CA2258439C - Ultrasonic lamb wave technique for measurement of pipe wall thickness at pipe supports - Google Patents
Ultrasonic lamb wave technique for measurement of pipe wall thickness at pipe supports Download PDFInfo
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- CA2258439C CA2258439C CA002258439A CA2258439A CA2258439C CA 2258439 C CA2258439 C CA 2258439C CA 002258439 A CA002258439 A CA 002258439A CA 2258439 A CA2258439 A CA 2258439A CA 2258439 C CA2258439 C CA 2258439C
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- pipe
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- wall thickness
- lamb wave
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B17/00—Measuring arrangements characterised by the use of infrasonic, sonic or ultrasonic vibrations
- G01B17/02—Measuring arrangements characterised by the use of infrasonic, sonic or ultrasonic vibrations for measuring thickness
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/04—Analysing solids
- G01N29/07—Analysing solids by measuring propagation velocity or propagation time of acoustic waves
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/02—Indexing codes associated with the analysed material
- G01N2291/028—Material parameters
- G01N2291/02854—Length, thickness
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/04—Wave modes and trajectories
- G01N2291/042—Wave modes
- G01N2291/0423—Surface waves, e.g. Rayleigh waves, Love waves
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/26—Scanned objects
- G01N2291/263—Surfaces
- G01N2291/2634—Surfaces cylindrical from outside
Abstract
This invention is a new inspection technique using ultrasonic Lamb waves to measure reduction of pipe wall thickness due to localized corrosion at pipe supports.
The technique uses two transducers in a pitch-catch mode to send and receive a selected Lamb wave. The wave travels along the pipe wall in the circumferential direction. The reduction of the pipe wall thickness at the pipe support changes the time-of-flight between the two transducers. By comparing time-of-flight data measured from the pipe support area and from areas adjacent to the pipe support, one can quantify the change of time-of-flight due to the pipe corrosion wall loss at the support. A mathematical model, a sizing algorithm and a PC program have been developed to quantitatively relate in time-of-flight to the minimum remaining wall thickness. With them, one can quantify the minimum remaining pipe wall thickness at a pipe support by simply measuring the change in the time-of-flight.
The technique uses two transducers in a pitch-catch mode to send and receive a selected Lamb wave. The wave travels along the pipe wall in the circumferential direction. The reduction of the pipe wall thickness at the pipe support changes the time-of-flight between the two transducers. By comparing time-of-flight data measured from the pipe support area and from areas adjacent to the pipe support, one can quantify the change of time-of-flight due to the pipe corrosion wall loss at the support. A mathematical model, a sizing algorithm and a PC program have been developed to quantitatively relate in time-of-flight to the minimum remaining wall thickness. With them, one can quantify the minimum remaining pipe wall thickness at a pipe support by simply measuring the change in the time-of-flight.
Description
ULTRASONIC LAMB WAVE TECHNIQUE FOR MEASUREMENT OF PIPE
WALL THICKNESS AT PIPE SUPPORTS
BACKGROUND OF THE INVENTION
1. Field of the Invention The invention is directed to methods and apparatus for ultrasonically measuring pipe wall thickness at pipe supports without lifting the pipe.
WALL THICKNESS AT PIPE SUPPORTS
BACKGROUND OF THE INVENTION
1. Field of the Invention The invention is directed to methods and apparatus for ultrasonically measuring pipe wall thickness at pipe supports without lifting the pipe.
2. Description of the Prior Art Prior to this invention, qualitative inspection using Lamb waves to detect pipe corrosion wall loss at pipe supports was available. Quality NDT, Inc., having an address at P 0 Box 1145, Breaux Bridge, Louisiana 70517, USA, offered such an inspection service.
There was, however, no sizing technique to allow quantitative measurements of pipe wall thickness at pipe supports.
SUMMARY OF THE INVENTION
This invention is a new inspection technique using ultrasonic Lamb waves to measure reduction of pipe wall thickness due to localized corrosion at pipe supports.
The technique uses two transducers in a pitch-catch mode to send and receive a selected Lamb wave. The wave travels along the pipe wall in the circumferential direction. The reduction of the pipe wall thickness at the pipe support changes the time-of-flight between the two transducers. By comparing time-of-flight data measured from the pipe support area and from an area adjacent to the pipe support, one can quantify the change of time-of-flight due to the pipe corrosion wall loss at the support. A mathematical model, a sizing algorithm and a PC program have been developed to quantitatively relate the change in time-of-flight to the minimum remaining wall thickness. With them, one can quantify the minimum remaining pipe wall thickness at a pipe support by simply measuring the change in the time-of-flight.
The invention may be summarized as a method for quantifying a minimum remaining wall thickness of a pipe at a pipe support without lifting the pipe from the support and without any need for calibration samples, comprising the steps of: a) installing a transmitting transducer and a receiving transducer on said pipe on opposite sides of said pipe support; b) transmitting a highly dispersive and monotonic part of an So mode Lamb wave, whose frequency multiplied by thickness values are equal to or less than the value corresponding to a group velocity minimum, into said pipe with said transmitting transducer; c) receiving said Lamb wave with said receiving transducer; d) measuring the time required for said transmitted Lamb wave to be received by said receiving transducer, said time being identified as the time-of-flight; e) without changing the instrument settings, but with said transmitting and receiving transducers repositioned to points adjacent said pipe support, repeating steps a) through d) to obtain a reference time-of-flight; f) determining a change in time-of-flight due to localized corrosion wall loss by subtracting the time-of-flight measured at the pipe support from the reference time-of-flight; and g) determining the pipe thickness at said pipe support by using said change in time-of-flight.
This is the only ultrasonic technique to the inventor's knowledge that can quantify the minimum pipe wall thickness at the pipe support without physically lifting the pipe for inspection. The quantification is absolute, i.e., = CA 02258439 2006-12-05 2a it is not based on comparison with calibration defects.
There is no need for calibration standards.
The principles of the technique can be used to quantify various piping defects, e.g., pits, general corrosion, corrosion at pipe supports, etc. The sizing algorithm for different defects is the same, although the mathematical model and the equations relating the defect depth to time-of-flight may need to be modified to take into account the different shapes of different defects. With modifications, this invention can be used as a general piping inspection tool.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic showing a method for Lamb wave inspection of a pipe at a pipe support.
Figure 2 shows the phase velocity and group velocity dispersion curves of So mode Lamb wave in steel.
Figure 3 shows progression of corrosion on a pipe at a pipe support.
Figure 4 shows the model for quantification of the minimum remaining wall thickness of a pipe at a pipe support.
Figure 5 shows an example of calculated time shift as a function of the minimum remaining wall thickness in percentage of the nominal wall thickness.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
As shown in Figure 1, a pitch-catch setup is used to measure the time-of-flight of Lamb wave propagation in the circumferential direction. A pipe 10, for which it is desired to determine the minimum wall thickness, hmin, is supported by a pipe support 11. A Lamb wave 12 is "pitched" by transmitter 13 and "caught" by receiver 14.
The change of material thickness affects the velocity of the wave propagation and thus the time-of-flight. By choosing a particular wave mode at a particular operating point, one can calculate the theoretical change of time-of-flight for the wave to propagate from transmitter 13 to receiver 14 due to the change of thickness. If the relationship between the thickness and the velocity is monotonic, one can uniquely determine the change of thickness from the change of time-of-flight. For example, one can choose the Sp mode (i.e. the lowest order symmetric mode) Lamb wave at an operating frequency corresponding to the point where the SO mode group velocity is at its minimum. Keeping the same operating frequency, one then has a monotonic increase of the group velocity with the decrease of the material thickness.
This means that any thickness reduction in the sound path will cause an increase of the Lamb wave 12 velocity and thus a decrease of the transit time for the Lamb wave 12 to propagate from the pitching transducer 13 to the catching transducer 14.
As an example, Figure 2 shows the phase velocity and group velocity of the Sp mode Lamb wave in a steel plate having a longitudinal wave velocity equal to 5.96 km/sec, a shear wave velocity equal to 3.26 km/sec, and a density equal to 7.93 g/cm3. The minimum of the group velocity occurs at f=h = 2.56 MHz=mm, where f is the Lamb wave frequency and h is the material thickness. If one uses the SO mode at the frequency giving the f=h value equal to 2.56 MHz=mm for the nominal thickness, then any reduction from the nominal thickness (e.g., due to localized corrosion at pipe support) will cause a decrease in the f=h value. For example, for inspecting a pipe whose thickness is 0.375" without wall loss due to pipe-support corrosion, one can use a frequency equal to 2.56/(0.375=25.4) = 0.27 MHz as the operating frequency.
This gives the group velocity of the SO mode Lamb wave equal to 1.77 km/sec when there is no wall loss, i.e., it is 100% of the nominal thickness. Should there be any wall loss, the "frequency-times-thickness" at value will decrease since the frequency is kept at a constant. As shown in Figure 2, such a decrease of the f=h value will lead to an increase of the group velocity and therefore a shorter time-of-flight between the transmitting 13 and receiving 14 transducers.
There are two common methods to excite a Lamb wave at a particular point of interest. One is to use an electromagnetic acoustic transducer (EMAT) and the other is to use a piezoelectric transducer (PZT) with a wedge.
With EMAT, one needs to choose a wire spacing that can generate a Lamb wave whose wavelength can satisfy the requirement for the phase velocity of the mode of interest at the operating point, as discussed in the following example. Using Figure 2, SO mode in the steel at f=h = 2.56 MHz=mm has a phase velocity equal to 4.0 km/sec. To operate on the SO mode at f=h = 2.56 MHz=
mm, one needs to use an EMAT wire spacing that gives the SO phase velocity equal to 4.0 km/sec at f=h = 2.56 MHz=
mm. For meander coils, one can use k = 2=d, where ~ is the wavelength and d is the wire spacing. Since Vp = f=k, we have Vp = 2=f=d (1) For a given nominal thickness, h is known. One can determine f to satisfy f=h = 2.56 MHz=mm. Using the f thus determined, one can then determine the wire spacing needed for the inspection. For example, to inspect a 0.375" thick pipe, one should use frequency f= 2.56/(0.375=25.4) = 0.27 MHz. The wire spacing should be 4.0/(2=0.27) = 7.4 mm (0.29"). If one does not have such a wire spacing, one can adjust the frequency f so that the operating point is close to the above and the signal is at its highest amplitude level.
If one uses a piezoelectric transducer with a wedge, the phase velocity is determined as Vp = Vi=sin= (Di) (2) where Vi is the velocity of the incident wave in the wedge material and 6i is the incident angle. For a given wedge material, Vi is fixed. One can adjust 6i to get the appropriate Vp value, as in the case for EMAT. That is, the only difference between EMAT and PZT is how we generate and determine the phase velocity. For EMAT we use Equation (1), and for PZT we use Equation (2). The rest of the calculations are the same.
Once a wave mode and an operating point are selected, one can quantify the minimum remaining wall thickness at a pipe support as follows:
Assume that the corrosion at the pipe support proceeds in a "tangential cut" fashion, as shown in Fig. 3. Mathematically, one can derive the following equations by using Figure 4. As shown in Figure 4, R is the outside radius of the pipe, i.e., R = D/2, where D is the outside diameter of the pipe; hmin is the minimum remaining wall thickness; h is the nominal wall thickness; emax is the maximum angle of corrosion measured from the center of the corroded area; d denotes the wall thickness within the corroded area at angle 0.
Express the minimum remaining wall thickness as a fraction of the nominal wall thickness by using a new variable x, where - ----------- -hmin x h (3) From simple trigonometry, it is not difficult to find that the maximum angle of corrosion, Amax., can be expressed as a function of x, h, and R:
0 =cos-1{1-(1-x) h max R ( 4 ) Similarly, one can find that the wall thickness d at angle 0 can be written explicitly as a function of 0, x, h, and R:
R-h+x.h d= cos(q) -R+h (5) Assume that the velocity of the Lamb wave at wall thickness d is Vg(f=d), where Vg is the group velocity of the Lamb wave as a function of the product of frequency and thickness and f is the Lamb wave frequency.
Similarly, the velocity of the Lamb wave at nominal wall thickness h is Vg(f=h). The time for the Lamb wave to propagate through the corroded area can be written as:
B d max R-h+-t=2. ~ Vg(f.d)2 dB (6) while the time for the Lamb wave to propagate through an equivalent area without the pipe-support corrosion can be written as:
max R_2 t0 =2. 0 Vg(f.h)d9 (7) Note: t0 is measured in the same manner as t except that the measurement is made at a point next to the pipe support.
The time shift due to localized corrosion at the pipe support is:
At = t - t0 (8) That is 0 d 0 h maxR-h+- max R--dO (9) Ot=2. J (f.d)dB-2. 0 Vg(f.h) 0 Vg Combining Equations (4), (5) and (9), one can find that the time shift is a function of x, h, R, f. Except for x, all the parameters are easily measurable in the field and can be used as input data for calculation of Lamb wave time shift with the equations. These other parameters can be measured as follows.
The nominal thickness, h, can be measured with an ultrasonic thickness gauge. The best mode of operation is to have the thickness measured from the bottom of the pipe in an area right next to the pipe support. If the bottom can not be reached, measure the thickness from as close to the pipe support as possible.
The outside radius, R, can be calculated from the nominal outside diameter, D. R = D/2. If the nominal outside diameter is not known, calculate R by using R = C/(2=7t), where C is the external circumference of the pipe, which can be measured with a tape measure.
The frequency, f, is the Lamb wave frequency used in the inspection. Preferably, this frequency is set at the value giving the f=h corresponding to the minimum group velocity of the SO mode Lamb wave, as discussed previously.
Using the h, R, and f values determined as above, one can then calculate At as a function of x. The calculated At vs x curve can then be used to determine the minimum remaining wall thickness from the measured time shift.
For example, Figure 5 shows the time shift At as a function of the remaining minimum wall thickness in percentage of the nominal wall thickness by using Equations (4), (5), and (9) with h = 0.437", R = 7", and f = 225 kHz (i.e. 0.225 MHz). This calculated curve can be used to determine the minimum remaining wall thickness of a pipe that has 0.437" nominal wall thickness and 14"
outside diameter, with the SO mode Lamb wave operated at 225 kHz. As shown in the figure, for each time shift, At, there is a corresponding minimum remaining wall thickness.
In the best mode of operation, one should use the frequency f that gives the f=h value corresponding to the minimum group velocity of the SO Lamb wave. By choosing such a frequency, the above correlation between the time shift, At, and the minimum remaining wall thickness as a fraction of the nominal wall thickness, x, is always one-on-one. That is, for each time shift value, there is only one possible minimum remaining wall thickness value.
If for any reason one cannot use the frequency in the best mode of operation, it is possible to have more than one minimum remaining wall thickness values corresponding to one time shift value. In such a case, one should not use the calculated curve in the time shift range where there is more than one minimum remaining wall thickness value. The part of the curve that has a one-on-one correspondence is still valid, and can be used to determine the minimum remaining wall thickness from measured time shift.
A FORTRAN program has been written for calculations per Equations (3), (4), (5) and (9). This program calculates time shift as a function of the minimum remaining wall thickness. The program is for Sp mode Lamb wave inspection of pipes at pipe supports. Vg as a function of f=h for the SO mode in a steel plate is included in the program.
Ultrasonic System Requirements:
The combination of EMAT and Automated Ultrasonic Testing (AUT) is preferred for the above inspection. EMAT
There was, however, no sizing technique to allow quantitative measurements of pipe wall thickness at pipe supports.
SUMMARY OF THE INVENTION
This invention is a new inspection technique using ultrasonic Lamb waves to measure reduction of pipe wall thickness due to localized corrosion at pipe supports.
The technique uses two transducers in a pitch-catch mode to send and receive a selected Lamb wave. The wave travels along the pipe wall in the circumferential direction. The reduction of the pipe wall thickness at the pipe support changes the time-of-flight between the two transducers. By comparing time-of-flight data measured from the pipe support area and from an area adjacent to the pipe support, one can quantify the change of time-of-flight due to the pipe corrosion wall loss at the support. A mathematical model, a sizing algorithm and a PC program have been developed to quantitatively relate the change in time-of-flight to the minimum remaining wall thickness. With them, one can quantify the minimum remaining pipe wall thickness at a pipe support by simply measuring the change in the time-of-flight.
The invention may be summarized as a method for quantifying a minimum remaining wall thickness of a pipe at a pipe support without lifting the pipe from the support and without any need for calibration samples, comprising the steps of: a) installing a transmitting transducer and a receiving transducer on said pipe on opposite sides of said pipe support; b) transmitting a highly dispersive and monotonic part of an So mode Lamb wave, whose frequency multiplied by thickness values are equal to or less than the value corresponding to a group velocity minimum, into said pipe with said transmitting transducer; c) receiving said Lamb wave with said receiving transducer; d) measuring the time required for said transmitted Lamb wave to be received by said receiving transducer, said time being identified as the time-of-flight; e) without changing the instrument settings, but with said transmitting and receiving transducers repositioned to points adjacent said pipe support, repeating steps a) through d) to obtain a reference time-of-flight; f) determining a change in time-of-flight due to localized corrosion wall loss by subtracting the time-of-flight measured at the pipe support from the reference time-of-flight; and g) determining the pipe thickness at said pipe support by using said change in time-of-flight.
This is the only ultrasonic technique to the inventor's knowledge that can quantify the minimum pipe wall thickness at the pipe support without physically lifting the pipe for inspection. The quantification is absolute, i.e., = CA 02258439 2006-12-05 2a it is not based on comparison with calibration defects.
There is no need for calibration standards.
The principles of the technique can be used to quantify various piping defects, e.g., pits, general corrosion, corrosion at pipe supports, etc. The sizing algorithm for different defects is the same, although the mathematical model and the equations relating the defect depth to time-of-flight may need to be modified to take into account the different shapes of different defects. With modifications, this invention can be used as a general piping inspection tool.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic showing a method for Lamb wave inspection of a pipe at a pipe support.
Figure 2 shows the phase velocity and group velocity dispersion curves of So mode Lamb wave in steel.
Figure 3 shows progression of corrosion on a pipe at a pipe support.
Figure 4 shows the model for quantification of the minimum remaining wall thickness of a pipe at a pipe support.
Figure 5 shows an example of calculated time shift as a function of the minimum remaining wall thickness in percentage of the nominal wall thickness.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
As shown in Figure 1, a pitch-catch setup is used to measure the time-of-flight of Lamb wave propagation in the circumferential direction. A pipe 10, for which it is desired to determine the minimum wall thickness, hmin, is supported by a pipe support 11. A Lamb wave 12 is "pitched" by transmitter 13 and "caught" by receiver 14.
The change of material thickness affects the velocity of the wave propagation and thus the time-of-flight. By choosing a particular wave mode at a particular operating point, one can calculate the theoretical change of time-of-flight for the wave to propagate from transmitter 13 to receiver 14 due to the change of thickness. If the relationship between the thickness and the velocity is monotonic, one can uniquely determine the change of thickness from the change of time-of-flight. For example, one can choose the Sp mode (i.e. the lowest order symmetric mode) Lamb wave at an operating frequency corresponding to the point where the SO mode group velocity is at its minimum. Keeping the same operating frequency, one then has a monotonic increase of the group velocity with the decrease of the material thickness.
This means that any thickness reduction in the sound path will cause an increase of the Lamb wave 12 velocity and thus a decrease of the transit time for the Lamb wave 12 to propagate from the pitching transducer 13 to the catching transducer 14.
As an example, Figure 2 shows the phase velocity and group velocity of the Sp mode Lamb wave in a steel plate having a longitudinal wave velocity equal to 5.96 km/sec, a shear wave velocity equal to 3.26 km/sec, and a density equal to 7.93 g/cm3. The minimum of the group velocity occurs at f=h = 2.56 MHz=mm, where f is the Lamb wave frequency and h is the material thickness. If one uses the SO mode at the frequency giving the f=h value equal to 2.56 MHz=mm for the nominal thickness, then any reduction from the nominal thickness (e.g., due to localized corrosion at pipe support) will cause a decrease in the f=h value. For example, for inspecting a pipe whose thickness is 0.375" without wall loss due to pipe-support corrosion, one can use a frequency equal to 2.56/(0.375=25.4) = 0.27 MHz as the operating frequency.
This gives the group velocity of the SO mode Lamb wave equal to 1.77 km/sec when there is no wall loss, i.e., it is 100% of the nominal thickness. Should there be any wall loss, the "frequency-times-thickness" at value will decrease since the frequency is kept at a constant. As shown in Figure 2, such a decrease of the f=h value will lead to an increase of the group velocity and therefore a shorter time-of-flight between the transmitting 13 and receiving 14 transducers.
There are two common methods to excite a Lamb wave at a particular point of interest. One is to use an electromagnetic acoustic transducer (EMAT) and the other is to use a piezoelectric transducer (PZT) with a wedge.
With EMAT, one needs to choose a wire spacing that can generate a Lamb wave whose wavelength can satisfy the requirement for the phase velocity of the mode of interest at the operating point, as discussed in the following example. Using Figure 2, SO mode in the steel at f=h = 2.56 MHz=mm has a phase velocity equal to 4.0 km/sec. To operate on the SO mode at f=h = 2.56 MHz=
mm, one needs to use an EMAT wire spacing that gives the SO phase velocity equal to 4.0 km/sec at f=h = 2.56 MHz=
mm. For meander coils, one can use k = 2=d, where ~ is the wavelength and d is the wire spacing. Since Vp = f=k, we have Vp = 2=f=d (1) For a given nominal thickness, h is known. One can determine f to satisfy f=h = 2.56 MHz=mm. Using the f thus determined, one can then determine the wire spacing needed for the inspection. For example, to inspect a 0.375" thick pipe, one should use frequency f= 2.56/(0.375=25.4) = 0.27 MHz. The wire spacing should be 4.0/(2=0.27) = 7.4 mm (0.29"). If one does not have such a wire spacing, one can adjust the frequency f so that the operating point is close to the above and the signal is at its highest amplitude level.
If one uses a piezoelectric transducer with a wedge, the phase velocity is determined as Vp = Vi=sin= (Di) (2) where Vi is the velocity of the incident wave in the wedge material and 6i is the incident angle. For a given wedge material, Vi is fixed. One can adjust 6i to get the appropriate Vp value, as in the case for EMAT. That is, the only difference between EMAT and PZT is how we generate and determine the phase velocity. For EMAT we use Equation (1), and for PZT we use Equation (2). The rest of the calculations are the same.
Once a wave mode and an operating point are selected, one can quantify the minimum remaining wall thickness at a pipe support as follows:
Assume that the corrosion at the pipe support proceeds in a "tangential cut" fashion, as shown in Fig. 3. Mathematically, one can derive the following equations by using Figure 4. As shown in Figure 4, R is the outside radius of the pipe, i.e., R = D/2, where D is the outside diameter of the pipe; hmin is the minimum remaining wall thickness; h is the nominal wall thickness; emax is the maximum angle of corrosion measured from the center of the corroded area; d denotes the wall thickness within the corroded area at angle 0.
Express the minimum remaining wall thickness as a fraction of the nominal wall thickness by using a new variable x, where - ----------- -hmin x h (3) From simple trigonometry, it is not difficult to find that the maximum angle of corrosion, Amax., can be expressed as a function of x, h, and R:
0 =cos-1{1-(1-x) h max R ( 4 ) Similarly, one can find that the wall thickness d at angle 0 can be written explicitly as a function of 0, x, h, and R:
R-h+x.h d= cos(q) -R+h (5) Assume that the velocity of the Lamb wave at wall thickness d is Vg(f=d), where Vg is the group velocity of the Lamb wave as a function of the product of frequency and thickness and f is the Lamb wave frequency.
Similarly, the velocity of the Lamb wave at nominal wall thickness h is Vg(f=h). The time for the Lamb wave to propagate through the corroded area can be written as:
B d max R-h+-t=2. ~ Vg(f.d)2 dB (6) while the time for the Lamb wave to propagate through an equivalent area without the pipe-support corrosion can be written as:
max R_2 t0 =2. 0 Vg(f.h)d9 (7) Note: t0 is measured in the same manner as t except that the measurement is made at a point next to the pipe support.
The time shift due to localized corrosion at the pipe support is:
At = t - t0 (8) That is 0 d 0 h maxR-h+- max R--dO (9) Ot=2. J (f.d)dB-2. 0 Vg(f.h) 0 Vg Combining Equations (4), (5) and (9), one can find that the time shift is a function of x, h, R, f. Except for x, all the parameters are easily measurable in the field and can be used as input data for calculation of Lamb wave time shift with the equations. These other parameters can be measured as follows.
The nominal thickness, h, can be measured with an ultrasonic thickness gauge. The best mode of operation is to have the thickness measured from the bottom of the pipe in an area right next to the pipe support. If the bottom can not be reached, measure the thickness from as close to the pipe support as possible.
The outside radius, R, can be calculated from the nominal outside diameter, D. R = D/2. If the nominal outside diameter is not known, calculate R by using R = C/(2=7t), where C is the external circumference of the pipe, which can be measured with a tape measure.
The frequency, f, is the Lamb wave frequency used in the inspection. Preferably, this frequency is set at the value giving the f=h corresponding to the minimum group velocity of the SO mode Lamb wave, as discussed previously.
Using the h, R, and f values determined as above, one can then calculate At as a function of x. The calculated At vs x curve can then be used to determine the minimum remaining wall thickness from the measured time shift.
For example, Figure 5 shows the time shift At as a function of the remaining minimum wall thickness in percentage of the nominal wall thickness by using Equations (4), (5), and (9) with h = 0.437", R = 7", and f = 225 kHz (i.e. 0.225 MHz). This calculated curve can be used to determine the minimum remaining wall thickness of a pipe that has 0.437" nominal wall thickness and 14"
outside diameter, with the SO mode Lamb wave operated at 225 kHz. As shown in the figure, for each time shift, At, there is a corresponding minimum remaining wall thickness.
In the best mode of operation, one should use the frequency f that gives the f=h value corresponding to the minimum group velocity of the SO Lamb wave. By choosing such a frequency, the above correlation between the time shift, At, and the minimum remaining wall thickness as a fraction of the nominal wall thickness, x, is always one-on-one. That is, for each time shift value, there is only one possible minimum remaining wall thickness value.
If for any reason one cannot use the frequency in the best mode of operation, it is possible to have more than one minimum remaining wall thickness values corresponding to one time shift value. In such a case, one should not use the calculated curve in the time shift range where there is more than one minimum remaining wall thickness value. The part of the curve that has a one-on-one correspondence is still valid, and can be used to determine the minimum remaining wall thickness from measured time shift.
A FORTRAN program has been written for calculations per Equations (3), (4), (5) and (9). This program calculates time shift as a function of the minimum remaining wall thickness. The program is for Sp mode Lamb wave inspection of pipes at pipe supports. Vg as a function of f=h for the SO mode in a steel plate is included in the program.
Ultrasonic System Requirements:
The combination of EMAT and Automated Ultrasonic Testing (AUT) is preferred for the above inspection. EMAT
is preferred because it does not require extensive surface preparation and it can work through a thin coating. Such capabilities make scanning along pipe for a long distance possible. AUT is preferred because scanning along a pipe is desired. For Lamb wave measurements, AUT
system should be able to operate in the frequency range between 100 kHz and 500 kHz. In addition, the system should allow measurements of time shift with an accuracy of at least 0.1 sec.
The system should be able to handle pitch-catch measurements, as shown in Fig. 1. It is also desirable to have pulse-echo capability, so that one can use pulse-echo techniques to detect such defects as pits and cracks which is not considered in the above discussion.
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system should be able to operate in the frequency range between 100 kHz and 500 kHz. In addition, the system should allow measurements of time shift with an accuracy of at least 0.1 sec.
The system should be able to handle pitch-catch measurements, as shown in Fig. 1. It is also desirable to have pulse-echo capability, so that one can use pulse-echo techniques to detect such defects as pits and cracks which is not considered in the above discussion.
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Claims (2)
1. A method for quantifying a minimum remaining wall thickness of a pipe at a pipe support without lifting the pipe from the support and without any need for calibration samples, comprising the steps of:
a) installing a transmitting transducer and a receiving transducer on said pipe on opposite sides of said pipe support;
b) transmitting a highly dispersive and monotonic part of an S o mode Lamb wave, whose frequency multiplied by thickness values are equal to or less than the value corresponding to a group velocity minimum, into said pipe with said transmitting transducer;
c) receiving said Lamb wave with said receiving transducer;
d) measuring the time required for said transmitted Lamb wave to be received by said receiving transducer, said time being identified as the time-of-flight;
e) without changing the instrument settings, but with said transmitting and receiving transducers repositioned to points adjacent said pipe support, repeating steps a) through d) to obtain a reference time-of-flight;
f) determining a change in time-of-flight due to localized corrosion wall loss by subtracting the time-of-flight measured at the pipe support from the reference time-of-flight; and g) determining the pipe thickness at said pipe support by using said change in time-of-flight.
a) installing a transmitting transducer and a receiving transducer on said pipe on opposite sides of said pipe support;
b) transmitting a highly dispersive and monotonic part of an S o mode Lamb wave, whose frequency multiplied by thickness values are equal to or less than the value corresponding to a group velocity minimum, into said pipe with said transmitting transducer;
c) receiving said Lamb wave with said receiving transducer;
d) measuring the time required for said transmitted Lamb wave to be received by said receiving transducer, said time being identified as the time-of-flight;
e) without changing the instrument settings, but with said transmitting and receiving transducers repositioned to points adjacent said pipe support, repeating steps a) through d) to obtain a reference time-of-flight;
f) determining a change in time-of-flight due to localized corrosion wall loss by subtracting the time-of-flight measured at the pipe support from the reference time-of-flight; and g) determining the pipe thickness at said pipe support by using said change in time-of-flight.
2. The method of claim 1 wherein a model, assuming that corrosion at the pipe support progresses in a tangential cut manner, is used to relate said change in time-of-flight to the minimum remaining wall thickness.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US09/007391 | 1998-01-15 | ||
| US09/007,391 US5965818A (en) | 1998-01-15 | 1998-01-15 | Ultrasonic Lamb wave technique for measurement of pipe wall thickness at pipe supports |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2258439A1 CA2258439A1 (en) | 1999-07-15 |
| CA2258439C true CA2258439C (en) | 2007-10-16 |
Family
ID=21725877
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002258439A Expired - Fee Related CA2258439C (en) | 1998-01-15 | 1999-01-13 | Ultrasonic lamb wave technique for measurement of pipe wall thickness at pipe supports |
Country Status (2)
| Country | Link |
|---|---|
| US (1) | US5965818A (en) |
| CA (1) | CA2258439C (en) |
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| JP3832142B2 (en) * | 1999-06-24 | 2006-10-11 | 株式会社日立製作所 | Pipe thickness reduction management system |
| US6363787B1 (en) * | 1999-12-13 | 2002-04-02 | Bechtel Bwxt Idaho Llc | Apparatus and method for measuring the thickness of a coating |
| US6561032B1 (en) | 2000-05-15 | 2003-05-13 | National Research Council Of Canada | Non-destructive measurement of pipe wall thickness |
| US6497159B1 (en) | 2000-06-12 | 2002-12-24 | Hydro-Quebec | Bracelet for moving ultrasonic sensors along a pipe |
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| EP1959229A1 (en) * | 2007-02-19 | 2008-08-20 | Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno | Ultrasonic surface monitoring |
| EP2120046A1 (en) | 2008-05-13 | 2009-11-18 | Nederlandse Organisatie voor toegepast-natuurwetenschappelijk Onderzoek TNO | Ultrasonic modelling |
| WO2009139627A1 (en) * | 2008-05-13 | 2009-11-19 | Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno | Ultrasonic modelling |
| US9182081B2 (en) * | 2008-06-30 | 2015-11-10 | Bp Corporation North America Inc. | Rapid data-based data adequacy procedure for pipeline integrity assessment |
| JP2011529182A (en) * | 2008-07-22 | 2011-12-01 | ネーデルランツ オルガニサティー フォール トゥーゲパストナトゥールヴェテンシャッペリーク オンデルズーク テーエンオー | Corrosion monitoring |
| US7941282B2 (en) | 2008-08-01 | 2011-05-10 | Bp Exploration Operating Company Limited | Estimating worst case corrosion in a pipeline |
| US8600702B2 (en) | 2008-12-30 | 2013-12-03 | United States Pipe And Foundry Company, Llc | Non-destructive thickness measurement systems and methods |
| CN101539540B (en) * | 2009-04-09 | 2011-07-06 | 江苏省电力试验研究院有限公司 | Ultrasonic guided wave testing method of corrosion of partially implanted pole body of steel pipe pole |
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| US9518959B2 (en) * | 2013-08-12 | 2016-12-13 | Texas Instruments Incorporated | Structural health monitoring system and method |
| WO2016032457A1 (en) * | 2014-08-27 | 2016-03-03 | Leeo, Inc. | Fluid-flow monitor |
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| DE102015100066A1 (en) * | 2015-01-06 | 2016-07-07 | Rosen Swiss Ag | Method for determining the state of elastomer or other coatings of a metal article coated at least on one side |
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| US11305133B2 (en) * | 2017-03-31 | 2022-04-19 | The Board Of Trustees Of The Leland Stanford Junior University | Efficient acoustic energy transfer through skull via excitation of Lamb waves |
| CN107632068B (en) * | 2017-09-12 | 2020-03-20 | 国网湖南省电力公司 | Method for detecting thinning amount of boiler heating surface pipe |
| US10859462B2 (en) | 2018-09-04 | 2020-12-08 | Mueller International, Llc | Hydrant cap leak detector with oriented sensor |
| GB2577920A (en) * | 2018-10-10 | 2020-04-15 | Guided Ultrasonics Ltd | Determining thickness of an elongate or extended structure |
| US11342656B2 (en) | 2018-12-28 | 2022-05-24 | Mueller International, Llc | Nozzle cap encapsulated antenna system |
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| US11542690B2 (en) | 2020-05-14 | 2023-01-03 | Mueller International, Llc | Hydrant nozzle cap adapter |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4307616A (en) * | 1979-12-26 | 1981-12-29 | Rockwell International Corporation | Signal processing technique for ultrasonic inspection |
| US5438872A (en) * | 1991-06-21 | 1995-08-08 | Canon Kabushiki Kaisha | Measuring method and apparatus using a lamb wave |
| US5619423A (en) * | 1994-01-21 | 1997-04-08 | Scrantz; Leonard | System, method and apparatus for the ultrasonic inspection of liquid filled tubulars and vessels |
-
1998
- 1998-01-15 US US09/007,391 patent/US5965818A/en not_active Expired - Fee Related
-
1999
- 1999-01-13 CA CA002258439A patent/CA2258439C/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| US5965818A (en) | 1999-10-12 |
| CA2258439A1 (en) | 1999-07-15 |
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