GB2149116A - Method and apparatus for measuring wall thickness - Google Patents

Abstract

A current pulse is applied to a magnetic reluctance coil (16) disposed adjacent the metal wall (11), and a first signal is obtained which is representative of the inductance of the coil. A second signal is obtained by an ultrasonic acoustic pulse transducer (23) which is representative of the distance between the coil (16) and the wall (11). The first and second signals are used to determine the thickness of the wall and hence the presence of any wall thickness anomaly compensated for variations in distances between the coil (16) and the wall (11). The coil inductance varies according to whether a pit in the wall is located at the side of the wall nearer or further away from the coil. The ultrasonic pulses may also be directed at an angle (33) to determine the presence of the anomalies. The apparatus is particularly applicable for use in a steel pipeline. <IMAGE>

Description

SPECIFICATION Method and apparatus for measuring wall thickness This invention relates to methods and apparatus for determining the thickness of a metallic wall, and hence for determining the presence of any wall thickness anomaly in the wall. The wall may be the steel wall of a pipeline.

Magnetic methods have been used previously for locating anomalies in the walls of pipes. Such methods have been found quantitatively and qualitatively inaccurate. It has been found that the difference in the magnetic permeability of the steel pipe and of the surrounding medium can cause these inaccuracies when magnetic sensor pads bounce and vibrate away from the pipe wall as a result of passing weld joints and other obstructions. Also, magnetic fields induced into the pipe by current conducting brushes or permanent magnets are often unstable.

According to the present invention there is provided a method for determining the thickness of a metallic wall, characterised by: applying a current pulse to a magnetic reluctance coil disposed adjacent said wall; obtaining a first signal representative of the inductance of the pulsed coil: obtaining a second signal representative of the distance between said coil and said wall; and utilising said first and second signals to determine the thickness of said wall and hence the presence of any wall thickness anomaly in the wall portion adjacent the coil.

The invention also provides apparatus for determining the thickness of a metallic wall, characterised by: a magnetic reluctance coil (16) for positioning adjacent said wall (11); a current source (47, 42) for applying a current pulse to said coil; first signal means (54) for obtaining a first signal representative of the inductance of the pulsed coil; second signal means (75) for obtaining a second signal representative of the distance between the coil and the wall; and means for utilising said first and second signals to determine the thickness of said wall and hence the presence of any wall thickness anomaly in the wall portion adjacent the coil.

It will be appreciated that the reluctance of the magnetic circuit linking the coil and the metal wall depends both on the distance between the coil and the wall and also on the thickness of the wall. Thus the sensed inductance of the pulsed coil also depends on both said distance and said thickness. A separate measurement of the distance between the coil and the wall therefore permits the inductance measurement to be utilized to determine the wall thickness and any anomalies therein.

Where the apparatus is moved along a wall, for example in a pipeline, the distance measurement permits compensation of the inductance measurement with respect to distance variations, so as to provide more accurate wall thickness measurements.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic longitudinal section through apparatus according to the invention for use in making wall thickness measurements along a pipe; Figure 2 is a schematic circuit diagram of electrical circuits used in the apparatus of Fig.

1; Figures 3-6 are oscilloscope traces of pulse signals generated under particular conditions in accordance with the invention; and Figure 7 is a graph showing inductance as the ordinate and the thickness of a wall as the abscissa. There are two curves shown, one for a measurement with a wall thickness variation on the same side of the wall as the measuring coil, and the other for a measurement with the wall thickness variation on the other side from the measuring coil.

Fig. 1 shows a combined magnetic and acoustic wall thickness measuring apparatus according to the invention. It will be appreciated that this apparatus can be employed for detecting and identifying wall thickness anomalies, for example pits which might be external or internal relative to the pipe of a pipeline, an offshore oil platform riser, a storage tank bottom or any other inaccessible steel or iron structure. The inspection apparatus can be used as a single unit or in multiples, depending upon the area and configuration of the wall structure that is to be inspected. For a pipline instrument, multiple units can be arranged around the internal periphery of the pipe. A selector network controls electronic stepping from one unit to another in sequence so as to scan the entire circumference of the wall of the pipe. Such an arrangement of units would be similar to that shown in U.S.Patent No. 4,022,055 issued May 10, 1977.

With reference to Fig. 1, a pipe wall 11 which has an internal pit 1 2 that may have been caused by corrosion or the like, and the presence of which is to be determined.

An instrument 1 5 comprises a reluctance coil 1 6 mounted in a block of supporting material 1 9 with the axis of coil 1 6 transverse to the wall 11 of the pipe. A thin surface layer 20 of wear-resistant material bears against the inside surface of the pipe 11 as the instrument is used when surveying pipe wall conditions.

Mounted in a fixed position relative to the reluctance coil 16, there is an ultrasonic transducer 23 that has acoustic material lenses 24 and 25 one on each face of transducer 23 in order to focus the acoustic energy pulses transmitted and received by the transducer.

Acoustic pulses are generated and transmitted by the transducer 23 when it is electrically pulsed in a manner that is known to those skilled in the art. The acoustic pulses are focused into column form and transmitted along the paths shown by dashed lines 28 and 29.

An acoustic reflector 32 has a reflecting surface set at an angle so that the column 29 of acoustic energy is reflected along a path transverse to the pipe wall 11. The reflecting surface of a second reflector 33 has an angle such that its column of acoustic energy (path 28) is directed at an angle of incidence relative to the pipewall 11 which is greater than the critical angle of refraction of the wall.

Therefore any reflected acoustic energy along path 28 indicates the presence of a pit or other anomaly, such as the pit 1 2. Such reflected acoustic energy returns along the acoustic path 28 and generates a signal at the transducer 23 which indicates the presence of the anomaly. In the absence of such anomaly no reflected acoustic energy will return along path 28 because of the angle of incidence indicated.

At the other face of the transducer 23, the acoustic energy pulses are transmitted so as to reflect from the reflector 32 and be directed at right angles to the surface of the pipe 11. Consequently, there will be reflected energy returned from both the inside surface and from the outside surface of the pipe wall 11. Such acoustic signals provide a measure of the distance from the instrument to the inside surface of the pipe wall 11, and also provide, incidentally, a measure of the wall thickness to check against the magnetic measurement to be described.

The instrument 1 5 includes portions 36 and 37 which have respective wear-resistant surface layers 38 and 39. These surface layers bear against the inside surface of the wall 11 during normal operation.

Fig. 2 illustrates circuits for pulsing the reluctance coil 1 6 and the acoustic transducer 23. Fig. 2 also illustrates a plurality of similar circuits 80-80N which are employed when the apparatus includes a plurality of instrument 15, for example for inspecting a pipe or pipeline. These instruments 1 5 would be positioned around the entire inner periphery of the pipe wall 11 in a manner similar to that described in U.S. Patent 4,022,055.

In each instrument 1 5 a silicon controlled rectifier (SCR) 42 is connected in series with the reluctance coil 16. The SCR 42 has a circuit connection from its cathode leading to one end of the coil 16. The anode of SCR 42 is connected via a resistor 43 to a voltage source 44 that is indicated by the + V caption. A capacitor 47 is charged through the resistor 43 from the source 44, and the other side of the capacitor is grounded as indicated.

When the coil 1 6 is to be pulsed, the SCR 42 is triggered by a trigger signal applied to its gate electrode via a resistor 48 and a buffer 49. The buffer 49 acts to pass the trigger signal which originates from a selector network 52. Triggering of the SCR 42 permits the energy stored in charged capacitor 47 to discharge through the SCR 42 and the coil 1 6 in series therewith and to ground via a resistor 58. The resulting signal at point 53 is amplified by an amplifier 54 and converted to digital form, if desired, by an analog to digital converter 57.

The transducer 23 is pulsed substantially at the same time as the reluctance coil 1 6 is pulsed, in order to provide for measurement of the distance of the wall 11 from the instrument 1 5 and thus from the coil 1 6. The circuit for pulsing the transducer 23 is similar to that for the coil 1 6. A voltage source terminal 61 has a resistor 62 connected between it and one terminal of a capacitor 63, the other terminal of which is grounded.

Another SCR has its control electrode connected via a resistor 67 to another buffer 68.

Buffer 68 is connected to the selector network 52 where a trigger signal for the SCR 66 also originates.

When the SCR 66 is triggered, the energy stored in charged capacitor 63 discharges via a resistor 71 and an inductance 72 connected in parallel with one another and also in parallel with the transducer 23. Consequently, this pulse of discharge current stimulates the transducer 23 to transmit an acoustic pulse as desired. That current pulse signal and the current pulse signal generated by the transducer upon reception of reflected return acoustic signals are amplified by an amplifier 75. The output of amplifier 75 may be connected to an analog/digital converter 76.

In a usual mode of operation of the instrument 15, the SCR 66 is triggered first, and the SCR 42 is triggered at a later time, for example substantially at the time of arrival of the ultrasonic reflected acoustic signals back at the transducer 23. In that mode, the measurement of the distance of the instrument 1 5 and therefore of the coil 16 from the wall 11 of the pipe being surveyed, is made substantially at the same position along the pipe wall as the position where the magnetic reluctance is measured. Consequently, the accuracy of the reluctance measurement may be verified or a compensating adjustment may be applied to the pulsed signal from the coil 1 6.

Figs. 3, 4, 5 and 6 illustrate pulses recorded by an oscilloscope when a reluctance coil 1 6 and resistor were energised in a laboratory simulation of the Fig. 2 circuit. The trace A in each case represents the applied voltage pulse that is generated when the SCR 42 is triggered and the trace B in each case indicates the IR voltage of the reluctance circuit, i.e. trace B is a measure of the inductance of the pulsed coil 1 6.

The vertical oscillograph scale represents one volt per vertical division in the case of trace A and represents two milli-volts per vertical division in the case of trace B. The time base of the oscillographs is ten micro seconds per horizontal division. The signal repetition rate is ten kilo-Hertz.

The Fig. 3 oscillograph was made with coil 1 6 notionally adjacent a full thickness of material simulating the wall 11. In the actual simulation the full wall thickness was 0.297 cm.

Fig. 4 illustrates another simulation with the same scales and time base. This case employed a wall thickness anomaly in the form of a depression or pit that was 1.6 cm across and was located on the opposite side of the simulated wall from the coil. The pit had a depth such that the wall was 0.1 52 cm. thick in the area of the pit.

Figs. 5 and 6 illustrate additional simulations which were made with the wall thickness reduced further in each case. In Fig. 5 the same scales and time base were employed with the pit being 1.6 cm. across and again located on the opposite side of the simulated wall, the pit depth being such that the wall was 0.102cm. thick in the area of the pit. In Fig. 6 the only change was to make the depth of the pit such that the wall was 0.064 cm.

The three simulations illustrated in Figs. 4, 5 and 6 were all carried out with the coil 1 6 located on the opposite side of the wall material being tested from the pit. In other words, the pit was on the other side of the wall from the reluctance coil 16. As a result, while it was expected that a pit (i.e. reduction in thickness of the wall) would cause a reduction in the sensed inductance of the coil 16, it was surprising that the opposite effect was discovered. It appears that a possible explanation for this phenomenon is that the lines of magnetic flux may be concentrated in the thinner metal wall section which causes an increase in the flux density nearer the coil centre.

Furthermore, it was found that when the pits that were being detected were located on the same side of the material wall as the coil, then the sensed coil inductance was reduced instead of being increased. Thus a manner of determining which side of the wall a pit was located on, has been invented. This situation is illustrated in Fig. 7 where the graph shows two curves 81 and 82. These were made from measurements of the inductance (on the ordinate of the graph) against the thickness of the wall (on the abscissa of the graph). These curves meet at a point 85 which represents the full thickness of the wall being measured.

The curve 81 shows the inductance values when the points that determine the curve were measured with the reduced thicknesses made by a simulated pit in each case, located on the opposite side of the wall from the coil 1 6 that was employed to make the measurements. On the other hand, the curve 82 shows the values of inductance that were found when the same thickness reductions were tested but with the simulated pit located on the same side of the wall as the coil.

Therefore, it may be noted that a pit can be located and also its location as to whether it is on the same side of the wall being measured or the opposite side thereof, will be indicated bythe signals generated.

It will be appreciated that a method according to this invention may be carried out by various types of apparatus and the method has the benefits indicated above that make it more accurate and reliable than the previous magnetic surveying methods. Thus, a method, according to this invention, includes the step of applying a pulse of energy to a magnetic reluctance coil that is located with the metallic wall in proximity to the coil. The inductance of the pulsed coil is measured and thereafter, or simultaneously therewith, the distance between the pulsed coil and the metallic wall material is determined. The latter is carried out by employing the transducer and ultrasonic distance measuring technique described above.

A mathematical explanation of the magnetic method employed now follows. The self-induced emf E5 in the pulsed reluctance coil 1 6 is a measure of the coil inductance and is an indication of the quantity of permeable material present at the time of the measurement in the proximity of the reluctance coil.The emf E5 is related to the permeability, ju, by the following formulae: df E5 = - (1) dt Ni - (2) R R=- (3) ,uA where f = magnetic flux linking the coil and the wall i = current flowing in the coil N = number of turns in the coil I = length of the flux path (magnetic circuit) A = cross-sectional area of the flux path IL = permeability of the wall material R = reluctance of the magnetic circuit.

The foregoing equations shows that a predictable d/dt requires a knowri l/,u ratio and when IL is very small as in the case for media other than steel, 1 must be kept small also.

The permeabilities of oil, water and gas are several orders of magnitude smaller than that of steel, a common material for the walls of pipes and pipelines.

Claims (8)

1. A method for determining the thickness of a metallic wall, comprising: applying a current pulse to a magnetic reluctance coil disposed adjacent said wall; obtaining a first signal representative of the inductance of the pulsed coil; obtaining a second signal representative of the distance between said coil and said wall; and utilising said first and second signals to determine the thickness of said wall and hence the presence of any wall thickness anomaly in the wall portion adjacent the coil.

2. A method according to claim 1 wherein said second signal is obtained by transmitting an ultrasonic acoustic pulse at the wall from a point fixed relative to the coil, and receiving a reflected pulse from said wall.

3. Apparatus for determining the thickness of a metallic wall, comprising: a magnetic reluctance coil for positioning adjacent said wall; a current source for applying a current pulse to said coil; first signal means for obtaining a first signal representative of the inductance of the pulsed coil; second signal means for obtaining a second signal representative of the distance between the coil and the wall; and means for utilising said first and second signals to determine the thickness of said wall and hence the presence of any wall thickness anomaly in the wall portion adjacent the coil.

4. Aparatus according to claim 3 wherein said second signal means comprises an ultrasonic acoustic pulse transducer fixed relative to the coil, a circuit for applying a voltage pulse to the transducer so that the transducer transmits an ultrasonic acoustic pulse at the wall, and means connected to the transducer to detect the transmitted pulse signal and a received pulse signal reflected by said wall.

5. Apparatus according to claim 3 or claim 4 wherein said coil is mounted with the axis thereof transverse to said wall.

6. Apparatus according to claim 4 including means for directing acoustic pulses transmitted by said transducer both along a path perpendicular to said wall and also along a path making an angle of incidence at said wall greater than the critical angle of refraction, whereby to permit detection of any anomaly in said wall.

7. Apparatus for determining the thickness of a metallic wall, substantially as described herein with reference to the accompanying drawings.

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