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
------- |