GB2088554A - Pipeline route surveying device - Google Patents

Abstract

A pig or train of pigs (1,2,3) for surveying the route of a pipeline (4) in situ has distance measuring wheels (18, 18') for measuring the distance travelled through the pipeline (4), and means such as a magnetic or gyrocompass, or an accelerometer (not shown) for measuring changes of direction of the pipeline (4) independently of the measuring wheels (18), whereby the complete route of the pipeline (4) can be determined. The pigs may have centralising rollers (Figure 3 not shown), and may be self-propelled or driven by fluid pressure. <IMAGE>

Description

SPECIFICATION Pipeline route surveying device The present invention relates to a device for use in surveying pipeline routes and, more particularly, to such a device which comprises a pig or a train of pigs for travelling through a pipeline.

It is necessary to ensure that the actual profile and plan of a pipeline, that is to say its total route in both the horizontal and vertical planes, are known, regardless of whether the pipeline is above ground, under ground or under water. During the construction of a pipeline of course conventional methods can be used, but more sphisticated methods of determining the route of a pipeline are necessary after construction has been completed, especially when the pipeline is buried or is under water.

The importance of surveying the route of a pipeline will be appreciated from the fact that external factors may affect the efficiency of the pipeline if distortion occurs. In particular ground subsidence can introduce added stresses and strains to the pipeline which may even result in fracture. In underground pipelines settlement can promote stress corrosion cracking in steel pipelines when the steel is exposed to certain types of chemical environments, principally carbonate-bicarbonate solutions and when that stress is sufficiently high. Underwater pipelines installed on the seabed may have spans of varying lengths which are unsupported over the seabed and underwater currents may effect the positioning and hence the stresses imposed on the pipeline.Of course, it is also important to know the exact location and configuration of a pipeline for purposes of servicing and inspection and also where additional construction is to be carried out close to a pipeline.

Existing surveying techniques of operational pipelines include carrying out infra-red surveys from aircraft in order to trace the pipeline route, and side scan sonars and undersea visual surveys for investigating undersea pipelines. However, whilst providing a general survey of a pipeline neither of these methods is particularly accurate and they are also complicated and expensive to carry out.

Pipeline pigs are well known and are frequently used to carry a television camera so that internal inspection can be carried out. It is also known to monitor curvature in a pipeline by passing a pig through it, the pig having a series of rollers arranged around its periphery and the differential outputs from sensors connected to the rollers providing an indication of distance travelled by the different rollers and hence curvature. However, it will be appreciated that unevenness in the wall of the pipeline or accumulations of dirt or the like can cause errors in the readings and it is therefore necessary often to pass the pig through the pipeline several times in order to obtain satisfactory data from which to calculate pipeline curvature.

In order to overcome these problems and in accordance with the present invention, a pipeline route surveying device comprises a pig or train of pigs for travelling through a pipeline to survey its route, the pig or train of pigs having means for measuring the distance travelled through the pipeline, and means for measuring changes of direction of the pipeline independently of the means for measuring the distance travelled whereby the complete route of the pipeline can be determined. By separating the distance measuring function from the direction measuring function increased accuracy can be obtained, particularly, as is preferably the case, tilt angle, pendulum, other gravity sensing transducers, accelerometers, gyroscopes or magnetometers are used to determine changes of direction in the motion of the pig through the pipeline.

Preferably, the instrumentation will be enclosed in a pressure tight cylinder and powered by a set of battery cells. Clearly, depending on the diameter of the pipeline which it is intended to survey then it may also be possible to include a range of instruments, together with recorders and perhaps even a power supply unit although, normally, the pig will be driven by compressed air, gas or liquid. The instrumentation can be enclosed in one pig or a train of pigs and this again will depend on the diameter of the pipeline and the radii of the bends which are expected to be encountered. Conventional pipelines of diameters from 2" to 60" could be surveyed by use of a pig according to the invention and for short runs of pipes it may be possible to include an electric motor for driving the pig.

The pig or train of pigs will preferably be supported by rubber, polyurethane or other suitable materials in such a way as to centralise the device along the axis of the pipeline, to reduce or absorb vibrations and shock, and to ensure sufficient seal between the pipeline and the pig in order to enable propulsion. Rollers or wheels can also be used to engage the internal surface of the pipeline to centralise the pig.

Preferably, to provide a measure of the distance travelled a set of wheels is arranged about the periphery of the pig and means for averaging signals from sensors measuring the rotation of the wheels is provided to eliminate errors occuring in individual wheels as a result of localised deformation of the pipeline, unevenness, or accumulations of dirt or the like.

An accelerometer may be mounted axially of the device to monitor speed variations and for use in rectifying error signals produced as a result of acceleration and retardation of the pig or train of pigs.

The particular type of transducers and the complexity of the instrumentation included in any given device will depend upon the precise application. It is important to ensure that a precise navigational system is installed in the pig and although magnetometers may be used, particularly where the pipelines are of a non-ferromagnetic material, with steel pipelines conventional magnetometers may be inappropriate due to anomalies introduced by permea bilityvariations of the steel structure of the pipeline.

With non-ferromagnetic pipelines it is possible to measure changes of direction by virtue of north seeking compasses and magnetometers incorporated in a train of pigs, the outputs of these devices being fed, in use, to differential amplifiers to provide a zero reading when all the outputs indicate the same axial straight line. As the pipeline tends to curve in one direction or another the output from the differential amplifiers may be arranged to be a direct measurement of the resultant orientations of the separate parts of the train and thus provide an indication of the change of direction.

Gyrocompasses may be employed, the gyrocom pass incorporating a gyroscope incorporating a microprocessor for the purpose of correcting drift and eliminating errors due to the devices course and speed along its path. The processing of signals could be performed either instantly or after retrieval of the pig, depending on the structure of the pipeline and the gravitational errors.

Alternatively, dual axis rate gyro-transducers could be incorporated or a high accuracy displace ment gyro incorporating a dual axis electromagnetic pendulum as its vertical reference, together with a system of amplifiers to correctfor pitch and roll changes. Preferably, the gyro axis will be aligned along the roll axis of the pig or train of pigs in order to reduce error resulting from roll.

High accuracy angular accelerometers may also be used for the measurement of small angular accelerations of the pig or train of pigs and it is also envisaged that a tri-axial combination of gyroscopes and accelerometers could be incorporated in a highly accurate inertial navigational system to provide the necessary measurements of changes of direction of motion of the pig.

Two examples of a surveying device in accordance with the present invention will now be described with reference to the accompanying drawings, in which :- Figure 1 is a diagrammatic view of one example of a train of pigs in a pipeline, with the centralising wheels omitted for clarity; Figure2 is a diagrammatic view similar to Figure 1 of a second example but with the centralising wheels shown; Figure 3 shows the orientation of a set of centralising wheels and is a diagrammatic section taken generally on the line 3-3 in Figure 2 with some parts omitted; Figure 4 is a diagrammatic perspective view showing the centralising wheels on one carriage; Figure 5 illustrates one of the centralising discs used in the second example; Figure 6 is a section taken on the line 6-6 in Figure 1; and Figure 7is a circuit diagram for use with either of the examples illustrated in Figures 1 and 2.

The surveying device illustrated in Figure 1 comprises a train of three pigs 1,2,3 positioned in a curved part of a pipeline 4. The leading end of each pig includes a polyurethane or rubber centralising cap 5 which is frusto-conical in cross-section with its radially outermost surface sealing against the inner surface of the pipeline 4. In this example, the rear end of each pig 1,2,3 is also formed by a similar polyurethane or rubber centralising cap 5' which is shown in more detail in Figure 6. Connecting rods 6 are mounted on the rear caps 5' of the pigs 1, 2 and are connected by means of respective universal joints 7 to the leading caps 5 of the pigs 2, 3. By this means, the three pigs are connected together in series. At least the pigs 2, 3 carry centralising wheels similar to those shown and described with reference to Figure 2.

The device illustrated in Figure 2 also comprises a train of pigs 8, 9, 10, which are connected together with rods 6 and universal joints 7 as in the previous example. The leading pig 8 has a similar form to the pig 1 of the previous example, but the trailing pigs 9, 10 instead of having centralising caps 5, 5' are provided with polyurethane or rubber centralising discs 11. The discs 11 have a generally concave form, as may be seen in Figure 2, and, as is shown in Figure 5, each disc has a number of radially outward ly extending slits 12. The purpose of the discs 11 is to allow the fluid propellant, which may be compressed air, to pass through the centralising discs 11 and to act directly on the rear cap 5' of the leading pig 8.

Thus, the leading pig in this example pulls the two trailing pigs 9, 10 through the pipeline. Furthermore, in the case where some fluid is already present in the pipeline, the discs 11 minimize the opposing force exerted on the pigs by the fluid in the pipeline 4.

Further centralisation of the two trailing pigs 9, 10 in the second example and, if necessary, the pigs 1, 2, 3 of the first example is achieved by means of six radially positioned carriages 13 each of which carries a set of four centralising wheels 14. Only two carriages 13 are illustrated for each pig 9, 10 in Figure 2 but the overall arrangement may be more clearly seen in Figure 3. A single carriage 13 is illustrated in even greater detail in Figure 4. Each carriage 13 comprises four arms 15 pivoted to the pig 10 and supporting the wheels 14, and four further arms 16 connecting the four wheels 14 together. The carriage 13 is spring loaded by means of two tension springs 17 although these could be alternatively provided by shock absorbers particularly in the case where a gas pipeline is being surveyed.

In the example illustrated in Figure 2, the means for measuring distance travelled by the device comprises a pair of distance measuring wheels 18.

These are mounted to the rear cap 5' of the leading pig 8 and are biased into contact with the inner surface of the pipeline 4. Transducers 19 (Figure 7) sense the rotation of the measuring wheels 18 and provide an output corresponding to the distance travelled by the device as will be explained later. In the example shown in Figure 1, one pair of distance measuring wheels 18 are mounted to the front cap 5 of the leading pig 1 and a further set of distance measuring wheels 18' are mounted to the rear cap 5' of the trailing pig 3. This arrangement provides a more accurate method of determining distance travelled by correcting for any errors due to slippage of one or more of the wheels 18, 18'.

The centralising wheels 14 are used in these examples to sense speed variations of the pig as well as to quantify vibration and distortion during movement of the pig, this information being used to discriminate against unwanted signals other than the profile and route variations of the pipeline 4.

Transducers 20 (Figure 7) are mounted to detect rotation of each of the wheels 14 for this purpose.

As has been previously mentioned, the means for measuring changes of direction of the pipeline 4 may be provided by conventional instrumentation sealed into the body of one or more of the pigs 1,2,3 and 8,9, 10 respectively. For example, where a non-ferromagnetic pipeline 4 is being surveyed this instrumentation may include compasses which set a control direction of North with which the angle made by the axis of each pig is compared. Thus, in the example shown in Figure 1, North is indicated by the arrow 21 and compasses (not shown) in the leading and trailing pigs 1,3 which, of course, also indicate the north direction are angled at angles of ss1, t32 to the axes 22,23 of the pigs respectively. By comparison of these angles, the resultant orientation of the pipeline can be determined by conventional methods.

The instrumentation may further comprise roll sensors and pitch sensors which compensate for deviations of the surveying device and also a tri-axial accelerometer 24 which is used to eliminate signals from the centralising carriage 13 erroneously emitted when the device slows or stops due to fluid already present in the pipeline.

In the case where only a single pig is used, the centralising carriage wheels 14 can be used both to measure the distance along the pipeline and also the speed variations and this may be achieved in one example by sensing motion of the leading wheels 14 of each set to indicate distance, and sensing motion of the trailing wheels of each set to measure speed variation.

The signals generated by the various transducers can be processed in a conventional method, for example as shown in Figure 7. This circuit would be used primarily with the example shown in Figure 2.

The signals from transducers 19 are fed to an averager 25 and subsequently the average signal is fed to a conventional signal processing device 26.

Signals from the transducers 20 are, in a similar way, fed to an averager 27 and subsequently to the processing device 26. As has been previously mentioned, the tri-axial accelerometer 24 is provided to eliminate erroneous speed variation signals and the signals from the accelerometer 24 are fed in a conventional manner through filters 28 to a comparator 29 and subsequently to the processing device 26. Signals from the various compasses or other direction measuring instruments 30 are fed directly to the processing device 26. A further refinement is the provision of linear displacement transducers 31 (one of which is shown in Figure 7) connected to the centralising carriages 13 for quantifying vibration and distortion, and signals from these transducers are also fed to the signal processing device 26. The signal processing device 26 provides a resultant signal or signals indicative of the distance travelled by the surveying device and the various turns made by the surveying device in its course through the pipeline which are due solely to the pipeline 4 and these signals are fed to a conventional recorder 32.

In the example shown in Figure 2, the circuit shown in Figure 7 is sealed in the body of the central pig 9.

Claims (10)

1. A pipeline route surveying device comprising a pig or train of pigs for travelling through a pipeline to survey its route, the pig or train of pigs having means for measuring the distance travelled through the pipeline, and means for measuring changes of direction of the pipeline independently of the means for measuring the distance travelled whereby the complete route of the pipeline can be determined.

2. A device according to claim 1, wherein the means for measuring the distance travelled through the pipeline comprises a set of wheels arranged about the periphery of the pig, and means for averaging signals from sensors measuring the rotation of the wheels to eliminate errors occurring in individual wheels.

3. A device according to claim 1 or claim 2, wherein an accelerometer is mounted axially of the device to monitor speed variations and for use in rectifying error signals produced as a result of acceleration and retardation of the pig or train of pigs.

4. A device according to any of the preceding claims for use in a non-ferromagnetic material pipeline, wherein the means for measuring changes in direction of the pipeline comprises a number of magnetometers and north seeking compasses, the outputs of these devices being fed, in use, to differential amplifiers to provide a zero reading when all the outputs indicate the same axial straight line.

5. A device according to any of the preceding claims, wherein the means for measuring changes of direction of the pipeline comprises dual axis rate gyro-transducers.

6. A device according to claim 5, wherein the gyro axis is aligned along the roll axis of the pig or train of pigs.

7. A device according to any of the preceding claims, wherein instrumentation is enclosed in a pressure type cylinder and is powered by a set of battery cells.

8. A device according to any of the preceding claims, wherein the pig or train of pigs is supported by rubber or polyurethane to centralise the device along the axis of the pipeline, to reduce or absorb vibrations and shock, and to ensure sufficient seal between the pipeline and the pig or train of pigs, in use, in order to enable propulsion.

9. A device according to any of the preceding claims, further including an electric motor for driving the pig or train of pigs.

10. A device according to claim 1, substantially as described with reference to the accompanying drawing.