This invention relates to joint detection, in particular the detection of joints between adjacent sections of tubing used in flowlines and to position or depth control using joint detection.

Typically the flowlines used for carrying oil, gas or other product in the oil and gas industry are made up of a plurality of sections which are joined together at threaded joints. In some flowlines, especially production or drill strings within wells it can be very difficult to determine the position of a tool or any other item within the well.

For example, in the case of a well, a tool or another component may be lowered down the well on a wireline or using coiled tubing. Some crude measure of the position of the supported object within the well may be determined by noting how much wireline or coiled tubing has been fed out. However, the accuracy of such techniques is very limited especially in the case of coiled tubing which can tend

to buckle or collapse in various sections such that the amount fed out gives no true picture of the position of the supported object.

It has been realised by the applicants that an accurate knowledge of the position of a tool or another item within a well (or other flowline) can be determined by counting the number of sections through which the tool or other item has passed since its introduction.

Such a technique however, needs a method for detecting joints. lt will also be appreciated that a method for detecting joints in a flowline could have other uses besides forming part of such a technique.

It is an object of this invention to provide a joint detection system and/, or method which in at least some cases may be used in position or depth control.

According to a first aspect of the invention there is provided a method of detecting joints in flowline systems having metallic structure comprising a fluid conduit disposed within a casing, the conduit comprising a plurality of longitudinally arranged sections meeting at respective joints and the transverse external dimensions of the joints exceeding the transverse external dimensions

of the remainder of the conduit, the method comprising the steps of: a) electrically contacting with the metallic structure at a first and a second location, the two locations being longitudinally spaced from one another, and causing an electrical signal to pass between the spaced locations via the metallic structure ; b) repeating step a) a plurality of times, each time contacting with the metallic structure at one of a plurality of different sets of longitudinally spaced locations; c) monitoring the impedance of the signal path through the metallic structure for each set of spaced locations; and d) determining the position of joints in the conduit in dependence on monitored variations in the impedance of the signal path through the metallic structure.

This method allows the position of joints to be determined in flowlines such as wells where direct inspection is impossible. The detection of joints can aid in determining position within a well. The above method can function because the conduit will tend to contact with the casing via its joints since these generally have a greater diameter. Because of this, different conduction paths exist in the region of joints which lead to changes in impedance that can be detected.

It will be appreciated that there is not necessarily any need to determine the absolute value of the impedance of the signal path, it can be enough just to look for variations. Typically as one or both of the spaced contact locations reaches the region of a joint a local maximum in impedance is seen.

The impedance of the signal path may be monitored in any one of a number of different ways, for example, the voltage seen across a known resistor connected in series with the signal path may be monitored or the voltage seen between the spaced locations may be monitored. A four terminal technique may be employed for applying a signal and detecting the desired parameter.

Contact with the metallic structure maybe maintained whilst one or both of the locations at which contact is made is moved, for example contact may be made via a contact that can slide along the surface of the metallic structure. In such cases the impedance may be measured at effectively an infinite number of sets of spaced locations.

In one set of embodiments, a tool arranged to travel inside the fluid conduit is provided. The tool comprises at least one contact portion towards each end. In carrying out the method using such a tool the contact portions can provide the spaced connections to the metallic structure. Movement of the tool within the

conduit facilitates the use of different sets of spaced locations. The tool may have two contact portions towards each end of the tool, a first portion at each end may be used in applying signals and a second portion may be used in receiving signals.

The tool may comprise signal generating means for applying signals to metallic structure and may comprise monitoring means for monitoring the impedance of the signal path.

Where a tool is used, the step of determining the position of a joint, at least initially, comprises the step of determining the position of the joint relative to the tool.

Means for monitoring the impedance of the signal path may be provided at the tool or remote from the tool. Similarly means for determining the position and/or presence of a joint may be provided at the tool or remote from the tool.

According to a second aspect of the invention there is provided a joint detection system for detecting joints in flowline systems having metallic structure comprising a fluid conduit disposed within a casing, the conduit comprising a plurality of longitudinally arranged sections meeting at respective

joints and the transverse external dimensions of the joints exceeding the transverse external dimensions of the remainder of the conduit, the system comprising: a tool having at least two spaced contact portions for electrically contacting with the metallic structure at a first and a second location, the two locations being longitudinally spaced from one another, the tool funher comprising signal generating means for causing an electrical signal to pass between the spaced locations via the metallic structure ; and the system further comprising: monitoring means for monitoring the impedance of the signal path through the metallic structure ; and means for determining the position of joints in the conduit relative to the tool in dependence on monitored variations in the impedance of the signal path through the metallic structure as detected with the tool in a plurality of different positions relative to the metallic structure.

According to another aspect of the present invention there is provided a position control method for controlling the position of an object within a fuid conduit disposed within a casing comprising the steps of; detecting joints in the conduit using any one of the methods defined above ; determining the position of the object in dependence on the detected joints ; and modifying the position of the object.

According to yet another aspect of the present invention there is provided a position control system for controlling the position of a tool within a fluid conduit disposed within a casing comprising a joint detection system as defined above; means for determining the position of the tool in dependence on the detected joints; and means for modifying the position of the tool.

The position control method or system may be a depth control system or method.

The position of the tool/object may be determined by counting the number of joints which have been passed by the tool/object as the tool/object travels through the conduit from a known starting point. This point might be the point at which the tool/object is inserted into the conduit.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which:- Figure] is a schematic sectional view showing a fluid conduit disposed within a casing and a joint detection tool disposed within the conduit ; Figure 2 shows a simplified equivalent circuit of the situation where the joint

detection tool of Figure 1 straddles a joint; and Figure 3 shows a simplified equivalent circuit of the situation where the joint detection tool of Figure 1 is positioned between two joints.

Figure 1 shows a conduit I disposed within a surrounding casing 2. The conduit J and casing 2 make up part of the metallic structure of a bowline system, in this case a well. The conduit 1, which in this case is a production string. is arranged to carry products out of the well and comprises a plurality of conduit sections la which are longitudinally arranged and joined to one another at respective joints lb. Each of the conduit sections la flares towards its ends such that appropriate opposite threads may be provided at each end so that the conduit sections may be threaded together. This flaring at the ends of each conduit section means that the overall diameter of the conduit I in the region of the joints lb is significantly greater than in the remainder of the conduit.

Thus in a normal situation, where there is nothing to prevent the conduit from moving freely within the casing 2, the conduit will tend to come into contact with the casing and more particularly the contact points between the conduit and the casing will be at the joints lb. This occurs because of the interplay between the lengths of the conduit sections 1 a, the flexibility of the conduit 1

as a whole and the amount which the joints lb protrude from the remainder of the conduit.

For the sake of simplicity in Figure 1, the conduit I is shown as being off- centre within the casing 2 such that the joints lb along one side of the conduit I contact along one side of the casing 2. It will be appreciated that in practice the conduit 1 may adopt a more sinuous path through the casing 2, but at a vast majority (or even at all) of the joints there will be contact between the conduit 1 and the casing 2.

It will be appreciated that both the casing 2 and conduit 1 are metallic (typically steel) and in normal circumstances there will be no insulation provided on either the external surface of the conduit I or the internal surface of the casing 2. Thus, anywhere where the conduit I contacts with the casing 2 there is a reasonably good electrical connection.

A joint detecting tool 3 disposed within the conduit 1 is shown in one position, stradling a joint lb, in solid lines and in another position, between joints lb. in dotted lines in Figure 1. The joint detecting tool 3 comprises a conductive central body 31 and a pair of conductive centralisers 32, one being disposed at each end of the conductive body 31. Within the conductive body 31 is provided

an electronics module 33 which performs a number of functions.

The electronics module 33 comprises a signal generator (not shown) arranged for applying a signal to the conductive body 31 of the tool. Any such signal travels along the conductive body 31 and out through the conductive centralisers 32 which are arranged to remain in physical and electrical contact with the internal surface of the conduit Thus, any signal applied to the conductive body 31 by the electronics module 33 can be fed into the conduit I and the remainder of the metallic structure. Once in the metallic structure there are a number of different paths which the signals may follow. These signal paths depend on the position of the tool 3 within the conduit.

With the tool 3 in the position stradling a joint lb shown in solid lines in Figure 1, the two main current paths are shown by arrows labelled 1, and The first path 1, is directly between the contact portions 32 via the conduit 1, whereas the second 12 is via the length of two conduit sections 1 a, the contacts between two non-adjacent joints lb and the casing 2, and a portion of the casing 2 which has a length substantially equal to two lengths of conduit section lb.

Figure I also shows possible current paths for a signal applied to the conduit I

when the tool 3 is in the position between joints lb as shown by dotted lines in Figure 1. When the tool 3 is in the position shown in dotted lines, again two main current paths exist, and these are shown by the arrows l,. and l3.

The first current path 1, in this case is substantially the same as the first current path 11 where the tool 3 straddles a joint lb. However, the second important current path] 3 is significantly shorter and is of correspondingly lower resistance than the corresponding second path 12 mentioned above. In this case, the second important current path 3 consists of one length of conduit section la, the contacts between adjacent joints lb and the casing 2, and a length of casing 2 corresponding to one section of conduit 1.

It should be said that the relative lengths of the tool 3 and the conduit sections ] a have been deliberately distorted in Figure 1 to aid clarity. In practice the length of the tool 3 would be small compared with the length of each conduit section la such that the distance between the conductive centralisers 32 at each end of the tool 3 is small compared with the length of a conduit section.

The electronics module 33 also comprises detection means (not shown) for detecting the potential difference between the points where the conductive centralisers 32 meet the internal surface of the conduit 1. In the present

embodiment, suitable wiring (not shown) is provided between a potential difference sensor at the electronics module 33 and the contact points to allow this measurement to take place. In general terms it is desirable if a four terminal technique can be used for the application of signals to the conduit 1 and the detection of a potential difference caused by the application of a signal.

In some cases an entirely separate set of contacts might be provided for picking up the potential difference to be detected and these contacts might be provided by a separate set of conductive centralisers.

Figures 2 and 3 respectively show simplified equivalent circuits which exist when the tool is in the"straddling position"shown in solid lines in Figure 1, and the"in between position"shown in dotted lines in Figure 1. In the equivalent circuits, Rp represents the resistance of the portion of conduit 1 between the conductive centralisers, whereas, R represents the resistance of a length of casing 2 equal to the length of one conduit section Ja, and Rs represents the resistance of one conduit section la. Figures 2 to 3 help to illustrate how different resistance will be seen by the signals applied to the metallic structure depending on the position of the tool 3.

In operation, the tool 3 is moved through the conduit 1 and the impedance of the signal path through the metallic structure seen by the applied signals is

monitored by monitoring the changes in potential difference as seen at the contacts. It is of little consequence whether continuous measurements are made as the tool 3 moves within the conduit] or whether discrete measurements are made with the tool 3 in a series of different positions. In either case variations in the potential difference and hence impedance can be monitored. The electronics module 33 further comprises analysing means (not shown) for analysing the variations in monitored impedance to thereby determine when the tool 3 is adjacent to, or passes a joint lb.

The fact that a joint lb has been reached can be used in a number of different ways.

For example, the tool 3 may be arranged to communicate the fact that a joint lb has been reached to a remote location and/or the tool 3 may be arranged to count joints and to carry out a particular action only when a specific number of joints have been detected. In this way for example, the tool 3 may be used to deploy an item at a particular location determined by virtue of the number of joints which have been passed.

The analysing means of the electronics module 33 may also be arranged to detect a change in length of sections within the conduit. It will be appreciated

that if the sections are all of equal length, then as the tool 3 travels within the conduit 1, local peaks in impedance will be seen as the joints are passed, and these local peaks will appear at a steady frequency. However, if a shorter or longer section is traversed, then there will be a shorter period or longer period before the next peak is reached and this can be used to decide that a shorter or longer conduit section has been reached. Again, this can be useful for detecting that the tool is in a particular position, since certain special sections of the conduit may have different lengths. As well as noting that a different time period has occurred to give the indication of a non-standard conduit length, a difference in signal strength might also be detected. ln general terms, a shorter conduit length would be expected to give a stronger signal, whilst the tool 3 is traversing that section (i. e. in a position similar to that shown in dotted lines in Figure 1). Of course, a longer conduit section would be expected to give a smaller signal (indicative of a higher impedance).

Although the specific method described above, which makes use of a tool travelling with the conduit, is considered to be one of the most practicable ways of carrying out the method, it is not the only way in which the method can be carried out. For example, in principle the method will work with a device which traverses outside of a casing. Further, it is not essential that there is a travelling tool having a spaced pair of contacts. Systems might be

envisaged in which one of a pair of contacts is stationary, whilst the other moves and/or is placed at different positions on the metallic structure.

Furthermore, there is no requirement to have any signal detecting means or analysing means at the tool. Signals applied by a tool of the same general type shown in Figure 1 might be detected remotely as the tool moves, and changes in the received signal strength taken as indicative of the tool's position.

In at least some cases the fact that there is no contact between the casing and a particular joint might be tolerated. For example if an established time interval is found for traversing one section, an occurrence where local peaks are seen spaced by twice the time interval could be taken as indicative that a joint has been missed. This is especially the case if it is known that all conduit sections are of uniform length in that region.

In one particular application, the above method and system may be for position control or depth control. That is to say the position of the tool in the conduit, for example its depth in the case of a well, may be determined using the information gained by detecting joints and the position of the tool may be adjusted and controlled as appropriate. Of course the position of another object within the conduit may be determined and controlled if this is connected to the tool. Most typically the position determining process will involve maintaining a

count of the number of joints which have been passed by the tool since its insertion into the conduit. Thus for example the position relative to a well head (or depth) can be monitored and controlled.

Where the tool is connected to/suspended from a wireline or coiled tubing the means for adjusting the position of the tool will comprise the mechanism for feeding out/drawing in the wireline/tubing.

In an alternative method and system, signal generating means are provided at the tool for applying the signals to the conduit via contacts at each end of the tool so that signals are injected into the system, and monitoring means are provided at a location remote from the tool. Such a system is typically used in a cased well and the monitoring means located at the head of the well. In this case the monitoring means will have a first terminal contacting with the metallic structure of the well at or towards the wellhead and a second terminal connected to ground. The variations in impedance of the signal path downhole due to the tool moving past joints is then detected via the change in the signals flowing in the metallic structure at the wellhead. In this way the passing of a joint and/or the position of the tool may be detected from the wellhead without the need for any dedicated conductor connecting the tool to the wellhead.