GB2458460A

GB2458460A - Power and data communication in underwater pipes

Info

Publication number: GB2458460A
Application number: GB0804926A
Authority: GB
Inventors: Kamal Babour; Russell Smith; Azedine Kacher; Stephane Vannuffelen; Pascal Panetta
Original assignee: Gemalto Terminals Ltd; Schlumberger Holdings Ltd
Current assignee: Gemalto Terminals Ltd; Schlumberger Holdings Ltd
Priority date: 2008-03-17
Filing date: 2008-03-17
Publication date: 2009-09-23
Also published as: WO2009115798A1; GB0804926D0

Abstract

A system and method is provided for communicating electrical power or data signals along a pipe extending underwater, such as a subsea riser or umbilical. The pipe includes an inner, electrically-insulating sheath defining a conduit 31 and an outer, electrically-insulating layer 33 surrounding the inner sheath so as to define a conducting annulus 32 in-between. An electric current generator (47, fig 4) is positioned on the pipe and generates a current 34 in the annulus. A device 37, such as a pressure sensor, is positioned outside the outer layer of the pipe at a first location remote from the generator. The annulus is in electrical communication with the water 36 at a second location (50, fig 5) on the pipe distant from the generator such that an electrical return path 35 extends through the water between the second location and the position of the generator, optionally via a surface vessel (62, fig 6). The device is operable to draw power or data from the annulus in a contactless manner, preferably by an inductive coupling.

Description

Power and Data Communication Along Underwater Pipes

Technical field

[0001] This invention relates to systems and methods of communicating power and/or data along an underwater pipe. In particular, the invention provides techniques to utilise the structure of a widely available flexible pipe or conduit for such purposes.

Background art

[0002] This invention finds particular application in the domain of subsea installations of the type used in the offshore oil and gas industry. Figure 1 shows a typical topology of a subsea field comprising surface vessels such as semisubmersible rigs 10 and FPSOs 11, which are connected to seabed installations such as manifolds 12 and well heads 13. The connections between the vessels and the seabed installations utilize a combination of rigid risers and flow lines 14 and flexible pipes and umbilicals 15. A typical subsea field includes several systems such as production or injection trees, manifolds, production chokes, etc., which need to be controlled and monitored for effective operation.

[0003] Power and data communication between seabed and surface production facilities is usually achieved using umbilicals. Umbilicals are bundles of electrical cables and hydraulic pipes that connect the different subsea installations with surface. They carry power (hydraulic and/or electrical) and communication (electrical and/or optical). Deployment of an umbilical is often costly and the total number of umbilicals that can connect to surface is limited. Power and communication availability on the seabed is therefore restricted and usually umbilicals are used to connect to production trees and to manifolds. The limited availability of power and communication reduces the possibilities of deploying sensing systems for installation integrity surveillance and production management.

[0004] Flexible pipes of the type used for risers and flowline are complex pipe structures used to carry hydrocarbons in a subsea environment. A typical construction of such a flexible pipe is shown in Figure 2 as a five-layer structure comprising a carcass 21, an inner liner 22, pressure armour 23, tension armour 24 and an outer sheath 25.

[0005] The carcass 21 is typically an interlocked metallic construction that can be used as an innermost layer to prevent total or partial collapse of the internal pressure sheath or pipe (not shown) due to pipe decompression, external pressure, tensile armour pressure, and mechanical crushing loads. It may be used externally to protect the external surface of the pipe.

The carcass defines a bore or conduit through which the production fluid flows. The carcass also provides to the pipe with its flexibility.

[0006] The inner liner 22 is typically a polymer sheath that surrounds the carcass and provides fluid isolation.

[0007] The pressure armour 23 is typically a structural layer with a lay angle close to 90 degrees, which increases the resistance of the flexible pipe to internal and external pressure and mechanical crushing loads. The layer also structurally supports the internal-pressure sheath and typically consists of an interlocked metallic construction, which may be backed up by a flat metallic spiral layer.

[0008] The tension armour 24 is typically a structural layer with a lay angle typically between 20 degrees and 55 degrees, which consists of helically wound metallic wires, and is used to totally or partially sustain tensile loads and internal pressure. Tensile armour layers are typically counter [0009] The outer sheath 25 is typically an external polymer layer. This layer protects the pipe internal structure against contact with the external environment. In subsea environment, the surrounding seawater can generate significant corrosion problems. Therefore the outer sheath isolates the pipe inner structure from the surrounding water.

[0010] A flexible pipe 20 can also include anti-wear layers, an intermediate sheath and an insulation layer. Further details of construction and properties are widely published in industry standards.

[0011] The pipe annulus is the space between the inner liner 22 and outer sheath 25. Any gas and/or liquid that enter the annulus through the liner and/or sheath are generally free to move and mix in the annulus.

[0012] Flexible risers often operate in very challenging environment as water depths become progressively deeper. Monitoring of the integrity of the installation in such environments is therefore very important. Due to the increasingly long lengths of installed pipes, fatigue management is also becoming more important. Different systems have been proposed for the monitoring of the strain on flexible risers, and industry reports summarise potential failure modes of flexible risers.

[0013] Bonded flexible pipe is not as widely used as un-bonded flexible pipe but has perfect insulation layers between the metallic armour wires (used in the armour layers 23, 24) and any seawater that may enter the annulus. In bonded flexible pipe, armour wires are embedded in vulcanized fibre reinforced rubber (similar to a steel reinforcement in a car tyre).

[0014] Rigid insulated flow lines are also widely used. In this case, the pipe is typically made out of an inner metallic rigid pipe coated with a thermally insulating polymer layer. This polymer layer also provides electrical insulation. A lot of subsea flow lines are also coated with cement. Cement also makes a very good electrical insulator.

[0015] It is an object of the invention to provide a means for using the existing structure of the underwater pipe for power and/or data communication.

This is based on the recognition that the annulus in common underwater pipe structures comprises a conductive layer between two insulating layers.

Disclosure of the invention

[0016] A first aspect of this invention provides a system for communicating electrical power and/or data signals along a pipe extending underwater, such as a subsea umbilical or other flexible or rigid pipe, wherein the pipe comprises an inner, electrically-insulating sheath defining a conduit (such as an inner liner around a carcass) and an outer, electrically-insulating layer (an outer sheath) surrounding the inner sheath so as to define an Lf annulus in-between, the system comprising: -an electric current generator located at a predetermined position on the pipe and operable to generate a current in the annulus; and -a device positioned outside the outer layer of the pipe at a first location distant from the generator; wherein the annulus is in electrical communication with the water at a second location on the pipe distant from the generator such that an electrical return path extends through the water between the second location and the position of generator; and the device is operable to draw power and/or data from the current generated in the annulus by the generator.

[0017] The first position (the location of the device) is typically between the location of the generator and the second position (the point of current return on the pipe). The annulus is also in electrical communication with the water at a third location at the position of the generator or distant from the generator on the opposite side to that of the first location.

[0018] The pipe typically extends from the surface to an underwater location and the generator and/or the third location are located at or near the surface and the first and second locations are located below the surface. When the pipe extends between a surface vessel or installation and an underwater installation, the surface vessel or installation can provide the electrical communication at the third location, and/or the underwater installation can provide the electrical communication at the second location. End fittings for the pipe can provide the electrical communication with the annulus. When a surface vessel is used, the hull of the vessel can form part of the return path.

[0019] The generator and/or the device will typically comprise a coil, which embraces the pipe, power and/or data signals being communicated between the annulus and the coil by inductive coupling. In one embodiment, the coil comprises two half-coils connected on one side by a hinge and with a releasable coupling at the other to allow the coil to be positioned around the pipe. An electronic circuit can be connected across the coupling to complete the connection of the two half-coils.

S

[0020] The device typically comprises one or more sensors that are provided with power and/or data from the annulus. In one embodiment, the sensor makes a direct physical measurement in the conduit or annulus, such as a pressure measurement.

[0021] An array of devices can be positioned at different locations along the pipe.

[0022] In another embodiment, the device can be moveable along the outside of the pipe so as to make measurements at different locations. In this case it can comprise a contactless sensor system for making measurements of parameters inside the pipe. It is also preferred that the device further comprises a contactiess power system for drawing power and data from the annulus; and a series of actuators for operably anchoring to the outside of the pipe; and for moving the device axially along the pipe.

[0023] In one particular embodiment, the device comprises an underwater vehicle including a releasable clamp for connecting around the pipe to allow power and/or data to be exchanged.

[0024] A second aspect of the invention provides a method of communicating electrical power and/or data signals along a pipe extending underwater, wherein the pipe comprises an inner, electrically-insulating sheath defining a conduit and an outer, electrically-insulating layer surrounding the inner sheath so as to define an annulus in-between, the method comprising: -generating an electric current in the annulus at a predetermined position on the pipe; -positioning a device outside the outer layer of the pipe at a first location distant from the generator; -providing an electrical return path extending through the water from a second location distant from the generator where the annulus is in electrical communication with the water to the position of generator; and -operating the device to draw power and/or data from the current generated in the annulus by the generator.

[0025] This method is preferably performed using a system according to the first aspect of the invention.

L

[0026] The device can be used to measure a parameter at or near the second location, or to make a series of measurements at different locations along the pipe.

[0027] Where the device comprises an underwater vehicle, the method can comprise moving the vehicle to the second location; connecting a power and data communication device around the pipe at the second location; communicating power and data from the annulus to the vehicle; detaching the power and communication device from the pipe; and moving the vehicle away from the pipe.

[0028] Further aspects of the invention will be apparent from the following

description.

Brief description of the drawings

[0029] Figure 1 shows a typical topography of a subsea installation; Figure 2 shows a known flexible pipe construction; Figure 3 shows the overall layout of an embodiment of the invention; Figures 4a and 4b show embodiments of the invention that use inductive coupling; Figures 5a-5c show an embodiment of the invention in which pipe end fittings provide the current return path; Figure 6 shows the current return path for the embodiment of Figure 5; Figures 7a and 7b show an example of a retrofit implementation of the invention; Figure 8 shows an implementation of a semi-distributed system; Figure 9 shows a system according to the invention used as a contact-less power supply for subsea vehicle; Figures lOa-lOd show an embodiment of the invention including a moveable sensing platform; Figures ha and 1 lb show embodiments for making direct measurements on the fluid in the pipe and end fitting; Figures 12a and 12b illustrate a system for detecting damage to the pipe; and Figure 13 shows a system for power and communication in a subsea control system.

Mode(s) for carrying out the invention [0030] Figure 3 shows schematically the overall layout of an embodiment of the invention based on a subsea multilayered pipe 30. The pipe 30 includes a main conduit 31 surrounded by several layers. The pipe layers include a conductive layer 32 linking the two ends of the pipe (for example the metal constituting armour layers provides a conductive path), and an electrically insulating layer 33 between the conductive layer 32 and the surrounding seawater.

[0031] A means is provided for generating a current 34 through the pipe conductive layer 32. The ends of the conductive layer 33 are arranged in such a way that a current return path 35 is provided through the surrounding water 36.

[0032] A preferred embodiment comprises a subsea device 37 including means 38 for retrieving electrical power from the current flowing through the pipe in a contactless manner, and means 39 for transmitting information through the pipe 30 from its location, using the current 34 flowing through the pipe 30. The subsea device can include one or more sensors depending on applications.

[0033] Because the invention is based on the use of common pipe constructions, it can often be retrofitted on existing installations.

[0034] A system of the type described above can have a number of applications, including: -Implementation of distributed sensing system along a subsea pipe.

-Implementation of distributed power recharge along a subsea pipe on which a subsea vehicle (such as AUV or ROV) can be powered or recharged.

-Power and communication for a sensing system at the end of the pipe.

-Control of subsea equipment.

-A sensing system to detect damage in the insulating layer.

[0035] The particularly preferred implementation of the invention is based on contact-less retrieval power or of communicating using the current flowing through the pipe. One such implementation comprises use of inductive coupling as is shown in Figures 4a and 4b. In this approach, an ac current is generated through the pipe 40 and end fittings of the pipe are arranged in such way that a current return 41 is provided through the seawater in which the pipe 40 is located.

[0036] The subsea device includes a receiver coil 42 includes a magnetic core wrapped with an electrical wire forming N turns around the magnetic core.

The assembly constituted by the pipe 40 and the receiver coil 42 forms a I:N electrical transformer. The magnetic core collects the magnetic field generated by the current passing through the pipe 40 and it induces a current in the wire coil surrounding it. An electronic system 43 can then be connected to the receiver coil 42. This is powered by the current generated in the receiving coil 42 without physical contact to the conductive layer in the pipe 40.

[0037] The current in the pipe 40 can be generated in different ways. In the embodiment of Figure 4a, a current source 44 is directly connected to the conductive layer of the pipe 40. In the embodiment of Figure 4b, an inductive coupling system 45 is used. In this case, an emitter coil 46 (similar to the receiver coil 42) is connected to a generator 47. The emitter coil 46 induces a current in the pipe 40. This configuration has certain advantages in the case of flexible risers. The emitter coil 46 can be located away from the top section of the pipe 40. There are often restrictions in terms of electrical equipment that can be used in the top section area (connecting the pipe to the vessel) due to the risk of explosion. Therefore having the emission coil below the end fitting, and possibly under water, simplifies the actual implementation.

[0038] In the embodiments described above, pipe ends are configured so that the current return path is through the seawater. Figure 5a shows an implementation using the end fittings 50, 51 for a flexible pipe 52 and the current flows through these sections. The end fittings 50, 51 comprise a connector 53 for connection to the vessel or subsea installation, and a pipe termination 54 including identification and handling collars 55, 56.

The top end fitting 51 further comprises a stiffener 57 to relieve stress on the connection point with the vessel hull. Figure 5b shows the connection of the various pipe layers 21-25 (see Figure 2 above) to the bottom end fitting 50. Figure 5c shows the corresponding connection for the top end fitting 51.

[0039] The bottom end fitting 50 is a metallic part in direct contact with seawater.

The top end fitting 51 is connected to the vessel pipe structure (not shown) and therefore the vessel metallic structure. The most likely return path 60 for the current is therefore from the bottom end fitting 50, through the sea water 61 to the vessel hull 62 and then to the top fitting 51 as is shown in Figures 5b, 5c and 6. It is also possible to have an electrode located directly in the water and connected to the top end fitting.

[0040] One advantage of the invention is the ability to retrofit the system on an existing installation. Figures 7a and 7b show one example of such an implementation. The retrofit system includes a clamp structure that can be attached around the pipe 70. The clamp is made out of two half-core shells 71 that are connected by a pivot 72. The pivot 72 allows the two shells 71 to be opened or closed around the pipe 70. The clamp also includes a locking mechanism 73 including a screw and a locking nut.

Once the clamp is installed, the screw and locking nut are engaged and bolted together so as to generate the mechanical tension required to hold the clamp on the pipe 70.

[0041] Each shell 71 includes a magnetic core 74 along its circumference. A wire is wrapped around the magnetic core 74 in order to form the coils. The wire and magnetic core are embedded in the shell structure 71. The shell 71 can be made out of composite material (glass or carbon fibre in an epoxy matrix) or in a bulk polymer material (injection moulding for

example).

[0042] The coils in each shell 71 are connected through the pivot section 72 by means of a flexible wire 75.

[0043] The coils in each shell are also connected across the locking mechanism (when closed, see Figure 7b) through wet mate connectors 76. These connect to an electronic system 77, the precise nature of which depends on the particular application as is discussed below.

[0044] Inductive coupling can also be used for data communication along the insulated pipe. An electronic circuit can be used to generate a modulation of the generator (coil) current. A preferred implementation involves modulation for a digital communication technique. One important communication feature is the ability to send a clock signal along the communication line. This clock signal can be used to synchronize measurements between different sensors.

[0045] One embodiment of the invention comprises a number of different stations implemented on the subsea installation. These can be configured in a network with possibility for sensors to communicate with each other and with surface. The chosen network architecture will depend on the application. Access methods to the network may include: contention, LAN switching, token passing, demand priority or pooling.

[0046] The invention can be used to deploy semi-distributed sensing system along a flexible pipe. An overview of an implementation of a semi-distributed system is shown in Figure 8, which is based on the generalised topology of Figure 1. The different sensing systems 80 can be located at different points of the installation on the various pipes already used, without the need for a dedicated umbilical for each sensor. Several types of application can be targeted using such a system.

[0047] A first application comprises a semi-distributed tension, bending, and torsion sensor along a subsea installation. Certain types of strain sensors have been proposed that can be retrofitted on a subsea installation. These systems are very useful for the monitoring of the mechanical stress in critical part of a subsea installation. They are typically deployed in the area of high loading on the installation. One issue associated with their deployment is the power and communication demands that usually require the use of an umbilical. This invention allows the implementation of such monitoring system without the need of additional umbilicals. Another advantage is the possibility to send a clock signal allows a precise synchronization between the different sensors.

[0048] A second application is a semi-distributed hoop strain sensor installation for slug detection. The use of semi-distributed hoop strain sensors has I' been previously proposed for slug detection and management. A system according to this invention can be used for power and communication of such a sensor network.

[0049] A third application is a semi-distributed accelerometer/inclinometer/gyro along a subsea installation. The use of an accelerometer/inclinometer plafform for the monitoring of subsea risers has been previously proposed.

Similar to direct strain measurement sensors, the deployment of such a system is normally limited by availability of power and communication, which require the use of umbilicals. The use of a system according to the invention allows the deployment of a higher number of sensors and more precise shape monitoring.

[0050] A fourth application is a semi-distributed passive microphone; monitoring noise on a subsea pipe can be useful to understand flow conditions. One application of interest is the monitoring of solid particles in the pipe.

Passive microphones can also be used to monitor cleaning or smart pigs moving in a subsea installation. The use of acoustic measurement for the sand monitoring and pig detection has been previously proposed. A system according to the invention can be used to power such sensors.

[0051] A fifth application is a semi-distributed network of acoustic sources and microphones. Acoustic wave emission is a powerful way of assessing pipe condition. Practical implementations have been previously proposed.

A system according to the invention has the advantage of allowing pipe characterization at long distances. It allows a estimation of the condition of the inside of the pipe. Such a system is also interesting in the case of flexible risers in order to monitor the condition of the outer sheath. Any damage occurring between the emitter and the receiver will be picked-up by the system. The use of a system according to the invention for power and communication simplifies the connectivity between the sensors.

Previous proposals show the use of cable between the emitter and receivers. This configuration makes retrofitting difficult. With contact-less power and communication provided by the present invention, the actual implementation becomes much simpler.

[0052] The preceding are specific examples of the use of systems according to the invention. Other uses are also within its scope. The system can be used to power and communicate with various types of sensor that can be installed on the pipe and that sense parameters of the pipe, of the fluid in the pipe and annulus, (including composition) or of the environment (temperature, pressure, seawater pH, flow rate and direction, etc.).

[0053] The system according to the invention can also be used as a contact-less power supply for subsea vehicle (such as an AUV or an ROV) or other moveable device as is shown in Figure 9. One possible application, for example, is recharging the battery of an AUV. Another possible application would be a sensing platform attached to the pipe and moving along it.

[0054] Figure 9 shows an AUV 90 with internal battery that requires periodic recharges. The AUV 90 hooks-up on the pipe 91 for its recharge by means of a power/communication clamp 92 (see for example, Figure 7a and 7b) and then is able to detach from the pipe 90 for further autonomous

operation in the subsea field.

[0055] Figures 1 Oa-1 Od show an embodiment of the invention including a sensing platform 100 that is moveable along the pipe 101. The sensing platform includes a tractor 102, a sensor hub 103 and a contact-less power and communication module 104. The tractor 102 comprises radial actuators 106 for releasably anchoring the platform to the pipe 101, and axial actuators 107 for extending and contracting the tractor chassis 108 as the radial actuators are sequentially operated to move the platform along the pipe 100 (as is shown in Figures lOa, lOb, lOc and lOd). the Chassis 108 is used as carrier for the sensor hub 103 and power/communication module 104. The tractor 102 and sensors are powered from the module 104. The tractor 102 moves and records its position along the pipe while the sensing platform 100 can be used to sense the internal condition of the pipe. Possible types of sensor include radiographic, X-Ray imaging tomographic, eddy current, electromagnetic, etc. The tool can used to scan pipe condition along its length.

[0056] Systems according to the invention can be used to power sensing systems located at the end of the pipe. Types of applications considered appropriate include: monitoring of the connection of the connectors at the end of an injection line, monitoring of fluid condition at an end-fitting, and monitoring of the pressure in the flexible pipe annulus at pipe end fitting.

[0057] One problem in the implementation of an injection line is the reliability of the pipe just before its connection to the subsea installation. The flexible pipe can be subjected to significant torsion, bending and tension in the connector section. Therefore monitoring of the condition of the connector is interesting to increase installation reliability.

[0058] Direct measurement and recording of flexible pipe torsion, tension and bending in the connector section allow an estimation of umbilical fatigue.

[0059] Implementation of microphone or temperature sensors also allows leak detection. Microphones detect the noise generated by the leak, while temperature sensors detect the cooling effect due to the decompression of

gas for example.

[0060] Another possible application of the invention is the implementation of sensors in direct contact with the fluid (such as pressure sensors) in the pipe. Figure ha shows an embodiment for making direct measurements on the fluid in the pipe. This system comprises a pressure sensor 110 connected through the end fithng 111 so as to directly measure the pressure of the fluids in the conduit. A power/communication module 112 is provided around the flexible pipe 113 above its connection to the end fitting 111 and is connected to the sensor by a cable 114.

[0061] A variation of this application is shown in Figure lIb and comprises a pressure sensor 115 located in the end fitting 111 where it connects to the flexible pipe 113. This sensor can be used to monitor pressure variation in the pipe annulus 116 and detect leaks in the annulus. With no leaks, the pressure in the annulus 116 is expected to be around atmospheric. If a leak occurs, there will be liquid build-up in the annulus, leading to a pressure increase.

[0062] The proposed invention can also be used to detect and locate damage in the insulating layer of the pipe or the conducting layers of the circuit. A practical application of this is the detection of the seawater ingress through the flexible pipe outer sheath and large-scale corrosion or breakage in the armour layers. Figures 12a and 12b illustrate such a system.

[0063] If damage occurs to the insulating layer 120, the inner metallic layer 121 (in the annulus) will be put in electrical contact with the seawater 122 as illustrated in Figure 12a. If a current is flowing through the metallic layer 121, the damaged area 123 will create a short circuit and will prevent the electrical current from flowing further along the layer 121. If a receiving coil 124 is located above the damaged area 123, a current will be generated in the manner described above. However, for a coil 125 located below the damaged area 123, no current will be generated as no current in flowing in the layer 121.

[0064] One system for this application comprises an electronic circuit connected to the coils in order to detect the presence of a current in the coil and send a signal back to surface if the current flows. In one embodiment, the electronic circuit can be a simple emitter powered by the current generated by the coil that broadcasts a coil identification signal. If a coil is located above the damaged area 123, a current will flow, and the circuit will power up and broadcast its ID signal, which can be received at surface. If the coil is located below the damaged area 123, the lack of current will mean that no signal will be transmitted back to surface.

Therefore by analyzing the coil ID signals received at surface, it is possible to detect between which coils the damaged area is located.

[0065] Figure 12b shows one implementation of such a system in which several coils 126 are deployed along the pipe 127 in a semi-distributed configuration for a precise localization of the damage. The sensors status can be continuously logged over time so that the occurrence of damage can be detected as well as its location.

[0066] In many existing subsea installations, control of the various installation components has mainly been effected using hydraulic systems, with hydraulic power being provided from surface through conduits in umbilicals. However, the industry is moving more towards the use of electrical control with low power electrical actuators actuating production valves, chokes, etc. [0067] This invention can be used to provide a system for power and communication in such a control system. Figure 13 shows an illustration of the use of an embodiment of the invention for such purposes. In this example, the system according to the invention is used to control and power electrical actuators such as electrical motors that used to control a production choke in a subsea production tree 130. A power and data clamp 131 is positioned on the flowline 132 connected to the production tree 130 in the manner described above. A power and control cable 133 connects the clamp 131 to the tree 130 where it is connected to the control system of the actuator (not shown). This system can also be used to communicate with sensing equipment such as pressure sensors or multi-phase flow meters located in the subsea installation.

[0068] Changes may be made to the various systems described above while staying within the scope of the invention.

Claims

Claims 1. A system for communicating electrical power and/or data signals along a pipe extending underwater, wherein the pipe comprises an inner, electrically-insulating sheath defining a conduit and an outer, electrically-insulating layer surrounding the inner sheath so as to define an annulus in-between, the system comprising: -an electric current generator located at a predetermined position on the pipe and operable to generate a current in the annulus; and -a device positioned outside the outer layer of the pipe at a first location distant from the generator; wherein the annulus is in electrical communication with the water at a second location on the pipe distant from the generator such that an electrical return path extends through the water between the second location and the position of generator; and the device is operable to draw power and/or data from the current generated in the annulus by the generator.
2. A system as claimed in claim 1, wherein the first position is between the location of the generator and the second position.
3. A system as claimed in claim I or 2, wherein the annulus is also in electrical communication with the water at a third location at the position of the generator or distant from the generator on the opposite side to that of the first location.
4. A system as claimed in claim 1, 2 or 3, wherein the pipe extends from the surface to an underwater location, the generator and/or the third location being located at or near the surface and the first and second location being located below the surface.
5. A system as claimed in claim 4, wherein the pipe extends between a surface vessel or installation and an underwater installation, the surface vessel or installation providing the electrical communication at the third location and/or the underwater installation providing the electrical communication at the second location.
6. A system as claimed in claim 5, wherein end fittings for the pipe comprise the electrical communication with the annulus.
7. A system as claimed in claim 5 or 6, wherein the pipe extends between a surface vessel and an underwater location, the hull of the vessel forming part of the return path.
8. A system as claimed in any preceding claim, wherein the generator and/or the device comprise a coil which embraces the pipe, power and/or data signals being communicated between the annulus and the coil by inductive coupling.
9. A system as claimed in claim 8, wherein the coil comprises two half-coils connected on one side by a hinge and with a releasable coupling at the other to allow the coil to be positioned around the pipe.
1O.A system as claimed in claim 9, wherein an electronic circuit is connected across the coupling to complete the connection of the two half-coils.
11.A system as claimed in any preceding claim, wherein the device comprises one or more sensors that are provided with power and/or data from the annulus.
12.A system as claimed in claim 11, wherein the sensor makes a direct physical measurement in the conduit or annulus.
13.A system as claimed in claim 12, wherein the direct physical measurement is a pressure measurement.
14.A system as claimed in any of claims 11-13, comprising an array of devices positioned at different locations along the pipe.
15. A system as claimed in claim 11, wherein the device is moveable along the outside of the pipe so as to make measurements at different locations.
16.A system as claimed in claim 15, wherein the device comprises a contactless sensor system for making measurements of parameters inside the pipe.
17.A system as claimed in claim 16, wherein the device further comprises a contactless power system for drawing power and data from the annulus; and a series of actuators for operably anchoring to the outside of the pipe; and for moving the device axially along the pipe.
18. A system as claimed in any of claims 1-10, wherein the device comprises an underwater vehicle including a releasable clamp for connecting around the pipe to allow power and/or data to be exchanged. 1%
19.A method of communicating electrical power and/or data signals along a pipe extending underwater, wherein the pipe comprises an inner, electrically-insulating sheath defining a conduit and an outer, electrically-insulating layer surrounding the inner sheath so as to define an annulus in-between, the method comprising: -generating an electric current in the annulus at a predetermined position on the pipe; -positioning a device outside the outer layer of the pipe at a first location distant from the generator; -providing an electrical return path extending through the water from a second location distant from the generator where the annulus is in electrical communication with the water to the position of generator; and -operating the device to draw power and/or data from the current generated in the annulus by the generator.
20.A method as claimed in claim 19 when performed using a system as claimed in any of claims 1-18.
21.A method as claimed in claim 19 or 20, further comprising using the device to measure a parameter at or near the second location.
22.A method as claimed in claim 21, comprising making a series of measurements at different locations along the pipe.
23.A method as claimed in claim 22 comprising providing an array of devices distributed along the pipe to make the measurements.
24.A method as claimed in claim 22, comprising moving the device along the pipe to make the measurements.
25.A method as claimed in claim 19, wherein the device comprises an underwater vehicle, the method comprising moving the vehicle to the second location; connecting a power and data communication device around the pipe at the second location; communicating power and data from the annulus to the vehicle; detaching the power and communication device from the pipe; and moving the vehicle away from the pipe.