EP1354432B1

EP1354432B1 - Data transmission in pipeline systems

Info

Publication number: EP1354432B1
Application number: EP01272712A
Authority: EP
Inventors: Steven Martin Hudson; Daniel Joinson
Original assignee: Expro North Sea Ltd
Current assignee: Expro North Sea Ltd
Priority date: 2001-01-03
Filing date: 2001-12-20
Publication date: 2006-04-05
Anticipated expiration: 2021-12-20
Also published as: ATE322773T1; NO327388B1; NO20033024D0; NO20033024L; WO2002054636A3; BR0116638A; WO2002054636A2; DE60118613D1; AU2002216222A1; EA200300612A1; CA2431707C; CA2431707A1; GB0100107D0; EP1354432A2

Abstract

A data transmission system for transmitting from a tool (505) in a well (501) when suspended from a deployment member (503). The system allows transmission whilst on the move within surrounding metal tubing (C).

Description

This invention relates to data transmission systems and methods of data transmission for use in pipeline systems, in particular wells.
It is useful to be able to take measurements when drilling for oil and gas and during the operation of producing wells. However, it is difficult to transmit data from downhole locations to the surface and the difficulty increases with depth. At present there is a requirement for data transmission from 3000 metres or more below the surface.
Of the signalling techniques currently available those which make use of the metallic structure of the well itself are particularly preferred as they remove the need to install separate wirelines. Most non-wireline systems make use of the production string and casing as a single conducting channel and use earth as the return path. Some attempts have been made to use the casing and string as separate conduction paths but this is fraught with problems because of the difficulties in isolating the string from the casing throughout its length and in particular at the wellhead because of the loads involved. Other methods include "mud-pulsing" which is not only difficult to implement and expensive but also gives a poor data rate.
Whichever system is used, the range is limited because of the inherent losses involved and the need to keep currents at reasonable levels. Further, to the applicant's knowledge no practical non-wireline systems are currently available for signalling from locations on the string within the casing. The communication system described in the applicant's earlier application EP-A-0, 646, 304, for example, works in open hole conditions and can transmit a signal along a cased section. However it is generally accepted that such a system cannot be used in practice to transmit from a position within a cased section.
In pipeline systems it is also desirable to be able to transmit signals from an apparatus within a flowline and/or the associated casing to an apparatus in the same region of the system but outside the flowline and/or casing. However, it is generally accepted that this is difficult to achieve.
It is an object of the present invention to provide communications systems which alleviate at least some of the problems associated with the prior art.
According to a first aspect of the present invention there is provided a data transmission system in which metallic structure of a well is used as a signal channel and earth is used as return, characterised by an elongate deployment member arranged to move within and relative to a surrounding portion of metallic structure, a local unit supported on the deployment member and having receiving and/or transmitting means coupled to the deployment member for receiving signals from and/or transmitting signals along the signal channel, and spacer means arranged to ensure that the deployment member and the surrounding portion of metallic structure are spaced from one another for at least a selected minimum distance in the region of the local unit, said minimum distance being selected to give desired reception and/or transmission characteristics.
According to a second aspect of the present invention there is provided a method of data transmission in which metallic structure of a pipeline system is used as a signal channel and earth is used as return characterised by the steps of:

arranging a signal coupling loop having first and second conducting portions, electrically connected to one another at spaced locations, the metallic structure comprising the first the conducting portion, and a portion of an elongate deploying member which is arranged to move within and relative to a surrounding portion of metallic structure comprising the second conducting portion;
applying a signal to one of the conducting portions to generate a potential difference between earth and the metallic structure in the region of the loop and cause a signal to be propagated along the metallic structure away from the loop; and
ensuring that the spaced locations are separated by at least a minimum distance selected to give desired transmission characteristics.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 schematically shows a subsea well including a data transmission system which aids in understanding the invention;
Figure 2 schematically shows a portion of the well shown in Figure 1 at which a relay station is disposed;
Figure 3 shows a simplified equivalent circuit of a typical length of production string and casing of the well shown in Figure 1;
Figure 4 shows a simplified equivalent circuit of the portion of the well shown in Figure 2 during reception of a signal;
Figure 5 shows a simplified equivalent circuit of the portion of the well shown in Figure 2 during transmission of a signal;
Figure 6 shows an alternative coupling method;
Figure 7 is a schematic view of part of a second system which aids in understanding the invention;
Figure 8 schematically shows a third system which aids in understanding the present invention;
Figure 9 shows an equivalent circuit for the arrangement shown in Figure 8; and
Figure 10 schematically shows an embodiment of the present invention.

production string

tree

casing

tree

string

casing

string

casing

string

casing

string

casing

The space between the string 1 and casing 3 is filled with brine (or alternatively another fluid which is denser than water) to help reduce the pressure acting on the packing ring 4 provided between the casing 3 and string 1 as they enter the formation F. The presence of the brine introduces a further conduction path between the string 1 and the casing 3.
The effect of the glancing contacts and conduction through the brine mean that in general corresponding points of the string 1 and casing 3 will reach the same potential and the string 1 and casing 3 must be treated as a single conductor.
The well also comprises a number of data logging stations 5 provided on the string 1 at open well locations, that is within the formation. The data transmission system is arranged to allow data to be transmitted between the data logging stations 5 and the mudline or beyond by using the metallic structure of the well 1,3 as a signal channel. The distance between the data logging stations and the mudline may be in excess of 3000 metres. Data is received at and transmitted from the data logging stations 5 using existing non-wireline open well techniques, for example those described in the applicant's earlier application EP-A-0,646,304. Whilst these techniques work in the open well and can transmit a signal along the cased section they cannot be used in practice to transmit from a position within the cased section. Only if the length of the cased section is not too great can signals be received directly at and sent directly from the mudline using the non-wireline techniques described in the above mentioned application; range and data rate being essentially determined by signal to noise ratio.
In the present system however, the strength of the signal and/or range of the system is improved by providing a relay station 6 partway along the cased portion of the production string 1. Referring particularly to Figure 2, the relay station 6 comprises transceiver means including an isolation joint 7 provided in the production string, signal generating means 8a used during transmission and signal measuring means 8b used during reception. Both the signal generating means and the signal measuring means are connected across the isolation joint 7. A plurality of insulating annular spacers 9 are provided around the production string 1 over a distance of the order of 100 metres in the region of the isolation joint 7. The distance over which the spacers 9 are provided is chosen such that signals can be effectively received and transmitted. The actual distance will depend on a number of factors relating to the components of the transmission system and the well itself.
The spacers 9 are of a half shell type which are bolted together around the string 1. An insulating layer 9a is provided between each spacer and the string 1. In Figure 2, a side view of one of the spacers 9 is shown and the remainder of the spacers 9 are shown in cross-section. The spacers 9 are arranged and positioned such that at each spacer 9 the string 1 is held towards the centre of the casing 3 and such that the string 1 will not contact with the casing 3 at any position between adjacent spacers 9. Beyond the last spacer 9 at each end of the plurality of spacers 9, the string 1 makes glancing contact 10 with the casing 3 as shown in Figure 2. The distance between each last spacer 9 and the respective glancing contact 10 will be random but its lower limit will be determined by characteristics of the well and spacers 9. Thus the spacers 9 ensure that there is no contact between the string 1 and casing 3 for at least a selected minimum distance.
In general terms the transmission and receiving characteristics of the system improve as the spacing between the glancing contacts 10 is increased. However, there is a trade off against the cost involved in lengthening the minimum distance. In general the actual spacing between the glancing contacts 10 will be greater than the minimum distance but this simply serves to improve the system.
The portions of the string 1 and casing 3 between the glancing contacts 10 are hereinafter referred to as the isolated portion of the string 1a and the corresponding portion of the casing 3a.
Figure 3 shows an equivalent (lumped parameter) circuit for a typical length of the production string 1 and casing 3. The string 1 and casing 3 are respectively represented by series of resistors R_s and R_c. The leakage paths between the string 1 and casing 3 are represented by a series of resistors R_g+b and the leakage paths between the casing 3 and remote earth E are represented by resistors R_e and capacitors C_e. If a signal is applied to the string 1 or casing 3 the strength of the signal will decrease with distance away from the source due to the losses through the leakage paths to remote earth E. Further, as mentioned above the potential of the string 1 and casing 3 will tend to equalise.
Figure 4 shows a simplified equivalent circuit for the portions of the production string 1a and casing 3a in the region of the relay station 6 during reception of a signal. Except those 10 at either end of the portions 1a, 3a, the leakage paths due to glancing contacts have been removed. Thus the resistors R_g+b are replaced by resistors R_b of much higher value representing the leakage through brine alone. The resistance through the brine in the region of the relay station 6 is so large compared with that provided by the glancing contacts 10 at the ends of the isolated portion of string 1a that the effect of the brine can essentially be ignored.
During reception of a signal, because there is no current path through the string portion 1a due to the isolation joint 7 and because the string portion 1a is effectively isolated from the corresponding casing portion 3a, all of the signal losses for that section of the metallic structure will be from the casing 3a. In this circumstance there will be little potential drop along the two halves of the isolated string portion 1a which essentially provide a direct contact with the glancing contacts 10 at the end of the portions 1a,3a. This means that the potential difference between two longitudinally spaced locations on the casing can be detected and hence a signal extracted from the metallic structure. The fact that all of the signal is forced along the casing 3 in the region of the relay station 6 can serve to increase the potential difference between the two spaced locations on the casing 3.
Figure 5 shows a simplified equivalent circuit for the portions of the production string 1a and casing 3a in the region of the relay station 6 during transmission. As above the leakage paths due to glancing contacts have been removed except those 10 at either end of the portions 1a, 3a. Thus the resistors R_g+b are replaced by resistors R_b of much higher value representing the leakage through brine alone. The resistance through the brine in the region relay station 6 is so large compared with that provided by the glancing contacts 10 at the ends of the isolated portion of string 1a that the effect of the brine can be ignored. Thus during transmission a current loop path can be considered to exist consisting of the isolated portion of the string 1a, the corresponding portion of the casing 3a and the glancing connection points 10. The two ends of this loop are of course also connected to the remainder of the string 1 and casing 3. The signal generating means 8a causes a current I to flow around the loop path. This flow of current I causes a potential difference to be set up between the glancing contacts 10 at opposite ends of the isolated portion of string 1a. This potential difference will be I x sumRc, where sumRc equals the total resistance of the casing between the glancing contacts 10.
Assuming that the isolation joint 7 is provided at the centre of the isolated portion of the string 1a and the system settles in balance relative to earth, the magnitude of the potential difference between metallic structure and earth at each end of the isolated portion 1a will be (I x sumRc)/2. Because a potential difference exists between the positions of the glancing contacts 10 and earth, a signal will tend to travel along the string 1 and casing 3 in each direction away from the relay station 6.
Desired data, for example that received from a data logging station, can be transmitted along the string 1 and casing 3 away from the relay station by encoding a suitable signal onto the string 1 by means of the mechanism described above. The resulting signal propagates away from the current loop path along the string and casing as a single conductor. The signal circuit is completed by an earth return and no wirelines are required. Thus all of the problems associated with the provision of wirelines, especially downhole, can be avoided.
Appropriate receiving means at the mudline or at another relay station (not shown) are used to detect the signal applied to the string 1 and casing 3 and extract the desired data. The receiving means may make use of an inductive coupling or be arranged to measure signals with respect to a separate earth reference.
Thus the range of the signal transmission system can be dramatically increased by providing a suitable number of relay stations within the casing 3. The relay stations are bi-directional so that the transmission range when transmitting signals down into the well as well as out of the well is increased.
With the isolation joint located centrally within the isolated portion 1a, the signals in each direction away from the relay station 6 will have substantially equal strength. However, if the isolation joint 7 is disposed towards one end of the isolated portion 1a, the potential difference generated at the other end of the isolated portion 1a will tend to be greater than (I x sumRc)/2. Thus if it is desired to increase the strength of the signal in one direction the isolation joint 7 may be disposed accordingly.
In an alternative the isolated portion of the production string 1a is provided with an insulating coating to further reduce conduction between the isolated portion 1a and the corresponding portion of the casing 3a.
Figure 6 shows a coil 201 provided on a toroidal core 202 disposed around the production string portion 1a for use in an alternative method of applying a signal to and/or tapping a signal from the production string 1. In this case inductive coupling is relied on and no isolation joint is used. During transmission the coil 201 is used to induce a current in the string 1 and the current loop path described above acts as a single turn transformer winding. During reception, a signal on the production string 1 induces a corresponding current in the coil 201 which can be detected. This method of reception does not rely on there being an isolated portion 1a of production string. This coupling method gives an advantage that it is possible to optimise impedance matching by appropriately choosing the turns ratio.
Figure 7 shows a further system suitable for use in a well of the type described above which comprises two pigs 301 connected by an electrically conductive strop 302 and disposed within the production string 1 which may or may not be cased. A first of the pigs 301 comprises a local station 303 having an isolation member 7 provided in series with the strop 302 and signal generating means 8a and signal measuring means 8b connected across the isolation member 7. Each of the pigs 301 has a contact 304 for contacting with an internal surface of the string 1.
Signals may be transmitted and received in this system in substantially the same way as described above in relation to the first system. During transmission the strop 302, a portion of the string 1a and the contacts 304 form a current loop path. When current is caused to flow around the loop by the signal generating means 8a a potential difference between the string 1 and earth can be generated at each contact 304 allowing a signal to be transmitted. During reception of a signal, the strop 302 and contacts 304 allow the potential difference between two longitudinally spaced points on the string 1 to be measured so that a signal can be extracted from the string 1.
In this system signals may be sent to and from the first pig 301. In particular, signals may be sent from the pig 301 which allow the location of the pig 301 to be determined and/or which represent a quantity, such as wall thickness, measured by the pig 301.
In implementing this system it is desirable to minimise the impedance of the conductive strop 302 and the contacts 304 between the pigs 301 and the production string 1. Wire brushes (not shown) provided around the pigs 301 for cleaning purposes may be used as the contacts 304.
One possible mechanism for determining the location of the pig 301 would be to arrange trigger means at spaced locations along a pipeline which cause the pig 301 to send an appropriate signal. Another method would be to determine the time difference of arrival of the signal at each end of the pipeline.
It will be appreciated that this system may be used whether the pigs 301 are within a cased or uncased section of string. Further the system may be used in other pipeline systems besides wells.
In alternatives more than two pigs may be used. Three pigs connected by two conductive members may be used and the local unit disposed at the central pig. This can facilitate equalisation of the transmission characteristics in both directions away from the local unit.
Figure 8 schematically shows a third system which is for transmitting data from inside a section of a cased well to a substantially adjacent position outside of the casing.
Referring to Figure 8 a metallic production string 401 is surrounded by a metallic casing 403 which form part of a cased well. An isolation joint 407 is provided in the string 401 and an internal unit 408 including transmitting means (not shown) is connected across the isolation joint 407. At equally spaced distances from the isolation joint 407, generally annular electrically conductive packers 411 are provided between the string 401 and casing 403. The electrically conductive packers 411 are spaced by a selected distance L and provide a good electrical connection between the production string 401 and the casing 403.
The portion 401a of the production string 401 between the spaced pair of packers 411 is provided with an insulating coating 409. The coating 409 helps to ensure that there is no conduction path or at least only a very poor conduction path between the string 401 and casing 403 at all points between the packers 411.
An external unit 413 comprising receiving means (not shown) and a toroid 415 is provided outside of the casing 403 at a position which is between the pair of spaced packers 411. The toroid 415 surrounds the casing 403 and is arranged to act as an inductive coupling means such that any net magnetic flux flowing through the toroid generates a signal which can be detected by the receiving means (not shown).
The system is arranged to be used to transmit signals from the internal unit 408 to the external unit 413 by the mechanism described below.
The insulated portion of the production string 401a, a corresponding portion of the casing 403a, and the pair of conductive packers 411 form a current loop path around which current may flow. However, the loop is imperfect such that there are other current flow paths and losses will occur. There can be considered to be a leakage loop via earth which accounts for the losses.
The current flow, at an arbitrary instant, around the current loop path as well as along the leakage paths is shown by arrows in Figure 8. I_s represents the current flowing through the insulated portion 401a of production string 401, I_c represents the current flowing in the corresponding portion of the casing 403a and I_e represents the leakage current to earth.
At the particular instant represented by the arrows in Figure 8, current I_s flows up the production string 401 away from the isolation joint 407, a portion of the current passes through the conductive packer 411 to the casing 403 but a further portion of the current continues up the string with subsequent losses to earth. At the casing 403 the path splits again and a proportion of the current I_c continues around the current loop path while the remainder travels along the casing 403 away from the current loop path and contributes to the leakage to earth. At the lower end of the insulated portion of the string 401a, current from the casing I_c returns to the string 401 via the respective conductive packer 411 and leakage currents from earth I_e join this flow back towards the isolation joint 407.
Figure 9 shows a simplified equivalent circuit for the current loop path and the leakages to earth. The resistances of the portion of the production string 401a, the corresponding portion of the casing 403a and earth are represented by a resistors R_s,R_c,R_e respectively.
From the equivalent circuit and the above description, it can be seen that I_s = I_c + I_e. It follows that the current I_s flowing through the insulated portion of the production string 401a does not equal the current I_c flowing through the corresponding portion of the casing 403a. This in turn means that there is a net magnetic flux generated by the current flowing around the loop path. The loop path is encircled by the toroid 415 and hence the toroid 415 is linked by the net flux. Therefore, as current flows around the loop, the existence of, and variations in, that current may be detected by monitoring signals generated in the toroid 415.
It therefore becomes possible to communicate between the internal and external units 408,413 by injecting appropriate signals onto the production string 401 and monitoring the signals generated in the toroid 415.
For this technique to work it is important that not all of the current I_s which is injected into the production string 401 continues around the current loop. That is to say, significant and appropriate leakages to earth and/or away from the current loop must be provided for. In practice such leakages will tend to occur because of the existence of the remainder of the metallic structure of the well and because the casing 403 will be in contact with earth or another conductive medium, such as sea water.
The level of signal obtained in the toroid 415 can be adjusted by making appropriate design choices. For example, the position of the toroid along the insulated portion of the string 401a and the position of the isolation joint 407 may be selected. Further, the spacing L between the conductive packers 411 may be changed, as may the length of the insulated portion of the production string 401a. The aim is to maximise the receivable signal by increasing the resistance of the casing loop R_c relative to the leakage resistance R_e as far as is practicable. In the first instance this may be achieved by increasing the spacing between the conductive packers. Theoretically there will come a point where spacing between the packers is electrically optimised, since increased spacing, at some stage, will begin to significantly increase the resistance of the leakage path R_e. Generally however, other practical considerations will prevent this electrical optimised spacing being reached. The exact nature and conductive properties of the packers 411 may also be selected to vary performance.
Although the position of the toroid along the current loop path/insulated portion 401a is not crucial, the best results are likely to be achieved towards a central position to balance signals generated during positive and negative going cycles and avoid any undesirable edge effects.
It will be noted that this system does not require insulation between the production string 401 and the casing 403 along the whole of the well's length, it is merely preferable along the length chosen to give the necessary transmitting characteristics.
Although this technique has been described with reference to a cased portion of a well, it will be appreciated that the technique is equally appropriate for other situations where it is desired to signal from within a conductive member which surrounds the transmitter. For example, the system can be used to signal from within the casing of flow lines other than production strings and from within flow lines themselves providing that a suitable inner conductor is provided.
In a particular case this system can be used with apparatus along the lines of that shown in and described with reference to Figure 7. That is to say the current loop path may be formed by a portion of a flow line 1, two pigs 301 and an interconnecting conductive strop 302. If a toroid is then provided around the flow line 1 it will be possible to pick-up signals generated by the transmitting means 8a located in the pig 301 as it passes through the region of the toroid.
It can be noted that this system makes use of the same phenomenon as described above with reference to the first and second systems. However, in the present system it is the effects which occur in the current loop path itself which are used rather than the current which leaks away from the current loop path along the production string and casing 1,3.
It should also be noted that the implementation of the present system will, at least in some circumstances, be compatible with the previously described systems. Thus systems may be provided in which signalling along the metallic structure to a remote location and signalling from within the casing to adjacent equipment outside of the casing is possible.
Although not shown in the drawings, the casing 3 of a well is typically made up of screwed together sections. In alternative implementations, some or all of the joints between the casing sections may be treated so as to cause a level of discontinuity in conductivity of the casing. This can typically be achieved by coating the mating surfaces at each joint with an isolating medium which does not prejudice the sealing requirements for the casing.
Introducing such discontinuities can significantly change the electrical characteristics of the well as a whole. At least in some circumstances this may lead to improved performance of the relevant systems described above. For example the range of transmission systems shown in Figures 1 and 2 may be improved. Improvements can be achieved whether the discontinuities are provided in the region of the current loop path, i.e. between the spaced connections or away from that region. The tendency is to force more of the signal into the string rather than the casing and to increase the proportion of the signal which travels away from the region of the loop.
In the case of the system shown in figure 8, the inclusion of an isolation medium between sections of the casing in the region between the spaced connections particularly aids performance as it reduces the screening effect of the casing. Looked at another way, it tends to increase the impedance of the string-casing loop and thus increase the difference between the current flowing in the string I_s and in the casing I_c.
It should be noted that, although as mentioned above, the present systems may function better if discontinuities exist between mating sections of casing this is not a requirement for operation. Thus the system may be such that the casing is substantially electrically continuous along its whole length or at least in the region of the loop. This is true for the casing of a well and the casing of any other pipeline as well for as any corresponding surrounding outer member such as the string in the system shown in figure 7.
Figure 10 shows a pipeline system embodying the present invention. In particular, Figure 10 shows two adjacent wells 501, 502. In this case a first of the wells 501 is being studied, whereas a second of the wells 502 is merely acting as part of an earth return circuit.
A deployment member 503 comprising a length of coiled tubing is disposed within the first well 501. In accordance with standard practice in the field of oil and gas wells, this coiled tubing 503 is arranged to be movable relative to the casing C of the well 501. Therefore, the tubing 503 and anything supported on it may be moved up and down the length of the well 501.
The end of the coiled tubing 503 is provided with a conductive centraliser 504 which both serves to keep that end of the coiled tubing 503 away from the casing and to provide electrical contact between the coiled tubing 503 and the casing C.
A local unit 505 is supported on the coiled tubing 503 in a region near the conductive centraliser 504. The local unit 505 comprises transmitting and receiving means 506 and a toroid 507 provided around the coiled tubing 503. These components are arranged so that signals may be transmitted from, and received at, the local unit 505 via the coiled tubing 503.
The metallic structure, including the respective casings C of the first and second wells 501, .502 is connected via a cable 508. A surface unit 509 is provided adjacent the cable 508 and comprises transmitting and receiving means 510 and a toroid 511 disposed around the cable 508.
In operation, the embodiment of the present invention functions in a way similar to the systems described above with reference to figures 1 to 7. In particular, the mechanisms described above allow the transmission of signals to and from the local unit 505 which is disposed in casing C.
It should be noted that although the coiled tubing 503 is shown to be displaced from the casing C along its length, in practice it will make glancing contact with the casing at a number of locations between the local unit 505 and the surface. On the other hand the conductive centraliser 504 ensures that there is a selected minimum spacing (in this embodiment the selected minimum spacing may be as little as 10 metres) between the connection provided by the centraliser 504 and the glancing connection nearest to the local unit 505. Thus current flow behaviour substantially the same as that described with reference to Figures 1 to 7 will occur allowing the local unit 505 to both inject signals onto the coiled tubing 503 and extract signals from the coiled tubing 503.
It will be appreciated that away from the region of the local unit 505 the coiled tubing 503 and casing C will essentially act as a single conductor and that the coiled tubing is a relatively good electrical conductor and typically metallic.
In the arrangement shown in Figure 10, the adjacent well 502 provides a convenient earthing point to allow completion of the signal circuit, but it will be appreciated that it is not essential to use a second well to provide the earth connection.
The present embodiment facilitates the extraction of data from various positions within a well 501. Although not shown in detail, the local unit 505 will generally comprise a number of sensors for measuring parameters such as pressure and temperature. The system allows the results of such measurements to be encoded onto signals which are transmitted away from the local unit 505 and received at the surface unit 509. It will be immediately apparent that as more coiled tubing 503 is fed into the well 501, the local unit 505 will traverse down the well 501 and measurements may be made and output from each location through which the local unit 505 passes. The system is such that signalling may be achieved whilst the local unit is on the move and/or when the local unit is stationary.
Although this embodiment has been described with particular reference to the use of coiled tubing within a cased section of a well it will be appreciated that the system may also be used in other situations where there is a conductive elongate deployment means which is arranged to move within and relative to a surrounding conductive member.
In this application the phrase conductive centraliser should be construed broadly to include any electrically conductive device which serves to keep the inner conductive portion away from the surrounding conductive portion.

Claims

A data transmission system in which metallic structure (C) of a well is used as a signal channel and earth is used as return, characterised by an elongate deployment member (503) arranged to move within and relative to a surrounding portion of metallic structure (C), a local unit (505) supported on the deployment member and having receiving and/or transmitting means (506) coupled to the deployment member for receiving signals from and/or transmitting signals along the signal channel, and spacer means (504) arranged to ensure that the deployment member (503) and the surrounding portion of metallic structure (C) are spaced from one another for at least a selected minimum distance in the region of the local unit (505), said minimum distance being selected to give desired reception and/or transmission characteristics.
A data transmission system according to claim 1 in which the spacer means (504) comprises a conductive centralising means (504) arranged to keep the deployment member (503) away from the surrounding portion of metallic structure (C) for a predetermined minimum distance whilst also providing connection between the conducting portions at one of the spaced locations.
A data transmission system according to claim 1 or claim 2 in which the deployment member (503) comprises coiled tubing (503).
A method of data transmission in which metallic structure (C) of a pipeline system is used as a signal channel and earth is used as return characterised by the steps of:
arranging a signal coupling loop having first and second conducting portions (503), (C) electrically connected to one another at spaced locations, the metallic structure (C) comprising the first the conducting portion, and a portion of an elongate deploying member (503) which is arranged to move within and relative to a surrounding portion of metallic structure comprising the second conducting portion;

applying a signal to one of the conducting portions (503) to generate a potential difference between earth and the metallic structure (C) in the region of the loop and cause a signal to be propagated along the metallic structure away from the loop; and

ensuring that the spaced locations are separated by at least a minimum distance selected to give desired transmission characteristics.
A method according to claim 4 comprising the step of transmitting signals as the deploying member (503) is moving relative to the surrounding portion of metallic structure (C).