Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF DETECTING CORROSION IN PIPELINES AND THE
LIKE BY COMPARATIVE PULSE PROPAGATION ANALYSIS
BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates to a system,
apparatus and method for testing elongate objects,
such as pipe, and is directed toward the problem
of detecting corrosion, defects or other anomalies
to the pipe under conditions where access and/or
visual inspection of the pipe is either not
possible or impractical.
b) Background Art
In petroleum processing and petrochemical
plants and other industrial environments, it is
common to have numerous pipes extending between
various locations in the plant, with these pipes
carrying fluid or gas (e.g.petroleum products),
often under high heat and pressure. These pipes
are commonly made of steel, and can have an inside
diameter ranging anywhere from two to sixty
inches, or even outside of this range. The
exterior of these pipes are often insulated, with
the insulating layers being as great as
approximately 1/8 to 5 inches in thickness, or
outside of this range.
For a number of reason, (safety,
environmental considerations, avoiding costly
shut-downs, etc.), the integrity of these pipes
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must be maintained. Defects in the pipe can occur
for a number of reasons. One is that moisture can
collect between the insulating layer and the pipe,
thus causing corrosion (i.e.rust). Visual
inspection of the steel pipe that is encapsulated
in insulation is not possible unless the layers of
insulation are removed, and then replaced.
However, this is expensive and time consuming, and
as a practical matter it would be economically
unfeasible to accomplish the inspections with
reasonable frequency.
It is the object of the present invention to
provide a means of inspecting pipes under the
circumstances given above in a manner that
corrosion, other defects and/or anomalies can be
detected with a relatively high degree of
reliability, and in a manner that the various
difficulties of inspection, such as those
mentioned above, can be diminished and/or
alleviated.
SUMMARY OF THE INVENTION
The method of the present invention enables
corrosion on an elecromagnetical permeable
elongate member, such has a pipe, to be detected
quite effectively. More specifically, this method
enables much of the irrelevant information
(reflections, electromagnetic noise) to be
elimiated from the wave form, and then the wave
forms processed in a particular manner to enable
clearer identification of variations in the wave
form that would indicate corrosion.
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In the method of the present invention, a
nearside and far side electric or electromagnetic
pulses (waves) are transmitted from, respectively,
nearside and farside spaced transmitting locations
on the elongate member. The pulses (waves) travel
toward one another to intersect at intersecting
locations on the elongate member.
The farside pulses are received as wave forms
at a receiving location after intersection with
related nearside pulses. The transmission of the
nearside and farside pulses are synchronized so
that the intersections of the near side and far
side pulses (waves) occur at spaced intersecting
locations on the elongate member.
The wave forms of at least two of the far
side pulses (waves) which are spaced from one
another are combined to form a composite wave
form. A variation of variations are ascertained-
from the composite wave form as a means of
detecting corrosion.
In the preferred form, one of the wave forms
of the two wave forms that are to be combined is
inverted and then added to the other of the
waveforms being combined to create a difference
waveform, and variations in the difference wave
form are ascertained as a means of detecting
corrosion.
Also, in the preferred form, the nearside
pulses which pass through points of intersection
that are adjacent to one another are considered to
be sequential nearside pulses, with the order of
sequence being the same as the order in which the
points of intersection are spaced along the
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elongate member. The combining of the nearside
waveforms is accomplished in a pattern such that
first and second adjacent wave forms are combined
to make a first composite waveform, the second
waveform and an adjacent third waveform are
combined to make a second composite waveform, the
third waveform is combined with an adjacent fourth
waveform to make a third composite waveform, with
the pattern repeating itself with subsequent pairs
of waveforms from adjacent farside pulses.
Adjacent composite wave forms are compared with
another as a means of detecting corrosion.
A reference wave form is established by
creating composite wave forms resulting from
pulses that intersect at non-corroded areas of the
elongate member, and identifying composite
waveforms that differ from the reference composite
waveform by a phase shift and/or dispersion and/or
amplitute and and/or wave distortion.
Corrosion that is present between two
adjacent points of intersection on the elongate
member is detected by comparing a composite wave
form resulting from combining the difference
waveform overlapping the point of intersection
with difference wave forms on opposite sides of
the overlapping composite waveform.
Also, corrosion that is present at a point of
intersection of two wave forms can be detected by
deriving two difference waveforms by combining the
waveform at the point of corrosion with adjacent
waveforms to form two difference wave forms which
are then compared.
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Also, two additional difference wave forms
that are on opposite sides of, and adjacent to,
the two difference waveforms which are analyzed to
detect the corrosion are compared with the two
5 difference waveforms which are combined at the
point of intersection as a means of detecting
corrosion.
Other features of the present invention would
become apparent from the following detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a somewhat schematic view of the
system of the present invention being in its
operative position where it is being used in
testing a length of insulated pipe;
Figure 2 is a graph illustrating one way in
which data can be taken and presented in
accordance with the present invention, this graph
plotting propagation time against distance from A
to B and again from B to A giving a reversed
profile;
Figures 2A and 2B are schematic drawings
showing the intersection of two pair of pulses at
adjacent spaced locations;
Figure 3 is a graph which displays a curve in
the lower part of the graph which represents a
composite wave form resulting from both the near
side of far side pulses traveling along the pipe
section under test, and the curves at the upper
part of Figure 1 showing resulting wave forms at
different locations, using the method of the
present invention;
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Figure 4 is a graph which is similar to the
upper part of the graph of Figure 3, displaying
separately a first resulting wave form identified
at the zero location shown in Figure 3;
Figure 5 is a graph similar to Figure 4, but
showing separately the resulting wave form
identified at the 25 location shown in Figure 3;
Figure 6 is a graph similar to both Figures 4
and 5, but showing separately the resulting wave
form at the 50 location of Figure 3;
Figures 7A through 71 are a series of
simplified illustrations of wave forms to
demonstrate certain principles of different wave
forms;
Figures 8A through 8E and Figures 9A through
9E are two series of Figures similar to those of
Figures 7A through 7E, and to illustrate further
the certain principles of difference wave forms;.
Figure 10 is an illustration of the paths of
the electromagnetic wave components traveling
along a section of pipe; and
Figures 11 - 16 are presentations of wave
forms illustrating the difference wave forms
produced in accordance with the method of the
present invention.
Figure 17 is a graph illustrating the wave
forms in the first step of third embodiment of the
present inventions;
Figure 18 is a graph similar to Figure 17,
showing the wave forms of a subsequent step in
this third embodiment;
Figure 19 is a schematic drawing of the place
for the antennas in this third embodiment;
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Figure 20 is a graph similar to Figures 17
and 18 illustrating a third step in the third
embodiment;
Figure 21 is a graph showing three of the
wave forms of Figure 20, drawn to a scale
emphasizing the vertical dimension of the waves;
Figure 22 is a graph similar to Figure 21
showing the wave forms of Figure 21 and also the
difference wave forms derived therefrom;
Figure 23 is a graph similar to Figure 21
showing three of the waves moved together;
Figure 24 is a graph showing a plurality of
difference wave forms, where two areas of
corrosion are being detected;
Figure 25 is a graph derived from the earlier
wave forms illustrating the difference in
amplitude of the difference waves where corrosion
exists;
Figure 26 is a graph based on Figure 24,
further emphasizing the differences in amplitude.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
The basic testing apparatus and method of the
present invention will now be described with
reference to Figure 1. There is shown a pipe 10
having a section 11 which is under test. This
pipe 10 is or may be a pipe or pipeline that would
typically be used in the petroleum or
petrochemical industry, where the pipe is made of
steel and surrounded by a coat or layer of
insulation.
The apparatus 12 of the present invention is
shown somewhat schematically in its operating
position, testing the section 11 of the pipe 10.
This apparatus 12 comprises a pulse generator 14,
a signal analyzer 16, and interactive computer 18,
and two transmitting/receiving antennas 20 and 22.
There are two cables 24 and 26 (or other signal or
pulse transmitting means) interconnecting the
antennas 20 and 22, respectively, to the pulse
generator 14. There is a second pair of cables or
other transmitting means 28 and 30 connected
between the cables 24 and 26, respective, and to
the signal analyzer 16.
When the transmitted pulse is received by one
or the other of the antennas 20 or 22, this pulse
is in turn transmitted to the signal analyzer.
Certain analysis can immediately take place in the
signal analyzer 16. Alternatively, the
information relating to the pulse can be stored
and analyzed at a later time. The computer
performs certain control functions in the proper
transmission and reception of the pulses and other
functions.
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There are several ways in which an apparatus,
such as the apparatus 12, can be used in detecting
corrosion in pipes, and two of these will be
discussed below.
There is a first method where a single pulse
is transmitted from the pulse generator 16 to one
or the other of the antennas 20 or 22 to cause the
wave form to travel from the location of that
antenna 20 or 22 along the pipe 10 to the location
of the other antenna 20 or 22 where the signal
from the wave form is received. The distance
between the sending location 20 and the receiving
location 22 is ascertained accurately, and the
timing of the time of transmission of the pulse
from the antenna 20 or 22 to the other antenna 20
or 22 is measured very accurately (desirably to a
fraction of a nanosecond or even to a very small
fraction of a nanosecond). If the section between
the two test locations 20 and 22 is non-corroded,
and if the pipe is uniform along its length, then
the pulse will arrive at the receiving location 22
in a wave form which is in the same general
pattern (except possibly for disturbances, such as
a near by magnetic field, electromagnetic noise,
etc.). Also, the rate of travel of the pulse
would remain substantially constant, provided the
pipe remains uncorroded and uniform.
However, when corrosion is encountered
between the points 20 and 22, the corrosion will
affect the wave form by retarding its velocity
diminishing its amplitude, and also possibly
changing the actual wave form itself.
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One method of utilizing this technique is to
send the pulse from the transmitting location to
the receiving location over an uncorroded section
of pipe of a know length and diameter, and known
characteristics, relative to its transmission of
electromagnetic waves. This would establish the
time of travel of the wave from the transmitting
to receiving location and the expected
configuration of the wave form at the receiving
location.
Then various sections of the pipe are tested,
as illustrated in Figure 1. When there is a delay
in the predicted arrival time of the wave form
and/or deviations from the reference wave form for
uncorroded pipe, then this will presumed to be due
to corrosion on the pipe. However, it should also
be understood that some other disturbance (e.g.
nearby electromagnetic noise, presence of some
other object that would disturb the
electromagnetic field) could also affect the wave
form, and this should be accounted for.
The second method which will be discussed
further in this text is what is termed the "dual
pulse" method, described in U.S. Patent 4,970,467.
In this method, the same apparatus as shown in
Figure 1 can be used. However, instead of using a
single pulse or series of single pulses, as in the
method described above, both antennas 20 and 22
are used as both transmitting and receiving
antennas in the same timeframe. Thus, as one
pulse is transmitted from the antenna 20, one is
also transmitted from the antenna 22. These
pulses travel toward one another and "collide" at
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some intermediate location along the pipe. This
meeting of the pulses will cause variations in
both of the wave forms as they move through the
area of collision toward the other antenna which
is its receiving location.
By properly coordinating the precise time at
which the pulses are transmitted from the two
locations 20 and 22, the point of collision along
the length of the pipe can be caused to occur at
any desired location along the length of the pipe.
Then by changing the relative time transmissions
of the pulses in small increments, this point of
collision can be stepped along the length of the
pipe.
As described in the above noted patent, when
the point of collision occurs at a location where
there is corrosion, the wave form resulting from
the collision will be different from a reference.
wave form which would occur where the collision
point is at a non-corroded section of pipe. Thus,
not only is there a means of detecting corrosion,
but also a means of determining the location of
such corrosion.
Also, the antennas 20 and 22 could be used
only as transmitting antennas and two additional
antennas could be used as receiving antennas.
Further, other transmitting and receiving devices
could be used, such as by making a direct
electrical connection to the pipe.
The present invention is particularly adapted
for extracting information from the wave forms
resulting from the dual pulse method described
above.
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A first embodiment of the method of the
present invention is described in the following
text, with reference to Figures 2A-2B through
Figures 9A-9E. A second embodiment is also
disclosed later herein, using in part the same
principles as the first embodiment, and this will
be described later with reference to Figures
- 16.
Reference is first made to Figures 2A and 2B
10 which are schematic illustrations of the operation
of the dual pulse method. In Figure 2A, there is
schematically shown a one hundred foot length of
pipe. It will be assumed that the pulse travels
along the length of the uncorroded pipe at the
rate of one foot per nanosecond.
In Figure 2A, the near side pulse is
transmitted into the pipe at the location NS (near
side location) at a point in time indicated at
zero. The second pulse is transmitted into the
pipe at the far side location (designated FS), and
in this particular example, it is assumed that the
second far side pulse is transmitted into the pipe
forty nanoseconds earlier than the time the near
side pulse is transmitted into the NS location.
Therefore, it can be seen that when the far
side pulse has traveled along the length of the
pipe for forty nanoseconds to reach a location
indicated at the sixty foot location, the near
side pulse is transmitted at time zero from the
near side location.
The near side and the far side pulses travel
toward one another, each traveling thirty feet
until they intersect at the thirty foot location
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on the one hundred foot pipe. At the
intersection, the two pulses interact with one
another, and the far side pulse continues it path
of travel to the near side (NS) location. Also,
the near side pulse after passing through the
point of intersection continues its course of
travel toward the far side (FS) location.
At this time, it is important to note that
each of the pulses is a somewhat complex wave
form. First, as a wave form travels along the
length of the pipe, it is subject to attenuation,
distortion, interference and dispersion. Further,
each wave can be considered as having what we
might term wave components made up of earlier and
later arrivals. There is a first arrival which
will travel the shortest course from the
transmitting to the receiving location. Thus, if
both the transmitter and the receiver are on top
of the pipe, the first arrival will travel along
the top surface of the pipe in a straight line.
Then there are second arrivals which are pulse
components which follow a helical path once around
the pipe to arrive at a short time later. Then
there are third, fourth, fifth,....etc. arrivals
which come at yet later times. Further, there are
quite commonly outside sources of interference,
such as sources of electromagnetic radiation,
nearby objects which may interact with the wave
form traveling along the pipe, and thus become
activated and in turn transmit their own
electromagnetic radiation back into the pipe under
test. Further, there are reflections and
refractions.
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To return to Figure 2A, let us first assume
that the one hundred foot section of pipe which is
under test is free of corrosion. After the near
side and far side pulses intersect at the thirty
foot location, there is a resulting wave form
which reaches the near side receiving location,
which is the composite of both the original near
side pulse and the far side pulse, with these
having been modified or affected to some extent by
reason of intersecting.
It has been found that if the intersection of
the near side and far side pulses takes place at a
location on a pipe which is noncorroded, then the
resulting pulse which travels through the location
of intersection will have certain characteristics
typical of a situation where the intersection
takes place at a non-corroded area of pipe.
However, if the intersection of the near side and
far side pulses take place at a location where
there is corrosion, the two pulses interact in a
rather different manner, and the resulting wave
form of each of the intersected pulses has
different characteristics.
However, the analysis of the wave form as a
means of detecting corrosion is difficult to
quantify. There are features such as rise time,
slope, amplitude, and phase change, all of these
being relevant characteristic of the wave form.
To illustrate this, reference is made to the
lower curve shown in Figure 3. This curve,
designated 50, is a composite curve which results
from the combination of both the near side and the
far side pulses. In this instance, one
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transmission takes place at the near side, and the
receiving antennae is also located at the near
side. The portion of the curve indicated at 52
represents the near side pulse being transmitted
into the pipe at the transmitting location.
The portion of the curve indicated at the
general area of 54 represents a portion of the
composite wave that arrives at the near side
receiving location, this being a combination of
the far side wave and near side wave components.
As indicated above, there are reflections,
refractions, late arrivals, etc., which complicate
the wave form.
Reference is now made to Figure 2B, which
shows a second dual pulse operation where the
transmission time of the far side pulse has been
delayed by four nanoseconds, so that it is
transmitted thirty six nanoseconds before the
transmission of the near side pulse. It can be
seen that after the far side pulse has traveled
thirty six feet, the transmission of the near side
pulse takes place. Thus when the near side pulse
is transmitted, the far side pulse is at the sixty
four foot location, and the two pulses intersect
at the thirty two foot location.
With the foregoing being presented, the
method of the present invention will now be
described. Let us assume that the dual pulse
testing method is being accomplished and that the
near side and far side pulses are timed (as
indicated in Figure 2A) so that there is
intersection at the thirty foot location on the
one hundred foot pipe. Let it further be assumed
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that the wave form which is received at the near
side location looks the same, or similar to, that
shown in the bottom part of Figure 3.
Now let us assume that a second testing
operation is to be initiated and the far side
pulse is delayed by four nanoseconds. However,
the transmission at the near side location remains
constant, in terms of time, and is still
transmitted at zero time. As illustrated in
Figure 2B, the intersection takes place at the
thirty two foot location. What this would
effectively mean is that the portion of the
composite curve which is attributable to the far
side pulse would have been delayed, relative to
the time of transmission of the near side pulse,
and that, with reference to the lower curve of
Figure 3, the portion of the composite wave form
contributed by the far side pulse would have
shifted to the right somewhat, from what is shown
in the lower curve of the graph of Figure 3.
In order to extract meaningful information
about the condition of the pipe, the following is
done. First, the composite wave form which
results from the transmission and intersection of
pulses as shown in Figure 2A is stored in the
memory. Next, the second composite wave form
resulting from the transmission of the near side
and far side pulses in accordance with Figure 2B
is also received. Then the second composite wave
form resulting from the test operation of Figure B
is subtracted from the composite wave resulting
from the test operation in Figure 2A.
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At this point, it is very important to keep
in mind that the near side pulse has in both
instances (in the operation of Figure 2A and the
operation of Figure 2B) been transmitted at zero
time. Thus, in both the Figure 2A and Figure 2B
operation, the near side pulse has not changed
position. From this, it becomes apparent that the
contribution of the near side pulse to the
composite wave form is essentially subtracted out
of the composite wave form resulting from the
operation of Figure 2A. Now, let us turn our
attention to the far side pulses of the test
operation of Figure 2A and Figure 2B. With the
far side pulse having been delayed by four
nanoseconds, the wave component of the far side
pulse has now shifted from the first location in
the first operation of Figure 2A four nanoseconds
to a second position composite curve of the second
test operation of Figure 2B.
With the entire first composite curve being
subtracted from the entire second composite curve,
there remains what can be termed a "difference
wave form". It has been found that if in the two
dual pulse operations where the intersecting
locations are stepped within a reasonably close
distance from one another, and if uncorroded pipe
is encountered at both intersecting locations, the
resultant difference wave form is a reasonably
well defined and identifiable peak.
Reference is now made to Figure 3. It can be
seen that the curve in the upper part of Figure 3
shows three separate peaks designated "zero",
"twenty five", and "fifty", respectively. Each of
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these peaks is the result of using the method of
the present invention where the point of
intersection for the two adjacent dual pulse
operations has been stepped by an interval of
about 5 feet.
To provide cleaner representations of the
wave forms, Figure 4 illustrates the single curve
indicated at "0" in Figure 3; Figure 5 illustrates
only the curve indicated at "25" in Figure 3; and
Figure 6 illustrates only the curve indicated at
"50" in Figure 3.
It should be noted that the wave forms
indicated at "0", "25", and "50" in the top part
of Figure 3 are actual wave forms extracted from
adjacent wave forms similar to the ones shown in
the lower part of Figure 3. It is important to
note that if the composite wave form is not
formatted correctly, the difference between
adjacent wave forms does not provide the "effect"
wave forms shown in Figure 3.
To review how to format the data correctly,
consider the two active wave forms on the pipe,
one from the near side (NS), and the other from
the far side (FS). When the data analyzer is
synchronized with the near side pulse, the near
side component of the composite pulse will not
move (i.e. shift position). However, the FS (far
side) pulse, which is synchronized to the master
clock, will move across the screen from left to
right and will modify the composite wave form for
each intersection along the pipe. When the
composite wave forms are subtracted from each
other, two significant things happen:
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1. The effect of the NS pulse, which has a
very large amplitude with respect to the FS pulse,
is cancelled, since this NS component in the
composite wave form is fixed in time.
2. The resulting difference wave form
represents the difference between the two adjacent
FS pulses that have intersected with the NS pulses
at two different points on the pipe.
When this difference occurs, then the
"effect" (time rise, slope, amplitude, dispersion
and absolute time, among many parameters that are
effected by corrosion) influence the shape of the
different wave form. The difference wave form
will be displaced in time with respect to other
adjacent pairs. This time displacement is a good
indication of the condition of the pipe, provided
it can be meaningfully interpreted. The
difference wave forms shown here are examples of.
the wave forms that are well defined, but are very
difficult to extract real time information. (See
Figures 4, 5 and 6. Note particularly Figure 6 at
the "knee" of the wave form is not well defined,
and could be selected anywhere from a point near
thirty two hundred to forty two hundred, a range
of one hundred nanoseconds. Automating a
selection of the absolute time location of the
knee is very difficult and sometimes impossible.
However, the peak is well defined. The peak is
not just a voltage difference between two
different response wave forms, but it is
determined by the shape factors involved with the
leading edge of the two adjacent wave forms.
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For example, if the two adjacent wave forms
are displaced more in time than any two other
adjacent wave forms, it will result in an increase
of amplitude. Hence, ?AE" (peak) is a function
of time. It is also a function of actual
amplitude difference between two different wave
forms. Also, if the leading edge of one wave form
is distorted as a result of corrosion, this
distortion will result in a change of amplitude in
the difference wave form and a shift in the
position of the peak with respect of time. When
the pipe is very good, the peak is very sharp and
the shape of the difference wave forms extremely
uniform. When the pipe has anomalies (e.g.
corrosion), the shape of the difference wave form
is significantly altered and the corrosion effect
(CE) displaced by major differences in the leading
edge. These differences result in an effective
peak shift that can be related directly to
pipeline quality.
This peak shift is much easier to instrument
and measure than other parameters. Also this peak
shift is an indicator of the cumulative effect of
all individual parameters and effect the
electromagnetic response, even if they might be
very difficult to measure individually. As the
pipe degrades, the peak distorts more readily
because of the complex contribution of all driving
forces. In a perfect system, every difference
pulse should be identical. Hence, measuring the
time associated with the first peak occurring
after an indefinite knee of a differential pair,
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provides an effective way of extracting critical
information and measuring the corrosion effect.
Obviously, a stable source is required and is
being used for this system. From the wave forms
included, it should be obvious that measuring the
peak is easier than measuring the time related to
the knee. When the peak is not well defined, it
will indicate different anomalies on the pipe.
The process of measuring the corrosion effect is
designed to impose the quality of data and reduce
the time required to collect and analyze the data
in the field.
To illustrate in a rather simplified fashion
certain aspects of the present invention, relative
to subtraction of one wave form from another,
reference is now made to Figures 7A through 7G,
and also Figures 7H and 71.
Figure 7A shows a rather simple wave form 60
which is drawn, for convenience, in straight
lines. Figure 7B shows the same wave form at 62,
but offset one unit from the wave form 60. Figure
7C shows the summation of the wave forms 60 and 62
as the wave form 64. It is possible to drive
meaningful information from the wave form in
Figure 7C where the wave forms are added, but it
is preferred to fist one of the wave forms and
then add the two together. This is done in
Figure 7D which shows the wave form 60, with the
offset wave form 62 inverted, and the summation of
the wave 60 and the inverted wave 62 accomplishes
a subtraction of the wave forms of Figure 7A and
7B. This results in the difference in the
difference wave form 66 shown in Figure E.
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As indicated above, these curves are somewhat
artificial, and in actuality, these simple wave
forms would not be formed with these straight
lines. Rather Figures 7H and 71 would be more
realistic, where we see the wave form 68 and a
very similar wave form 70 offset in the wave form
68. In 71, there is shown a difference wave form
72 which would result from subtracting the wave 70
from the wave form 68. It can be seen that the
difference wave form 72 forms in a rather well
defined peak.
For purposes of further analysis, in Figure
7F, the wave form 74 is shown, exactly in the same
form and position as the wave form 60 of Figure
7A. Then the same wave form is shown in Figure 7F
at 76, inverted and shifted two units from the
wave form 74. Then when the wave form 76 is
subtracted from the wave form 74, there is the
difference wave form 78 shown in Figure 7G. it
will be noted that with the wave form 76, spaced
two units away from the wave form 74 (instead of
one unit, as in Figures 7A and 7B)has an amplitude
which is twice the amplitude Of the difference
wave form 66. This illustrates that if the time
displacement of the wave forms increases, the
amplitude of the difference wave form would be
expected to increase. This is simply by way of
illustration, and relates to only one particular
facet of detecting corrosion from the difference
wave form.
For purposes of further analysis, reference
is made to Figure 8A through BE and to Figures 9A
through 9E.
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In Figure 8A, there is shown a wave form 80,
and in Figure 8B a second advanced wave form 82
which has been attenuated and delayed, presumably
because of encountering corrosion in the pipe.
Figure 8C shows the summation of these, this being
the wave form 84. Figure 8D shows the same wave
form 80 and the adjacent wave form 82 inverted.
Figure BE shows a difference wave form 86 which
results by subtracting the wave form 82 of Figure
BE from the wave form 80 of Figure 8A.
In Figure 9A, these same steps are followed.
Figure 9A shows a wave form 88 which is the same
as the wave form 80 of Figure 8A. Figure 9B shows
a second wave form 90 delayed by one unit, and
having a different slope along the leading edge-
Figure 9C shows the summation wave form at 92.
Figure 9D shows the wave form 88 and the second
wave form is inverted as shown at 96. Figure 9E
shows the difference wave form at 94.
In reviewing Figure 7A through 71, Figures 8A
through BE, and also Figures 9A through 9E, four
of the figures show difference wave forms, these
being the difference wave form 66 in Figure 7E,
the difference wave form 78 in Figure 7G, the
difference wave form 86 in Figure BE, and the
difference wave form 94 Figure 9E. It can be seen
that the different characteristics of these
difference wave forms emphasize the difference
between the adjacent wave forms.
It should be kept in mind that these wave
forms of Figures 7A-7I, 8A-8E and 9A-9E are not
the more complex composite wave forms such as
shown at 50. These are simplified wave forms
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provided simply to show some of the principles
involved.
What the method of the present invention
accomplished is the elimination of a great deal of
the irrelevant information. There is the tendency
of the near side pulse to swamp out the far side
pulse, mainly because the near side pulse has a
substantially larger amplitude, since it is closer
to the transmitting location. The components of
the near side pulses are substantially eliminated.
Beyond this, by subtracting the shifted wave
components attributable to the far side pulses
from one another is that a difference comparison
is provided. If the pipe is substantially uniform
along its length (non-corroded), and if the
intersecting point of the pulses is stepped in
even increments along the pipe, then the same or
very similar difference waves are expected to be
obtained. As indicated previously, the difference
curves shown in the upper part of Figure 3 are
quite similar, indicating no corrosion or possibly
minimal corrosion. Thus, the difference curves as
shown in Figures 4 through 6 provide the
meaningful information, without being cluttered by
extraneous wave components.
A second embodiment of the present invention
will now be described relative to Figures 10
through 16. By way of introduction, much of the
focus on the analysis of the wave forms to detect
corrosion has been directed toward the leading
edge of the wave form or at least the early
arrival portion of the wave form. To some
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extent, it has been recognized (or at least
conjectured) that valuable information would be
contained in the later arriving portions of the
wave form. However, the problem is how such
information could be identified and/or extracted.
As indicated earlier in this text, the
propagating wave form can be considered to be a
composite of a number of wave components made up
at least in part of early and late arrivals. To
illustrate this graphically, reference is made to
Figure 10 which shows a relatively short section
of pipe 100, where there is a transmitting lo-
cation 102 and receiving location 104. In this
instance, these locations 102 and 104 are both at
the top of the pipe and aligned. The straight
line lengthwise axis between the points 102 and
104 is indicated at 106. Since this axis 106 is
the shortest path between the points 102 and 104,
the first arrival path would be along the path
indicated at 108, which is coincident with the
axis 106.
In addition to the first arrival wave
component 108, there are two second arrival wave
components, the travel paths of which are
indicated at 110 and 112. It can be seen that
each of these are helical paths, which travel
longitudinally and through a helical curve of
360 . Then the third arrivals are indicated at
114 and 116, and these also are helical paths, but
with a total circumferential component of travel
of 7200.
Obviously, the second arrival has a longer
path of travel than the first arrival, the third
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arrival has a yet longer path of travel than the
second arrival, etc. If there is corrosion on the
pipe, at least some of these later arrival pulse
components will pass through the area or areas of
corrosion and that path component will be delayed,
attenuated, and/or otherwise modified.
With the foregoing being given as background
information, reference is now made to Figure 11,
which illustrates the wave forms obtained by the
second embodiment of the present invention. In
Figure 11, the vertical axis represents voltage
(measured in volts), and the horizontal axis
measures time, with each increment representing
ten nanoseconds. Thus, the numeral one thousand
actually represents one hundred nanoseconds, the
numeral two thousand represents two hundred
nanoseconds, etc. It can be seen that the wave
forms presented in Figure 11 extend over a full
five hundred nanoseconds.
The particular tests from which these curves
were developed were done over a pipe section one
hundred and sixty feet length (i.e. the
transmitting location was one hundred and sixty
feet away from the receiving location). Further,
the dual pulse method was utilized, as indicated
above. Since the entire pipe is encircled by
electromagnetic energy, the effect of corrosion
anywhere on the pipe will appear in the difference
wave form obtained by intersection of the pulses
at the location of corrosion.
The first steps in the second embodiment in
the method of the present invention are
substantially the same as those of the first
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embodiment. More specifically, a first testing
operation was performed by transmitting the near
side and far side pulse in timed relationship so
that these would meet at a predetermined point of
intersection. There is a composite wave form
resulting from this first test operation and that
is stored. Then, as described in the presentation
of the first embodiment, there is a second
operation in timing of the far side pulse so that
it was either advanced or delayed so that the
point of intersection was shifted, and the result
was a composite wave being recorded that had
components of the far side pulse shifted somewhat
from the previous composite wave. As described
in the first embodiment of this method of the
present invention, one of the composite wave forms
is subtracted from the other to get a difference
wave form.
These steps are performed in the second
embodiment of the present invention, and it will
be recognized, of course, that these are
substantially the same steps as described in the
first embodiment. From this point on in the
method of the second embodiment, a further
analysis is conducted as will be described below.
For purposes of description, we shall
consider the sequence of the difference wave forms
and designate these as difference wave form 1,
difference wave form 2, difference wave form 3,
etc.. It will be evident that the difference
wave form 1 results from processing composite wave
forms 1 and 2 which result from the first and
second dual pulse test operations; difference
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wave form 2 is a result of processing the
composite wave forms 2 and 3 resulting from the
second and third dual pulse operations; etc.
In Figure 11, there is first plotted the
difference wave form 120, and this wave form is
about five hundred nanoseconds in length. The
next step is to plot the second difference wave
form 122, but the second difference wave form 122
is shifted to the left, and is also lowered
somewhat so that the second difference wave form
122 is aligned with and a short distance below,
the first difference wave form 120. It will be
observed in the wave form representation of
Figure 11 that the two wave forms 120 and 122
match each other rather closely. These two
difference wave forms 120 and 122 were derived
from adjacent composite wave forms, and both of
these composite wave forms resulted from a dual
pulse operation where the pulse is intersected at
a noncorroded area (or at least a very lightly
corroded area) of the pipe under test. As
indicated at the right side of Figure 11, the
upper composite wave results from a difference
wave form where a reference point of intersection
was that at the 125.3 feet mark, while the second
difference wave form 122 was made up of adjacent
composite wave forms at a reference location
129.6.
Figure 12 shows two other reference wave
forms 124 and 126, resulting from two adjacent
pair of composite wave forms at reference
locations at the 34.6 and 38.9 foot locations on
the pipe section under test. These composite wave
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forms also resulted from the far side and near
side pulses of each test operation intersecting at
a noncorroded (or very lightly corroded) area of
the pipe section under test.
Reference is now made to Figure 13, where
there is shown as the upper wave form the same
wave form 126, as shown in Figure 12, this
difference wave form having a reference location
of 38.9 feet on the pipe. The lower wave form
128 has a reference of 43.2. This was at a
somewhat corroded pipe section having a corrosion
index of 1.0262. (The corrosion index is a scale
which is utilized by the inventor in rating areas
of corrosion. A rating of 1.000 would be no
corrosion and the higher the number, the greater
the severity of corrosion).
It will be noted in Figure 13 that the lower
wave form 128 has at two areas something of a
phase shift, indicated at 130 and 132.
Reference is now made to Figure 14, where
there are two adjacent reference wave forms at
locations along the pipe section at the 151.2 foot
location and the 155.5 foot location. There was a
corrosion index of 1.055, which is higher than in
Figures 11, 12 and 13. These wave forms are
indicated at 134 and 136. It can be seen that at
the location 138 there is a rather substantial
phase shift.
Next, attention is directed toward Figure 15
which shows as the upper wave form the same wave
form 136 which is the lower wave form in Figure
14, and a new difference wave form 140 taken at a
reference location of 160 feet on the pipe. These
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wave forms resulted from composite wave forms
developed with the intersecting locations being at
more highly corroded areas. Several features
should be noted. In observing the peak location
at 142, and the two peak locations at 144 and 146,
it can be seen that there are substantial
amplitude differences with regard to the second
and third peaks between these curves 136 and 140.
In addition there is significant phase shift
indicative of corrosion anomalies.
It is enlightening to observe the difference
curve 134 which is at the reference location 151.2
(Figure 14) and the curve 140 which is at the
reference location 160 (Figure 15). It can be
seen by matching up the curves 134 and 140 that
these correspond fairly closely to one another,
at least much more closely to one another than
each matches up with the curve 136. This would
indicate that the corrosion area is more likely in
the area of the reference location 155.5,
presumably somewhere between the 153 to the 158
area.
Finally, reference is made to Figure 16,
where there are two difference wave forms 150 and
152. It can be seen that the difference wave
form 152 has substantial similarities to the curve
136 (see Figures 14 and 15). Further, it can be
seen the match up between the wave forms 150 and
152 are rather similar to the matching of the wave
forms of 134 and 136 in Figure 14. More
specifically, it can be seen that the second and
third peaks 154 and 156, respectively, of the
wave form 152 are of nearly equal amplitude, and
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then there is the phase shift at the area 158.
This is a pattern quite similar to that shown at
area 138 in Figure 14.
Thus, it can be seen that with the method of
the second embodiment of the present invention,
the presence of corrosion is detected by a method
which might be termed "whole wave analysis", which
involves looking not only at the leading portion
of the wave, but also a much greater time span of
the wave form which also contains significant
information. It also becomes apparent that
valuable information is obtained from portions of
the wave form as far along the wave form as two
hundred to four hundred nanoseconds or longer from
the first arrival of the electromagnetic pulse.
Further, the location of the corrosion can be
located within reasonably close tolerances by
properly synchronizing the pulses so that the
point of intersection is known.
Also, it should be noted that these readings
were taken on the same section of pipe, but with
the intersection being moved to different
locations. Therefore, all of the wave forms
developed for the data of Figures 11 through 16
passed over the same pipe section. The key
difference is that the point of intersection was
moved. When the point of intersection of the wave
forms which were combined to make the difference
wave forms were at an area of corrosion, the
variations of the different wave forms become
apparent.
A third embodiment of the method of the
present invention will now be described. In this
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third embodiment, as in the prior embodiments, the
pulses will be transmitted from the far side and
near side locations, and in this particular
embodiment, the wave form which is to be analyzed
to detect corrosion is the far side pulse arriving
at a receiving location adjacent to the
transmitting location at the near side. Also, in
this third embodiment , the time intervals between
the transmissions of the far side pulse will
remain constant. Thus, to synchronize the pulses
so that the point of intersection stepped along
the length of the pipe, for each transmission, the
near side pulses shall be advanced in timing by a
short increment so that the point of intersection
of the pulses will be stepped in a left to right
direction across Figure 18. In the first step of
this further embodiment, the far side transmitter
is shut down, and a series of pulses are
transmitted from the near side and these are
picked up by the near end receiving antenna that
is closely adjacent to the near side transmitter.
The timing of the transmission of the near side
pulses is timed relative to a sender except that
each subsequent transmission is advanced one
additional increment of time, and in this
particular example we will assume that it is being
advanced by four nanoseconds for each pulse
transmission.
The first step in the method of this
preferred embodiment will now be described with
reference to Figures 17 and 19. In Figure 19,
there is shown a section of pipe 170 having a Near
side transmitting antenna 172 and a far side
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transmitting antenna 174. There is a receiving
antenna 176 spaced from the transmitting antenna
172 a short distance toward the far side
transmitting antenna 174.
Initially, the far side transmitter remains
inactive so that the far side transmitting antenna
174 is not transmitting any signal. The near side
transmitter is activated to transmit a series of
timed pulses which are synchronized with regular
time intervals. This is done in a manner that
each subsequent pulse is advanced four nanoseconds
relative to the preceding pulse.
For example, if the near side pulses are to
be transmitted every two seconds, less the time of
the advance of the timing, the first pulse would
be sent at 0 seconds. The second pulse would be
transmitted four nanoseconds before the two second
interval. The third pulse would be sent eight
nanoseconds sooner than the four second interval.
The first pulse would be sent twelve seconds
before the 8 second interval, etc.
Thus, as can be seen in figure 17, the first
pulse is transmitted at 0 nanoseconds, the next
pulse indicating as having a four nanosecond
advance, the third pulse at 8 nanoseconds advance,
with these advances continuing on down to the 18th
pulse which has been advanced by 72 nanoseconds
relative to the initial pulse at 0.
Each pulse from the near side antenna 172
passes by the receiving antenna 176, and the pulse
is recorded, with the wave forms indicated at 178.
It is to be recognized that in most all instances,
there is a certain amount of outside
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electromagnetic, electrical noise, echoes,
refractions, etc. that tend to obscure or
"clutter" the signal. Each of these pulses 178 is
recorded in the memories of the control unit,
including all the various extraneous influences
on the signal, plus the portion of the signal
attributable to the pulse itself. As will be
discussed subsequently herein, these pulses 178
that are recorded are used as reference pulses
which are subtracted in a subsequent step in the
method of the third embodiment.
The next step will now be described with
reference to Figure 18. The far side transmitter
is activated so that regularly timed pulses are
transmitted from the antenna 174 into the pipe
section 170 i.e. without any advancing or delay in
the timing. Each transmission at the near side is
synchronized with the transmission at the far
side. However, each time the near side transmits
a pulse, the next pulse from the near side is
advanced four nanoseconds from the designated time
period from the previous pulse. Thus, at the zero
location in Figure 18, the far side pulse is
transmitted at a time period so that the pulses
from the near side and far side antennas 174
intersect at the location of the receiving antenna
176. The next pair of pulses are transmitted with
the Near side pulse being advanced by four
seconds, so the point of intersection is spaced
two nanoseconds closer to the far side. The third
pulse is advanced by eight seconds so that the
intersection of the spaced and additional two
nanoseconds toward the far side.
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It can be seen that the first pair of pulses
intersecting at the antenna 176 that the peaks of
theses pulses come close to coinciding. It can be
seen that subsequent pairs of pulses are
transmitted and with the near side pulses being
advanced four nanoseconds on each transmission,
relative to the far side pulse 180, the pattern of
the wave which is received at the antenna 176
comprises a first peak 182 which is attributable
to the near side pulse passing by the antenna 176,
and a second peak 184 which is attributable to the
near side pulse reaching the antenna at a later
time.
In this third embodiment, the far side pulse
that is received by the antenna 176 is the one
which is analyzed to determine whether the
corrosion exists. To accomplish this, the
following procedure is followed. Each of the wave
forms 186 that result from the second step of this
method are also stored in memory. Then the wave
forms 178 (showed in Figure 17) are each
subtracted from the corresponding wave forms shown
in Figure 18 with the resulting wave forms being
shown in at 188, Figure 20.
What has occurred is that when the wave forms
of Figure 17 are subtracted from the corresponding
wave forms of Figure 18, the wave form from the
near side, along with the extraneous noise,
echoes, etc. is cancelled out so that what is left
is the wave form 188 that essentially represents
the far side wave form which is "uncluttered."
The overall result is that this facilitates the
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detection of variations in the wave form that
originated from the far side.
It should be noted that if there is a
relatively smaller amount of corrosion, its effect
on the wave forms which intersect at the location
of the corrosion is more difficult to detect. By
performing the first three steps as described with
reference to figures 17, 18 and 20, these more
subtle variations in the wave form resulting from
the far side pulse intersecting with the near side
pulse at the area of corrosion can be detected
more easily.
Figure 21 shows four adjacent wave forms 190
which are the same wave forms 188 of Figure 20,
except that vertical dimension has been increased
substantially so that the slope of these wave
forms 190 is steeper. It can be seen in Figure 21
that each of these four wave forms 190 are very
similar to one another. This would indicate that
there is little or no corrosion in the area where
the wave forms 190 have intersected.
Now a fourth step in the method of the
present invention is performed to further enhance
the ability to analyze the wave forms to detect
corrosion, and this will be explained first with
reference to Figure 22. Figure 22 represents four
adjacent wave forms resulting from four pulse
transmissions which follow one after the other in
sequence. These waves are designated 192-1,
192-2, 192-3 and 192-4. Each wave form is
subtracted from the preceding wave to obtain a
difference wave form.
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This is accomplished by first inverting the
wave form 192-2 and then adding this inverted wave
form to the wave form 192-1 to obtain a difference
wave form which is 194-1 (this 194-1 being the
difference wave form of the two wave forms 192-1
and 192-2). In a similar manner, a second
difference wave form 194-2 is obtained by
inverting the wave form 192-3 and adding this to
the wave form 192-2 to obtain the difference wave
form 194-2. The third difference wave form 194-3
is obtained in the same way by inverting the wave
form 194-4 and adding this to the wave form
192-3.
It can be seen in Figure 22 that each of the
three different wave forms 194-1, 194-2 and 194-3
are very similar to one another and have
substantially the same amplitude.
Figure 23 illustrates another technique
utilized in this third embodiment. The four wave
forms 194-1 through 194-4 are moved closer
together, while leaving the wave forms unchanged.
By moving these wave forms closer together, it is
much easier to detect variations in the wave
forms. Also, the different wave forms which would
result from the arrangement of the wave forms in
Figure 23 would be a much smaller anthitude. The
effect of this is, however, that differences in
amplitude between the peaks does not decrease when
the wave forms are moved closer together. This
further accentuates the differences in the wave
forms.
To illustrate the wave forms where corrosion
is being detected, reference is now made to
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Figure 24. There are shown 18 adjacent difference
wave forms such as shown at 194-1, 194-2, and 194-
3 in Figure 22. It can be seen that the fourth
wave form 196 and the fifth wave form 198 are
configured rather differently than the adjacent
wave forms shown immediately above and below these
two wave forms 196 and 198. It will be noted that
between the initial "hump" 199 and the second
"hump" 200 of the wave form 196 there is a
substantial dip at 201.
In addition, it will be noted that the second peak
or "hump" 202 of the wave form 198 has a much
smaller amplitude. Further, it can be seen that
the peak 203 of the first rise or "hump" 204 of
the wave form 198 is shifted to the right. An
alignment line 204 is drawn to show the shift from
the alignment line at the left.
It will also be noted that there is a shift
in the peak 205 of a wave form which is the
twelfth wave form from the top. This is also an
indication of a corrosion, but a smaller degree of
corrosion in comparison with the corrosion
detected in the area of the fifth and sixth wave
forms.
Figure 25 is a graph where the points of peak
amplitude for adjacent difference wave forms has
been prepared by drawing lines connecting adjacent
peak points. It can be seen that the peak
indicator at 206 is much greater than the rest of
the peak points, and this would indicate an area
of corrosion. The peak indicated at 208, while
not having the height of the peak at 206, still
rises above the others. This would indicate that
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corrosion would likely be encountered at the
location at the pipe represented by the point 208
which would be the peak of the difference wave
form of two adjacent wave forms where the
corrosion was at or near the location of
intersection.
Figure 26 is a graph similar to Figure 25,
where the amplitude of the points in Figure 25
have been amplified in a matter to further
accentuate the differences.
It is to be understood that the terms "near
side" and "far side" can be reversed. Further, it
is to be understood that while the third
embodiment has been described, with the far side
pulse being the pulse which is analyzed, and the
near side pulse which has been advanced to cause
the point of intersection to be stepped along the
elongate number (pipe), this arrangement could be
reversed. Further, in an actual testing
operation, both the near side pulses arriving at
the far side, and the far side pulses arriving at
the near side could each be received and analyzed.
Also, it is to be understood that various
modifications to be made in the present invention
without departing from the basic teachings
thereof. Further, the terminology used in this
description should, in the following claims, be
given an interpretation commensurate with the
scope of the invention and should not be
interpreted as being limited to the specific
procedures and operating components described
herein.
