GB2147102A - Acoustic pulse-echo wall thickness method and apparatus - Google Patents

Abstract

A method and apparatus for determining the thickness of a wall and the presence of any anomaly in said wall. A piezoelectric transducer 16 transmits acoustic pulses simultaneously in opposite directions from its parallel opposite faces. One of said pulses is reflected by a reflector 28 into a path normal to said wall for determining wall thickness, and the other of said pulses is reflected by a reflector 27 into a path at an oblique angle to said wall for determining any anomaly in said wall. For use in a pipeline. (Fig. 7) a plurality of such instruments (82) are disposed around a unit and are scanned circumferentially as the unit travels through the pipeline. The supporting arrangement for the transducer 16 and the reflectors 27, 28 is also described (Figs. 3, 4, 5). <IMAGE>

Description

SPECIFICATION Acoustic pulse-echo well thickness method and apparatus This invention relates to an acoustic pulse-echo method and apparatus for determining the thickness of a wall and the presence of any anomaly in said wall.

Heretofore, acoustic pulse-echo measurements have been employed for non-destructive wall thickness measurements. U.S. patents 3,995,1 79 and 4,022,055 describe piezoelectric transducer structure for making such wall thickness measurements. U.S. patent 3,995,179 relates to the need for dissipating or damping the acoustic energies from one side of a piezoelectric crystal in order to avoid the interfering signals generated at the back face of the crystal. U.S. patent 4,022,055 describes a structure for splitting the energy that is transmitted from a transducer in order to have one portion directed normally to the wall to be measured while the other portion is directed at an acute angle for discovering anomalies that may exist in the wall of a pipe or the like being measured.

U.S. Patent No. 3,106,839 discloses a wheel with a rubber tire exterior which was designed to ride upon the surface of a rail such as a railroad rail. Energies from a transducer are reflected down to the same location at the base of the wheel. Therefore, it is not applicable to wall thickness measurement and simultaneous anomaly surveying.

The present invention provides a method for determining the thickness of a wall and the presence of any anomaly in said wall, comprising: generating acoustic pulses, transmitting said pulses at a wall, and receiving pulses returning along said paths from said wall, characterised in that: said acoustic pulses are generated simultaneously in opposite directions; one of said pulses is reflected into a path normal to said wall for determining wall thickness; and the other of said pulses is reflected into a path an an obliquie angle to said wall for determining any anomaly in said wall.

The present invention also provides apparatus for determining the thickness of a wall and the presence of any anomaly in said wall, comprising: means for generating acoustic pulses and transmitting said pulses at a wall, and means for receiving pulses returning along said paths from said wall, characterised in that: said generating means comprises a generator (16) to generate simultaneous pulses in opposite directions; a first reflector (28,50) to reflect one said pulse into a path normal to said wall; and a second reflector (27,51) to reflect the other of said pulses into a path at an oblique angle to said wall.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of wall thickness measuring apparatus according to the invention; Figure 2 illustrates received echo signals in use of the apparatus of Fig. 1; Figures 3, 4 and 5 illustrate three views of a unitary instrument according to the invention, Fig 3 being a longitudinai cross-section taken along the line 3-3 of Fig. 4; Figure 4 is a plan view of the instrument illustrated in Figs. 3-5; Figure 5 is an end elevation of the instrument illustrated in Figs. 3-5; Figure 6 is a circuit diagram illustrating the circuits employed where a plurality of instruments are employed; and Figure 7 is a fragmentary cross-sectional schematic indicating the use of a plurality of instruments around the periphery of a surveying instrument which may traverse the inside of a pipe.

Fig. 1 is a schematic diagram of apparatus used in a method of acoustic pulse-echo wall thickness testing according to this invention. There is shown a fragmentary portion in crosssection of a pipe 11 that has a wall 1 2. The wall 1 2 may have an anomaly such as a pitted spot 1 5 therein. There is a transducer 1 6 that is preferably a piezoelectric crystal. It has two parallel faces, each of which are coated with a lens made of an epoxy material for acoustic matching to the fluid (not shown) within the pipe 11.Thus, there are lenses 1 9 and 20 illustrated, one located on each face of the transducer 1 6. When the transducer is actuated to produce acoustic energy by means of applying an electrical pulse, there is simultaneously generated an acoustic pulse which is transmitted from each face of the transducer 1 6. This is indicated by a pair of dashed lines 23 and 24.

The acoustic pulses travel along the paths indicated by the dashed lines 23 and 24 having been transmitted through the lenses 1 9 and 20, respectively. A pair of metal reflector elements 27 and 28 reflect the pulses travelling on paths 23 and 24 from angled faces 29 and 30, respectively. The faces are oriented so that the face 30 of reflector 28 is at 45 relative to the wall 12 of the pipe 11, while the face 29 of the reflector 27 is at an angle greater than 45 relative to the wall 1 2. Consequently, the pulse on path 24 is reflected so as to travel on a path 31 that is normal to the wall 12, while the pulse on path 23 is reflected onto a path 32, which path is oblique to the wall 1 2.

The pulse-echo technique is well known. It employs ultrasonic energies so that short time duration signals may be employed.

A short time duration, unitary pulse of ultrasonic acoustic energy is created by applying a short time duration electrical voltage to the crystal of transducer 1 6, in a conventional manner.

For example, see the disclosure of the U.S. Patent 3,995,1 79 mentioned above. The crystal material is preferably lead metaniobate, and in the schematic illustration it would be a flat disc shape having silvered faces (not shown) which act as the electrodes for causing the deformation of the crystal upon application of the electrical energy pulse.

Fig. 2 illustrates oscillograph traces which are developed in use. An upper trace 35 has returning pulses 36 and 37 thereon. These indicated the thickness of the wall 1 2 as shown by the time T, indicated at the bottom of the oscillograph illustration. Pulse 36 is the first returned (reflected) energy from the inside of the wall 12, and pulse 37 is that reflected from the outside of the wall 12. The acoustic pulse creating these reflections traveled over the paths 24 and 31.

The oscillograph trace 35 shows the signals that are received when the relative positions of the elements and the spot 1 5 on the pipe wall are such that there is no anomaly in the path of the other acoustic pulse traveling over the paths 23 and 32 and being reflected by the reflector element 27. Thus, there is only a low amplitude signal 38 reflected by the surface roughness of the inside of the wall 1 2.

However, when an anomaly such as the spot 1 5 is present in the path 32, the oscillograph trace is like lower trace 40 in Fig. 2. It contains pulses 41 and 42 that are returned along the path 31 which is normal to the wall 12. And also, after a longer interval of time T2, there is an additional pulse 43 which is caused by the reflected energy from the depth of the spot 1 5 returning along the paths 32 and 23 and going back to the face of the transducer 16. There also may have been a preceding surface roughness signal 44.

It may be noted that in contrast with the operation of a system according to the above noted U.S. Patent 4,022,055, the signal strength obtained is greatly increased by using the arrangement according to this invention. Thus, whereas in that patent the two pulse beams were acquired by having a split reflector in order to obtain two different angles of incidence, the present invention provides full signal strength going in the two opposite directions while obtaining the same angles of incidence to the pipe wall and, of course, with greatly increased signal strength. For example, the following tables compare the output voltages in millivolts obtained by the dual reflector transducer according to this invention, to the output voltages of a split reflector transducer as described in the above noted U.S. patent 4,022,055.

TABLE I Signal Output, millivolts Inside Wall Outside Wall Transducer Pipe Reflection Reflection Type Size 260 80 D 15 cm.

135 55 S 320 50 D 20 cm.

210 40 S 270 75 D 25 cm.

120 30 S D = Dual Reflector S = Split Reflector With reference to Figs. 3, 4 and 5, there is illustrated a unit according to this invention. An elongate body 46 has mouted centrally thereof a piezoelectric transducer disc 47. Spaced longitudinally one on either side from the transducer disc 47, there are a pair of polished metal reflector elements 50 and 51. These are cylindrical in shape with polished reflecting faces 54 and 55, respectively, situated at angles of 45 and something more than 45 relative to the longitudinal axis of the body 46. These angles are used in order to obtain reflection paths like those indicated in Fig. 1. The reflectors 50 and 51 rest in a hollowed upper surface 58 located along the top of the body 46. There are bolts 61 and 62 which go through holes in the reflector cylinders 50 and 51 to hold them securely in place.

The transducer crystal 47 is mounted in any feasible manner such as by being supported in a rim 65 that is suspended from an upper cantilevered roof element 66. Roof 66 has an elongate window or opening 69 for permitting free passage of the acoustic pulse energies from the transducer 47 to and from the reflector face 55. There is a resilient material pad 70 between the rim 65 (at the top of the transducer 47) and an upper plate 71 formed by an extension of the roof 66. The cantilevered roof 66 is held in place by being secured under the head of the bolt 62 as the roof lies over the top of the reflector 51. There are access holes 74 for accommodating electrical terminal connectors (not shown) which have the circuit connections (not shown) brought out via a short groove 75 to keep such connecting wires out of the path of acoustic energies.It will be understood that the structure of the electrodes for transducer 47 is well known and is as described in the above U.S. patents 3,995,179 and 4,022,055. Thus, the electrodes include silvered faces (not shown) on the transducer 47.

With reference to Figs. 6 and 7, the apparatus according to this invention is particularly well adapted for use in surveying the wall structure of a pipeline. And, in such case there will be employed a plurality of units like that shown and described above in connection with Figs. 3, 4 and 5. Thus, in Fig. 7 there is shown a plurality of instruments 82 which are indicated schematically in cross-section. They are mounted on a concentric structure 78 within a pipe 79.

The plurality of instruments 82 are mounted longitudinally with the tranducers of each oriented so that the faces thereof are transverse to the pipe wall 79. It will be noted that the instruments 82 are situated around the periphery of the concentric structure 78 on which they are mounted so that the inside surface of the pipe 79 may be scanned and any anomalies such as a dashed line anomaly 83 (shown in Fig. 7) will be detected when some of the instruments are actuated in the scanning sequence.

Each of the instruments 82 has included therewith an electrical circuit arrangement like that illustrated in Fig. 6. Thus, each has a pulse generating circuit which includes a circuit connection 86 that leads from an electrode of the transducer crystal 47. There is a common circuit connection 11 8 that is connected to a controlled electrode 90 of a silicon controlled rectifier (SCR) 91. The SCR 91 acts to pass a voltage pulse from a charged capacitor 94 to the transducer crystal 47 whenever it is triggered by a signal applied to a circuit 95 so as to trip the SCR 91 into conduction. The capacitor 94 is maintained charged by a relatively high DC potential which is maintained at a terminal 98 with a resistor 99 between the terminal 98 and the high potential plate of capacitor 94.

Referring to the pulse generating circuit of Fig. 6 again, it will be noted that there is a common ground circuit 102 that is connected to another electrode 103 of the transducer crystal 47. Also, the circuit 102 has one side of a variable inductor 106 as well as one end of a resistor 107 connected thereto. In addition, there is a resistor 110 that is in the control circuit 95. The control circuit goes via the resistor 110 from the output of an amplifier 111. And, the output of a selector circuit 114 goes to the input of the amplifier 111 over a circuit connection 11 5.

It will be understood that after each pair of acoustic pulses is transmitted by application of the voltage pulse from capacitor 94 to the transducer crystal 47, a sufficient period of time is allowed before the next electrical pulse is applied so as to permit the reflected acoustic pulses traveling over both paths e.g. path 24 and 31, (Fig. 1) in addition to path 23 and 32, to reach the crystal 47 of the transducer. And, it will be understood that when these reflected pulses of acoustic energy reach the crystal 47 there will be electrical signals generated and carried over a circuit connection 11 8. Circuit connection 11 8 goes to an amplifier (not shown) from which it may go to an oscilloscope (not shown) to develop oscillograph signals like those illustrated in Fig. 2.

It will be appreciated that there is an individual control and reflected-pulse amplifier circuit for each of the transducers of the instruments 82. This is indicated in Fig. 6 where there are rectangles 1 21 and 1 22 which represent additional circuits like that described above in connection with the crystal 47. There will, of course, be one such circuit for each of the instruments 82 indicated in Fig. 7.

It will also be understood that the time periods involved in sending and receiving individual acoustic pulses and reflected return pulses are relatively short so that the complete scan of the instruments 82 may be carried out around the concentric structure 78 rapidly enough to provide adequate indication and measurement of the pipe wall conditions along a pipeline.

Claims (8)

1. A method for determining the thickness of a wall and the presence of any anomaly in said wall, comprising: generating acoustic pulses, transmitting said pulses along a path at a wall, and receiving pulses returning along said path from said wall, wherein: said acoustic pulses are generated simultaneously in opposite directions; one of said pulses is reflected into a path normal to said wall for determining wall thickness; and the other of said pulses is reflected into a path at an oblique angle to said wall for determining any anomaly in said wall.

2. Apparatus for determining the thickness of a wall and the presence of any anomaly in said wall, comprising: means for generating acoustic pulses and transmitting said pulses along a path at a wall, and means for receiving pulses returning along said paths from said wall, wherein: said generating means comprises a generator to generate simultaneous pulses in opposite directions; a first reflector to reflect one said pulse into a path normal to said wall; and a second reflector to reflect the other of said pulses into a path at an oblique angle to said wall.

3. Apparatus according to claim 2 wherein said generator comprises a piezoelectric transducer having parallel opposite faces.

4. Apparatus according to claim 3 wherein said parallel faces of said transducer are disposed transverse to said wall, said reflectors are spaced apart from said transducer on opposite sides thereof, said first reflector is disposed at 45 to said wall, and said second reflector is disposed at an acute angle greater than 45 to said wall.

5. Apparatus according to any one of claims 2 to 4 wherein each said reflector comprises a polished metal reflector.

6. Apparatus according to any one of claims 2 to 5 wherein said wall is a wall of a pipe, and wherein said apparatus comprises a plurality of said generators, reflectors and receivers, and means for scanning said receivers circumferentially of the pipe.

7. A method according to claim 1 and substantially as described herein with reference to the accompanying drawings.

8. Apparatus for determining the thickness of a wall and the presence of any anomaly in said wall, substantially as described herein with reference to the accompanying drawings.