OA11986A

OA11986A - Data transmission in pipeline systems.

Info

Publication number: OA11986A
Application number: OA1200200006A
Authority: OA
Inventors: Steven Martin Hudson; Daniel Joinson
Original assignee: Flight Refueling Ltd
Priority date: 1999-07-07
Filing date: 2000-06-30
Publication date: 2006-04-18
Also published as: CA2378329C; CN1372615A; EP1194678B1; KR20020030075A; EA200101247A1; AU5694500A; EP1194678A1; WO2001004461A1; JP2003504543A; BR0012635A; AP2001002381A0; MXPA02000007A; DE60019290D1; ATE292743T1; CA2378329A1; NO20020041L; NO20020041D0; NO320860B1

Abstract

A first set of apparatus is arranged for transmitting data from a point in a cased section of a well (1, 3) to a remote location. The apparatus may be used as a relay station (6) to increase operational depth. Signals are applied to and received from the string (1) at the relay station (6) and a selected length of the string (1) is provided with insulating spacer means (9) on either side of the relay station to ensure that the string (1) and casing (3) are effectively isolated for a selected minimum distance. This enables potential differences to be both applied to and detected from the string (1), thus allowing data transmission and reception. A second set of apparatus (figure 8) is arranged for transmitting from an internal unit (408) inside a cased section of the well (401, 403) to the immediate surrounding area outside the casing (403). The internal unit (408) injects current into the string (401). A toroid (415) which surrounds the casing (403) is provided to pick up signals. Spaced connections between the string (401) and casing (403) are provided by conductive packers (411). A mismatch in the current flowing in the string (401) and casing (403) is generated so that a non-zero flux is seen by the toroid and hence a signal can be received.

Description

1198 6 1

Data transmission in pipeline Systems

This invention relates to data transmission Systems,methods o£ data transmission, signal receiving apparatus 5 and methods of receiving signais ail for use in pipelineSystems, in particular wells.

It is useful to be able to take measurements whendrilling for oil and gas and during the operation ofproducing wells. However, it is difficult to transmit 10 data from downhole locations to the surface and thedifficulty increases with depth. At présent there is arequirement for data transmission from 3000 métrés ormore below the surface.

Of the signalling techniques currently available 15 those which make use of the metallic structure of thewell itself are particularly preferred as they remove theneed to install separate wirelines. Most non-wirelineSystems make use of the production string and casing asa single conducting channel and use earth as the return 20 path. Some attempts hâve been made to use the casing andstring as separate conduction paths but this is fraughtwith problems because of the difficulties in isolatingthe string from the casing throughout its length and inparticular at the wellhead because of the loads involved. 25 Other methods include "mud-pulsing" which is not onlydifficult to implement and expensive but also gives apoor data rate.

Whichever System is used, the range is limitedbecause of the inhérent losses involved and the need to 30 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 119 8 6 2 described in the applicant's earlier application EP-A-0,646,304, for example, works in open hole conditions andcan transmit a signal along a cased section. However itis generally accepted that such a System cannot be used 5 in practice to transmit from a position within a casedsection.

In pipeline Systems it is also désirable to be ableto transmit signais from an apparatus within a flowlineand/or the associated casing to an apparatus in the same 10 région of the System but outside the flowline and/orcasing. However, it is generally accepted that this isdifficult to achieve.

It is an object of the présent invention to providecommunications Systems which alleviate at least some of 15 the problems associated with the prior art.

According to a f irst aspect of the présent invention there is provided a data transmission System in whichmetallic structure of a pipeline System is used as asignal channel and earth is used as return comprising 20 means for forming a current loop path having first andsecond conducting portions electrically connected to oneanother at spaced locations, the metallic structurecomprising at least one of the conducting portions, anda local unit having transmitting means for applying a 25 signal to one of the conducting portions whereby in.usecurrent flows around said loop generating a potentialdifférence between earth and the metallic structure inthe région of the loop and causing a signal to bepropagated along the metallic structure away from the 30 loop, wherein the means for forming the current loop path is arranged to ensure that the spaced locations are separated by at least a minimum distance selected to give desired transmission characteristics. 119 8 6 3

According to a second aspect of the présentinvention there is provided a method of data transmissionin which metallic structure of a pipeline System is usedas a signal channel and earth is used as returncomprising the sreps of: forming a current loop path having first and secondconducting portions electrically connected to one anotherat spaced locations, the metallic structure comprisingat least one of the conducting portions; applying a signal to one of the conducting portions tocause a current to flow around said loop to generate apotential différence between earth and the metallicstructure in the région of the loop and cause a signalto be propagated along the metallic structure away fromthe loop; and ensuring that the spaced locations are separated by atleast a minimum distance selected to give desiredtransmiss ion characteris tics.

The pipeline System may comprise an inner flow lineand a surrounding casing. Typically the pipeline Systemcomprises a well having a production string andsurrounding casing.

The current flowing around the loop path inoperation can be considered to make the System act as adipole transmitter.

Receiving means may be provided at a location remotefrom said current loop path for receiving the signaispropagated along the metallic structure.

The above arrangement has the advantages thatwirelines can be avoided and a signal which will bedétectable can be injected onto the metallic structurein practical situations using realistic current levelseven when signalling along a production string from a 1 19 8 6 4 position in which the string is located within a casing.Away from the région of the current loop path, themetallic structure as whole may be treated as a singleconduction channel.

The minimum distance can be chosen to suit thecircumstances such that an acceptable level of signal isdétectable at the desired location reraote from the localunit, for example at the well liead. A typical selectedminimum distance may be 100 métrés. It is preferred thatthe selected minimum distance is small relative to theoverall length of the structure/well,

Preferably one of the conducting portions comprisesa portion of a production string. The transmitting meansmay be arranged to apply signais to the productionstring.

In some embodiments one conducting portion comprisesa portion of a flow line, for example a production stringand the other conducting portion comprises a surroundingportion of casing. In such embodiments the means forforming a current loop path may comprise insulating, spacer means for keeping the flow line spaced from thesurrounding casing for the selected minimum distance.An insulating coating may be provided on the flow lineand/or casing over the portion corresponding to theselected minimum distance. The spaced connections betweenthe first and second conducting portions to complété thecurrent loop path may comprise glaneing contacts betweenthe flow line and casing beyond the selected région. Itwill be appreciated that the costs involved in improvingisolation between the flow line and casing over theselected minimum distance will be significantly lowerthan those involved in trying to isolate the string and casing along their whole length. 1198 6 5

In other embodiments one conducting portioncomprises a portion of a pipeline or flowline and theother conducting portion comprises at least oneelectrically conductive elongate member connecting atleast two pigs disposed within the pipeline or flowline.In such embodiments the spaced connections to complététhe current loop path may be provided at the pigs. Thelocal unit may be provided at one of the pigs. Preferablythe transmitting means is arranged to apply signais tothe elongate member.

The local unit may comprise sensor means formeasuring conditions in the région of the unit. The localunit may comprise receiving means for receiving incomingsignais transmitted along the metallic structure orotherwise. The local unit may be arranged to act as arelay station. It will be appreciated that the relaystation may be disposed on a cased section of productionstring and thus be used to improve the range of the datatransmission System.

Preferably the transmitting means applies signaissubs tant ially at the midpoint of the respectiveconducting portion. This tends to equalise the signalpropagation characteristics away from the local unit inboth directions along the metallic structure and isparticularly suitable if the local unit is to functionas a bi-directional relay station.

On the other hand, if it is desired to increase thesignal transmission in one direction, the transmittingmeans may be arranged to apply signais at a point towardsone end, preferably the opposite end, of the respectiveconducting portion.

The transmitting means and/or the receiving means may comprise an isolation member disposed in sériés with 119 8 6 6 the respective conducting portion. The transmitting meansmay comprise a signal generating means connected acrossthe isolation member. The receiving means may comprisea signal measuring means, for example voltage measuring 5 means, connected across the isolation member. Where therespective conducting portion comprises the productionstring the isolation member may be an isolation jointdisposed in the string.

The transmitting means and/or the receiving means10 may comprise inductive coupling means disposed around therespective conducting portion. The current loop path mayact as a single turn winding of a transformer. Theinductive coupling means may comprise a coil wound on agenerally toroidal core which encircles the respective 15 conducting portion.

According to a third aspect of the présent inventionthere is provided signal receiving apparatus for use witha data transmission System in which metallic structureof a pipeline System is used as a signal channel and 20 earth is used as return, comprising a local unit havingreceiving means, means for providing electrical contactbetween the local unit and at least two spaced locationson a portion of the metallic structure and means forensuring that the two spaced locations are separated by 25 at least a minimum distance selected to give desiredréception characteristics.

According to a fourth aspect of the présentinvention there is provided a method for receiving asignal from the metallic structure of a pipeline System 30 which is used as a signal channel in a data transmissionSystem with earth as return, comprising the steps ofproviding a local unit having receiving means; providingelectrical contact between the local unit and at least 1 19 8 6 7 two spaced locations on a portion of the metallicstructure; and ensuring that the spaced locations areseparated by at least a minimum distance selected suchto give desired réception characteristics.

When a signal is transmitted along the metallicstructure of a pipeline system the magnitude of thesignal generally decreases as distance from the signalsource is increased. This is mainly due to the graduaileakage to earth of the signal. Thus when a signal istravelling along the metallic structure there is apotential différence between any two longitudinallyspaced points and it has been appreciated that providinga connection to two such points enables a signal to beextracted from the metallic structure. Thé minimumdistance required dépends on the signal level withrespect to earth at the locations concerned and thesensitivity/noise performance of the receiving means.

The means for providing electrical contact at spacedlocations may comprise a portion of the production stringand insulating spacer means provided to keep said stringportion spaced from the corresponding portion ofsurrounding casing. An isolation joint may be providedin the string in the région of the local unit and asignal measuring means connected across it. In this case,because the string is effectively isolated from thecasing, ail of the signal losses for that section of themetallic structure will be from the casing and there willbe little potential drop along that portion of thé stringso that the potential différence between the spacedlocations can be detected.

The means for providing electrical contact at spaced locations may comprise at least one electrically conductive elongate member connecting at least two pigs 1198 6 8 disposed within the production string.

According to a fifth aspect of the présent inventionthere is provided signal receiving apparatus for use witha data transmission System in which metallic structureof a pipeline System is used as a signal channel,comprising a local unit having receiving means whichcomprises an inductive coupling.

The signal channel may be split into two or morebranches in the région of the local unit and theinductive coupling disposed around one of said branches.

Preferably the inductive coupling is disposed arounda production string disposed within a casing. One branchmay comprise the production string and another branch maycomprise the casing.

The inductive coupling may comprise a toroiddisposed around said one of the ehannels and/or aproduction string.

According to a further aspect of the présentinvention there is provided a data transmission Systemin which metallic structure of a well including aproduction string and casing is used as a signal channeland earth is used as return compris ing a local unithaving receiving and/or transraitting means coupled to thestring for receiving signais from and/or transmittingsignais along the signal channel and insulating spacermeans arranged to ensure that the production string andcasing are spaced from one another for at least aselected minimum distance in the région of thè localunit, said minimum distance being selected to givedesired réception and/or transmission characteristics.

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 1198 6 9 coated with an isolating medium. This can change theelectrical characteristics of the métal structure andenhance performance.

Many of the additional features described followingthe earlier aspects of the invention are equallyappropriate for use in conjunction with said furtheraspect of the invention.

According to another aspect of the invention thereis provided a data transmission System for use inpipeline Systems which comprises, means for forming a current loop path compris ing aportion of an inner conductive member and a correspondingportion of an outer conductive member electricallyconnected to one another at two spaced locations, theouter conducting member surrounding the inner conductivemember and being part of the metallic structure of apipeline System? an internai unit disposed within the outer member andhaving transmission means for injecting a signal into thecurrent loop path; and an external unit disposed outside the outer membercomprising inductive coupling means arranged to be linkedby flux generated by current flowing around the looppath, the arrangement being such that in use the currentflowing in said portion of the inner member does notmatch the current flowing in the corresponding portionof the outer member whereby signais are generated in theinductive coupling means so allowing communication fromthe internai unit to the external unit.

According to yet another aspect of the présentinvention there is provided a method of data transmissionSystem for use in pipeline Systems which comprises the 10 119 8 6 steps of: forming a current loop path comprising aportion of an inner conductive member and a correspondingportion of an outer conductive member electricallyconnected to one another at two spaced locations, theouter conducting member surrounding the inner conductivemember and being part of the metallic structure of apipeline system; injecting a signal into the current loop path from aninternai unit disposed within the outer member; anddisposing an external unit outside the outer member whichunit comprises inductive coupling nteans arranged to belinked by flux générated by current flowing around theloop path, and the arrangement being such that in use the currentflowing in said portion of the inner member does notmatch the current flowing in the corresponding portionof the outer member whereby signais are generated in theinductive coupling means so allowing communication fromthe internai unit to the external unit.

Generally the inner and outer members will begenerally co-axially arranged elongate members, the outermember being generally tubular.

The spaced locations may separated by a selectedminimum distance. Preferably the minimum distance isselected to give desired transmission characteristics.In some embodiments, the data transmission system may bearranged for use in pipeline Systems comprising aconductive flowline which acts as the outer member anda dedicated inner conductive member may be provided. Insuch a case the inner conductive member my comprise aconductive strop connected between two pigs.

The electrical connections between the dedicated inner conductor and a flow line may be provided at the pigs. 119 8 6 11

Cleaning brushes located on the pigs may act as contactswith the inner surface of the flowline.

In other embodiments, the data transmission Systemmay be arranged for use in pipeline Systems comprisingan inner conductive flowline and an outer conductivecasing. In such a case the outer member may comprise thecasing and the inner member may comprise the flowline.

The outer member, particularly when a casing, maycomprise a plurality of separate sections, which may bescrewed together. Mating surfaces at one or more jointbetween adjacent sections may be coated with an isolatingmedium. This can change the electrical characteristicsof the métal structure and enhance performance. It ispreferred that no completely isolated joint is disposedin the casing between the spaced locations at which thecasing and flowline electrically contact one another.

The electrical connections between the flowline andcasing may comprise glancing contacts and/or conductivepackers. Where the spaced connections consist of glancingcontacts it is possible to select a minimum séparationbetween the connections. Where conductive packers areused the actual spacing between the packers, and hencethe connections, may be chosen. The means for forming thecurrent loop path may comprise an insulating layerprovided on the outer surface of the inner flow lineand/or the inner surface of the outer casing. The meansfor forming the current loop path may comprise insulatingspacer means.

The positions and/or nature of the connectionsand/or means used for insulating the portion of theflowline from the corresponding portion of the casing maybe chosen to give desired transmission characteristics.

Preferably the transmission means is arranged to 119 8 6 12 apply signais to the inner flowline. An isolation jointmay be provided in the flowline and the transmissionmeans may be arranged to signal across the isolationjoint.

The inductive coupling means may comprise a toroiddisposed around the casing in the région of the currentloop. Preferably the inductive coupling means is disposedtowards a midpoint between the spaced connections.

Typically the pipeline System comprises a casedsection of a well, the production string being theflowline in such a case.

According to yet another aspect of the présentinvention there is provided apparatus for use with ametallic structure in carrying out any one of the aboveaspects of the invention.

Embodiments of the présent invention will now bedescribed by way of example only with reference to theaccompanying drawings in which:

Figure 1 schematically shows a subsea well includinga data transmission System which comprises a firstembodiment of the invention;

Figure 2 schematically shows a portion of the wellshown in Figure 1 at which a relay station is disposed;

Figure 3 shows a simplified équivalent circuit ofa typical length of production string and casing of. thewell shown in Figure 1;

Figure 4 shows a simplified équivalent circuit ofthe portion of the well shown in Figure 2 duringréception of a signal;

Figure 5 shows a simplified équivalent circuit of the portion of the well shown in Figure 2 during transmission of a signal;

Figure 6 shows an alternative coupling method; ί1 19 8 6 13

Figure 7 is a schematic view of part of a secondembodiment of the invention;

Figure 8 schematically shows a third embodiment ofthe présent invention; and

Figure 9 shows an équivalent circuit for thearrangement shown in Figura 8.

Figures 1 and 2 schematically show a subsea wellincluding a wireless or non-wireline data transmissionSystem. The well comprises a production string 1 forextracting product from a formation F. The productionstring 1 joins a tree 2 at the mudline and is surroundedby casing 3 between the tree 2 and the formation F. Thestring 1 and casing 3 form part of the metallic structureof the well. Although Figure 1 shows the string 1 asbeing disposed centrally within the casing 3, in practicethe string 1 and casing 3 will make glancing contact withone another at numerous positions along their lengths.In general there is nothing to prevent such glancingcontact and the string 1 will follow a sinuous, forexample a helical, path within the casing 3.

The space between the string 1 and casing 3 isfilled with brine (or alternatively another fluid whichis denser than water) to help reduce the pressure actingon the packing ring 4 provided between the casing 3 andstring 1 as they enter the formation F. The presence ofthe brine introduces a further conduction path betweenthe string 1 and the casing 3.

The effect of the glancing contacts and conductionthrough the brine mean that in general correspondingpoints of the string 1 and casing 3 will reach the samepotentiel and the string 1 and casing 3 must be treatedas a single conductor.

The well also comprises a number of data logging 119 8 6 14 stations 5 provided on the string 1 at open welllocations, that is within the formation. The datatransmission System is arranged to allow data to betransraitted between the data logging stations 5 and the 5 mudline or beyond by using the metallic structure of thewell 1,3 as a signal channel. The distance between thedata logging stations and the mudline may be in excessof 3000 métrés. Data is received at and transmitted fromthe data logging stations 5 using existing non-wireline 10 open well techniques, for example those described in theapplicant's earlier application EP-A-0,646,304. Whilstthese techniques work in the open well and can transmita signal along the cased section they cannot be used inpractice to transmit from a position within the cased 15 section. Only if the length of the cased section is nottoo great can signais be received directly at and sentdirectly from the mudline using the non-wirelinetechniques described in the above mentioned application;range and data rate being essentially deterrained by 20 signal to noise ratio.

In the présent embodiment however, the strength of the signal and/or range of the System is improved byproviding a relay station 6 partway along the casedportion of the production string 1. Referring 25 particularly to Figure 2, the relay station 6 comprisestransceiver means including an isolation joint 7 providedin the production string, signal generating means 8a usedduring transmission and signal measuring means 8b usedduring réception. Both the signal generating means and 30 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 métrés in the région 119 8 6 15 of the isolation joint 7. The distance over which thespacers 9 are provided is chosen such that signais canbe effectively received and transmitted. The actualdistance will dépend on a number of factors relating tothe components of the transmission System and the wellitself.

The spacers 9 are of a half shell type which arebolted together around the string 1. An insulating layer9a is provided between each spacer and the string 1. InFigure 2, a side view of one of the spacers 9 is shownand the remainder of the spacers 9 are shown in cross-section. The spacers 9 are arranged and positioned suchthat at each spacer 9 the string 1 is held towards thecentre of the casing 3 and such that the string 1 willnot contact with the casing 3 at any position betweenadjacent spacers 9. Beyond the last spacer 9 at each endof the plurality of spacers 9, the string 1 makesglancing contact 10 with the casing 3 as shown in Figure 2. The distance between each last spacer 9 and therespective glancing contact 10 will be random but itslower limit will be determined by characteristics of thewell and spacers 9. Thus the spacers 9 ensure that thereis no contact between the string 1 and casing 3 for atleast a selected minimum distance.

In general terms the transmission and receiv.ingcharacteristics of the System improve as the spacingbetween the glancing contacts 10 is increased. However,there is a trade off against the cost involved inlengthening the minimum distance. In general the actualspacing between the glancing contacts 10 will be greaterthan the minimum distance but this simply serves toimprove the System.

The portions of the string 1 and casing 3 between 1 198 6 16 the glancing contacts 10 are hereinafter referred to asthe isolated portion of the string la and thecorresponding portion of the casing 3a.

Figure 3 shows an équivalent (luinped parameter)circuit for a typical length of the production string 1and casing 3. The string 1 and casing 3 are respectivelyrepresented by sériés of resistors Rs and Rc. Theleakage paths between the string 1 and casing 3 arerepresented by a sériés of resistors Rg+b and the leakagepaths between the casing 3 and remote earth E arerepresented by resistors Re and capacitors C„. If asignal is applied to the string ”*T“ or casing 3 thestrength of the signal will decrease with distance awayfrom the source due to the losses through the leakagepaths to remote earth E. Further, as mentioned above thepotential of the string 1 and casing 3 will tend toequalise.

Figure 4 shows a simplified équivalent circuit forthe portions of the production string la and casing 3ain the région of the relay station 6 during réception ofa signal. Except those 10 at either end of the portionsla, 3a, the leakage paths due to glancing contacts hâvebeen removed. Thus the resistors Rg+b are replaced byresistors Rb of much higher value representing theleakage through brine alone. The résistance through thebrine in the région of the relay station 6 is so largecompared with that provided by the glancing contacts 10at the ends of the isolated portion of string la that theeffect of the brine can essentially be ignored.

During réception of a signal, because there is no current path through the string portion la due to the isolation joint 7 and because the string portion la is effectively isolated from the corresponding casing 17 portion 3a, ail of the signal losses for that section ofthe metallic structure will be from the casing 3a. Inthis circumstance there will be little potential dropalong the two halves of the isolated string portion lawhich essentially provide a direct contact with theglancing contacts 10 at the end of the portions la,3a.This means that the potential différence between twolongitudinally spaced locations on the casing can bedetected and hence a signal extracted from the metallicstructure. The fact that ail of the signal is forcedalong the casing 3 in the région of the relay station 6can serve to increase the potential différence betweenthe two spaced locations on the casing 3.

Figure 5 shows a simplified équivalent circuit forthe portions of the production string la and casing 3ain the région of the relay station 6 during transmission.As above the leakage paths due to glancing contacts hâvebeen removed except those 10 at either end of theportions la, 3a. Thus the resistors Rg+b are replaced byresistors Rb of much higher value representing theleakage through brine alone. The résistance through thebrine in the région relay station 6 is so large comparedwith that provided by the glancing contacts 10 at theends of the isolated portion of string la that the effectof the brine can be ignored. Thus during transmission acurrent loop path can be considered to exist consistingof the isolated portion of the string la, thecorresponding portion of the casing 3a and the glancingconnection points 10. The two ends of this loop are ofcourse also connected to the remainder of the string 1and casing 3. The signal generating means 8a causes acurrent I to flow around the loop path. This flow ofcurrent I causes a potential différence to be set up I »98 6 18 between the glancing contacts 10 at opposite ends of theisolated portion of string la. This potential différencewill be I x sumRc, where sumRc equals the totalrésistance of the casing between the glancing contacts10.

Assuming that the isolation joint 7 is provided atthe centre of the isolated portion of the string la andthe System settles in balance relative to earth, themagnitude of the potential différence between metallicstructure and earth at each end of the isolated portionla will be (I x sumRc)/2. Because a potential différenceexists between the positions of the glancing contacts 10and earth, a signal will tend to travel along the string1 and casing 3 in each direction away from the relaystation 6.

Desired data, for example that received from a datalogging station, can be transmitted along the string 1and casing 3 away from the relay station by encoding asuitable signal onto the string 1 by means of themechanism described above. The resulting signalpropagates away from the current loop path along thestring and casing as a single conductor. The signalcircuit is completed by an earth return and no wirelinesare required. Thus ail of the problems associated withthe provision of wirelines, especially downhole, can beavoided.

Appropriate receiving means at the mudline or atanother relay station (not shown) are used to detect thesignal applied to the string 1 and casing 3 and extractthe desired data. The receiving means may make use of aninductive coupling or be arranged to measure signais withrespect to a separate earth reference.

Thus the range of the signal transmission System can (119 8 6 19 be draina tic al ly increased by providing a suitable numberof relay stations within the casing 3. The relay stationsare bi-directional so that the transmission range whentransmitting signais down into the well as well as out 5 of the well is increased.

With the isolation joint located centrally within the isolated portion la, the signais in each directionaway from the relay station 6 will hâve substantiallyequal strength. However, if the isolation joint 7 is 10 disposed towards one end of the isolated portion la, thepotential différence generated at the other end of theisolated portion la will tend to be greater than (I xsumRc)/2. Thus if it is desired to increase the strengthof the signal in one direction the isolation joint 7 may 15 be disposed accordingly.

In an alternative the isolated portion of the production string la is provided with an insulatingcoating to further reduce conduction between the isolatedportion la and the corresponding portion of the casing 20 3a.

Figure 6 shows a coil 201 provided on a toroidalcore 202 disposed around the production string portionla for use in an alternative method of applying a signalto and/or tapping a signal from the production string 1. 25 In this case inductive coupling is relied on and noisolation joint is used. During transmission the coil 201is used to induce a current in the string 1 and thecurrent loop path described above acts as a single turntransformer winding. During réception, a signal on the 30 production string 1 induces a corresponding current in the coil 201 which can be detected. This method o£ réception does not rely on there being an isolated portion la of production string. This coupling method 119.8 6 20 gives an advantage that it is possible to optimiseimpédance matching by appropriatelv choosing the turnsratio.

Figure 7 shows a further embodiment of the inventionsuitable for use in a well of the type described abovewhich comprises two pigs 301 connected by an electricallyconductive strop 302 and disposed within the productionstring 1 which may or may not be cased. A first of thepigs 301 comprises a local station 303 having anisolation member 7 provided in sériés with the strop 302and signal generating means 8a and signal measuring means8b connected across the isolation member 7. Each of thepigs 301 has a contact 304 for contacting with aninternai surface of the string 1.

Signais may be transmitted and received in thisembodiment in substantially the same way as describedabove in relation to the first embodiment. Duringtransmission the strop 302, a portion of the string laand the contacts 304 forra a current loop path. Whencurrent is caused to flow around the loop by the signalgenerating means 8a a potential différence between thestring 1 and earth can be generated at each contact 304allowing a signal to be transmitted. During réception ofa signal, the strop 302 and contacts 304 allow thepotential différence between two longitudinally spacedpoints on the string 1 to be measured so that a signalcan be extracted from the string 1.

In this embodiment signais may be sent to and fromthe first pig 301. In particular, signais may be sentfrom the pig 301 which allow the location of the pig 301to be determined and/or which represent a quantity, suchas wall thickness, measured by the pig 301.

In implementing this embodiment it is désirable to 1 198 6 21 minimise the impédance of the conductive strop 302 andthe contacts 304 between the pigs 301 and the productionstring 1. Wire brushes (not shown) provided around thepigs 301 for cleaning purposes may be used as thecontacts 304.

One possible mechanism for determining the locationof the pig 301 would be to arrange trigger means atspaced locations along a pipeline which cause the pig 301to send an appropriate signal. Another method would beto détermine the time différence of arrivai of the signalat each end of the pipeline.

It will be appreciated that this System may be usedwhether the pigs 301 are within a cased or uncasedsection of string. Further the System may be used inother pipeline Systems besides wells.

In alternatives more than two pigs may be used.Three pigs connected by two conductive members may beused and the local unit disposed at the central pig. Thiscan facilitate equalisation of the transmissioncharacteristics in both directions away from the localunit.

Figure 8 schematically shows a third embodiment ofthe présent invention which is a System for transmittingdata from inside a section of a cased well to asubstantially adjacent position outside of the casing.

Referring to Figure 8 a metallic production string401 is surrounded by a metallic casing 403 which formpart of a cased well. An isolation joint 40J7 is providedin the string 401 and an internai unit 408 includingtransmitting means (not shown) is connected across theisolation joint 407. At equally spaced distances fromthe isolation joint 407, generally annular electricallyconductive packers 411 are provided between the string

I 1)986 22 401 and casing 403. The electrically conductive packers411 are spaced by a selected distance L and provide agood electrical connection between the production string401 and the casing 403. 5 The portion 401a of the production string 401 between the spaced pair of packers 411 is provided withan insulating coating 409. The coating 409 helps toensure that there is no conduction path or at least onlya very poor conduction path between the string 401 and 10 casing 403 at ail points between the packers 411.

An external unit 413 comprising receiving means (not shown) and a toroid 415 is provided outside of the casing403 at a position which is between the pair of spacedpackers 411. The toroid 415 surrounds the casing 403 and 15 is arranged to act as an inductive coupling means suchthat any net magnetic flux flowing through the toroidgenerates a signal which can be detected by the receivingmeans (not shown).

The system is arranged to be used to transmit 20 signais from the internai unit 408 to the external unit413 by the mechanism described below.

The insulated portion of the production string 401a,a corresponding portion of the casing 403a, and the pairof conductive packers 411 form a current loop path around 25 which current may flow. However, the loop is imperfectsuch that there are other current flow paths and losseswill occur. There can be considered to be a leakage loopvia earth which accounts for the losses.

The current flow, at an arbitrary instant, around 30 the current loop path as well as along the leakage pathsis shown by arrows in Figure 8. I, represents the currentflowing through the insulated portion 401a of productionstring 401, Ic represents the current flowing in the 119 8 6 23 corresponding portion of the casing 403a and Iereprésenta the leakage current to earth.

At the particular instant represented by the arrowsin Figure 8, current Is flows up the production string401 away from the isolation joint 407, a portion of thecurrent passes through the conductive packer 411 to thecasing 403 but a further portion of the current continuesup the string with subséquent losses to earth. At thecasing 403 the path splits again and a proportion of thecurrent I. continues around the current loop path whilethe remainder travels along the casing 403 away from thecurrent loop path and contributes to the leakage toearth. At the lower end of the insulated portion of thestring 401a, current from the casing Ic returns to thestring 401 via the respective conductive packer 411 andleakage currents from earth I. join this flow backtowards the isolation joint 407.

Figure 9 shows a simplified équivalent circuit forthe current loop path and the leakages to earth. Therésistances of the portion of the production string 401a,the corresponding portion of the casing 403a and earthare represented by a resistors Rs,Re,R. respectively.

From the équivalent circuit and the abovedescription, it can be seen that I, = Ic + I,. It followsthat the current I, flowing through the insulated portionof the production string 401a does not equal the currentIe flowing through the corresponding portion of thecasing 403a. This in turn means that there is a netmagnetic flux generated by the current flowing around theloop path. The loop path is encircled by the toroid 415and hence the toroid 415 is linked by the net flux.Therefore, as current flows around the loop, theexistence of, and variations in, that current may be 1 1986 24 detected by monitoring signais generated in the toroid415.

It therefore becomes possible to communicatebetween the internai and external units 408,413 byinjecting appropriate signais onto the production string401 and monitoring the signais generated in the toroid415.

For this technique to work it is important that notail of the current I, which is injected into theproduction string 401 continues around the current loop.That is to say, significant and appropriate leakages toearth and/or away front the current loop must be providedfor. In practice such leakages will tend to occurbecause of the existence of the remainder of the metallicstructure of the well and because the casing 403 will bein contact with earth or another conductive medium, suchas sea water.

The level of signal obtained in the toroid 415 canbe adjusted by making appropriate design choices. Forexample, the position of the toroid along the insulatedportion of the string 401a and the position of theisolation joint 407 may be selected. Further, thespacing L between the conductive packers 411 may bechanged, as may the length of the insulated portion ofthe production string 401a. The aim is to maximise thereceivable signal by increasing the résistance of thecasing loop Re relative to the leakage résistance R. asfar as is practicable. In the first"instance this maybe achieved by increasing the spacing between theconductive packers. Theoretically there will corne a pointwhére spacing between the packers is electricallyoptimised, since increased spacing, at some stage, willbegin to significantly increase the résistance of the 119 8 6 25 leakage path Re. Generally however, other practicalconsidérations will prevent this electrical optimisedspacing being reached. The exact nature and conductiveproperties of the packers 411 may also be selected to 5 vary performance.

Although the position of the toroid along thecurrent loop path/insulated portion 401a is not crucial,the best results are likely to be achieved towards acentral position to balance signais generated during 10 positive and négative going cycles and avoid anyundesirable edge effects.

It will be noted that this System does not requireinsulation between the production string 401 and thecasing 403 along the whole of the well's length, it is 15 merely préférable along the length chosen to give thenecessary transmitting characteristics.

Although this technique has been described withreference to a cased portion of a well, it will beappreciated that the technique is equally appropriate for 20 other situations where it is desired to signal fromwithin a conductive merober which surrounds thetransmitter. For example, the System can be used tosignal from within the casing of flow Unes other thanproduction strings and from within flow Unes themselves 25 providing that a suitable inner conductor is provided.

In a particular case this System can be used withapparatus along the Unes of that shown in and describedwith reference to Figure 7. That is to say the· currentloop path may be formed by a portion of a flow line 1, 30 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 signais generated by the transmitting means 8a located in the pig 301 as it passes 119 8 6 26 through the région of the toroid.

Xt can be noted that this embodiment makes use ofthe same phenomenon as described above with reference tothe first and second embodiments. However, in theprésent embodiment it is the effects which occur in thecurrent loop path itself which are used rather than thecurrent which leaks away from the current loop path alongthe production string and casing 1,3.

It should also be noted that the implémentation ofthe présent embodiment will, at least in somecircums tances, be compatible with the previouslydescribed embodiments. Thus Systems may be provided inwhich signalling along the metallic structure to a remotelocation and signalling from within the casing toadjacent equipment outside of the casing is possible.

Although not shown in the drawings, the casing 3 ofa well is typically made up of screwed together sections.In alternative implémentations of the invention, some orail of the joints between the casincj sections may betreated so as to cause a level of discontinuity inconductivity of the casing. This can typically beachieved by coating the mating surfaces at each jointwith an isolating medium which does not préjudice thesealing requirements for the casing.

Introducing such discontinuities can significantlychange the electrical characteristics of the well as awhole. At least in some circumstances this may lead toimproved performance of the relevant embodimentsdescribed above. For example the range of transmissionSystems shown in Figures 1 and 2 may be improved.Improvements can be achieved whether the discontinuitiesare provided in the région of the current loop path, i.e.between the spaced connections or away from that région. ”986 27

The tendency is to force more of the signal into thestring rather than the casing and to increase theproportion of the signal which travels away from therégion of the loop. 5 In the case of the System shown in Figure 8, the inclusion of an isolation medium between sections of thecasing in the région between the spaced connectionsparticularly aids performance as it reduces the screeningeffect of the casing» Looked at another way, it tends 10 to increase the impédance of the string-casing loop andthus increase the différence between the current flowingin the string I4 and in the casing Ic-

It should be noted that, although as mentionedabove, the présent embodiments, and présent invention in 15 general, may function better if discontinuties exist·between mating sections of casing this is not arequirement for operation. Thus the System may be suchthat the casing is substantially electrically continousalong its whole length or at least in the région of the 20 loop- This is true for the casing of a well and thecasing of any other pipeline as well as for anycorresponding surrounding outer member such as the stringin the embodiment' shown in figure 7.

Claims

119 8 6 28 CLAIMS

1. A data transmission System in which metallicstructure of a pipeline System is used as a signal,channel and earth is used as return, comprising means S for forming a currant loop path for usa in applyingsignais to the signal channel and earth returncircuit, the loop having first and second conductingportions electrically connected to one another at afirst location and electrically connected to one 10 another at a second location,, the second location being spaced from the first location, and the metallicstructure comprising at least one of the conductingportions, and a local unit having transmitting meansfor applying a signal to one of the conducting 15 portions whereby in use a potential différence is generated between earth and the metallic structure inthe région of the loop which causes a signal to bepropagated along the signal channel provided by themetallic structure away from the loop, wherein the 20 means for forming the loop is arranged to ensure thatthe spaced locations are separated by at least aminimum distance selected to give desired transmissioncharacteristics. 25

2. A data transmission System according to Claim 1 in which the pipeline System comprises an inner flow ·line and a surrounding casing wherein, one conductingportion comprises a portion of the flow line and theother conducting portion comprises a surrounding 30 portion of the casing.

3. A data transmission System according to Claim 2in which the means for forming the loop comprises 119 8 6 29 insulating spacer raeans for keeping the flow linespaced from the surrounding casing for the selectedminimum distance. 5

4. A data transmission System according to Claira 2 or Claim 3 in which the spaced connections between thefirst and second conducting portions comprise glancingcontacts between the flow line and casing beyond theselected région. 10

5. A data transmission System according to anypreceding claim in which the local unit comprisesreceiving means for receiving incoming signaistransmitted along the metallic structure. 15

6. A data transmission System according to Claim 5in which the local unit is arranged to act as a relaystation. 20

7. A data transmission System according to any preceding claim in which the transmitting means isarranged to apply signais substantially at themidpoint of the respective conducting portion. 25

8. A data transmission System according to any preceding claim in which the transmitting meanscomprises an isolation member disposed in sériés withthe respective conducting portion and a signalgenerating means connected across the isolation 30 member.

9. A data transmission System according to any ofClaims 1 to 7 in which the transmitting means 119 8 6 30 comprises inductive coupling means disposed around therespective conducting portion.

10. A data transmission System according to Claim 15 in which one conducting portion comprises a portion of a pipeline or flowline and the other conductingportion comprises at least one electrically conductiveelongate member connecting at least two pigs disposedwithin the pipeline or flowline and wherein the spaced 10 connections to complété the current loop path areprovided at the pigs. 15 20 25 30

11. A data transmission System in which metallicstructure of a well including a production string andcasing is used as a signal channel and earth is usedas return comprising a local unit having receivingand/or transmitting means coupled to the string forreceiving signais from and/or transmitting signaisalong the signal channel and insulating spacer meansarranged to ensure that the production string andcasing are spaced from one another for at least aselected minimum distance in the région of the localunit, said minimum distance being selected to givedesired réception and/or transmission characteristics.

12. A data transmission System according to any oneof Claims 2 to 4 and 11 in which the casing comprisesa plurality of separate sections, and mating surfacesat one or more joint betveen adjacent sections arecoated with an isolating medium.

13. A method of data transmission in which metallicstructure of a pipeline System is used as a signal 1198 6 31 channel and earth is used as return comprising thesteps of: arranging a current loop path. for use in applyingsignais to the signal channel and earth return 5 circuit, the loop having first and second conductingportions electrically connected to one another at afirst location and electrically connected to oneanother at a second location, the second locationbeing spaced from the first location, and the métallie 10 structure comprising at least one of the conductingportions ? applying a signal to one of the conducting portionsto generate a potential différence between earth andthe metallic structure in the région of the loop and 15 cause a signal to be propagated along the signal channel provided by the metallic structure away fromthe loop; and ensuring that the spaced locations are separated by atleast a minimum distance selected to give desired 20 transmission characteristics.

14. Signal receiving apparatus for use with a datatransmission System in which metallic structure of apipeline System is used as a signal channel and earth 25 is used as return, comprising a local unit havingreceiving means, means for providing electricalcontact between the local unit and at least two spacedlocations on a portion of the metallic structure andmeans for ensuring that the two spaced locations are 30 separated by at least a minimum distance selected togive desired réception characteristics, wherein saidmeans for providing electrical contact include a firstelectrically conductive member portion for connecting 32 1198 6 the local unit to a fixât of the two locations and asecond electrically conductive member portion forconnecting the local unit to a second of the twolocations. 5

15. A method for receiving a signal from the metallicstructure of a pipeline system which is used as asignal channel in a data transmission system withearth as return, comprising the steps of providing a 10 local uni-t having receiving means; providing electrical contact between the local unit and at least two spaced locations on a portion of the metallic structure, the electrical contact being provided via a first electrically conductive member portion for« 15 connecting the local unit to a first of the two locations and a second electrically conductive memberportion for connecting the local unit to a second ofthe two locations; and ensuring that the spacedlocations are separated by at least a minimum distance 20 selected such to give desired réceptioncharacteristics.

16. A data transmission system for use in pipelineSystems which transmission system comprises: 25' means for forming a currant loop path comprising aportion of an inner conductive member and acorresponding portion of an outer conductive memberelectrically connected to one another at two spacedlocations, the outer conducting member surrounding the 30 inner conductive member and being part of the metallicstructure of a pipeline system; an internai unit disposed within the outer member andhàving transmission means for injecting a signal into 1198 6 33 the loop; and an externai unit disposed oubside the outer member andcomprising inductive·coupling means arranged to be.linked by flux generated by current in the loop, 5 the arrangement being such that in use the currentflowing in said portion o£ the inner member does notmatch the current flowing in the corresponding portionq£ the outer member whereby signais are generated inthe inductive coupling means so allowing communication 10 from the internai unit to the externai unit.

17. A data transmission System according to Claim 16in which the spaced locations are separated by atleast selected minimum distance chosen to give desired 15 transmission characteristics.

18. A data transmission System according to Claim 16or Claim 17 which is arranged for use in a pipelineSystem comprising an inner eonductive flowline and an 20 outer eonductive casing, said outer member comprisingpart of the casing and said inner member comprisingpart of the flowline.

19. A data transmission System according to Claim 18 25 in which the casing comprises a plurality of separate sections and mating surfaces at one or more jointbetween adjacent sections are coated with an isolatingmedium. 30

2 0. A data transmission System according to Claim 18 or Claim 19 in which the electrical connectionsbetwèen the flowline and casing comprise glancingcontacts and/or eonductive packers. 1198 6 34

21. A data transmission System aeeording to any oneof Claims 16 to 20 in which the means for forming theloop comprises insulating spacer means. 5

22. A method of data transmission for use in pipeline Systems which method comprises the steps of:forming a current loop path comprising a portion of aninner conductive member and a corresponding portion ofan outer conductive member electrically connected to 10 one another at two spaced locations, the outer conducting member surrounding the inner conductivemember and being part of the metallic structure of apipeline System? injeoting a signal into the loop from an internai unit 15 disposed within the outer member; and disposing an external unit outside the outer memberwhich unit comprises inductive coupling means arrangeato be linked by flux generated by current flowing inthe loop, 20 and the arrangement being such that the current flowing in said portion of the inner member does notmatch the current flowing in the corresponding portionof the outer member whereby signais are generated inthe inductive coupling means so allowing communication 25 from the internai unit to the external unit.

23. À method of data transmission aeeording to Claim22 in which the spaced locations are separated by atleast selected minimum distance chosen to give desired 30 transmission characteristics.

24. Appararus for use with a metallic structure toprovide a System aeeording to any one of Claims 1 to 1198 6 35 12 and 16 to 21 or carry out a method according to anyone of Claims 13, 15 and 22,