United States Environmental Protection Agency Risk Reduction Engineering Laboratory Cincinnati, OH 45268 Research and Development EPA/600/SR-92/143 October 1992 EPA Project Summary Acoustic Location of Leaks in Pressurized Underground Petroleum Pipelines Eric G. Eckert and Joseph W. Maresca, Jr. Experiments were conducted at the Underground Storage Tank (UST) Test Apparatus Pipeline in which three acoustic sensors separated by a maxi- mum distance of 38.1 m (125 ft) were used to monitor signals produced by 11.4-, 5.7-, and 3.8-L/h (3.0-, 1.5-, and 1.0-gal/h) leaks in the wall of a 5-cm- (2-in.-) diameter pressurized petroleum pipeline. The line pressures and hole diameters used in the experiments ranged from 69 to 138 kPa (10 to 20 psi) and 0.4 to 0.7 mm (0.01 to 0.03 in.), respectively. Application of a leak lo- cation algorithm based on the tech- nique of coherence function analysis resulted in mean differences between predicted and actual leak locations of approximately 10 cm. The standard de- viations of the location estimates were approximately 30 cm. This is a signifi- cant improvement (i.e., smaller leaks over longer distances) over the cross- correlation-based techniques currently being used. Spectra computed from leak-on and leak-off time series indicate that the majority of acoustic energy received in the far field of the leak is concentrated in a frequency band from 1 to 4 kHz. The strength of the signal within this band was proportional to the leak flow rate and line pressure. Energy propa- gation from leak to sensor was ob- served via three types of wave motion: longitudinal waves in the product and longitudinal and transverse waves in the steel. The similarity between the measured wave speed and the nominal speed of sound in a gasoline suggests that longitudinal waves in the product dominate the spectrum of received acoustic energy. The effects of multiple- mode wave propagation and the reflec- tion of acoustic signals within the pipeline were observed as non-random fluctuations in the measured phase difference between sensor pairs. Additional experiments with smaller holes and higher pressures (138 to 345 kPa [20 to 50 psi]) are required to de- termine the smallest leaks that can be located over distances of several hun- dred feet. The current experiments in- dicate that improved phase-unwrapping algorithms or lower noise instrumenta- tion, or both, are required to optimize system performance. This Project Summary was developed by EPA's Risk Reduction Engineering Laboratory, Cincinnati, OH, to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering Information at back). Introduction Underground pressurized pipelines are frequently used to transfer liquid products for many industrial applications. Some of these pipelines are associated with the underground storage tanks typically found at retail gasoline stations and others with tanks at industrial storage facilities; they can contain petroleum products or a vari- ety of other chemicals. The many systems used to detect leaks in underground pres- surized pipelines are designed for use on pipelines that are typically 5 cm (2 in.) in diameter and generally 15.2 to 61.0 m (50 Printed on Recycled Paper ------- to 200 ft) in length. EPA regulations (40 CFR Part 280 Subpart D) require that the leak detection equipment used to test a pipeline on a monthly basis be capable of detecting leaks at least as small as 0.76 L/h (0.2 gal/h) with a probability of detec- tion (P0) of 95% and a probability of false alarm (PFA) of 5%. If the equipment is used to test the line annually, it must be able to detect leaks as small as 0.38 L/h (0.1 gal/h); in the regulations, this type of test is designated as a line tightness test. If a leak is found, remediation must follow, and the first step is to locate the leak. Presently, two methods are used, but neither is totally acceptable. The first method is to systematically uncover the line and perform a visual inspection for leaks. Although this method works, it is time consuming, disruptive to operations, and costly. In addition, the line is subject to damage during the excavation process. The second method is to use a helium- or halogen-tracer technique, but both of these have operational and accuracy problems. There is a need for a nondestructive method of leak location that is accurate, relatively simple to use, and applicable to a wide variety of pipelines and pipeline products. One method of expediting the remedia- tion process is to apply remote sensing techniques to the pipeline as a means of accurately locating the leak. Passive- acoustic measurements, combined with advanced signal-processing methods, may provide a means for locating small leaks in limited-access pipeline delivery systems. Although passive acoustics has been used for some time to determine the spatial location of leaks, this concept has not been applied to underground petroleum pipelines. With the use of cross-correla- tion techniques, leaks of approximately 113 L/h (30 gal/h) have been successfully located in water-filled pipelines that are pressurized to 827 kPa (120 psi) and are less than 30 m long. Cross-correlation analysis works well when the signal is very strong or the background noise is not excessive. When the acoustic signal is weak in relation to the level of background noise or has a finite frequency bandwidth, more sophisti- cated signal processing techniques are available. Advanced signal processing is required if any of the following objectives are to be achieved: (1) the detection of leaks smaller than several gallons per hour, (2) a reduction in the number of false alarms and missed detections due to op- erational or ambient noise, and (3) an increase in the distance between sensors bracketing the leak. One such advanced technique is coherence function analysis. The best way to locate the signal source is to apply coherence function analysis to signals measured by two or more trans- ducers. Coherence function analysis, which estimates the correspondence between two measurements as a function of fre- quency, is analogous to the squared cor- relation coefficient but is a far more pow- erful tool in estimating and locating sig- nals. The coherence magnitude measures the strength of the correspondence, and the coherence phase measures the rela- tive time delay. In contrast, the correlation coefficient is a measure of correspondence that is the result of an integration over all frequencies. If the correspondence is fre- quency^dependent, or if the_ phase de^ pendence of the correspondence is a nonlinear function of frequency, the corre- lation is degraded. By contrast, coherence is a direct measure of the complex fre- quency correspondence between two measurements, and, therefore, preserves the actual correspondence between the two measurements of the signal. The last 5 to 10 years have seen sig- nificant advances in commercially available acoustic sensors, in powerful computers that are both small and inexpensive, and in digital signal processing. This means that an acoustic leak location system can be made available in a portable package, a possibility that makes it an attractive and viable option. Acoustic systems are attractive from an operational standpoint because the test is short (a few minutes) and the sensors can be mounted directly on the outside of the pipeline. Acoustic systems have direct application to the 15.2- to 61.0-m (50 to 200 ft) pipelines found at retail service stations because the sensors can be placed at each end of the line. The objective of this work was to esti- mate, by means of passive acoustic sen- sors mounted on the outside wall of the pipeline, the accuracy of locating a leak in a pressurized petroleum pipeline as a function of leak rate and distance between acoustic sensors. There are no regulatory requirements for leak location. For rapid repair of the pipelines and rapid remedia: tion of any contamination that might have occurred from the leak, however, it would be highly desirable if the leak could be located within 10% of the length of the pipeline in the case of a line longer than 30.5 m (100 ft), or within 3.0 m (10 ft) in the case of a line shorter than 30.5 m (100 ft). This limits the excavation to only a small fraction of the line. Theoretical estimates suggest that when coherence analysis is used, acoustic sensing tech- niques can detect a leak within 35 cm of its actual location. Results Experiments were conducted on a pipe- line at the UST Test Apparatus in which 3 acoustic sensors separated by a maximum distance of 38.1 m (125 ft) were used to monitor signals produced by 3.8-, 5.7-, and 11.4-L/h (1.0-, 1.5-, and 3.0-gal/h) gasoline leaks. These flow rates were generated through drilled holes 0.4 to 0.7 mm in diameter. The three-transducer system enabled the propagation speed of acoustic waves to be measured for par- ticular combinations of product, pipeline geometry, and analysis frequency band. Data recorded at the higher leak flow rates (5.7 and 11.4 L/h, [1,5 and,3.0 gal/h]) correspond to full line pressure (103 to 138 kPa [15 to 20 psi]); data recorded at the lower flow rate (3.8 L/h [1.0 gal/h]) were obtained under partial line pressure (69 kPa [10 psi]) because of the limitation imposed by the minimum available hole diameter (0.4 mm). Application of a leak location algorithm based on the technique of coherence function analysis resulted in mean differences between predicted and actual leak locations of 8.7 cm (at 11.4 L/ h [3.0 gal/h]), 3.6 cm (at 5.7 L/h [1.5 gal/ h]), and -11.6 cm (at 3.8 L/h [1.0 gal/h]). Standard deviations of the location esti- mates were 26.1 cm (at 11.4 L/h [3.0 gal/ h]), 26.3 cm (at 5.7 L/h [1.5 gal/h]), and 39.1 cm (at 3.8 L/h [1.0 gal/h]). The mean propagation speed was 915 m/s with a standard deviation of 146 m/s. Data recorded in the presence of a 1.9- L/h (0.5-gal/h) leak were obtained as part of an investigation of signal strength as a function of line pressure for a fixed-diam- eter hole (0.4 mm). The 1.9-L/h (0.5-gal/h) leak produced a detectable signal; how- ever, because of the reduced line pressure, the algorithm, as applied,'yielded no loca- tion estimates. Spectra computed from leak-on and leak-off time series indicate that the ma- jority of acoustic energy received in the far field of the leak is concentrated in a frequency band from 1 to 4 kHz. The strength of the acoustic signal within this band was proportional to the leak flow rate and line pressure, as expected. En- ergy propagation from leak to sensor was via three forms of wave motion: longitudi- nal waves in the product and both trans- verse and longitudinal waves in the steel. Isolation of each of these propagation modes was achieved through the use of gasoline and CO2 as the product fluids and through the generation of impulsive cali- bration signals. Though each of these ------- propagation modes is believed to contrib- ute to the overall received signal, longitu- dinal wave motion in the product was clearly the dominant propagation mode for liquid-filled pipelines. The effects of multiple-mode wave propagation and the reflection of acoustic signals within the pipeline were observed 'as non-random fluctuations in the measured phase differ- ence between sensor pairs. Accurate leak location requires the identification of frequency bands within which a high degree of similarity is main- tained between acoustic signals propa- gated along different paths from leak to sensor. Coherence function analysis pro- vides the best means of gauging this similarity and, thus, separating useful in- formation opncerningjhe leak location from ambient or system noise. While the signal- to-noise ratio (SNR) was observed to be generally high within the entire 1 - to 4-kHz frequency band, continuous regions of high coherence appropriate for source location were typically 100 to 500 Hz in width. Several data sets recorded in the pres- ence of the 11.4-L/h (3-gal/h) leak exhib- ited high coherence over a 2-kHz band- width. Location estimates obtained by means of the cross-correlation technique showed that without the detailed knowl- edge of signal similarity provided by the coherence function, cross-correlation analysis cannot locate small leaks with acceptable accuracy. The observed cor- respondence between measured and pre- dicted phase shifts within the 1 - to 4-kHz frequency band demonstrates the need to develop a more sophisticated location al- gorithm such that a greater fraction of the information contained in coherent leak signals may be processed. Buried pipelines provide a generally quiet ambient environment in which to per- form acoustic measurements. Since the SNR for a given leak largely determines the ability of a passive acoustic system to locate the leak, the system noise level should be determined by ambient acoustic noise rather than by electronic noise. The combination of sensors (CTI-SOs*) and pre- amplifiers (Panametrics 5660-Cs) used in this work was incapable of resolving the low levels of ambient acoustic noise asso- ciated with the pipeline at the UST Test Apparatus. Improved system performance may be attained through the use of trans- ducers with greater sensitivity in the low- frequency range (1 to 10 kHz) and low- noise preamplifiers. Conclusions Passive acoustic measurements, com- bined with advanced signal processing techniques based on coherence analysis, offer a promising method for the location of small leaks in pressurized petroleum pipelines found at retail gasoline service stations and industrial petroleum storage facilities. Although the results presented in this work represent a significant im- provement over previous pipeline leak lo- cation efforts, additional research and de- velopment are required before system performance can be optimized. Location of leaks of several tenths of a gallon per hour over distances of several hundred feet should ultimately be possible. Recommendations The full capability of the location algo- rithm was not evaluated in these tests. * Mention of trade names or commercial products does not constitute endorsement or recommendation for use. The smallest hole used to generate a leak in the experiments was 0.4 mm. A line pressure of 138 kPa (20 psi) resulted in a leak rate of 3.8 L/h (1.0 gal/h). It is recom- mended that additional experiments be performed with smaller holes at higher line pressures (138 to 345 kPa [20 to 50 psi]) to determine the minimum leak rate that can be reliably located. The current work indicates that further improvement can be realized through the application of better phase-unwrapping algorithms and better instrumentation. A better under- standing of the underlying physics of pipeline acoustics, including the propaga- tion modes and source mechanisms of the acoustic leak signal, will help optimize the algorithms and the hardware. It is recommended that the following,,work be performed to extend the technology: • develop a location algorithm ca- pable of processing the coherence phase over an arbitrarily wide-fre- quency band, • characterize the wave propagation modes excited by the acoustic leak signal and the degree to which each mode enhances or degrades the leak location estimate, • reduce system noise through trans- ducers specifically designed for low- frequency, high-sensitivity applica- tions and through low-noise, audio- range preamplifiers, and • automate the data acquisition sys- tem and signal processing algo- rithm, and evaluate system perfor- mance on a variety of actual pipe- lines. The full report was submitted in fulfill- ment of Contract No. 68-03-3409 by Vista Research, Inc., under the sponsorship of the U. S. Environmental Protection Agency. •U.S. Government Printing Office: 1992— 648-060/60133 ------- Eric G. Eckert and Joseph W. Maresca, Jr., are with Vista Research, Inc., Mountain View, CA 94042 R. W. Hlltgeris the EPA Project Officer (see below). The complete report, entitled "Acoustic Location of Leaks in Pressurized Under- ground Petroleum Pipelines," (Order No. PB92- 207 687/AS; Cost: $19.00, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Risk Reduction Engineering Laboratory U.S. Environmental Protection Agency Edison, NJ 08837 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati. OH 45268 Official Business Penalty for Private Use $300 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-35 EPA/600/SR-92/143 ------- |