GB2105041A - Pipeline traveller detection system - Google Patents

Abstract

A pipeline traveller detection system comprises a traveller 12 (Fig. 2) in the form of a cylindrical slug of brass containing a pair of magnets 14, 15 disposed along the axis of the traveller with like poles adjacent each other to exert weak fields in the pipe wall in opposite directions with a null between them. The traveller is arranged to be pushed along an under-sea oil pipeline 11 (Fig. 1) of carbon steel by the oil flow and a detector 17, attached to the pipe, comprises a housing 18 of the same carbon steel containing a pick-off coil 20 wound on a high permeability (e.g. ferrite) yoke 21 which has pole pieces adjacent the pipe wall. The coil feeds an amplifier and comparator 27, 28 by which voltages induced in the coil by passage of the traveller are detected. The strength of the magnets 14, 15 are kept small to produce very little field outside the pipe, i.e. it is not saturated, but the flux that is leaked combined with the rapid change of flux due to the opposed magnet arrangement and traveller speed produces a detectable signal without attracting ferromagnetic debris to the pipe. <IMAGE>

Description

SPECIFICATION Pipeline traveller detection system This invention relates to the detection of the presence of a pipeline traveller at a particular location along a pipeline.

It has been proposed to detect pipeline travellers or other bodies containing ferromag netic components by means of electromagnetic coils wound around a pipe section formed of a non-ferromagnetic material such as stainless steel and examples of such are found in British Patent Specification Nos.

943,064 and 1,407,530.

The use of a non-ferromagnetic section in a carbon steel pipeline is not readily acceptable to the oil industry, particularly where environmental conditions e.g. under sea, exaggerate galvonic corrosion between dissimilar metals.

It has been proposed in British Patent specification no. 1,397,542 to detect bodies within a buried steel pipeline by employing a traveller containing a permanent magnet configuration of such strength that the field thereof effectively saturates and extends beyond the pipe wall for detection by an external magnetometer.

However the use of such a strong magnetic field is undesirable in an exposed or submerged pipeline as leading to the attraction and accretion of ferromagnetic debris.

It is an object of the present invention to provide a pipeline traveller detection system for a pipeline of ferromagnetic material which mitigates the above outlined disadvantages of known system.

According to the present invention a pipeline traveller detection system for a ferromagnetic pipeline comprises a traveller of nonmagnetisable material, adapted to be moved along the interior of the pipeline, carrying a magnet assembly having poles between which poles magnetic flux extends by way of the wall of the pipe at a value below that at which the wall is saturated, and a pick-off arrangement, adapted to be mounted adjacent the pipe, comprising a pick-off coil wound on a yoke of high magnetic permeability material having pole pieces disposed to lie adjacent the pipe wall and spaced from each other in the direction of the flux in the wall and signal detection means operable to detect a voltage induced in the pick-off coil by passage of the magnetic flux in the adjacent pipe wall due to the traveller.

Preferably the magnet assembly has poles displaced in the direction of travel such that the flux extends along the wall. The magnet assembly may comprise a pair of magnets displaced in the direction of motion of the traveller and with one pair of like poles of the magnets adjacent each other.

The magnets are conveniently bar magnets which may be embedded in a cylindrical traveller to facilitate passage through the pipeline.

The pick-off arrangement may be mounted permanently on the pipeline by preferably is releasably mounted so that it may be moved to different locations.

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 partly cut away perspective view of a traveller detection system according to the present invention including a circuit schematic of the signal detection means thereof, Figure 2 is a partly cut-away perspective of a preferred form of traveller, and Figure 3 is a partly cut away perspective, similar to Fig. 2, of an alternative form of traveller.

Referring to Figs. 1 and 2, a pipeline laid on the sea-bed for the transport of oil is formed of joined sections of carbon steel pipe, one section of which is shown at 11. A traveller 1 2 is formed from a cylindrical slug of brass dimensioned for sliding passage along the pipe propelled by the pressure of pipe contents bearing against it.The slug is arranged to fit substantially coaxially within the pipe and contains substantially at the longitudinal axis 13, a magnet assembly comprising a pair of permanent bar magnets 14, 1 5 displaced in the direction of motion of the traveller, that is, along the axis 1 3 and with the longitudinal axis of each bar on the axis 1 3. The magnets are disposed such that like poles of the two magnets are adjacent each other and spaced by about 20% to 50% of the magnet lengths whereby the two magnetic fields extend out of the traveller 1 2 between the poles of each magnet with a null, or zero, field in plane 16, transverse to the axis 13, between the adjacent like magnetic poles.

It will be appreciated that when the traveller is contained within the pipeline the magnetic fields extends into the pipe walls, which are of ferromagnetic material, to which the flux of each field portion running along the length of the magnet is substantially confined. The magnetic flux strength of each magnet is chosen to be relatively low such that the flux of each field does not 'saturate' the wall and permit considerable leakage of flux through the wall whereby ferromagnetic debris is attracted to the outside of the pipe. A flux density of not greater than 2% of that which approaches 'saturation' is acceptable.

However it has been discovered that at such low levels there is still some flux leakage detectable adjacent the outer pipe wall by pick-off means 1 7. The pick-off means 1 7 comprises a housing 1 8 formed of the same carbon-steel material of the pipeline (to avoid any dissimilar metal effects) arranged to be releasably clamped to the pipeline by any conventional arrangement shown generally at 1 9. The housing 1 8 is an enclosure with the base curved to fit against the pipe section 11 and sealed to exclude sea water. The housing 1 8 contains a pick-off coil 20 wound on a yoke 21 of high magnetic permeability material such as ferrite or mild steel.The yoke is of a U-shape having limbs 22, 23 extending towards the pipe and ending in pole-pieces 24, 25 attached to the base of the housing.

The yoke is disposed such that the limbs are spaced from each other in the direction of motion of the traveller, that is, along the pipeline.

The coil is connected to signal detection means 26 comprising an amplifier 27 and a comparator 28. The amplified signal is fed to one input of the comparator and a reference voltage from a source 29 fed to a second comparator input.

In operation the traveller causes the production of two oppositely directed magnetic fields on the pipe wall each of which is accompanied by a small amount of leakage through the wall. As the traveller passes the housing 1 8 some of this leakage flux extends into the base of the housing and through the yoke between its pole-pieces 24 and 25.

Motion of the traveller, and thus the magnetic field cause a rise and fall in flux level in the yoke which induces a voltage across coil 20.

If the amplified voltage is in excess of the threshold level the comparator gives an output at 30. It will be appreciated that the voltage generated depends on the rate of change of flux level and the passage of two fields aligned in opposing direction having only a small gap between magnet pole pieces, whereby the change of leakage flux direction takes place rapidly and causes a measurable voltage to be developed, even for relatively peak magnetic fields.

The reference voltage of source 29 is chosen to eliminate extraneously generated voltages due to the interaction between any magnetic 'noise' in the pipe 11 or housing 1 8 and any close ferromagnetic object.

While the use of two opposed magnets provides the sensitivity of the preferred system, it may be operated with a traveller containing a single magnet as shown in Fig. 3, which has been found able to produce a voltage in excess of the threshold for a traveller velocity of 0.06m/sec while a typical traveller velocity might be 0.5 m/sec.

The magnet or magnets are preferably aligned with the longitudinal axis of the pipe section through which they pass. The magnet or magnets may be carried at any angle with respect to the longItudinal axis by assymetrical distribution of the flux density around the pipe wall may result. For magnets mounted at right angles to the longitudinal axis there would be two angular positions which would give a null response but the use of two detectors at 90 to each other disposed around the pipe would overcome this.

The magnets which are shown in Fig. 2 and 3 as permanent magnets may be electromagnets powered from a source on the traveller or another body to which the traveller is attached. The magnets need not be in the barform illustrated by may take any configuration providing the pole dispositions desired.

The traveller 1 2 is conveniently made of brass which in the form of a solid slug is suited for pipes of the order of 10 cms.

diameter but may be formed of any non solid construction or of any non-magnetisable substance capable of supporting the magnet means and passage through the pipeline.

The housing 1 8 serves only to exclude seawater from the pick-off coil 20 and yoke 21 and may be formed of other material and/or may be isolated from contact with the pipe material by an inert material such as a glass reinforced plastics.

The pick-off means 1 7 is described above as releasable from the pipe to enable it to be used in different locations, for example during periodic maintainance programmes, but may, if desired be permanently attached to the pipeline at a desired location.

The detection system of the present invention while described above in relation to a carbon steel pipeline way be employed advantagously with a pipeline of any ferromagnetic material and may, of course, be employed with a pipeline of any non ferromagnetic material.

Claims (14)

1. A pipeline traveller detection system for a ferromagnetic pipeline comprising a traveller of non-magnetisable material, adapted to be moved along the interior of the pipeline, carrying a magnet assembly between which poles magnetic flux extends by way of the wall of the pipe at a value below that at which the wall is saturated, and a pick-off arrangement, adapted to be mounted adjacent the pipe, comprising a pick-off coil wound on a yoke of high magnetic permeability material having pole pieces disposed to lie adjacent the pipe wall and spaced from each other in the direction of the flux in the wall and signal detection means operable to detect a voltage induced in the pick-off coil by passage of the magnetic flux in the adjacent pipe wall due to the traveller.

2. A pipeline traveller detection system as clained in claim 1 in which the magnet assembly has magnetic poles displaced in the direction of travel of the traveller and the yoke has pole pieces spaced in the direction of motion of the traveller.

3. A pipeline traveller detection system as claimed in claim 2 in which the magnet assembly comprises a pair of magnets displaced in the direction of motion of the travel ler and with one pair of like poles of the magnets adjacent each other.

4. A pipeline traveller detection system as claimed in anyone of claims 1 to 3 in which the traveller comprises a cylindrical slug of non-magnetisable material in which the magnet assembly is embedded.

5. A pipeline traveller detection system as claimed in claim 4 in which the slug is a sliding fit within the pipeline and arranged to support the magnet assembly substantially at the longitudinal axis of the pipeline.

6. A pipeline traveller detection system as claimed in any one of the preceding claims in which the traveller is formed of brass.

7. A pipeline traveller detection system as claimed in any one of the preceding claims in which each magnet of the magnet assembly is a permanent magnet.

8. A pipeline traveller detection system as claimed in any of of the preceding claims in which the pick-off arrangement is adapted to be releasably mounted on the pipeline.

9. A pipeline traveller detection system as claimed in any one of the preceding claims in which the pick-off arrangement includes a housing containing the pick-off coil and carrying the yoke such that the pole-pieces thereof contact the housing wall adapted to lie adjacent the pipeline wall.

10. A pipeline traveller detection system as claimed in claim 9 in which the housing wall is arranged to contact the wall of the pipeline directly.

11. A pipeline traveller detection system as claimed in claim 9 or claim 10 in which the housing is formed of the same material as the pipeline.

1 2. A pipeline traveller detection system as claimed in any one of the preceding claims in which the yoke is formed of a ferrite material.

13. A pipeline traveller detection system as claimed in any one of the preceding claims in which the signal detection means comprises a comparator operable to detect a change in voltage generated at the pick-off coil in excess of a predetermined threshold level.

14. A pipeline traveller detection system for a ferromagnetic pipeline substantially as herein described with reference to, and as shown in Figs. 1 and 2 or Figs. 1 and 3 of the accompanying drawings.