First surveys run with electromagnetic acoustic transducers.
This article introduces some parts of the data evaluation process used and presents first results obtained from these two field tests. An extensive follow-up validation program is in progress.
Commonly, non-destructive ILI tools are based on technologies such as magnetic flux leakage (MFL), ultrasonic testing (UT) or eddy current systems. However, none of these techniques is applicable to the detection of SCC, especially in gas pipelines. Rosen has developed a new type of ultrasonic sensor that is based on an electro-magnetic acoustic transducer (EMAT) (Klann and Beuker, 2006). By utilizing physical effects such as the Lorentz force and magnetostriction, this technology allows, unlike conventional UT, contact free-generation and observation of ultrasonic signals. Since the pipeline serves as its own transducer, this new approach works independently of a coupling medium between the sensors and the pipeline to be inspected. A 16-inch tool was manufactured and equipped with EMAT sensors (Fig. 1).
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EMAT Module Arrangement
Following a high-resolution approach (Fig. 2), numerous EMAT modules were arranged on the inline inspection tool.
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Fig. 3 shows the basic arrangement of the EMAT modules used to inspect a distinct area (pixel) of the pipeline. The ultrasonic waves do not travel around the whole circumference of the pipeline before they are observed by a receiver. Rather, the acoustic waves only travel a short distance between the EMAT sender and the receiver thereby allowing comparatively simple data evaluation and avoiding false alarms. The sensor arrangement required to inspect one pixel of the pipeline consists of one EMAT sender (left hand side) and two EMAT receivers (one on the left hand side, one on the right hand side). The EMAT sender generates a tailored shear horizontal wave which is characterized by distinct frequencies and therefore especially sensitive to near-surface defects.
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The generated wave propagates from the EMAT sender on the left-hand side toward the EMAT receiver on the right-hand side. If no cracks are present, this wave reaches the receiver and is recorded as a so-called transmission signal. However, if there is a crack-like defect between the EMAT sender and the opposite EMAT receiver, parts of the signal energy are reflected in the direction of the EMAT sender. There, this signal is recorded as a so-called echo signal by the second EMAT receiver. This means that two acoustic data channels exist for each pixel, namely one echo and one transmission channel.
From these data channels, numerous signal parameters can be extracted, e.g. signal frequencies, signal amplitude, traveling time of the acoustic wave and so on. Unlike an MFL measurement, not only one value (magnetization level) is recorded at one particular pipeline position, but several vectors (time signals), thus providing much more information.
Additional information, e.g. lift-off between the EMAT modules and the pipeline, is stored in separate data channels. This independent data storage ensures that echo and transmission data can unambiguously be evaluated with respect to the success of the physical measurement.
Transmission Signal Evaluation
The transmission channel contains information about the wave that directly propagates from the EMAT sender to the transmission receiver. The overall amplitude of this wave depends on the amount of lift-off, the presence of a defect, and the existence (and type) of external coating. The latter dependency can be used to detect coating disbondment, since a coating generally damps the acoustic wave. Hence, if the damping effect is missing due to a reduction in the bonding quality of the coating, a significant increase in the signal amplitude can be observed. Distinct examples for several cases are shown below.
Echo Signal Evaluation
From Fig. 3 it can be seen that an echo signal will only be recorded if a significant amount of energy is reflected into the EMAT echo receiver. Since the echo receiver is active for a short time interval, only signals that are reflected from a specific position relative to the sensor within the pipeline are detected. Consequently, other signals emitted from adjacent EMAT senders or late reflections from other positions within the pipeline can easily be excluded during the data evaluation process.
Due to the arrangement of the EMAT modules, the system is especially suitable for the detection of features with an axial dimension. A detailed analysis of significant echo signals--signal amplitude, arrival time and frequency content--provides valuable information about the type of defect detected.
First inspections employing the [RoCD.sup.2] 16-inch tool were performed in May 2006, one in a gas pipeline and another in an oil pipeline. Both runs were evaluated using automated algorithms developed during the project. For example, girth welds can be detected easily since they cause typical signal characteristics in different data channels (transmission channel, echo channel, lift-off channel). Similarly, long seams can be observed in echo channels (increase) and transmission channels (decrease).
Fig. 4 shows a C-scan view of one part of the gas pipeline. For this plot, the EMAT echo signals obtained from each particular pipeline position were integrated to form one single value. A C-scan can thereby be generated. Recorded wave signals obtained from three particular positions within the pipeline are shown as examples in the insets of Fig. 4. The high-resolution approach covers the full circumference with 36 channels. As can be seen from Fig. 4, several significant signal increases can be observed. For example, since girth welds are good reflectors of acoustic waves, they can easily be identified in the echo data. At the end of the joint shown in Fig. 4 (lower right corner), another significant echo signal can be observed. This echo is generated by a linear anomaly in the pipeline.
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Fig. 5 consists of four individual illustrations which provide additional information on the particular anomaly shown in Fig. 4. While the two upper graphic representations show integrated data of the echo (left) and the transmission data (right) as a function of the circumferential position and the log distance, the two lower images show non-integrated vector data of one specific channel (the channel at 75 degrees). The lower left illustration shows the echo time signals as a function of the log distance, whereas the lower right image represents the corresponding signal spectra.
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Time domain signal analysis allows collection of information about the orientation of the defect in relation to the pipe axis. This means that the echo channels are sensitive to defects in both the axial and circumferential direction.
Fig. 6 shows transmission data of another joint as a C-scan view. Decreased signal amplitudes can be observed at the girth welds and the long seam, since ultrasonic energy is reflected at these positions. As a result of the reflection, this energy does not reach the transmission receiver. The regular shape (stripes) in the transmission signals is generated by the tape coating. Red-colored areas, e.g. at the beginning and end of the joint, indicate a weaker or even loose coating. This means that the transmission channels are--on the one hand--sensitive to larger reflectors (signal decrease) and--on the other hand--to different coating qualities (signal increase if coating is weaker).
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A novel high-resolution EMAT technology has been developed and implemented on an ILI-tool by Rosen. The intelligent inspection tool has been run successfully in two different 16-inch pipelines, a gas pipeline and an oil pipeline, both operated by Saudi Aramco. The promising results of the first inspection survey are being further validated. The data evaluation can rely on multi-dimensional data sets, which allows a continuous improvement, on the basis of the scheduled validation program.
Klann and Beuker, 2006, "Pipeline Inspection With The High Resolution EMAT ILI--Tool: Report On Full-Scale Testing And Field Trials," IPC2006-10156, in: Proceedings of IPC 2006, 6th International Pipeline Conference, Sept. 25-29, 2006, Calgary, Alberta, Canada
By Dr. Joerg Damaschke and Thomas Beuker, Rosen Technology & Research Center, Germany
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|Author:||Damaschke, Joerg; Beuker, Thomas|
|Publication:||Pipeline & Gas Journal|
|Date:||Oct 1, 2006|
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