EP1194678B1

EP1194678B1 - Data transmission in pipeline systems

Info

Publication number: EP1194678B1
Application number: EP00942241A
Authority: EP
Inventors: Steven Martin Hudson; Daniel Joinson
Original assignee: Expro North Sea Ltd
Current assignee: Expro North Sea Ltd
Priority date: 1999-07-07
Filing date: 2000-06-30
Publication date: 2005-04-06
Anticipated expiration: 2020-06-30
Also published as: OA11986A; JP2003504543A; NO20020041D0; NO20020041L; DE60019290D1; EP1194678A1; EA200101247A1; KR20020030075A; CA2378329A1; AP2001002381A0; WO2001004461A1; MXPA02000007A; ATE292743T1; CA2378329C; AU5694500A; BR0012635A; CN1372615A; NO320860B1

Description

This invention relates to data transmission systems, methods of data transmission, signal receiving apparatus and methods of receiving signals all 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.

US 2,364,957 discloses apparatus for signalling from downhole in a well. An oscillator located downhole is caused to output electrical signals which travel through the surrounding earth to a receiver at the surface.

US 3,129,394 discloses a communication system for use on a buried, insulated pipe. A coaxial mode of transmission is achieved with the surrounding ground acting as the outer conductor. As such the insulated pipe is used as a signal channel and earth is used as return in US 3,129,394.

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 pipeline system is used as a signal channel and earth is used as return, comprising means for forming a current loop path for use in applying signals to the signal channel and earth return circuit, the loop having first and second conducting portions electrically connected to one another at a first location and electrically connected to one another at a second location, the second location being spaced from the first location, and the metallic structure comprising at least one of the conducting portions, and a local unit having transmitting means for applying a signal to one of the conducting portions whereby in use a potential difference is generated between earth and the metallic structure in the region of the loop which causes a signal to be propagated along the signal channel provided by the metallic structure away from the loop, wherein the means for forming the loop is arranged to ensure that the spaced locations are separated by at least a minimum distance selected to give desired transmission characteristics and the transmitting means comprises inductive coupling means disposed around the respective conducting portion.

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 comprising the steps of:

arranging a current loop path for use in applying signals to the signal channel and earth return circuit, the loop having first and second conducting portions electrically connected to one another at a first location and electrically connected to one another at a second location, the second location being spaced from the first location, and the metallic structure comprising at least one of the conducting portions;

applying a signal to one of the conducting portions using inductive coupling means disposed around the respective conducting portion 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 signal channel provided by 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.

The pipeline system may comprise an inner flow line and a surrounding casing. Typically the pipeline system comprises a well having a production string and surrounding casing.

The current flowing around the loop path in operation can be considered to make the system act as a dipole transmitter.

Receiving means may be provided at a location remote from said current loop path for receiving the signals propagated along the metallic structure.

The above arrangement has the advantages that wirelines can be avoided and a signal which will be detectable can be injected onto the metallic structure in practical situations using realistic current levels even when signalling along a production string from a position in which the string is located within a casing. Away from the region of the current loop path, the metallic structure as whole may be treated as a single conduction channel.

The minimum distance can be chosen to suit the circumstances such that an acceptable level of signal is detectable at the desired location remote from the local unit, for example at the well head. A typical selected minimum distance may be 100 metres. It is preferred that the selected minimum distance is small relative to the overall length of the structure/well.

Preferably one of the conducting portions comprises a portion of a production string. The transmitting means may be arranged to apply signals to the production string.

In some embodiments one conducting portion comprises a portion of a flow line, for example a production string and the other conducting portion comprises a surrounding portion of casing. In such embodiments the means for forming a current loop path may comprise insulating spacer means for keeping the flow line spaced from the surrounding casing for the selected minimum distance. An insulating coating may be provided on the flow line and/or casing over the portion corresponding to the selected minimum distance. The spaced connections between the first and second conducting portions to complete the current loop path may comprise glancing contacts between the flow line and casing beyond the selected region. It will be appreciated that the costs involved in improving isolation between the flow line and casing over the selected minimum distance will be significantly lower than those involved in trying to isolate the string and casing along their whole length.

In other embodiments one conducting portion comprises a portion of a pipeline or flowline and the other conducting portion comprises at least one electrically conductive elongate member connecting at least two pigs disposed within the pipeline or flowline. In such embodiments the spaced connections to complete the current loop path may be provided at the pigs. The local unit may be provided at one of the pigs. Preferably the transmitting means is arranged to apply signals to the elongate member.

The local unit may comprise sensor means for measuring conditions in the region of the unit. The local unit may comprise receiving means for receiving incoming signals transmitted along the metallic structure or otherwise. The local unit may be arranged to act as a relay station. It will be appreciated that the relay station may be disposed on a cased section of production string and thus be used to improve the range of the data transmission system.

Preferably the transmitting means applies signals substantially at the midpoint of the respective conducting portion. This tends to equalise the signal propagation characteristics away from the local unit in both directions along the metallic structure and is particularly suitable if the local unit is to function as a bi-directional relay station.

On the other hand, if it is desired to increase the signal transmission in one direction, the transmitting means may be arranged to apply signals at a point towards one end, preferably the opposite end, of the respective conducting portion.

The current loop path may act as a single turn winding of a transformer. The inductive coupling means may comprise a coil wound on a generally toroidal core which encircles the respective conducting portion.

When a signal is transmitted along the metallic structure of a pipeline system the magnitude of the signal generally decreases as distance from the signal source is increased. This is mainly due to the gradual leakage to earth of the signal. Thus when a signal is travelling along the metallic structure there is a potential difference between any two longitudinally spaced points and it has been appreciated that providing a connection to two such points enables a signal to be extracted from the metallic structure. The minimum distance required depends on the signal level with respect to earth at the locations concerned and the sensitivity/noise performance of the receiving means.

The casing may comprise a plurality of separate sections, which may be screwed together. Mating surfaces at one or more joint between adjacent sections may be coated with an isolating medium. This can change the electrical characteristics of the metal structure and enhance performance.

Preferably the transmission means is arranged to apply signals to the inner flowline.

Typically the pipeline system comprises a cased section of a well, the production string being the flowline in such a case.

Embodiments 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;

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, and

Figure 6 shows a coupling method used in an embodiment of the invention.

Figures 1 and 2 schematically show a subsea well including a wireless or non-wireline data transmission system. The invention is embodied in this system but for the use of a different coupling technique which is explained below with reference to Figure 6. The well comprises a production string 1 for extracting product from a formation F. The production string 1 joins a tree 2 at the mudline and is surrounded by casing 3 between the tree 2 and the formation F. The string 1 and casing 3 form part of the metallic structure of the well. Although Figure 1 shows the string 1 as being disposed centrally within the casing 3, in practice the string 1 and casing 3 will make glancing contact with one another at numerous positions along their lengths. In general there is nothing to prevent such glancing contact and the string 1 will follow a sinuous, for example a helical, path within the casing 3.

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_o 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 Ba 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 a method of applying a signal to and/or tapping a signal from the production string 1 which embodies the present invention. 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.

Although not shown in the drawings, the casing 3 of a well is typically made up of screwed together sections.

In alternative implementations of the invention, 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 embodiments 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.

It should be noted that, although as mentioned above, the present embodiments, and present invention in general, may function better if discontinuties 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 continous 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 as for any corresponding surrounding outer member.

Claims

A data transmission system in which metallic structure (1, 2, 3) of a pipeline system is used as a signal channel and earth is used as return, comprising means for forming a current loop path (9, 1, 3) for use in applying signals to the signal channel and earth return circuit, the loop having first and second conducting portions (1, 3) electrically connected to one another at a first location (10) and electrically connected to one another at a second location (10), the second location being spaced from the first location, and the metallic structure comprising at least one of the conducting portions, and a local unit (6) having transmitting means (8a) for applying a signal to one of the conducting portions whereby in use a potential difference is generated between earth and the metallic structure in the region of the loop which causes a signal to be propagated along the signal channel provided by the metallic structure away from the loop, wherein the means for forming the loop (9, 1, 3) is arranged to ensure that the spaced locations are separated by at least a minimum distance selected to give desired transmission characteristics and the transmitting means (8a) comprises inductive coupling means (201, 202) disposed around the respective conducting portion.
A data transmission system according to Claim 1 in which the pipeline system comprises an inner flow line (1) and a surrounding casing (3) wherein, one conducting portion comprises a portion of the flow line and the other conducting portion comprises a surrounding portion of the casing.
A data transmission, system according to Claim 2 in which the means for forming the loop comprises insulating spacer means (9) for keeping the flow line (1) spaced from the surrounding casing (3) for the selected minimum distance.
A data transmission system according to Claim 2 or Claim 3 in which the spaced connections between the first and second conducting portions comprise glancing contacts (10) between the flow line and casing beyond the selected region.
A data transmission system according to any preceding claim in which the local unit (6) comprises receiving means (8b) for receiving incoming signals transmitted along the metallic structure.
A data transmission system according to Claim 5 in which the local unit (6) is arranged to act as a relay station.
A data transmission system according to claim 5 or claim 6 in which the receiving means (8b) comprises the inductive coupling means.
A data transmission system according to any preceding claim in which the transmitting means (8a) is arranged to apply signals substantially at the midpoint of the respective conducting portion.
A data transmission system according to any one of Claims 2 to 4 in which the casing (3) comprises a plurality of separate sections, and mating surfaces at one or more joint between adjacent sections are coated with an isolating medium.
A method of data transmission in which metallic structure of a pipeline system (1, 2, 3) is used as a signal channel and earth is used as return comprising the steps of:

arranging a current loop path (1, 3, 10) for use in applying signals to the signal channel and earth return circuit, the loop having first and second conducting portions (1, 3) electrically connected to one another at a first location (10) and electrically connected to one another at a second location (10), the second location being spaced from the first location, and the metallic structure comprising at least one of the conducting portions;

applying a signal to one of the conducting portions using inductive coupling means (201, 202) disposed around the respective conducting portion 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 signal channel provided by 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.