Abstract:
A method for the nondestructive testing of pipeline, and internal or external coatings, using guided ultrasonic waves excited from the inside or outside diameter, wherein at least two transducers are employed to send and receive ultrasound, wherein a data normalization scheme is employed to account for transducer variability, and wherein the hardware arrangement has a number of pulser channels and a number of receiver channels for the collection and storage of signals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application 60/987,864 filed on Nov. 14, 2007, the entirety of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     Aspects of the invention relate to pipeline inspection. More specifically, aspects of the invention relate to providing a guided wave pipeline inspection system and method with enhanced coating disbond detection, corrosion detection and crack sizing capabilities. 
     BACKGROUND INFORMATION 
     Corrosion is one of the primary preventable causes of failure in transmission pipelines. Protective coatings serve as the first barrier against this phenomenon. Once a coating is compromised, it is only a matter of time before corrosion begins. 
     The Magnetic-Flux Leakage (MFL) method is currently the dominant technology for the in-line inspection (ILI) of transmission pipeline. Because the MFL ILI method depends on the ferrous nature of the material being inspected, it is only capable of detecting the effect of the corrosion problem and not the cause; a failure of the protective coating. Furthermore, MFL methods can potentially miss crack-like defects and have a large tolerance and low certainty for sizing metal-loss defects, such as corrosion. 
     The ability to reliably detect coating disbonds would represent a significant contribution to the pipeline integrity management sector as it would enable preventative maintenance action prior to the development of corrosive metal loss defects. In the event that defects are already present, a method of reliably detecting cracking and more accurately sizing metal-loss defects would also be a welcome contribution. 
     To prevent the onset of corrosion and to identify critical regions where corrosion may have already initiated, there is a need for a reliable, robust, multi-feature coating disbond detection technique that is applicable over a range of coating types and thicknesses. For regions that already contain defects, there is a present need for more accurate detection and sizing capabilities. 
     Furthermore, there is a need for a coating disbond detection technique and apparatus that will not be confounded by moderate wall thinning, diametral variations, and wetted/soiled coating interfaces and for a defect sizing technique that is highly sensitive to wall losses and cracking. 
     Furthermore, there is a need for a sensor arrangement and data acquisition technique that is relatively low cost and capable of withstanding the rigors encountered in an ILI environment. 
     SUMMARY 
     It is therefore an objective of an aspect of the invention to provide a reliable method and apparatus for detecting coating disbonds using a multi-feature algorithm. 
     It is also an objective of an aspect of the invention to provide a more accurate and reliable method for crack and corrosion sizing than is presently available through the employment of guided-wave focusing in real-time, synthetically, and/or geometrically and through the employment of pattern recognition approaches. 
     It is also an objective of an aspect of the invention to provide said capabilities in a package that is capable of withstanding the rigors of a typical ILI environment. 
     It is also an objective of an aspect of the invention to provide disbond detection capabilities for a range of coating types and thicknesses without being confounded by moderate wall thinning, diametral variations, and wetted/soiled interfaces. 
     Another objective of an aspect of the invention is to improve upon conventional technology through the employment of newly identified coating disbond detection features, newly developed data processing techniques for ILI using guided waves, newly developed guided-wave focusing capabilities for ILI, and newly developed pattern recognition algorithms. 
     The objectives are achieved as illustrated and described. In an example embodiment of the invention, an ultrasonic guided-wave nondestructive testing method for coating disbond detection is presented, comprising: a mode selection routine, a self-calibrating sensor arrangement, a set of extracted wave features, and signal enhancement methods, wherein the bond state of internal or external pipe coatings, or both, may be characterized. 
     In a further example embodiment, the system is configured to acquire data at fixed intervals while moving through a pipe. 
     In a further example embodiment, the system configured for disbond detection uses a guided wave mode and frequency that exhibits dominant in-plane particle displacement, so to be sensitive to coating presence, or lack thereof, and is insensitive to moderate wall thinning, diametral variation, and wetted/soiled interfaces. 
     In a further example embodiment, the system for disbond detection may be configured such that a minimum of two sensors are required for the full circumferential inspection of a pipe at a particular axial location when employing circumferential guided waves. Sensors may comprise EMATs, piezoelectric transducers, or any other device capable of generating ultrasound in the pipe wall. Examples of EMATs may be, as non-limiting examples, Lorentz Type and Magnetostrictive Type sensors. The EMAT&#39;s may be pulsed or permanent magnet types. 
     In another example embodiment, the system for disbond detection may be configured such that a minimum of three sensors are required for the partial circumferential inspection of a pipe over some axial distance, as determined by the sensor separation, when employing axial guided waves. In this example embodiment, multiple sensor sets are required to achieve full circumferential inspection over some axial distance determined by the sensor separation. 
     In a further example embodiment, the system for disbond detection is configured such that all extracted signal features are measured relative to the induced wave as received by a receiver juxtaposed to the transmitting sensor. 
     In a yet further example embodiment, the system for disbond detection is configured such that the arrangement configured to send, receive and store signals uses an algorithm that minimizes data collected from the pipe. 
     In another example embodiment, the system for disbond detection may be configured such that a receiving sensor is exactly opposed the transmitting sensor, wherein constructive interference of the CW and CCW propagating waves result in the formation of a single received wave packet for bare or perfectly coated pipe. Deviations from perfect constructive interference can be used to characterize the coating bond state. 
     The objectives are achieved as illustrated and described. In an example embodiment of the invention, an ultrasonic guided-wave nondestructive testing method for crack and metal-loss defect sizing using a real-time, synthetic, and/or geometric focusing technique is presented, comprising at least two transducers; an arrangement configured to send, receive, and store signals, wherein the arrangement has a number of pulser channels and a number of receiver channels, wherein the arrangement has at least one multiplexing arrangement for multiplexing signals from the arrangement; and a time delay arrangement connected to the arrangement configured to send, receive and store signals, the time delay arrangement configured to calculate and/or recall delay times for activation of the at least two transducers and activate the arrangement configured to send and receive signals for the at least two transducers based upon the delay times. 
     In a further example embodiment, the array geometry may be arranged so to achieve focusing in the circumferential direction, axial direction, or in both directions. 
     In a still further example embodiment, the array may contain as few as two sensors and a maximum number limited by the smaller of two numbers: the maximum number physically permitted by the pipe geometry, or the maximum number permitted by the utilized electronics. 
     In a yet farther example embodiment, the time-delay and amplitude weightings required for real-time phased-array focusing may be computed onboard or may be stored in an onboard library, wherein focal points are predetermined or fed to the system from other data sources, which may include, but are not limited to, guided wave, MFL, and Eddy Current data. 
     Additionally, in an example embodiment, correction of the time-delay and amplitude weights may be made to account for the motion of the ILI tool. Corrections are based on the velocity of the tool relative to the desired focal point, wherein the altered path length can be used to adjust the time-delays and amplitude weights. This correction may not be required but may result in enhanced focusing results. 
     In a further example embodiment, the spatial orientation of the array sensors used for geometrical focusing is predetermined by the desired focal point and mode and frequency of inspection. 
     In an example embodiment of the invention, a pattern recognition approach may be employed which has, as inputs, occurrences and values of wave phenomena such as, but not limited to, mode conversion phenomena, scattering phenomena, transmission and reflection factors, phase features, and time-of-flight and amplitude features, wherein the algorithm inputs are compared to predetermined trends and, as output, a prediction is made concerning the type of defect detected, location of defect and the approximate size of one or more of the defect dimensions. The pattern recognition algorithm inputs are not limited to ultrasonic data, but may include data from any other test method, such as, but not limited to, MFL or Eddy Current data. 
     In another example embodiment of the invention, an ultrasonic guided-wave nondestructive testing method for a pipe is presented. In this example embodiment, the method provides for placing an array of transducers on the pipe; placing a self calibrating sensor arrangement on the pipe, generating a wave through the transducers from a mode selection routine operable on a computer to select a mode of excitement for the array of transducers; receiving the wave through the sensor arrangement and processing the wave through a sensor receiving routine operable on the computer, the computer configured to operate with the self-calibrating sensor arrangement and receive the wave from the array of transducers, and processing the wave through the computer through a signal enhancement process; wherein at least one of a bond state of internal and external pipe coatings are characterized through the signal enhancement process and any present coating disbonds are detected and sized approximately. 
     In another example embodiment, a guided-wave sensor array and system for the approximate sizing of corrosion and cracking in pipe using at least one of a real-time, synthetic, and geometric focusing technique is presented comprising: at least two transducers, an arrangement configured to send, receive, and store signals, wherein the arrangement has a number of pulser channels and a number of receiver channels, wherein the arrangement has at least one multiplexing arrangement for multiplexing signals from the arrangement; and a time delay arrangement connected to the arrangement configured to send, receive and store signals, the time delay arrangement configured to at least one of calculate and recall delay times for activation of the at least two transducers and activate the arrangement configured to send and receive signals for the at least two transducers based upon the delay times. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A ,  1 B, and  1 C are examples of phase velocity, group velocity, and attenuation dispersion curves, respectively, for circumferential shear horizontal (SH-) waves in bare and coated pipe. 
         FIGS. 2A ,  2 B, and  2 C are examples of wave structures for different circumferential SH-wave modes and frequencies in a coated pipe. 
         FIGS. 3A ,  3 B,  3 C, and  3 D contain illustrations of several different self-calibrating sensor arrangements for inspection using both circumferentially and axially oriented waves. 
         FIGS. 4A ,  4 B,  4 C, and  4 D illustrate how coating disbond detection and size estimation is done using a sensor configuration as seen in  FIG. 3A . Experimental results for the detection of differently sized coating disbonds in a 20 in. Schedule 10 pipe with a 3 mm coal-tar coating using the 130 kHz SH1 wave mode. 
         FIGS. 5A ,  5 B, and  5 C contain experimental results for the detection of differently sized coating disbonds in a 20 in. schedule 10 pipe with a 3 mm coal-tar coating using the 130 kHz SH1 wave mode. Frequency-based disbond detection features are detailed. 
         FIGS. 6A and 6B  provide an illustrative example of how disbond detection is done using the constructive interference method as illustrated in  FIG. 3D . 
         FIG. 7  shows several of the identified coating disbond detection features plotted on the same graph vs. disbond size, illustrating the disbond sizing estimate potential. 
         FIGS. 8A and 8B  illustrate the phased/synthetic array and geometric approach to focusing guided waves. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 3A ,  3 B,  3 C and  3 D, in an example embodiment of the invention, an apparatus for implementing a method is developed for the inspection of a coated pipe  10  in which a transmitting sensor  20  and a receiving sensor  30  are separated by some short fixed distance. Referring to  FIGS. 4A ,  4 B,  4 C and  4 D, all signal features used for disbond detection are calibrated to a reference pulse  40  that travels directly from the transmitter  20  to the receiver  30 . The pulse  50 , seen in  FIGS. 4B-4D , that arrives prior to the reference pulse  40  is electronic noise generated by the electromagnetic acoustic transducers (EMATs) used for excitation and reception. In an example embodiment, there is no specific requirement for the use of EMATs; any device that generates the desired guided-wave mode and frequency is applicable. Several coating disbond detection features are illustrated in the conceptual waveform shown in  FIG. 4A . The time-of-flight (“TOF”)  110  and amplitude disparity (“ΔA”)  120  between the reference pulse  40  and the first complete circumferential traversal  180  may be used to identify and approximately size coating disbonds, as illustrated in  FIGS. 4B-4D . Referring to  FIGS. 5A ,  5 B and  5 C, the time-frequency representation of the waveforms seen in  FIGS. 4B-4D  can be seen. Another disbond detection feature, the lost frequency content  210 , is a measure of the relative difference between the frequency content of the reference pulse  60  and the first complete circumferential traversal  200 . The remaining frequency content of the reference pulse  60  and first complete traversal  200  may be utilized. All disbond detection features described here may also be applied to the analysis of the partial circumferential traversal  170 , 190  though the results will not account for the region between the transmitter  20  and receiver  30 . The time-frequency domain signal of the complete traversal pulse  200  may also be used to aid in the identification of low-amplitude pulses that are embedded in the noise of the time-amplitude representation. 
     Referring to  FIG. 7 , in an example embodiment of the invention, the identified trends relating to coating disbond size are used to approximately size the coating disbonds. The change in time-of-flight (“ΔTOF”) trend  220 , the amplitude disparity trend  230 , or the lost frequency content trend  240  may be used individually to estimate coating disbond size or through any combination of addition, subtraction, multiplication, and division of the trends. The sizing estimate routine is not limited to the feature trends shown in  FIG. 7  but may employ trends from any of the features discussed herein. 
     Referring to  FIGS. 3A ,  3 B,  3 C and  3 D, multiple transmitter  20  and receiver  30  arrangements are possible. As seen in  FIG. 3A , an example embodiment of the coating disbond detection technique may require as few as two sensors. In this configuration, a single transmitter  20  is used to generate a wave that travels directly to the receiver  30  and then once completely around the circumference of the pipe  10 , arriving at the receiver  30  once again. Depending on the method of excitation, a wave traveling in the opposite direction may also be present but does not initially travel beneath the receiver and also does not traverse the entire circumference prior to first reception. In  FIG. 3A , the clockwise going wave travels directly to the receiver  30  and then completely traverses the circumference of the pipe  10 . The counter clockwise going wave does not. 
     For cases where high attenuation prevents sufficient propagation distances, or for cases in which increased circumferential resolution is desired, an example embodiment may employ the use of multiple transmitter  20  and receiver  30  pairs for disbond detection, as illustrated in  FIG. 3B . In this configuration, a wave excited by a transmitter  20  travels directly to the nearest receiver  30 , providing a reference pulse  40 , then propagating around the circumference of the pipe  10  when it is again received by the next closest receiver  30 . This configuration is not limited to two sensor pairs, as illustrated, but can include as many sensor pairs as can be physically accommodated given the geometrical constraints of the pipe  10 . 
     Referring to  FIG. 3C , another example embodiment of the invention in which full circumferential inspection for coating disbonds is accomplished with two sensors and propagation distances as short as one half of the circumference of the pipe  10  is illustrated. In this configuration, the transmitter  20  simultaneously generates a clock wise and counter clock wise traveling wave. For the case of a coated pipe  10  or a completely bare pipe  10 , the clock wise and counter clock wise traveling wave will constructively interfere in a region opposite the transmitter  20 . The receiver  30  is placed in this region. Referring to  FIGS. 6A and 6B , constructive interference, as would occur for a uniform coating or for a bare pipe  10 , will result in the reception of only one perceived wave packet  70  whose amplitude contains contributions from both the clock wise and counter clockwise traveling waves (wave packet envelope is illustrated in figure). If not completely attenuated, subsequent pulses  80  from multiple traversals of the pipe circumference  10  may be received and utilized for measurement. This is also true of the configurations seen in  FIGS. 3A and 3B . In the case of an asymmetrically distributed coating anomaly, the single perceived wave packet  70  will assume a multiple-peak packet  90  as provided in  FIG. 6B . Features that may be utilized to identify and estimate the size of the coating disbond include the time-of-first-arrival (“TOFA”)  100 , the time-of-flight (“TOF”)  110  between the first received wave packet  70 , 90  and subsequently received wave packets  80 , the amplitude disparity  120  between the multiple peaks of the first wave packet  90 , the arrival time disparity  130  between the multiple peaks of the first wave packet  90 , the average amplitude  140  of the multiple peaks of the first wave packet  90 , and the average TOFA  150  of the multiple peaks of the first wave packet  90 . These same features can be examined for any or all subsequently received wave packets  80 . In a further example embodiment, the amplitude  160 , TOFA  100  of the wave packet  70 , and TOF  110  between subsequently received waves  80 , or multiples thereof may be used as a baseline if acquired in a known bare pipe region or known region of well bonded coating. 
     Referring to  FIG. 1 , in an example embodiment of the invention, the selected mode and frequency used for coating disbond inspection should be sensitive to the presence of coating, or lack thereof and should not be sensitive to variables often encountered in an inspection environment. These variables could include moderate wall thinning, diametral variations, and wetted/soiled coating interfaces. In this embodiment, the phase velocity dispersion curves for the bare pipe  250  (black line) and the coated pipe  260  are plotted. In  FIG. 1 , the phase velocity, group velocity, and attenuation dispersion curves are shown for the SH mode group. A unique feature of this mode group is that it exhibits entirely in-plane particle displacement, that ensures that energy is not leaked into water that may occur at a disbond interface and minimizes losses into moisture containing soils. Lamb-type modes with dominant in-plane displacements also meet these criteria. Furthermore, the coated pipe curves  260  will converge, temporarily, to the bulk shear-wave velocity of the substrate to which the coating is applied, usually but not limited to steel. In these convergence regions  270 , the wave structure in the pipe wall is similar for both the bare pipe and coated pipe cases.  FIGS. 2A-2C  illustrate the wave structures for modes and frequencies within the convergence regions  270 . In these figures, the lines  300  correspond to particle displacement and the lines  310  correspond to shear stress. The ideal modes and frequencies have large particle displacements at the interface of the pipe  320  and the coating  330 . This ensures sensitivity to the presence of the coating. It is seen that all mode/frequency combinations in  FIG. 2  meet this criteria. Next, the group velocity dispersion curves seen in  FIG. 1B  show that, in these identified regions, the measurable wave velocity peaks at a value below the normal SH0 wave velocity in a bare pipe. Therefore, with the identified modes and frequencies, wave packets will travel more slowly in a coated pipe than in a bare pipe, resulting in a time-based disbond detection feature. Additionally, as seen in the attenuation dispersion curve shown in  FIG. 1C , the presence of coating results in wave attenuation, providing an amplitude-based disbond detection feature. Furthermore, for the mode  290  shown in  FIG. 1 , it is seen that attenuation increases as a function of frequency. Therefore, coating will cause the higher frequency content of pulse to be attenuated more quickly than the lower frequency content, thus providing a frequency-based disbond detection feature. Essentially the coating is acting as a high-frequency filter with the amount of high frequency content being filtered dependent on the amount of coating present. This same concept is applicable to any attenuation trend that is either entirely increasing or entirely decreasing with frequency. Lamb-type modes are applicable. 
     Referring to  FIGS. 8A and 8B , in an example embodiment of the invention, guided-wave focusing can be achieved through several methods. In the phased-array and synthetic focusing methods, an array of sensors  340  such as illustrated in  FIG. 8A  is employed, though the array may assume other shapes than the illustrated line. In the phased-array approach, time delayed and possible amplitude weighted pulses  360  are generated using the array sensors  340  which will converge constructively at some focal point  350 , which need not be in the center of the array but may be skewed in some direction. The time delays and amplitude weights are calculated based on the mode and frequency excited and the path  370  to the focal point  350 . All or some of the array sensors  340  may be used for the reception of wave reflections. In the case of a synthetic phased array, one sensor  340  is excited at a time and all other sensors  340  are used to receive reflections. Once all sender/receiver permutations are complete, time delays and amplitude weights are applied digitally, via post processing, to produce a focal point  350  at a desired location. The focal point  350  may then be digitally rastered to generate a C-scan image of the inspected area. In the case of a real-time phased-array, rastering may be employed but the size of the rastered region will depend on the data acquisition speed and ILI tool velocity. In  FIG. 8B  the geometric focusing concept is illustrated. In this method, the focal point  350  is determined by the relative spacing of the array sensors  340 , which are all excited simultaneously. 
     In an example embodiment, a method to determine wave propagation features that are sensitive to coating failures is presented. Multiple detection features are necessary in order to create a robust and reliable disbond detection routine. Several potential features, including time, amplitude and frequency-based features are discussed. 
     Wave attenuation is one feature for the detection of coating disbonds. Because most protective coatings are visco-elastic in nature, a portion of the wave energy may be absorbed into the coating layer, resulting in a decrease in the amplitude of the received wave. Disbonds will therefore appear as an increase in wave amplitude. This phenomenon can be seen in the ultrasonic guided wave RF waveforms. 
     Because signal amplitude will vary with EMT liftoff, material properties, and other factors, using the absolute amplitude measurement may not be reliable. To circumvent these issues, a relative amplitude comparison may be used to determine disbonds. The transmitting and receiving EMAT&#39;s are separated by some short fixed distance and all amplitudes are normalized to the “reference pulse” that travels directly from the transmitter to the receiver. In one non-limiting embodiment, the circumferential wave traveling in the counterclockwise direction is the first wave to traverse the entire circumference and is therefore used for coating bond assessment. 
     Another coating disbond detection feature arises when theoretical modeling results are considered. As provided in group velocity dispersion curves, there is a notable decrease in the maximum velocity at which a group of waves can propagate in a coated pipe as compared to the case of a bare pipe. Additionally, the number of propagating modes in the frequency range of interest increases significantly for the case of a coated pipe. The velocity difference between the bare pipe and the coated pipe cases can be used as a disbond detection feature. 
     When visco-elastic materials are involved, the attenuation of ultrasound guided waves becomes a frequency dependent problem. In an example embodiment, an elastic layer is assumed and a coating layer is truly visco-elastic and thus there will be some attenuation due to absorption. In the case of guided SH-waves, attenuation increases with frequency. As SH-waves are of primary interest, the tendency of attenuation to increase with frequency can be used as a coating disbond detection feature. The absence of coating will result in a frequency spectrum with higher frequency content. As the amount of well bonded coating increases, higher frequency content will be filtered out by absorption. Single sensor pair and multiple sensor pair systems may be used for amplitude based, time based and frequency based analysis techniques. 
     In the foregoing specification, the aspects of the invention have been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than in a restrictive sense.