Abstract:
In apparatus for controlling a fluid production well, a control device D, such as a choke, located downhole is operable selectively by first and second drive means ( 2,3 ). First and second power supply means and first and second control channels (ChA, ChB) for control signals are provided for the first and second drive means, the arrangement being such that if one of the power supplies fails, the respective drive means is operable via the other power supply. First and second means (DEM  1 A, DEM  1 B) for routing control signals from the first and second channels respectively to the first and second drive means are provided, the routing means being interconnected so that, in the event of a fault, control signals from the second channel are routed via the first routing means to the second drive means and/or control signals from the first channel are routed via the second routing means to the first drive means.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of United Kingdom Patent Application No. 0328440.3, filed on Dec. 9, 2003, which hereby is incorporated by reference in its entirety.  
       TECHNICAL FIELD OF THE INVENTION  
       [0002]     The present invention relates to controlling a fluid well, such as a hydrocarbon extraction well.  
       BACKGROUND OF THE INVENTION  
       [0003]     Subsea hydrocarbon extraction wells are controlled, typically, by hydraulically powered valves and fluid control chokes, downhole, with the control of the hydraulic power to such devices being effected by directional control valves (DCVs) which are electrically operated. The DCVs are typically housed in a control pod mounted on a well tree located on the sea bed above the well production tubing. The DCVs are, in turn, controlled by electronics, housed in a subsea electronics module (SEM) located in the control pod. The SEM is supplied with both electric power and control signals via an umbilical from a sea surface platform. Modern systems typically send the control signals by a communication system which superimposes them on the power feeds. The communication system is generally bi-directional in that not only are control signals to the fluid control devices required, but the outputs of sensors, such as pressure, temperature and flow sensors, are also required to be transmitted to the surface platform to provide the operator with well operation data. Well operators require high availability and reliability for both the power supply and the communication systems and in an effort to achieve this, the power feed, with its superimposed control and sensing signals, is duplicated within the umbilical, or even by a second umbilical, with further duplication of electronic modules in the control pod. Furthermore, future wells will use fluid control devices such as chokes which have dual redundant operating mechnisms that employ both an electrical and hydraulic drive, such that if one fails the other is still operable.  
         [0004]     However, these techniques only provide a limited protection against failure, with the situation becoming much more serious when a plurality of fluid control chokes are fitted to a well, as is the trend in modern wells.  
         [0005]     According to the present invention from one aspect, there is provided apparatus for controlling a fluid well comprising, a control device for location downhole and operable selectively by first and second drive means, there being first and second power supply means and first and second control channels for control signals for the first and second drive means, the arrangement being such that if one of the power supplies fails, the respective drive means is operable via the other power supply, the apparatus further comprising first and second means for routing control signals from the first and second channels respectively to the first and second drive means, the routing means being cross-connected so that, in the event of a fault, control signals from the second channel are routed via the first routing means to the second drive means and/or control signals from the first channel are routed via the second routing means to the first drive means.  
       SUMMARY OF THE INVENTION  
       [0006]     According to the present invention from another aspect, there is provided apparatus for controlling a fluid production well, comprising: 
        a) a control device for location downhole;     b) first drive means for operating the control device;     c) second drive means for operating the control device, the control device being operable selectively by the first and second drive means;     d) first power supply means;     e) second power supply means;     f) a first control channel, for control signals for the first drive means;     g) a second control channel, for control signals for the second drive means;     h) first switching means, for switching power and control signals to the first drive means; and     i) second switching means, for switching power and control signals to the second drive means; wherein 
            i) the first and second power supply means and the first and second control channels are connected to the first switching means and also to the second switching means, the arrangement being such that, in normal operation, power from the first power supply means powers the first drive means via the first switching means and power from the second power supply means powers the second drive means via the second switching means, and in the event of a fault, power from the first power supply means powers the second drive means or power from the second power supply means powers the first drive means; and     ii) the first switching means includes means for routing control signals from the first control channel to control the first drive means and the second switching means includes means for routing control signals from the second control channel to control the second drive means, the first and second routing means being cross-connected so that, in the event of a fault, control signals from the second channel are routed via the first routing means to the second drive means and/or control signals from the first channel are routed via the second routing means to the first drive means.    
               
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0019]      FIG. 1  shows a known form of choke drive assembly for a hydrocarbon extraction well;  
         [0020]      FIG. 2  shows a modification of what is shown in  FIG. 1 , being an example of the present invention;  
         [0021]      FIG. 3  shows in more detail one of the electronic modules of  FIG. 2 ;  
         [0022]      FIGS. 4 and 5  show the switching of relays in modules for two chokes; and  
         [0023]      FIG. 6  shows how redundancy is provided in control channels in an example of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]      FIG. 1  shows, diagrammatically, a known form of drive assembly for a fluid control device, typically a choke, mounted downhole in a hydrocarbon extraction well. The output of the drive is a shaft  1  with a linear motion which operates the fluid control device D, typically a choke having a sliding slotted sleeve that controls fluid flow. The linear action of the shaft  1  is derived from the rotary motion of a motor via a screw arrangement. The assembly (as in GB 2350659) has two motors  2  and  3  coupled with a mechanism  4 , that provides the required linear output from the shaft  1  from either motor. The two motors provide greater availability in the event of the failure of one of them and are controlled and powered via separate feeds from a control system at a surface platform to the well. In the example shown, motor  2  is electric and motor  3  is hydraulic, thus continuing to provide availability in the case of failure of either the electric or hydraulic power sources or their feeds. The control of both the electric motor  2  and the hydraulic motor  3  is through an electronic communication system.  
         [0025]     In the case of the hydraulic motor  3 , a hydraulic power supply on a line  5  is switched to the motor  3  by a DCV  6 , electrically operated by a downhole electronics module (DEM)  7 . The DEM  7  recognises and acts upon a digital message received from the control system via one of the feeds through an umbilical which is designated ‘Channel B’ (Ch B). Electric power to the DEM  7  is provided by a power supply unit (PSU)  8  which is provided with electric power via the same umbilical and is designated ‘Power B’ 
         [0026]     Likewise, in the case of the electric drive, the motor  2  is operated directly by another DEM  9 , which recognises and acts upon a digital message received from the control system via another feed in the umbilical and is designated ‘Channel A’ (Ch A). Electric power to the DEM  9  is provided by a PSU  10  which is fed with electric power via the same umbilical and is designated ‘Power A’.  
         [0027]     Although the known system described above provides considerable redundancy, it could be considered as less than adequate when a plurality of fluid control devices D such as chokes are fitted downhole, in that a failure of electrical links between the devices could render the well inoperative. Thus, a system is desired that continues to provide redundancy in the event of such failures. Since both control signals and electric power are equally important in sustaining well control, this invention provides a solution to the failure of either or both.  
         [0028]     In order to provide redundancy of power supply, an embodiment of the invention modifies the DEMs of  FIG. 1  as shown in  FIG. 2 , by the addition of power supply selection and isolation relay assemblies, the cross-connection between drives of the power supply units (PSUs), the feeding of both ‘Power A’ and ‘Power B’ to the relay assemblies and the feeding of both control channels Channel A and Channel B to the DEMs.  
         [0029]     Referring to  FIG. 2 , a modified DEM  1  for DCV  6  now includes a relay assembly  12  and likewise a modified DEM  13  for motor  2  includes a relay assembly  14 . Power from PSU 8  is applied to DEM  13  and power from PSU  10  is applied to DEM  1 , the latter using one of ‘Power A’ and ‘Power B’ and DEM  13  using the other of ‘Power A’ and ‘Power B’ in normal operation. Since the normal operation of the system is to operate each of the two drives from a different one of the two power sources, the cross-connected PSUs allow for selection of an alternative power source by a DEM.  
         [0030]     The reason for the cross-connection of the PSUs  8  and  10  is to retain operation of the control to enable switching to an alternative power supply source in the event of failure of either a power source or a PSU. To illustrate this further, assume it was the case that PSU  10  powers the control logic electronics within DEM 13  (rather than DEM  11 ) which controls the selection of ‘Power A’ or ‘Power B’ by the relay unit  14  to feed the PSU  10 . Also assume that ‘Power A’ is powering the system. Now, if PSU  10  fails then the power to the control logic electronics in DEM  13  would disappear and thus it would be unable to select, as an alternative, ‘Power B’ to continue operation. By cross-connecting the PSUs  8  and  10  between the two drive systems and ensuring that, in normal operation, the control logic electronics within DEM  13  operates the relay unit  14  such that PSU  10  is fed with, say, ‘Power A’ and the control logic electronics within DEM  11  operates the relay unit  12  such that the PSU  8  is fed with ‘Power B’, then, in the event of either a PSU or power source failure, the control logic elements are still powered by the other source and thus able to continue to receive commands to switch the power source to sustain operation of at least one drive.  
         [0031]      FIG. 3  shows the arrangement of the relays in the relay assembly of a modified DEM, i.e. item  12  or  14  of  FIG. 2 . A latching relay  15  provides selection of the power supply, i.e. ‘Power A’ or ‘Power B’, under the control of the electronic part of the DEM. A latching relay  16  provides switching or isolation of the power feed output to another relay assembly on another choke. A latching relay  17  provides switching or isolation of power to the PSU, i.e. item  8  or  10  of  FIG. 2 .  
         [0032]     As shown in  FIG. 2 , the choke carries two DEMs. When there is a plurality of chokes in a well, the relay assemblies are connected as illustrated in  FIG. 4 .  FIG. 4  shows relay assemblies  18   a  and  19   a  for a choke  20  and  18   b  and  19   b  for a choke  21 . Power supplies, ‘Power A’ and ‘Power B’, are connected to and routed through the two DEM relay assemblies as shown. The two output isolation relays  16   a  of assemblies  18   a  and  19   a  respectively are connected to an identical arrangement of relay assemblies in the second choke  21 .  FIG. 4  shows clearly which electronic section of the DEMs ( 11   a  and  13   a  for choke  20  and  11   b  and  13   b  for choke  21 ) controls each relay. The versatility of control and isolation allows availability of power to at least one of the choke drives in the event of a single power source failure or interconnection failure as illustrated by the example in  FIG. 5 .  
         [0033]      FIG. 5  shows, as an example, how power can be sustained to both DEMs in the second choke  21 , and any subsequent chokes (not shown in the figure), in the event of a short or open circuit of an electrical link  22  between the two chokes. With such a failure, a digital control message is sent to the DEM  11   a  in choke  20  which operates relay  16  a in the relay assembly  19   a  in the choke  20 . Operation of relay  16   a  isolates choke  21  from the faulty link. This action is followed by a digital control message being sent to the DEM  11   b  in choke  21  which operates the relay  15   b  in relay assembly  19   b  of choke  21 . This reconnects power from the ‘Power A’ so that both drives continue to operate in choke  21 .  
         [0034]     It follows from analysis of the circuit that the failure of any one power link between the chokes or a failure of a power source can be circumvented by suitable operation of the appropriate relays. However this power supply architecture is of limited value unless the same versatility is available, in the event of a failure, for the communication links that control the relays and command the choke drive operation.  
         [0035]      FIG. 6  shows a typical arrangement for the architecture of the communications system of a subsea well. Communication from the surface platform is duplicated via the umbilical (or via two umbilicals) as Ch A and Ch B. Typically, communication data is transmitted on the power feed and then extracted at the duplicated subsea electronic modules (SEM)  25  located on the well tree on the sea bed. The data is then transformed into the format required to communicate downhole to the choke drives by the interface units  26 . Each choke has two DEMs (DEM  1 A and DEM  1 B) which contain electronic circuitry that reconfigures the architecture in the case of a fault. These circuits are integrated into integrated circuits, each with four ports P 0 , P 1 , P 2 , P 3 .  
         [0036]     Under normal, no fault, conditions the communications operates in ‘loop mode’, with simplex traffic, of frames of data with a token system. Each integrated circuit operates such that an input to P 0  is retransmitted from both P 0  and P 3 , and an input to P 3  is retransmitted from P 3  and P 0 . Thus communication is passed round the loop such that any choke can be operated from one channel or the other, in the event of a failure of one link between the chokes.  
         [0037]     However, a much improved fault tolerant system is achieved by additional features in the integrated circuits with the ports cross-connected as shown. In the event of a fault, the integrated circuits are commanded to operate in half duplex mode and communicate as to the table below.  
                                       RECEIVE ON   REPLY ON   RE-TRANSMIT ON                   P0   P0   P3 P1       P1   P1   P2 P0       P2   P2   P3 P1       P3   P3   P2 P0                  
 
         [0038]     Thus in the half duplex mode each DEM will repeat data from the DEM above it, to the DEM below it, i.e. from port P 0  to P 3 . Similarly, each DEM will repeat data from the DEM below it to the DEM above, i.e. from port P 3  to P 0 . Data that is repeated on port P 3  of DEMs  3 A and  3 B can be ignored.  
         [0039]     Because of the local cross-loop in each choke, each DEM will actually receive data on two ports. However, the data is delayed by an extra 1.5 bits from the companion DEM and is then not used unless there is a fault. Thus, for example, if there is a fault in the cable (short or open circuit) between DEM  1 A and DEM  2 A, then DEM  2 A receives its data on P 2  from DEM  2 B. DEM  2  A will continue to re-transmit to P 3  and P 1 , so data arrives at DEM  3  A port P 0 . Similarly, if a fault occurs between DEM  1 B and DEM  2 B, then DEM  2 B receives its data on P 2  from DEM  2 A. DEM  2 B will continue to re-transmit to P 3  and P 1  so data arrives at DEM  3 B port P 0 . It should be noted that the local cross links in each choke are not in a high stress environment and are thus unlikely to fail. It follows that any single fault between chokes is tolerated and that multiple faults are also tolerated, provided there is only one fault between chokes.  
         [0040]     The combination of the described power and communication architecture substantially improves fault tolerance in the electrical control of subsea wells.