GB2501184A

GB2501184A - Monitoring corrosion of a pipe

Info

Publication number: GB2501184A
Application number: GB1306526.3A
Authority: GB
Inventors: Anthony Cole
Original assignee: Mi & Corr Ltd
Current assignee: Mi & Corr Ltd
Priority date: 2012-04-10
Filing date: 2013-04-10
Publication date: 2013-10-16
Anticipated expiration: 2033-04-10
Also published as: GB201206253D0; GB201306526D0; GB2501184B

Abstract

The present invention relates to corrosion monitoring apparatus 10 for installation between first and second ends of a pipeline P. The apparatus includes a monitoring spool 12 having a series of longitudinally arranged monitoring elements 18 that are arranged to carry an electrical current, and a series of corresponding longitudinally arranged electrical isolation elements 20 that are arranged to isolate the electrical current. The monitoring elements and electrical isolation elements are alternatelyarranged around the circumference of the apparatus in order to form a fluid passage of the monitoring spool through which fluid may pass between the first and second ends of the pipe. This allows the apparatus to detect the orientation of corrosion or wear in the apparatus and hence the pipeline by comparing electrical resistance in the series of monitoring elements.

Description

Apparatus and Method for Monitoring Corrosion of a Pipe The present invention relates to apparatus for monitoring corrosion of a pipe, particularly, but not exclusively, a pipe which is part of a pipeline used to transport S fluids.

Internal corrosion of metallic pipes causes many hundreds of millions of pounds of damage in a number of industries every year. Hundreds of millions of pounds are therefore also spent combating such corrosion in a variety of industries including for example oil and gas, nuclear and water treatment etc. Known methods of combatting internal corrosion include for example building in a sacrificial corrosion allowance (an excess of material which is intended to corrode at a controlled rate), deploying corrosion resistant alloys (which usually contain quantities of chromium, nickel, copper and other elements in addition to steel) or by introducing chemical corrosion inhibitors which reduce the effect of the corrosive species.

To ensure that the chosen corrosion mitigation technique is working effectively it is desirable to be able to monitor the corrosion rate in-situ. In the event that corrosion is occurring too rapidly, such in-situ monitoring allows intervention and re-evaluation of the mitigation technique. In-situ monitoring also allows chemical inhibition injection rates to be optimised with direct impact in reducing operational costs such as chemical, maintenance, shut-down, replacement and repair costs, as well as minimising lost production in oil and gas applications.

In-situ corrosion rate monitoring is currently often achieved by inserting probes through the wall of the pipeline, piping or vessel and measuring corrosion using one of several well established techniques such as electrical resistance or weight loss. These probes often intrude far into the pipeline, piping or vessel being installed or may be used in a "flush" system where the intention is that the probe or coupon remains flush with the wall of the pipeline, piping or vessel. Precise flush-placement is difficult to achieve, but in order to try and accomplish this, the physical dimension of the coupon or probe is often limited.

However, any form of monitoring which requires a breach in the wall of the vessel S creates a potential hazard (especially if the pipe operates at pressure) such that inserting and retrieving probes requires special precautions to prevent product leakage. In addition, in some instances, access to the pipe is restricted or may indeed be impossible; for example, in subsea pipelines, many buried onshore pipelines and in applications in underground oil and gas wells.

A further disadvantage of such intrusive methods of in-situ corrosion monitoring is that insertion of the probes into the fluid stream alters the flow characteristics thereof. This in turn can affect the corrosion rate determined by the probe thereby resulting in inaccurate readings. Moreover, in some instances, particularly in pipelines and tubulars of oil and gas producing wells, it is necessary to run tools internally through the pipeline, and in these cases probes can interfere with the running of such tools.

Alternative known corrosion monitoring techniques may be used to overcome some of the above problems. An example of such a known technique is described in European Patent Publication No. ER 2 270 483 Al. Such techniques rely on monitoring electrical resistance to measure corrosion by monitoring the increase in resistance of a metallic element as the wall thickness decreases due to corrosion.

However, these methods can be relatively insensitive and therefore often require a 26 significant amount of corrosion to take place before it can be detected.

Furthermore, such methods can be subject to inaccuracies associated with small fluctuations in temperature and conductive corrosion products.

Another method of detecting corrosion is described in WO 86/02728 Al; however, this does not provide any information on the orientation of any corrosion or wear around the pipe.

According to a first aspect of the present invention, there is provided corrosion monitoring apparatus for installation between first and second ends of a pipe, the corrosion monitoring apparatus comprising:- a monitoring spool comprising:-S a plurality of longitudinally arranged monitoring elements adapted to carry an electrical current; and a plurality of corresponding longitudinally arranged electrical isolation elements adapted to isolate said electrical current, the longitudinally arranged monitoring elements and longitudinally arranged electrical isolation elements being alternately arranged around the circumference of the apparatus in order to form a fluid passage of the monitoring spool through which fluid may pass between the first and second ends of the pipe such that the apparatus is adapted to detect the orientation of corrosion or wear therearound by a comparison of electrical resistance in the plurality of monitoring elements.

According to a second aspect of the present invention, there is provided a method of monitoring corrosion in a pipe, the method comprising the steps of:-providing a plurality of longitudinally arranged monitoring elements adapted to carry an electrical current; providing a plurality of corresponding longitudinally arranged electrical isolation elements adapted to isolate said electrical current, and alternately arranging said longitudinally arranged monitoring elements and said longitudinally arranged isolation elements in a circumferential arrangement to form a monitoring spool having a fluid passage through which fluid may pass; 26 installing said monitoring spool between first and second ends of a pipe such that fluid may pass between the first and second ends of the pipe through said fluid passage in said monitoring spool; and selectively detecting the orientation of corrosion or wear around the circumference of the monitoring spool by conducting a comparison of electrical resistance in the plurality of monitoring elements.

Further features and advantages of the present invention will be made apparent

from the claims and the following description.

Embodiments of the invention will now be described, by way of example only, with S reference to the accompanying drawings, in which:-Fig. 1 is a schematic partial cross sectional transverse view of a first embodiment of corrosion monitoring apparatus according to the present invention. The apparatus is shown installed in-line on a pipeline; Fig. 2 is a more detailed view of one end of the apparatus shown in Fig. 1 and illustrates electrical connections to a corrosion monitoring control unit; Fig. 3 is an isolated schematic view illustrating the relative dimensions of a monitoring spool contained within the apparatus of the present invention; Fig. 4 is a schematic end cross sectional view of monitoring and isolation elements arranged around the circumference of a monitoring spool member; Fig. 5 is a schematic perspective view of the monitoring spool of Fig. 3; Fig. 6 is a schematic partial cross sectional transverse view of a second embodiment of the corrosion monitoring apparatus according to the present invention. Modular spool members are provided in this embodiment; Fig. 7 is a schematic perspective view of the modular spool members shown in Fig. 6; Fig. 8 is a schematic view of a single isolated modular spool member; Fig. 9 is a schematic perspective view of a single isolated modular spool member; Fig. 10 is a schematic partial cross sectional transverse view of a third embodiment of the corrosion monitoring apparatus according to the present invention. Straight, as opposed to helical, monitoring elements are illustrated; Fig. 11 is a schematic perspective view of the modular spool members shown in Fig. 10; Fig. 12 is a schematic view of a single isolated modular spool member of Fig. 10; Fig. 13 is a schematic perspective view of a single isolated modular spool member of Fig. 10; and Fig. 14 illustrates side, plan, end and perspective views of a straight monitoring spool element.

S

A first embodiment of the corrosion monitoring apparatus is described below with particular reference to a pipeline P. The skilled reader will appreciate that the term pipeline means any tubular or pipework arrangement.

The term "corrosion" is not limited to chemical corrosion but instead covers other forms of degradation of the pipe such as; for example, wear due to particulate scouring of the pipe's inner surface.

As best illustrated in Fig. 1, a first embodiment of the corrosion monitoring apparatus 10 is attached in-line to first and second ends of a pipeline P. The apparatus 10 has an overall length D and is provided with a monitoring spool 12 which abuts against, and is connected to, flanged ends of the pipeline P (or alternatively may be welded directly thereto). Casing 14 surrounds the monitoring spool 12 and tapers onto the outer wall of the ends of the pipeline P such than an annulus is provided between the outer circumference of the monitoring spool 12 and the inner circumference of the casing 14. Insulation and / or support material 16 is provided in said annulus. The casing 14 contains any fluid pressure within the apparatus 10 as well as protecting the monitoring spool 12 during pipe-laying operations and during the lifetime of the apparatus 10. The tapered ends of the casing 14 also serve to avoid snagging during pipe-laying and other handling operations.

The monitoring spool 12 comprises a series of helical monitoring elements 18 alternately arranged with isolation elements 20 to form the tubular body of the monitoring spool 12. As shown in Fig. 3, each monitoring element 18 has a width E; in the embodiment shown, the isolation elements 20 also have a similar width; however, they could be smaller or larger if desired. The overall length C of the monitoring spool 12 is much greater than its diameter A in order to improve the accuracy of any corrosion readings obtained. In the present embodiment, the wall thickness B of each monitoring element 18 is arranged to match the wall thickness of the pipeline P such that both the inner and outer surfaces of the monitoring S spool 12 are flush with the inner and outer surfaces of the pipeline P when connected thereto. However, in an alternative arrangement although the monitoring spool 12 will have the same inner diameter as the pipeline P the wall thickness B of each monitoring element 18 may be much thinner than the wall thickness of the main pipeline P. This is possible because the monitoring spool 12 does not contain the pressure in the flow (the pressure is contained by the casing 14 provided therearound). The wall thickness B of the monitoring elements 18 can typically be as little as a few mm. This is desirable because thinner elements have a high electrical resistance; therefore, any degradation (corrosion or wear) resulting in metal being removed from the inner diameter of the monitoring spool 12 has a more pronounced effect on readings obtained.

The isolation elements 20 may comprise any material having the required strength properties and being able to provide adequate electrical isolation properties whilst being able to withstand the conditions present in the expected surrounding environment during use. Examples of such material include for example nylon, carbon fibre or ceramics; however, any material with similar properties may be used.

Isolating rings 22 are provided at either end of the monitoring spool 12 to electrically isolate the monitoring spool 12 from the pipeline P. Electrical connectors 24 are provided at each end of each monitoring element 18. As best illustrated in Figs. 2 and 3, the electrical connectors 24 allow electrical connections 26 to be connected thereto; these electrical connections 26 allow the apparatus to be connected to a corrosion monitoring control unit (which may be a self-contained computer controlled unit).

With reference to Fig. 3, the monitoring elements 18 and isolation elements 20 are provided with an angle of helix cx (relative to the longitudinal axis of the apparatus).

This angle a may be chosen at the design stage and may be anything above 0 and less than 90 degrees depending upon the required application. Many factors may S be considered when selecting the angle of helix cx; for example, in applications where the flow of fluid through the pipeline is expected to be homogenous, then it may be desirable to align the monitoring elements 18 with the longitudinal axis of the corrosion monitoring apparatus 10 (0 degrees of helix). On the other hand, where the flow of fluid through the pipeline is expected to be heterogeneous then it may be desirable to provide a greater degree of helix in the monitoring elements 18. Situations where a greater degree of helix in the monitoring elements 18 may be desirable include e.g. where particulates run along the bottom of the pipe (thereby causing a corresponding degree of wear damage along the bottom of the pipe) or in a low flow pipe where e.g. microbes cause localised corrosion, or where water drop out is occurring. In such instances comparing the relative increase in resistance of two ends of a single helical monitoring element 18 allows quick detection of the problem. Furthermore, this can be applied on bends where it is advantageous to expose one end of a single monitoring element 18 to the outside of the bend and the other to a more benign experience.

The number of monitoring and isolation elements making up the monitoring spool 12 is selected at the design stage depending upon the required application and can include as few as two of each.

26 The monitoring elements 18 are made from the same material as the pipeline P for reasons which will become apparent subsequently. Furthermore, the monitoring spool 12 provides a tubular bore along which the fluid flow may pass. In this regard, all surfaces of the monitoring elements 18 are electrically insulated with the exception of the innermost working surfaces, which form the tubular bore within the monitoring spool 12, and which are therefore in direct contact with the flow of fluid.

With reference to Fig. 4, in the present embodiment both longitudinal edges of each monitoring element 18 have grooves 29 which mate with corresponding tongues 30 on each isolation element 20. This provides a strong connection between the elements to form a secure tubular wall of the monitoring spool 12.

S Other alternative methods of connecting the elements (such as e.g. seam welds) may be used.

The corrosion monitoring apparatus 10 may be attached to the pipeline P in a number of ways; for example it can be welded to each free end of the pipeline P or can alternatively be attached via connecting flanges. This may be performed during construction of the pipeline. Alternatively, it may be installed post construction once the section of pipeline has been isolated, de-pressured, drained and appropriately prepared.

In use, as the flow of production fluid passes through the apparatus 10, pressure is contained by the outer casing 14. In other words, pressure in the annulus between the monitoring spool 12 and the outer casing 14 equalises with the pressure inside monitoring spool 12. This allows the casing 14 to take the pressure load rather than the monitoring spool 12.

In one method of monitoring corrosion of the pipeline P during use, a corrosion monitoring control unit (not shown) passes a small electrical current into each monitoring element 18 by way of electrical connections 26 connected to the electrical connectors 24 of the monitoring spool 12. As the electrical current 26 passes between the electrical connector 24 at one end of a monitoring element 18 to the electrical connector 24 at the other end of a monitoring element 18 the electrical resistance of that monitoring element 18 is read by the corrosion monitoring control unit. By rapidly switching the current so that current is provided in each monitoring element independently around the circumference of the monitoring spool 12, an individual resistance reading is obtained for each monitoring element 18.

Any corrosion or wear of a monitoring element 18 will result in a small reduction in the mass of material in that monitoring element 18. In turn, this will cause an increase in electrical resistance to be detected in that monitoring element 18. The corrosion monitoring control unit captures any such changes in electrical resistance S of an individual monitoring element 18 in real time and records such information for further analysis. A known technique (which is commonly applied to electrical resistance probes (ERPs)) can then be used to calculate the average corrosion rate based on the measured changes in electrical resistance in the monitoring elements 18. The data returned by this technique may be enhanced by the presence of a temperature probe.

Since the monitoring elements 18 are made from the same material as the pipeline F, any corrosion detected in the monitoring elements 18 indicates a corresponding amount of corrosion of the pipeline F to which the apparatus is connected. In this regard, since the inner walls of the monitoring spool 12 are flush with the inner walls of the pipeline F, and since there are no probes intruding into the internal bore of the apparatus 10, fluid flow will pass smoothly through the apparatus 10 with no or minimal disruption to the fluid flow pattern. This prevents any distortion of results caused by fluid flow turbulence (which can be problematic in prior art systems). The corrosion data obtained for each monitoring element 18 can therefore be confidently used to determine corrosion occurring in the pipeline F at any given time. In this regard, the relatively small mass of each individual monitoring element 18 allows even very small variations in electrical resistance to be readily detected. This is in contrast to prior art systems which can often only detect relatively large amounts of corrosion.

Another advantage of the present invention over prior art systems is that by allowing individual monitoring elements 18 and individual pairs of monitoring elements 18 to be interrogated, corrosion rates can be monitored around 360 degrees. Furthermore, the ability to independently interrogate each monitoring element 18 allows the orientation of any detected corrosion to be determined; this is a useful indication of the type of corrosion occurring and facilitates the application of more efficient and effective mitigation techniques to combat said corrosion.

In addition to allowing each individual monitoring element 18 to be interrogated as described above, when the apparatus 10 is connected to an appropriate corrosion monitoring control unit, there are a number of ways of extracting additional data on the pipeline P and the fluid passing therethrough. In this way, the apparatus 10 is able to provide a suite of corrosion monitoring techniques within a single device.

One example of additional information which may be captured as a result of the apparatus being able to interrogate individual monitoring elements 18 is now described. By passing an electrical current between a first electrical element 18 and a second electrical element 18 located at a different point around the circumference of the monitoring spool 18, the electrical resistance between any pair of individual monitoring elements 18 can be measured. This can be used as a useful indication of, for example, water drop out (in oil lines) and condensation (in gas lines).

Another example is where well-known electrochemical measurements, such as linear polarisation resistance (LPR) measurements, are made on any of the monitoring elements using any other monitoring element, or the pipe wall or an introduced cell as the reference. When taking [PR measurements a reference electrode, a working electrode and a test electrode must be exposed to the environment. A small electrical potential perturbation is applied to the test electrode and the resulting current response is measured and used to calculate the corrosion rate using known equations.

In many current systems, assumptions must be made about the proportionality constant in the equation used to calculate corrosion (which varies depending on the metal, environment, temperature etc.). This is because measuring these "constants" in-situ could cause slight corrosion damage and affect any [PR result.

However, since several monitoring elements 18 are exposed to the environment in the present invention, the "constants" can be determined using one monitoring element 18 and [PR data collected using a different monitoring element 18. The present arrangement therefore improves the accuracy of LPR testing.

S In addition, having several individual monitoring elements 18 in the monitoring spool 12 means that any of individual monitoring element 18 may be monitored using LPR and any of the other monitoring elements may be utilised as the reference and I or working electrodes.

Furthermore, electrochernical noise (ECN) measurements can be made in order to provide useful indicators of the effectiveness of corrosion mitigation using chemicals such as oxygen scavengers or inhibitors. ECN measurements may take the form of voltage or current measurements and they monitor corrosion by detecting small changes in either voltage or current flow associated with individual corrosion events. In general, when corrosion is occurring, many significant current and/or voltage perturbations are detected (i.e. there is a high level of electrochemical noise) and when corrosion is not occurring, or is occurring in a controlled manner, there are few significant perturbations (i.e. there is low level of electrochemical noise). With the apparatus and method of the present invention, it is possible to couple together any pair of individual monitoring elements 18 to determine the relative intensity of electrochemical noise and this allows the effectiveness of chemicals such as inhibitors and oxygen scavengers to be determined. This is particularly useful in situations where corrosion and/or the distribution of inhibitors may be uneven (for example at the end of a long pipeline where corrosion control chemicals may have dropped out, or partitioned, and collected toward the bottom of the pipeline, but be less concentrated toward the top of the pipeline).

Referring to Figs. 6 to 9, a second embodiment of the present invention will now be described. In order to minimise repetition, similar features of the apparatus described subsequently are numbered with a common two-digit reference numeral and are differentiated by a third digit placed before the two common digits. Such features are structured similarly, operate similarly, and/or have similar functions as previously described unless otherwise indicated.

In a second embodiment, the corrosion monitoring apparatus 110 has a monitoring S spool 112 comprising a series of monitoring modules 113. Each monitoring module 113 is coaxially aligned with an adjacent monitoring module 113 and is isolated therefrom by an intermediate isolation ring 123. As well as providing electrical isolation between each monitoring module 113, the intermediate isolation rings 123 may also provide structural reinforcement to each monitoring module 113. In this regard, the length and orientation of the monitoring elements 118, and hence the monitoring modules 113, is determined at the design stage and takes account of several factors such as operating pressures, likely corrosion mechanisms and the fluid expected in the pipeline or other pipe on which the apparatus is to be installed.

The monitoring elements 118 of one monitoring module 113 may be aligned with the corresponding monitoring elements 118 of the adjacent module 113 (as shown in Fig. 6) or alternatively may be staggered such that the end of each monitoring element 118 is adjacent the end of an isolation element 120 of the adjacent monitoring module 113 (as shown in Fig. 7).

As illustrated in Fig. 7, electrical connectors 124 are provided at each end, as well as toward the centre, of each monitoring element 118. Providing an additional electrical connector toward the centre of each monitoring element 118 allows an electrical resistance measurement to be taken either for the full length of the monitoring element 118 and / or for each half of the monitoring element 118. This allows the uniformity of any detected corrosion to be assessed.

With reference to Figs. 10 to 13, in a third embodiment, the corrosion monitoring apparatus 210 comprises similar components to the second embodiment described above. One difference being that each of the monitoring elements 218 and the isolation elements 220 are straight and therefore aligned with the longitudinal axis of the apparatus 210, as opposed to being helically arranged. This embodiment is particularly suited to monitoring homogenous fluids where no difference in corrosion / wear rate is expected around the circumference of the monitoring spool 212.

S

Referring to Fig. 14, a straight monitoring element 218 for use with the third embodiment of the present invention has a pair of electrical connectors 224 at its outer circumference. All surfaces except for the working surface W of the monitoring element 218 are electrically insulated.

As is well known, the electrical resistance of metals varies as the temperature of the metal varies. Since the intended application of the present invention makes it likely that the apparatus will be subjected to a varying range of temperatures during use, it is desirable to provide some means of compensating for temperature variations in order to avoid inaccuracies in the corrosion readings captured. This applies equally to all embodiments previously described.

One way of achieving this is to utilise the modularised monitoring spools of the second and third embodiments. The internal surface of one or more monitoring module 113, 213 may be coated to prevent contact with the flow of fluid (and hence prevent corrosion of the monitoring elements). This therefore provides a reference point for any variation in resistance with temperature. In other words, any variation in resistance in the active corrosion monitoring spool caused by temperature variation rather than corrosion can be actively cancelled from the readings by 26 correlating this against any variation in resistance detected by the control module (which must inherently be due to temperature variation rather than corrosion).

Although not shown in the drawings, an alternative method of performing similar temperature compensation in the apparatus of the present invention is to incorporate a secondary concentric temperature compensation spool around the monitoring spool and use this in substantially the same way as described above.

It is desirable for the corrosion monitoring apparatus 110, 210 to be relatively long; however, the longer each monitoring element 118, 218 is, the less accurate any reading returned from that monitoring element 118, 218 will be. Therefore, utilising a number of modules provides the required overall length in the apparatus 110, 210 whilst reducing the required length of each monitoring element 118, 218, thereby improving the accuracy of the readings obtained. Providing several modules in the corrosion monitoring apparatus 110, 210 also allows a cross check to be made if any monitoring element 118, 218 should start to give suspicious readings (perhaps due to it having become electrically disconnected); this therefore improves the reliability of the apparatus.

Furthermore, if the apparatus is to be provided around a bend for example, the corrosion characteristics before, at and beyond the bend may differ because the fluid flow pattern will be different at each of these locations. Splitting the monitored area into modules and taking measurements from each module allows the corrosion characteristics at such locations to be monitored independently, thereby improving the understanding of the corrosion situation.

A feature of the present invention is that monitoring devices may be provided in the annulus to monitor e.g. internal pressure, flow rate, particulates etc. Modifications and improvements may be made to the foregoing without departing from the scope of the invention, for example:-It will be appreciated that the present invention is scalable and adaptable such that it may be used in numerous pipeline applications; for example, it may be used on small diameter piping at for example a water or chemical plant or refinery.

Although not shown in the described embodiments, an entirely electrically isolated single reference element may be introduced into the monitoring spool to provide a reference throughout the life of the monitoring spool. The resistance of other elements in the monitoring spool can be measured against this single reference element to account for e.g. temperature fluctuations or spurious data caused by corrosion products capable of conducting electricity.

In the embodiments described, only one set of wires is shown attached to each S monitoring element 18. In an alternative embodiment, it may be beneficial to attach more than one set of wires such that, for example, current flow through the monitoring element can be achieved using one set of wires and voltage drop across that monitoring element can be measured using a different set of wires when carrying out electrical resistance measurements. This can lead to greater accuracy of measurement.

Claims

CLAIMS1. Corrosion monitoring apparatus for installation between first and second ends of a pipe, the corrosion monitoring apparatus comprising:- S a monitoring spool comprising:-a plurality of longitudinally arranged monitoring elements adapted to carry an electrical current; and a plurality of corresponding longitudinally arranged electrical isolation elements adapted to isolate said electrical current, the longitudinally arranged monitoring elements and longitudinally arranged electrical isolation elements being alternately arranged around the circumference of the apparatus in order to form a fluid passage of the monitoring spool through which fluid may pass between the first and second ends of the pipe such that the apparatus is adapted to detect the orientation of corrosion or wear therearound by a comparison of electrical resistance in the plurality of monitoring elements.
2. Apparatus according to claim 1, wherein a longitudinal axis of each longitudinal monitoring and electrical isolation element is substantially aligned with the longitudinal axis of the pipe.
3. Apparatus according to claim 1, wherein the longitudinal monitoring and electrical isolation elements comprise helical members having an angle of helix which is angled relative to the longitudinal axis of the pipe by more than 0 and less than 90 degrees.
4. Apparatus according to claim 3, wherein the angle of helix is between 5 and 75 degrees.
5. Apparatus according to any preceding claim, wherein the monitoring and electrical isolation elements each extend from one end of the monitoring spool to the other end of the monitoring spool, such that the apparatus comprises a single monitoring spool.
6. Apparatus according to any preceding claim, further comprising secondary monitoring elements adapted to provide a temperature compensation reference point.S
7. Apparatus according to claim 6, wherein the secondary monitoring elements form a secondary monitoring spool which surrounds, and is electrically isolated from, the first monitoring spool.
8. Apparatus according to any of claims 1 to 4, wherein each of the monitoring and electrical isolation elements each extend only partly along the monitoring spool and wherein a plurality of individual monitoring spool modules are provided along the length of the apparatus.
9. Apparatus according to claim 8, wherein a circumferential arrangement of monitoring and isolation elements on each monitoring spool module is staggered with respect to the circumferential arrangement of monitoring and isolation elements of the adjacent monitoring spool module.
10. Apparatus according to either of claims 8 or 9, wherein at least one of the monitoring spool modules comprises a protective inner coating to at least partially prevent corrosion of the monitoring elements in that monitoring spool module and hence provide a temperature compensation reference point for adjacent monitoring spool modules.
11. Apparatus according to any preceding claim, wherein tongue and groove arrangements are provided between the longitudinal edges of the monitoring and electrical isolation elements.
12. Apparatus according to any preceding claim, wherein the alternately arranged monitoring and electrical isolation elements of the monitoring spool have an inner diameter substantially similar to the inner diameter of the ends of pipe to which the apparatus is to be connected such that, when installed between the first and second ends of the pipe, the inner surface of the monitoring spool is substantially flush with the inner surface of the first and second ends of the pipe.
13. Apparatus according to any preceding claim, further comprising isolating members provided at either end of the single monitoring spool and / or the monitoring spool modules to electrically isolate the said single monitoring spool and! or monitoring spool modules from each other and / or from the pipe to which the apparatus is to be connected.
14. Apparatus according to any preceding claim, wherein the monitoring elements are provided with electrical connectors adapted to allow connection to a corrosion monitoring control unit.
15. Apparatus according to claim 14, wherein electrical end connectors are provided at each end of each monitoring element.
16. Apparatus according to claim 15, wherein at least an additional intermediate electrical connector is provided along the length of each monitoring element between the end connectors.
17. Apparatus according to any preceding claim, wherein the monitoring elements comprise substantially the same material as the pipe to which the apparatus is to be connected.
18. Apparatus according to any preceding claim, further comprising a protective pressure containing casing provided around the monitoring spool.
19. Apparatus according to claim 18, wherein the protective casing provides an annulus between the outer circumference of the monitoring spool and the inner circumference of the protective casing.
20. Apparatus according to claim 19, further comprising insulating material located in said annulus.
21. Apparatus according to either of claims 19 or 20, further comprising secondary S monitoring devices provided in the annulus to monitor at least one of internal pressure, flow rate, particulates etc.
22. Apparatus according to any preceding claim, having a length many times greater than its width.
23. A method of monitoring corrosion in a pipe, the method comprising the steps of: -providing a plurality of longitudinally arranged monitoring elements adapted to carry an electrical current; providing a plurality of corresponding longitudinally arranged electrical isolation elements adapted to isolate said electrical current, and alternately arranging said longitudinally arranged monitoring elements and said longitudinally arranged isolation elements in a circumferential arrangement to form a monitoring spool having a fluid passage through which fluid may pass; installing said monitoring spool between first and second ends of a pipe such that fluid may pass between the first and second ends of the pipe through said fluid passage in said monitoring spool; and selectively detecting the orientation of corrosion or wear around the circumference of the monitoring spool by conducting a comparison of electrical resistance in the 26 plurality of monitoring elements.
24. A method according to claim 23, further comprising aligning a longitudinal axis of each monitoring and electrical isolation element with the longitudinal axis of the pipe.
25. A method according to claim 23, further comprising providing the electrical isolation members and the monitoring elements with an angle of helix of between more than 0 and less than 90 degrees relative to the pipe longitudinal axis.
26. A method according to any of claims 23 to 24, further including the step of providing secondary monitoring elements adapted to provide a temperature compensation reference point.
27. A method according to claim 26, further comprising surrounding the first monitoring spool with the secondary monitoring elements.
28. A method according to any of claims 23 to 27, including the step of providing a plurality of monitoring modules comprising said monitoring spools and coaxially arranging said monitoring modules adjacent each other in the apparatus.
29. A method according to claim 28, comprising providing at least one monitoring module with a protective inner coating to at least partially prevent corrosion of the monitoring elements in said monitoring module, and obtaining a temperature compensation reference point from said protected monitoring module.
30. A method according to any of claims 23 to 29, comprising connecting a plurality of corrosion monitoring control units to said monitoring elements to provide a controlled electrical current in said monitoring elements and independently measuring the electrical resistance in each monitoring element of the monitoring spool.
31. A method according to any of claims 23 to 30, further comprising detecting the electrical resistance between individual monitoring elements around the circumference of the monitoring spool in order to determine characteristics of the fluid flowing through the apparatus.