Dispersion behavior of torsional guided waves in a small diameter steel gas pipe

Condition monitoring of gas pipes has been an important issue for gas companies. Failure to accurately identify condition of gas pipes result in numerous problems. Also, producing a condition monitoring system for buried pipelines is challenging. Small pipes (with diameters less than 50 mm) are considered here as most of the literature focuses on larger pipes. Guided wave theory will be introduced alongside a numerical simulation of the relevant dispersion curves of the system. This paper investigates the feasibility of using torsional guided waves for inspecting defects in buried pipes with small diameters. The pipes are assumed to be lossless and hence the effect of attenuation is ignored in the calculations. Upon finding the theoretical guided wave characteristics, experiments were conducted to see if the aim could be achieved in a realistic scenario. A steel pipe with a diameter of 34 mm and wall thickness of 5.5 mm is considered. High reverberation levels at high frequency propagations due to mode conversion are studied. Having only a limited number of transducers could be a reason for high reverberation at high frequencies

On-line measurements of contents inside pipes using guided ultrasonic waves

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Experimental Characterization of the Propagation of Acoustic Waves in Pipe Strings

This thesis explores the propagation of acoustic waves in pipe strings by designing a series of experimental procedures to characterize their behavior. It describes in detail four sets of experiments that were used to characterize the behavior of the acoustic waves as they propagate in pipe strings. First, the Existence Test tried to prove the existence of the dispersion phenomenon in pipe strings by measuring the speed of waves with different frequencies on a small scale version of the field test bed. Second, the Consistency Test tried to determine the circumstances that allow for reliable and consistent measurements as a preparation for the field tests by testing the transmitter, receiver, and coupling consistency. Third, the Dispersion Test aimed at discovering all of the existing vibrational modes and matching them with the theoretical solutions by performing a two-dimensional Fourier Transform. Finally, the Frequency Test attempted to describe the pipe strings response to both narrowband and broadband signals by proving the existence of passbands and stopbands

Monitoring system for long-distance pipelines subject to destructive attack

In an era of terrorism, it is important to protect critical pipeline infrastructure, especially in countries where life is strongly dependent on water and the economy on oil and gas. Structural health monitoring (SHM) using acoustic waves is one of the common solutions. However, considerable prior work has shown that pipes are cylindrical acoustic waveguides that support many dispersive, lossy modes; only the torsional T(0, 1) mode has zero dispersion. Although suitable transducers have been developed, these typically excite several modes, and even if they do not, bends and supports induce mode conversion. Moreover, the high-power transducers that could in principle be used to overcome noise and attenuation in long distance pipes present an obvious safety hazard with volatile products, making it difficult to distinguish signals and extract pipeline status information. The problem worsens as the pipe diameter increases or as the frequency rises (due to the increasing number of modes), if the pipe is buried (due to rising attenuation), or if the pipe carries a flowing product (because of additional acoustic noise). Any system is therefore likely to be short-range. This research proposes the use of distributed active sensor network to monitor long-range pipelines, by verifying continuity and sensing small disturbances. A 4-element cuboid Electromagnetic Acoustic Transducer (EMAT) is used to excite the longitudinal L(0,1) mode. Although the EMAT also excites other slower modes, long distance propagation allows their effects to be separated. Correlation detection is exploited to enhance signal-to-noise ratio (SNR), and code division multiplexing access (CDMA) is used to distinguish between nodes in a multi-node system. An extensive numerical search for multiphase quasi-orthogonal codes for different user numbers is conducted. The results suggest that side lobes degrade performance even with the highest possible discrimination factor. Golay complementary pairs (which can eliminate the side lobes completely, albeit at the price of a considerable reduction in speed) are therefore investigated as an alternative. Pipeline systems are first reviewed. Acoustic wave propagation is described using standard theory and a freeware modeling package. EMAT modeling is carried out by numerical calculation of electromagnetic fields. Signal propagation is investigated theoretically using a full system simulator that allows frequency-domain description of transducers, dispersion, multi-mode propagation, mode conversion and multiple reflections. Known codes for multiplexing are constructed using standard algorithms, and novel codes are discovered by an efficient directed search. Propagation of these codes in a dispersive system is simulated. Experiments are carried out using small, unburied air-filled copper pipes in a frequency range where the number of modes is small, and the attenuation and noise are low. Excellent agreement is obtained between theory and experiment. The propagation of pulses and multiplexed codes over distances up to 200 m are successfully demonstrated, and status changes introduced by removable reflectors are detected.Open Acces

Pipeline leak detection

In the present research two techniques are applied for leak detection in pipelines. The first method is a hardware-based technique which uses ultrasonic wave\u27s emission for pipeline inspection. Ultrasonic waves are propagated in the pipe walls and reflected signal from leakage will be used for pipe analysis. Several Pipes with various dimensions and characteristics are modeled by finite element method using ANSYS. Second order longitudinal modes of ultrasonic waves are emitted in their walls. For this purpose, excited frequency is calculated such that it excites the second order longitude mode. In order to investigate the behavior of emitted wave in contact with leakage, four sensors are used in outer surface of pipe. Waves are reflected when encountering leakage and the leak location is recognized knowing the wave emission speed and flight time of backscattered signals. Wavelet transform is used for processing these signals and recognizing leak location. This method is tested on several pipe models and it presents satisfactory results for short pipes. The second approach is a software-based method which works based on the transient model of the pipeline. In this method the outputs from simulated pipeline are compared to those measured from flow meters and if their difference goes beyond a threshold value, leak is detected. For leak localization a gradient pressure technique is applied which needs pressure slope measurements at inlet and outlet of the pipeline. Several cases with leak at various positions are studied. This method works well with high accuracy for long pipelines. --Abstract, page iii

Structural Health Monitoring of Large Structures Using Acoustic Emission-Case Histories

Acoustic emission (AE) techniques have successfully been used for assuring the structural integrity of large rocket motorcases since 1963 [...

Structural Health Monitoring for Small Diameter Pipes with Guided Ultrasonic Wave

The structural health monitoring (SHM) paradigm includes device design for data acquisition and signal processing for defect visualization. An excitation is applied to the interested structure to generate vibration which is measured and analyzed to reveal structural defects. Guided ultrasonic wave (GUW) has proved to be effective as an excitation for shell SHM. Device settings and corresponding signal processing methods have been proposed to detect defects over plate and plate-like structures. However, there is no effective SHM paradigm for small diameter pipes having salient curvatures. By utilizing GUW, we proposed three methods for SHM over pipes for different application backgrounds. The first method detects the defect axial position. Measurements are acquired by our innovative dual transducer array design and are processed with the Multiple Signal Classification (MUSIC) signal processing method. The defect position is predicated accurately and efficiently, which is very adaptive for SHM over long pipes. However, it fails to predicate the defect circumferential position. The second method detects both defect axial and circumferential positions. Measurements are acquired via the same transducer array setting as the previous method, and are processed by a novel method, Geodesic Distance MUSIC (GD-MUSIC) method, based on the assumption that the defect-reflected GUW only travels along the geodesic path embedded in pipe surfaces. It predicates the defect axial and circumferential positions accurately for single-defect cases. However, the detection accuracy is affected if it is applied to multiple-defect cases because the geodesic assumption does not hold, which inspires us to design the third method. The third method detects multiple defects as well as a single defect over pipes. After being acquired by the dual array setting, measurements are processed by an innovative statistical Predominant Component Voting MUSIC (PCV-MUSIC) method, which identifies defect locations on aggregation of a trove of inaccurate detection results. A pipe is unwrapped regarding different axial cuts to form different voters. Votes casted by voters are pipe images of voters obtained from MUSIC method. The polling result predicates both axial and circumferential defect positions even for multiple defects. Finally, all methods are validated by multiple simulation cases