GB2266370A - Apparatus for use in testing the surroundings of a buried pipe - Google Patents

Abstract

The apparatus 10 consists of a pig or pigs in train, which is pulled or propelled through a buried gas, oil, water, chemical or other pipe. The chassis 16 of the pig supports a spring system 32 which carry a mass 30 forming part of an electrodynamic vibrator 31 having an armature driven by alternating current. The armature output shaft 34 is secured to the chassis 16. The mass 30 vibrates, vertically and imposes a force on the pipe. The frequency spectrum of the applied acceleration is measured using an accelerometer 50 attached to the mass 30. The frequency spectrum of the response of the pipe is measured using an accelerometer 54 attached to the chassis 16. Pipe inertance is calculated as the quotient of pipe acceleration divided by the applied force. This is characteristic of the surroundings of the pipe and changes as different surroundings are encountered by the pig as it travels through the pipe. <IMAGE>

Description

APPARATUS FOR USE IN TESTING THE SURROUNDINGS OF A BURIED PIPE The invention relates to apparatus for use in testing the surroundings of a buried pipe.

One example of testing surroundings which is required arises in the case of buried distribution gas pipes. when a pipeline is buried in a ditch formed in the earth the ditch is ideally backfilled with a suitable material which will settle around the pipeline and provide a surrounding support for it. As long as the physical support of a pipeline is uniform, the possibility of the development of overstressed regions is reduced. On the other hand, if the surrounding support for a pipeline changes so that sections are not properly supported, regions of high stress can develop which ultimately can cause deformations, cracks or breaks in the pipeline.

The pipe is a structure which responds to applied dynamic force. The response of a pipe depends upon its own stiffness, due to its cross-section and material properties, its mass and the properties of materials being conveyed, and the stiffness, mass and damping properties of the surrounding pipe support. The pipe support not only includes the backfill material but also includes any surrounding coating material has been applied to the outside of the pipe and which, in some sections of the pipe at least, may have become damaged or may have been removed entirely due to subsequent operations around the pipe.

Another example of surrounding testing which is required arises in the case of transmission pipes for gas, oil, water and other chemicals.

These are particularly likely to have a surrounding~axternal coating of concrete when first installed especially in offshore locations in order to counteract the natural buoyancy of the pipe. In the case of offshore pipes especially, the support may have become eroded over a large region leaving the pipe unsupported. This is an example of "spanning" of the pipeline.

United Kingdom patent No. 1521252 describes the use of electromagnetic vibrators to define vibration inputs to an offshore structure and accelerometers to detect the vibrational response of the structure.

United Kingdom patent No.1521252 describes the use of electromagnetic vibrators to define vibration inputs to an offshore structure and accelerometers to detect the vibrational response of the structure.

United Kingdom patent application publication No.2083913A describes the derivation of a transfer function as the ratio of the response signal to the exciting signal in a structure of steel tubes joined together and supporting an oil-drilling platform, the structure being partly buried in the seabed.

The descriptions given in patent No.1521252 and application No.2083913A are of static arrangements of separate input and output devices mounted on the outside of tubular members.

United Kingdom patent No.2145772B describes a pig movable in a buried pipeline. The pig has a stepped wheel which rolls along the inside of the pipe and repeatedly strikes the pipeline. The vibrations created in the pipeline generate sound within the pipeline, which is- received by a microphone carried by the pig. The sound received is recorded in the pig and, when the test run has been completed, the sound recording is played back and analysed. If the pipeline is well supported by its backfill the vibrations quickly die out and produce a first type of recorded sound.

If the pipeline is badly supported, or is not supported, the sound may be greater intensity and of greater duration, thus giving a second type of recorded sound. However, there is no provision of means which can detect the motion response of the pipeline, nor is any transfer function derived.

According to the invention, apparatus for use in testing the surroundings of a buried pipe having a pipe wall and a longitudinal axis comprises a pig movable through the pipe, said pig comprising a chassis supported on means engageable with said wall, a mass, spring means supporting the mass on the chassis for vibratory motion, mechanism operable to drive said mass in said vibratory motion with an acceleration in a direction transverse to said axis, thereby to impose through said means a force acting on said pipe wall in said direction, a first transducer having in use a first output responsive to said acceleration and a second transducer having in use a second output responsive to a resultant acceleration of said pipe wall.

Preferably, said mass is a movable part of a two-part system comprising said movable part and a second part fixed in relation to said chassis, said two-part system including a magnet and an air-gap associated with one of said parts and a coil associated with the other of said parts, said coil being located in said air-gap and a magnetic field produced by said magnet extending across said air-gap, said mechanism including said magnet and said coil.

Preferably, said transducers are in the form of accelerometers.

Embodiments of apparatus will now be described with reference to the accompanying drawings, in which: Figure 1 is a plan of one embodiment of the apparatus; Figure 2 is a side elevation of the apparatus shown in Figure 1; Figure 3 is a transverse vertical section on the line Ill-Ill of Figure 2 showing the apparatus installed in a buried pipe; Figure 4 is a side-elevation of a second embodiment of the apparatus, also showing in diagrammatic form the main circuit features; and Figures 5 to 8 are diagrammatic representations of the mass 30, the mass of the chassis and of the pipe, described below, and of forces and accelerations acting upon each one of them, for the purposes of illustration of the relevant theory.

In Figures 1 to 3 there is shown apparatus 10 and in Figure 3 the apparatus 10 is shown installed in a 4 inch (102 millimeters) cast-iron distribution gas pipe 12 which is buried in the ground 14. The apparatus 10 is intended to be pulled through the pipe 12 and is designed continuously to subject the pipe 12 to a force having a suitably wide frequency bandwidth. The pipe 12 vibrates in response to the applied force. By measuring the applied force and the motion of the pipe it is possible to derive a transfer function (or ratio of the two). This is achieved by spectral analysis in the frequency domain. It is this transfer function which is characteristic of the surroundings of the pipe 12 and which will reveal such features, for example, as a change to relatively hard material or the absence of any surroundings sufficient to give the pipe support.

The quotient given by dividing the resultant acceleration of the pipe by the applied force is known as the "inertance" and preferably is measured according to the invention. As an alternative, the pipe impedance may be measured. This is the quotient found by dividing the applied force by the resulting pipe velocity. The inertance or impedance spectrum shows the variation of inertance with the frequency of applied force.

We prefer to measure the pipe inertance which can be shown to be [ m1 (x1 s x2) + rn3 ] -1, where m1 is the mass 30; m2 is the mass of the chassis 16; x1 is the acceleration of the mass 30; and x2 is the acceleration of the chassis 16. The mass 30 and the chassis 16 are described below.

The acceleration x1 is measured by the accelerometer 50 and the acceleration x, is measured by the accelerometer 54, which are described below and shown in Figures 1 to 3. A full derivation of the formula given above is given below.

The apparatus 10 shown in Figures 1 to 3 is a pig comprising a chassis 16; the chassis 16 has two leading runners 18, 20 and a trailing runner 22 which support the chassis 16 at three points in the pipe 12.

In an alternative form of chassis, the runners 16-20 would be replaced by wheels. The chassis 16 has a leading cone 24 and a trailing cone 26 to assist the pigs in negotiating pipe joints and other obstacles in the pipe 12. Wires (not shown) are connected to the leading and trailing ends of the apparatus and are led out of the pipe 12 to access pits or to glands in the side wall of the pipe 12.

A mass 30 is supported by spring means 32 for vibratory motion in a vertical plane. The mass 30 is, in fact, the yoke of a known type of vibrator 31 obtainable under the model number V100 series (models nos.

101-111) from Ling Dynamic Systems Limited of Baldock Road, Royston, Hertfordshire, England.

In the vibrator the yoke 30 internally forms an air gap with an internal permanent magnet in a manner similar to the system used in a moving coil loudspeaker. A coil is suspended in the air gap by means of a very light spring system. An armature is connected to the coil and has an output shaft shown at 34. This shaft 34 is connected to the chassis 16. When the coil is energised using alternating current of any waveform covering the frequency range of interest, the mass 30, or yoke assembly vibrates in the plane of the figure and is constrained in this motion by the spring means 32.

The spring means 32 consists of two identical springs 36, 38 which are clamped at one pair of similar ends 40 and are rigidly connected , at the second part of similar ends 42, to the mass 30. The springs 36, 38 each have a circular aperture 44 to accommodate the mass 30. The stiffness of the springs 36, 38 is designed to give a low frequency resonance of mass 30 on springs 36, 38 of around 10 - 15 Hz. Spring material is chosen to given good damping characteristics at high frequency.

An adjustment (not shown) at the end 40 allows adjustment of the ends of the springs 36, 38 in the plane of the figure to counteract sag of the mass 30 on the springs under the effect of gravity. The opposite ends of the springs 36, 38 are effectively vertically offset at rest so that oscillation of the mass 30 is truly about the median position of the vibrator armature.

It is important to mass balance the mass 30 about the vertical axis of the vibrator during assembly. This is achieved by adding weights to it until no rocking motions are observed while the apparatus is measuring its own inertance i.e. while a flat inertance spectrum is obtained, the apparatus being operated whilst suspended on a low frequency mount. The magnitude of the force imposed on the chassis 16, and on the pipe 12, is measured by an accelerometer 50. The accelerometer 50 is secured to a member 52 rigid with the mass 30. As alternatives to using an accelerometer 50, a piezoelectric disc load-cell could be used mounted between the armature output shaft and the chassis, or else the current fed to the coil of the vibrator could be measured. This current is directly proportional to the applied force.However, both alternatives will give inaccurate measurements of force applied to the pipe wall at frequencies in the vicinity of the resonance of the low frequency support of the moving mass 30 as described above.

The motion of the pipe is measured by a second accelerometer 54 mounted on the chassis 16.

It should be noted here that the runners 18, 20, 22 or preferably the mounting wheels (not shown) have to be designed to be mechanically stiff, with no internal structural resonances in the measurement frequency range.

In the apparatus just described, the vibrator imposes a force of up to 10 Newtons and has a maximum displacement amplitude of plus or minus 1.25 mm. The frequency bandwidth of this changing force can be engineered to be as desired. Low frequency performance is limited by the travel of the vibrator and the provision of low frequency support for the mass 30, to keep it physically in place within the constraints of the vibrator travel, ie. counteracting gravitational forces. The apparatus has a flat bandwidth from 30 hertz to 1.3 kilohertz and can accurately measure inertance in this frequency range, i.e. producing an inertance spectrum valid from 30 Hz to 1.3 kHz.

For the apparatus just described the force applied to the pipe and the motion response of the pipe can be measured at a single point. The corresponding transfer function would be known as "point inertance". For low frequency work, as here, only overall bending motions of the pipe can be considered and the apparatus as a whole can be considered as a point.

The pig described above may be one pig of a train of pigs. Figure 4 shows a diagrammatically two such pigs connected in train.

where such an arrangement is used, the point at which force is applied to the pipe may be at one pig and the point at which the pipe response is measured may be at another pig. In such a method the inertance measured would be transfer inertance; or the impedance measured would be cross impedance.

The plane in which the force is applied and the response measured need not be limited to a single plane. Alternatively, the force may be applied in a single plane, but the response may be measured in two or more planes.

At higher frequencies, the distortion (other than bending in a single plane) of the pipe cross-section, or "breathing" modes of the pipe will come into play. These to will be influenced by local bed, or surrounding, condition, and may be more significant than the bending modes found at lower frequencies. The breathing mode may be significantly affected by the presence, in the pipe, of the apparatus.

where more than one force generator (e.g. a vibrator) is used, and where there are transducers arranged to respond in more than one plane to the forces generated, it is preferred to drive all such generators, and to measure all such responses, simultaneously.

Figure 4 shows diagrammatically two pigs 60, 62 connected by a tow link 64 in train in a buried pipe 66, the two pigs 60 and 62 together forming a second embodiment of apparatus. Each pig 60, 62 is mounted on skids 68, 70, 72 and 74. The pigs 60, 62 are pulled through the pipe 66 by means of wires (not shown) attached to the pigs similarly to the method described in relation to the apparatus shown in Figures 1 to 3. The leading pig 60 (from the point of view of advance of the apparatus in the pipe 66) has a vibrator 76 with its mass 78 constrained to vibrate vertically by low-frequency springs 80, 82. The armature output shaft 84 of the vibrator 76 is connected to the chassis 86 of the pig 60.

The acceleration of the vibrating mass 78 is measured by an accelerometer 88 attached to the mass 78. The response of the pipe 66 is measured by an accelerometer 90 secured to the chassis 86 and by a second response accelerometer 92 secured to the chassis 94 of the trailing pig 62.

The trailing pig 62 also carries anti-aliasing filters 96 (to avoid corruption in the digital signal analyser) and a distance encoder wheel 98 which rolls along the pipe 66 and measures the distance travelled by the train. The distance travelled can be monitored externally by monitoring the umbilical in a variation for example.

Signals from the accelerometers 88, 90 and 92, are fed by wires 100 to the anti-aliasing filters 96. Further output signals from the filters 96 and from the distance encoder 98 are fed by an umbilical 102 out of the pipe 66 to a station at the ground surface. The umbilical 102 also contains wires along which are fed a drive signal to the vibrator 76 (shown at 104) from a power supply 106 at the station at the ground surface.

At the ground surface station the signals obtained by the apparatus are fed into a transfer function analyser 108 and the output from the distance encoder 98 is fed to a counter 110. The outputs from the analyser 108 and counter 110 are fed to a plotting device 112 (which may be a screen display). The drive current for the drive 104 is produced by a waveform generator 114 and fed along the umbilical 102 to the vibrator 76. In practice, a personal or other computer can provide the functions of the analyser 108, the counter 110, the plotting device 112 and the waveform generator 114.

It will be understood that the apparatus 10 described with reference to Figures 1 to 3 would also be connected to a personal computer by an umbilical similar to the arrangement just described for the apparatus of Figure 4.

The waveform generator 114 is capable of producing any type of waveform for driving the vibrator such as a swept sine wave or a random waveform, for example.

Although the two embodiments of apparatus described above have been described as being applicable to the testing of gas distribution pipes of as small a diameter as 4 inches, the apparatus may be modified to enable it to be used for testing gas transmission pipes or for testing water or oil transmission pipes, for example. In particular, the apparatus may be arranged to test for spanning in such pipes as well as testing the bed conditions generally. In such apparatus, the distance usually travelled by the apparatus is greater than can be handled by tow wires and the apparatus in that case would be in the form of a pig or pigs propelled by a difference in pressure of the product transported by the pipe across one or more annular drive cups which engage the wall of the pipe. The material of the pipe wall includes plastic, steel or any other material.

All of the facilities shown in Figure 4 would be arranged on the pig or pigs and the signals representing applied force and pipe response would be recorded for exampled on a tape recorder also carried by the pig.

THEORETICAL CONSIDERATIONS There will now follow a theoretical consideration of the forces and accelerations acting upon the apparatus of Figures 1 to 3 and the pipe.

Similar considerations apply to the apparatus of Figure 4.

In the discussion which follows, the dynamic characteristics of the pipe 12 and the surrounding ground 14 are idealised by a simple mass, mp, a spring having a rate kp and a damper having a characteristic cp.

The mass 30 is shown as m1. The mass of the chassis 16 is shown as m2.

The spring system 32 is shown having a spring rate k. The dynamic force acting on the mass m, and on the mass m2 due to the vibrator 31 is f(t), a time-varying force. The force acting on the mass m2 (and as a reaction also upon the pipe 12) is fc The motion of m, is x, and is shown as being positive in the upward direction. The motion of m2 and of the pipe 12 is x2, also shown as being positive in the upward direction.

The acceleration experienced by the mass ml is xl, the second derivation with respect to time. The acceleration experienced by the mass m2 is x2, also the second derivation with respect to time.

Figure 5 shows the system at equilibrium, with the vibrator 31 not energised. The force acting on m1 is zero. The force on m2 as a reaction from the pipe 12 is fc = (ml + m)g, where g is the acceleration due to gravity.

Figure 6 shows the system when the vibrator 31 is energised. It is necessary to closely examine the forces and accelerations acting on the various components, which is done in Figures 7 and 8.

The force acting on ml is f(t)-k(xl-x2) (Figure 7). We can therefore write: ml X1 = f(t)-k(x1-x2). (1) Figure 8 shows the forces acting on m2 and we can write: m2 #2 = fc + k(x1-x2) - f(t) (2) If m2 and the pipe are always considered as a single body i.e. m2 does not move relative to the pipe, then the force applied to m2 and the pipe together is f(t)-k(x1-x2) = m1#l from equation (1). So the inertance of m2 plus the pipe is x2 + The accelerometers 54 and 50, respectively, directly measure #2 and #1.

By combining equation (1) with equation (2) we get: m2 #2 = fc - m1 #1 By definition, the inertance of the pipe taken alone is #2 x2 #fc, so long as m2 moves as one with the pipe.

Therefore, the true inertance of the pipe is given by: #2 # (m2#2 + m1#l) = [m1(#1#2)+m2]-1 as stated above.

Claims (11)

1. Apparatus for use in testing the surroundings of a buried pipe having a pipe wall and a longitudinal axis comprising a pig movable through the pipe, said pig comprising a chassis supported on means engageable with said wall, a mass, spring means supporting the mass on the chassis for vibratory motion, mechanism operable to drive said mass in said vibratory motion with an acceleration in a direction transverse to said axis, thereby to impose through said means a force acting on said pipe wall in said direction, a first transducer having in use a first output responsive to said acceleration and a second transducer having in use a second output responsive to a resultant acceleration of said pipe wall.

2. Apparatus according to claim 1, said mass being a movable part of a two-part system comprising said movable part and a second part fixed in relation to said chassis, said two-part system including a magnet and an air-gap associated with one of said parts and a coil associated with the other of said parts, said coil being located in said air-gap and a magnetic field produced by said magnet extending across said air-gap, said mechanism including said magnet and said coil.

3. Apparatus according to claim 1 or claim 2, said transducers being in the form of accelerometers.

4. Apparatus according to claim 1, 2 or 3, said spring means comprising two spring members, each being a planar member, which members are secured at first like ends to said chassis and which are secured to second like ends to said mass, said members being spaced apart in said direction.

5. Apparatus according to any preceding claim, said two-part system being a known form of vibrator.

6. Apparatus according to any preceding claim, said pig being a first pig of a multi-pig train, the pigs being interconnected.

7. Apparatus according to claim 6, there being at least first and second pigs in said train, said direction in the case of each said pig being in a different plane transverse to said wall from each said direction in the case other pig in said train.

8. Apparatus according to any preceding claim, the apparatus being pulled through said pipe by a wire or cable attached to the pig or pigs and also being connected to an umbilical along which electrical energy is conveyed to said mechanism and outputs from said transducers are conveyed to a stationary station.

9. Apparatus according to any claim of claims 1 to 7, the apparatus being propelled through the pipe by pressure difference acting across an annular drive cup engaging said pipe wall, said apparatus including a source of power for powering said mechanism, means for recording outputs from said transducers, and a distance encoder.

10. Apparatus according to claim 1 substantially as herein described with reference to Figures 1 to 3 of the accompanying drawings.

11. Apparatus according to claim 1 substantially as herein described with reference to Figure 4 of the accompanying drawings.