equivalent. The time domain response captured using DSO is shown in Fig. 3. From this trace, we observe that the FBG sensor
Discussion
The novelty of the experimental approach and the challenges involved are discussed in this section. The conventional approach makes use of mode scattering from defects and works only for defects which are at least half the wavelength circumferentially. However, the method presented here provides a two-way solution which can both detect wall loss and also locate it circumferentially by measuring changes to the group velocity and amplitude distribution of the L(0,2) mode respectively. As the
Conclusions
In this paper, a novel technique based on fiber Bragg grating sensors for monitoring cross sectional irregularities in pipes based on L(0,2) mode detection is demonstrated. Eccentricity results in local thinning or thickening of pipe wall at diametrically opposite positions. Such a wall loss or eccentricity is measured by monitoring the group velocity and circumferential amplitude distribution of the L(0,2) mode. Experimental results demonstrate L(0,2) mode energy confinement in the thinner
Acknowledgment
The authors would like to acknowledge the support provided by the team of Detect Technologies, a IIT Madras incubated company in installing the MsS transducers. We would also like to acknowledge Bhuvana, IIT Madras for helping with the FBG fabrication.
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