A well instrumentation system, comprising: a power and data supply; and a plurality of functional units attached to the power and data supply and distributed throughout the well, characterised in that the power and data supply comprises first and second substantially identical cables, and in that each unit comprises a first power supply channel and a first data channel connected to the first cable, a second power supply channel and a second data channel connected to the second cable, and a functional module which draws power from the first power supply channel or the second power channel module, and data from the first data channel or the second data channel. The power and data supply can comprise a surface unit that can be selectably connected to either the first or second cable. The selection of connection of the surface unit to one or other cable is effective to select the corresponding power and data channels in the functional units.

CROSS-REFERENCE TO RELATED APPLICATIONS 
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   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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   THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
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   INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
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   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to power and data systems for use in permanent installations in wells such as oil and gas wells. In particular, the invention provides such systems which incorporate redundancy in order to improve overall system reliability. 
   2. Description of Related Art 
   It has been known for some time to install permanent monitoring devices and valves in wells such as oil and gas wells in order to monitor and control the operation of the well over its lifetime. Such installations require the provision of power and data communication systems for each device. One such system is the WellNet power distribution and communication system of Schlumberger, which is described in more detail in U.S. Pat. No. 6,369,718. 
   WellNet uses an armored twisted pair cable to transport both communication and power. In this configuration, shown schematically in  FIG. 1 , power is sent in common mode, often referred to as the “phantom wire”, and communication is sent in differential mode, on the twisted wires. 
     FIG. 2  shows a simplified representation of the theoretical principle. The current “Icom” goes from A to B in the primary winding of the surface transformer of the transmitter. The secondary of this transformer sends the modulation in the cable through one wire of the twisted pair with return on the other wire. The signal is picked by the primary of the downhole device transformer, down the cable. The cable armor is never involved in the communication path. Power “Ipow” is sent to the centre tap of the secondary of the transmitter transformer. The current “Ipow/2” flows in the same direction through each wire of the twisted pair. It goes out at the centre tap of the primary of the receiver transformer, enters the power converter of the downhole tool “Load”, and returns through the cable armor. The power supply uses both wires of the twisted pair in parallel, allowing large power transfer capabilities, even when using small wires. 
     FIG. 3  shows the manner in which the WellNet system is applied to multiple tools connected on the same cable. Each tool (Node) intercepts the twisted wires to the power and data in the manner described above. A termination resistance is located in the lower cable head of the lowermost device in the string to avoid signal reflections. In order to communicate between the surface controller and the downhole tools, a communication protocol is implemented, each node being addressable independently via the network. Each mode will include one or more electronics modules (WellNet modules) with appropriate firmware to manage this activity. 
   The system described above has a single cable. Therefore, damage to the cable can mean that the whole installation can become inoperative. One way to avoid this possibility is described in WO 00/46616, which shows a loop configuration for the cable. In this case, instead of terminating the cable at its lower end, the cable is returned to the surface of the well and is connectable to another power and data system. Each node can be provided with power and data from either surface supply, and each node is provided with switches to allow this to be selected. In the event of a problem on the cable, typically detected by the loss of signal, the switches at each node are operated until the location of the fault is identified. Following this, the switches on the nodes above the fault are set to take power and data from the original supply, and those of the nodes below the fault to take power and data from the second supply. Thus the effect of the fault can be limited to one or two devices rather than affecting the whole installation. However, this system requires that the cable run to the bottom of the well, and back to the surface, and that two supplies be provided. 
   It is an object of the present invention to provide a power and data system which does not require a looped cable in order to operate in the event of faults. 
   BRIEF SUMMARY OF THE INVENTION 
   In accordance with the present invention, there is provided a well instrumentation system, comprising: a power and data supply; and a plurality of functional units attached to the power and data supply and distributed throughout the well, characterised in that the power and data supply comprises first and second substantially identical cables, and in that each unit comprises a first power supply channel and a first data channel connected to the first cable, a second power supply channel and a second data channel connected to the second cable, and a functional module which draws power from the first power supply channel or the second power channel module, and data from the first data channel or the second data channel. 
   Preferably, the power and data supply comprises a surface unit that can be selectably connected to either the first or second cable. The selection of connection of the surface unit to one or other cable is effective to select the corresponding power and data channels in the functional units. 
   The present invention also comprises a functional unit for installation in a well, comprising a first power supply channel and a first data channel connectable to a first cable in the well, a second power supply channel and a second data channel connectable to a second cable in the well, and a functional module which draws power from the first power supply channel or the second power supply channel, and data from the first data channel or the second data channel. 
   Preferably, each functional unit is connected to the first and second cable via separate, isolated connectors, one on each cable. The functional units are functionally connected to the cables via respective input transformers. Each power channel is taken from the centre tap of the primary winding of the corresponding input transformer, and each data channel is taken from the secondary winding of the corresponding input transformer. 
   The outputs of the power channels are typically combined to provide a single power input for the functional module. Diodes can be provided in each channel to prevent power from an active channel affecting an inactive channel. A fuse can also be included in each channel to allow permanent disabling of a channel in the case of a fault. 
   Where an intermediate transformer is provided between the power and data channels and the functional module, the single power input can be provided to a tap in the secondary winding of the intermediate transformer. 
   The power channels can be configured to provide multiple power signals. In such cases, the channels can comprise DC/AC converters and step-down transformers with multiple outputs. 
   Each data channel is typically taken from the secondary winding of the corresponding input transformer. In one arrangement, the secondary windings of the input transformers are connected to each other. In such a case, the data channel is taken from a common connection to the connection between the secondary windings. It is preferred to provide resistors in the common connection at each transformer to provide an impedance barrier such that one channel does not affect the behaviour of the other channel due to the common connection, which will typically be taken between the resistors. 
   The data channels can contain transmitter/receiver electronics modules to allow two-way data communication. 
   One particularly preferred form of the invention uses only passive components in the data and power channels. 
   An alternative form of the functional unit includes switch devices in each channel, each switch device including one or more active components. 
   The functional module can include power regulator and controllers for the, or each, power input, and data communication modules, preferably with transmit and receive functionality. 
   The functional module will typically include sensor and/or actuator functions. One such module is a flow control valve, although other functions such as pressure or flow sensors can also be included according to requirements. Each functional module can also include its own power channel to allow direct connection to a single cable for power and data. 
   The outputs of the data channels are preferably combined to provide a single data input for the functional module. Where an intermediate transformer is provided, the single data input can be provided to the primary winding of the transformer. 


   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Examples of embodiments of the present invention are described below with reference to the accompanying drawings, in which: 
       FIG. 1  shows a schematic diagram of a prior art power and data system; 
       FIG. 2  shows a schematic diagram of the principle of operation of the system of  FIG. 1 ; 
       FIG. 3  shows a schematic diagram of a multi-tool installation using the system of  FIG. 1 ; 
       FIG. 4  shows a general view of an installation incorporating the present invention; 
       FIG. 5  shows one example of a sub falling within the scope of the present invention for use in the installation of  FIG. 4 ; 
       FIG. 6  shows an alternative form of sub to that shown in  FIG. 5 ; 
       FIG. 7  shows a version of the sub according to the invention using only passive components; 
       FIG. 8  shows the electrical circuit corresponding to the sub of  FIG. 7 ; 
       FIG. 9  shows an equivalent structure to the embodiment of  FIG. 7  with a short on one of the cables; 
       FIG. 10  shows a schematic view of a further embodiment of the present invention; 
       FIG. 11  shows further detail of the tool electronics cartridge shown in  FIG. 10 ; 
       FIG. 12  shows an embodiment of the invention in the form of a flow control valve; 
       FIG. 13  shows the electrical system block diagram for the embodiment of  FIG. 12 ; 
       FIG. 14  shows an embodiment of the invention using active switching components; and 
       FIG. 15  shows a further embodiment of the invention using active switching components. 
   


   DETAILED DESCRIPTION OF THE INVENTION 
   The proposed redundancy technique is shown schematically in  FIG. 4  and comprises deployment of two power and data cables  20 ,  22 , each connected to a power and data channel  24 ,  26  at the surface (which may be the sea bed in an offshore installation) with an or-wired condition. This ensures redundancy for the top channel. Only one cable is used at a time and ensures links to all downhole equipment  28 ,  28 ′,  28 ″, etc. In case of fault condition of this primary cable  20 , switching on the secondary cable  22  allows reconnection of all equipment  28 . This ensures redundancy for the downhole equipment. The structure comprises two interface cards  24 ,  26  at surface/subsea level, one for each cable  20 ,  22 , and two cable penetrators at the tubing head  30  to allow connection to the two cables  20 ,  22  installed in the well. All the downhole tools  28  are connected to the two cables  20 ,  22  for power and communication and can be accessed via either cable. Should a tool  28  or a cable  20 , fail, switching on the alternate cable  22  will give access to the tool chain. Each downhole tool  28  is equipped with a separate sub that allows enabling the power supply and the communication link on the proper cable. In case of cable default, the sub automatically recovers the power supply and ensures the communication link via the secondary cable. 
     FIG. 5  shows one example of a redundancy sub falling within the scope of the invention. The sub  30  is positioned between the downhole tool  32  and the cables  20 ,  22 . Each cable  20 ,  22  is provided with a separate connection box  34 ,  36  at the level of the sub  30 . The connection boxes  34 ,  36  are separated and separately insulated, such that failure caused by entrance of fluid into one box has no effect on the other. Connection wires  38 ,  40  pass from each connection box  34 ,  36  into the sub  30  via bulkhead connectors  42 ,  44  having pressure integrity at the pressures encountered in the borehole. The connection wires  38 ,  40  from each cable  20 ,  22  are connected to the primary winding  46 ,  48  of a respective transformer  50 ,  52  located in the sub  30 . A centre tap  54 ,  56  is taken from each primary winding  46 ,  48  to corresponding DC power supply circuitry  58 ,  60  which provides power at the appropriate level for the tool in question via a common power connection  62  which passes to the tool  32  from the sub  30  via a suitable bulkhead connector  64 . The secondary winding  66 ,  68  of each transformer  50 ,  52  is connected to a respective modem  70 ,  72  which outputs data signals which pass to the tool  32  via another bulkhead connector  74 . 
   In use, power and data signals are typically provided on one cable (e.g. cable  20 ), the other cable  22  carrying no signals whatsoever. In the sub  30 , the power signals pass through the power supply circuitry  58  to the tool  32 . Since the output of both sets of power supply circuitry  58  and  60  are combined before the power signal passes through the bulkhead connector  62  to the tool  32 , suitable protection (diodes  76 ,  78 ) is provided to prevent the live power channel powering up the dormant one. The data signals are demodulated in modem  70  and passed to the tool  32 . Since cable  22  is dormant, no data passes via modem  72 . Each tool in the installation will operate in this manner. 
   In the event of a failure on cable  20 , power and data signals on that cable are halted and the signals are passed on cable  22 . Consequently, power is passed to the tool via power supply circuit  60  and data via modem  72 . Again, all of the tools in the installation behave in this manner. The selection of which modem and power supply circuit is used is effected by the selection of the live cable, there being no active switching in the sub  30 . Also, the change in power and data supply is invisible to the tool since the power and signal arrive at the tool along common paths  62 ,  74  irrespective of which cable is used. 
     FIG. 6  shown an alternative form of sub to that shown in  FIG. 5 . In this case, the modems  70 ,  72  are replaced by amplifiers  80 ,  82  which also receive power signals  84 ,  86  from the power supply circuits  58 ,  60 . The output  88 ,  90  from an amplifier  80 ,  82  passes through a first bulkhead  92  to the primary winding  94  of an intermediate transformer  96 . The power signals  98 ,  100  pass through the first bulkhead  92  to a tap  102  on the secondary winding  104  and the output  106  from the intermediate transformer  96 , comprising both power and data signals, passes through a second bulkhead  108  to the tool  32 . The combined signal passed to the tool is separated into its respective data and power components in the normal manner for downhole tools. In this case, the tool  32  sees a power and data signal that is essentially the same as that as might appear from a single cable connection in a normal installation. Again, the selection of power and data channels associated with a particular cable is achieved by switching the supplies to the cable at the surface and it is irrelevant to the tool which channel is active. 
     FIG. 7  shows an embodiment of the invention that uses only passive components ( FIG. 8  shown only the electric circuit of  FIG. 7  with the structural component omitted for clarity). In this case, the circuit is similar to that of  FIG. 5 . The power supply feed is taken as a tap  110 ,  112  from the primary winding  46 ,  48  of the input transformer  50 ,  52 , through the first bulkhead  92 , to the secondary winding  104  of the intermediate transformer  96  (with diode protection  76 ,  78  on each channel) as before. The secondary windings  66 ,  68  of the input transformers  50 ,  52  are connected to each other  114 ,  116  and a feed  118  taken to the primary winding  94  of the intermediate transformer  96 . Resistors  120 ,  122  are provided on either side of the feed take-off  118  so as to provide an impedance barrier. In the event of a change in the transformer characteristics for one channel, e.g. a cable failure, short circuit or the like, the impedance barrier means that the behaviour of the other input transformer is not substantially changed. As with the embodiments discussed above, only one cable is active at any given time. If a short circuit, or other failure, is detected in the active cable, the other cable is selected as before.  FIG. 9  shows the effective configuration when a short circuit occurs in cable  22 . In this case, cable  20  is active and power and data arrive at the tool via input transformer  50 . The diode protection  78  on power channel  112  stops the channel  110  power supply charging that side of the circuit, and the resistors  122  on the side of input transformer  52  provide an impedance barrier so that the behaviour of the input transformer  50  is not modified. It is to be noted that the configuration of  FIGS. 7 ,  8  and  9  comprises entirely passive components. 
     FIG. 10  shows a schematic view of further embodiment of the present invention, configured for use with a downhole electric control valve for managing the flow in the well. The figure shows the structure of a redundancy sub A and part of the tool electronics cartridge B. As before, the redundancy sub A essentially comprises two substantially identical power and data channels, one for each cable, the outputs of which are mixed prior to passing through the bulkhead to the tool electronics cartridge. Each channel comprises an input transformer  124 ,  126  as before. Again, the power take off  128 ,  130  is from the primary winding of the input transformer  124 ,  126 , but in this case, since more than one power supply is needed by the tool for proper functioning, the power channel comprises a DC/AC converter  132 ,  134  and a step-down transformer  136 ,  138  which gives three output power supply channels PS 0 , PS 1 , PS 2  which pass to the tool electronics cartridge B via the bulkhead  140  after the two channels are joined, with diode protection, as before. The number of power supply channels can be less or more than three. 
   The data channel is capable of handling both transmit and receive data streams. The secondary winding of the input transformer  124 ,  126  is connected to a Tx/Rx circuit including an electronics module (e.g. a WellNet module)  142 ,  144 . The data channels from each cable are mixed at a Tx/Rx amplifier  146  and passed as a single feed to the tool electronics cartridge B, via the bulkhead  140 , as before. 
   In the tool electronics cartridge B, each power channel PS 0 , PS 1 , PS 2  is fed to a respective power regulator  148 ,  150 ,  152  and from there to an electronics module (e.g. WellNet module)  154  and motor controller  156  and thence on to the sensor and actuator parts of the tool (not shown). The data channel passes via the electronics module  154  to the motor controller  156  and sensors. 
   While the tool described here takes power and data via the redundancy sub A, the tool electronics cartridge B may also contain a conventional, single cable, power supply circuit as shown in  FIG. 11 . This comprises an input transformer  158 , DC/AC converter  160  and step-down transformer  162 . The data feed  164  is taken from the secondary winding of the input transformer  158  to the electronics module  154  of the cartridge, i.e. essentially the same structure as one channel of the redundancy sub A. This approach makes it unnecessary to have different tools for dual or single cable operation; in single cable operation, the tool is connected to the cable directly via the cartridge, in dual cable operation via the redundancy sub. 
   The present invention can be implemented as part of a flow control installation utilising, for example, a variable flow control valve system such as the TRFC-E (Tubing Retrievable Flow Control-Electrical) of Schlumberger.  FIG. 12  shows such an installation with two cables  20 ,  22 , the redundancy sub  30 , and the tool electronics cartridge  32  mounted on the valve assembly  160 . A typical installation will comprise several of these valves in a well, possibly in conjunction with pressure gauges, flow meters or other measurement devices. While  FIG. 12  shows the redundancy sub integrated with the flow control valve, it is also possible to implement the redundancy sub as a totally separate tool in its own right, which can be connected to any one of a number of permanently installed tools (e.g. WellNet tools).  FIG. 13  shows schematically the electronic elements of the installation, comprising the cables  20 ,  22 , cable connectors (one for each cable/channel)  34 ,  36  the redundancy sub  30 , a connection box  168  incorporating the bulkhead described above and electronics sections for DC power regulation  170 , interface and motor control  172 , and the motor and actuator itself  174 . The connection box architecture is described in U.S. Pat. No. 6,499,541. 
   Two further embodiments of the invention are shown in  FIGS. 14 and 15 . These comprise full redundancy subs under active control downhole. By sending appropriate signals to the sub using the WellNet protocol, each can be configured to connect to one or other of the cables according to requirements. Consequently, it is possible to power both cables and have some tools connected to one cable and other tools to the other cable. Thus it is possible to isolate faults in both cable while retaining full functionality of the installation. 
   In the embodiment of  FIG. 14 , the redundancy sub contains two control modules (WellNet modules)  176 ,  178 , one for each cable  20 ,  22 . By sending an appropriate signal on a given cable, the relevant module operates switches  180 ,  182  to connect that cable to the tool  32 , and instructs the other module to operate switches to disconnect its cable from the tool. The module can also operate other switches  184 ,  186  and  188 ,  190  to cross-connect the cables  20 ,  22  such that the power and data signals are transferred from one cable to the other below that sub. For example, if a fault is detected in cable  20  above the sub, the signal is sent on cable  22  for the module  178  to close the switch  182  to connect the tool  32  to cable  22 , and to instruct the other module  176  to open the switch  180  to disconnect the tool  32  from cable  20 . If it is also desired to continue operation on cable  20  below this tool, module  178  closes the switch  188  connecting cable  22  to cable  20  and opens the switch  190  for continued connection to the lower part of cable  22 . Module  176  operates to open the switches  184 ,  186  to stop signals passing up cable  20  or to module  176 . 
   The embodiment of  FIG. 15  has one controller module  192 ,  194 ,  196 ,  198  for each switch function as well as a module  200  for the tool in question. Each module  192 - 200  has a network address and can be controlled from the surface via either cable using the appropriate protocol (WellNet). 
   By providing redundancy in the power and data channels for each tool in a manner that is easily controlled from the surface, the present invention allows improved reliability of the installation as a whole, with less susceptibility to total failure from cable problems or the like.