WO2005056980A1

WO2005056980A1 - Controlling a fluid well

Info

Publication number: WO2005056980A1
Application number: PCT/GB2004/004961
Authority: WO
Inventors: Neil Douglas; Knut Asskildt; Sohrab Yaghmai; Svein Johannessen
Original assignee: Vetco Gray Controls Limited
Priority date: 2003-12-09
Filing date: 2004-11-25
Publication date: 2005-06-23
Also published as: GB0611818D0; GB0328440D0; EP1702136B1; EP1702136A1; GB2408987A; GB2427221A; GB2408987B; US20050121188A1; NO20061428L; GB2427221B

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) iOr control sig.,nals 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 1A, DEM 1B) 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

CONTROLLING A FLUID WELL

The present invention relates to controlling a fluid well, such as a hydrocarbon extraction- ell.

Subsea hydrocarbon extraction wells are controlled, typically, by liydraulically powered valves and fluid control chokes, downliole, with the control of the hydraulic power to such devices being effected by directional control valves (DCVs) which are electrically operated. The DCVs are t}pically 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. Modem 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 mechanisms that employ both an electrical and hydraulic drive, such that if one fails the other is still operable.

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 mode wells.

According to the present invention from one aspect there is provided apparatus for controlling a fluid well comprising a control device for location downliole and operable selectively by first and second drive means, there being first and second power supply means for powering the first and second drive means respectively, the 5 arrangement being such that if one of the power supplies fails, the respective drive means is operable via the other power supply.

In accordance with a second aspect of the invention there is provided apparatus for controlling a fluid well, comprising: 10 a) a control device for location downliole; 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 p ower supp ly means ; 15 e) second power supply means; f) first switching means, for switching power to the first drive means; and g) second switching means for switching power to the second drive means; wherein

L0 i) the first and second power supply means 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

15 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.

.0 According to the present invention from a third aspect, there is provided apparatus for controlling a fluid well comprising, a control device for location downliole 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.

According to the present invention from a fourth aspect, there is provided apparatus for controlling a fluid production well, comprising: a) a control device for location downliole; 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 -A. 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.

In accordance with a fifth aspect of the invention 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 control chamiels for control signals for the first and second drive means, 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.

In accordance with a sixth aspect of the invention there is provided apparatus for controlling a fluid well, comprising: a) a control device for location downliole; 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) a first control channel, for control signals for the first drive means; e) a second control, channel, for control signals for the second drive means; f) first switching means, for switching control signals to the first drive means; and g) second switching means, for switching control signals to the second drive means; wherein i) the first and second control channels are connected to the first switching means and also to the second switching means; and id) 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.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Fig. 1 shows a lαiown form of choke drive assembly for a hydrocarbon extraction well;

Fig. 2 shows a modification of what is shown in Fig. 1, being an example of the present invention; Fig. 3 shows in more detail one of the electronic modules of Fig. 2;

Figs. 4 and 5 show the switching of relays in modules for two chokes;

Fig. 6 shows how redundancy is provided in control channels in an example of the present invention;

Figs. 7-9 show alternative embodiments of the control channel arrangement; and

Fig. 10 shows a tjpical downliole electronics module (DEM) according to the invention.

Fig. 1 shows, diagrammatically, a lαiown form of drive assembly for a fluid control device, typically a choke, mounted downliole 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.

hi 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 downliole electronics module

(DEM) 7. The DEM 7 recognizes 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 Tower B ' .

Likewise, in the case of the electric drive, the motor 2 is operated directly by another DEM 9, which recognizes 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'.

10 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 cho es are fitted downliole, in that a failure of electrical links between the devices could render the well inoperative. Thus, a system is desired that continues to [5 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.

In order to provide redundancy of power supply, an embodiment of the invention '.0 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. 5 Referring to Fig. 2, a modified DEM 11 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 PSUS is applied to DEM 13 and power from PSU 10 is applied to DEM 11, the latter using one of 'Power A' and 'Power B^: and DEM 13 using the other of 0 'Power A' and Tower 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.

The reason for the cross-connection of the PSUs 8 and 10 is to retain operation of the 5 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 DEMI 3 (rather than DEM 11) which controls the selection of 'Power A' or Tower B' by the relay unit 14 to feed the PSU 10. Also assume that Tower A' is powering the system. Now, if

.0 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, Tower A' and 5 the control logic electronics within DEM 11 operates the relay unit 12 such that the PSU 8 is fed with Tower 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.

L0 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 Tower B', under the control of the electronic part of the DEM. A latching relay 16 provides switching or isolation of the power feed output

15 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.

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. 4O shows relay assemblies 18a and 19a for a choke 20 and 18b and 19b for a choke 21. Power supplies, Tower A' and Tower B^!, are connected to and routed through the two DEM relay assemblies as shown. The two output isolation relays 16a of assemblies 18a and 19a 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 (11a and 13a for choke 20 and l ib and 13b for cho e 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.

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 11a m choke 20 which operates relay 16a in the relay assembly 19a in the choke 20. Operation of relay 16a isolates choke 21 from the faulty link. This action is followed by a digital control message being sent to the DEM l ib in choke 21 which operates the relay 15b in relay assembly 19b of choke 21. This reconnects power from the Tower A⁵ so that both drives continue to operate in choke 21.

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.

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 u bilicals) 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 downliole to the choke drives by the interface units 26. Each choke has two DEMs (DEM 1 A and DEM IB) which contain electronic circuitry that reconfigures the architecture in the case of a fault. These circuits are integrated into integrated circuits, for example communication ASICS ("Application Specific Integrated Circuits"), each with four ' ports P0, PI, P2, P3.

Under normal, no fault, conditions the communications operates in 'loop mode', with full duplex traffic, of frames of data with a token system. Each integrated circuit operates such that an input to P0 is retransmitted from both P0 and P3, and an input to P3 is retransmitted from P3 and PO. 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.

However, a much improved fault tolerant system is achieved by additional features in the integrated circuits with the ports cross-connected as shown, hi the event of a fault, the integrated circuits are operated in half duplex mode. This is also the startup mode. In half duplex mode the integrated circuits communicate as to the table below:

RECEIVE ON REPLY ON RE-TRANSMIT ON

P0 P0 P3 PI PI PI P2 PO P2 P2 P3 PI P3 P3 P2 P0

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 P0 to P3. Similarly, each DEM will repeat data from the DEM below it to the DEM above, i.e. from port P3 to PO. Data that is repeated on port P3 of DEMs 3A and 3B can be ignored.

Because of the local cross-loop in each choice, 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 2A, then DEM 2A receives its data on P2 from DEM 2B. DEM 2 A will continue to re- transmit to P3 and PI, so data arrives at DEM 3 A port PO. Similarly, if a fault occurs between DEM 1 B and DEM 2B, then DEM 2B receives its data on P2 from DEM 2A. DEM 2B will continue to re-transmit to P3 and PI so data arrives at DEM 3B port PO. 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.

If normal communication is restored, the system automatically switches to full duplex mode.

Fig. 7 shows an alternative embodiment suitable for use where there may be only a single "penetration", i.e. a single line leading from the well head through the tubing hangar to the downliole electronics. In this case, communication through the hangar can be achieved by use of diplexers 27 and 28 to respectively combine and re-divide the combined control signals. With this embodiment the level of redundancy is reduced since there is only one connection through the hangar, but the substantial remaining redundancy is maintained.

A further variation is also shown in Fig. 7 wherein the last integrated circuit in the chain (i.e. for choke C) also as cross-connection of the ports P2 and PI. This provides for circumvention of further faults and thus prcwides even greater redundancy, i.e. maintainability at the expense of reliability. This variation is also equally applicable to the full duplex system of Fig. 6.

Fig. 8 shows an embodiment shovring an alternative arrangement for achieving redundancy by using a different interconnection of DEMs. Like the example shown in Fig. 6, two penetrations through the hangar are used, hi contrast to the system shown in Fig.6, the DEMS IB, 2B and 3B are reversed and the connections between Ports PO and P3 are changed, so that, for example, Port P3 of DEM 2B is directly connected to Port PO of DEM IB rather than Port PO of DEM 3B. The connections between Ports PI and P2 of the DEMs are maintained however, so that, for example, Port PI of DEM 2B is still connected to Port P2 of DEM 2A. The DEMs themselves function in an identical manner as in the previous embodiments. It can be seen that with this arrangement full redundancy is still provided.

Fig.9 shows a further embodiment with a similar DEM connection to that shown in Fig. 8, but in this embodiment there is only a single penetration from the well head through the tubing hangar, similar to the embodiment shown in Fig. 7. As in that embodiment, diplexers 27 and 28 are used to respectively combine and re-divide the combined control signals.

An arrangement of a typical DEM according to the invention is shown in Fig. 10, to show how the communications architecture provides redundancy of the control of the choke drives. The integrated circuit 29, in this case a communications ASIC, has ports P0 to P4 which interface with line drivers 30. The communications ASIC also interfaces with a second ASIC 31, a profibus ASIC with a standard profibus interface. This interface can be connected to other profibus ASICs if required to expand the system. The profibus ASIC has a number of interfaces which can provide facilities as required for downliole tasks. These include interfaces such as a serial interface and an analogue to digital facility which provides conversion of downliole pressure and temperature measurements to digital code for transmission. A further interface is a standard parallel output which is used to control the operation of the choke electric motor via the DC motor controller 32 and the DCVs via the driver unit 33 to operate the hydraulic motor.

Other variations are possible within the scope of the invention which will be apparent to the skilled practitioner. For example, a single loop system may be used instead of the double loop system described. A half duplex system only may be used, with no mode switching as described. The arrangement of the wiring through the tubing hangar can be made in a way which increases fault tolerance, for example the four pairs used for communication can be passed through the tubing hangar in -1 o-

two tubes, such that pairs Al aid Bl pass through a first tube and pairs A2 and B2 pass through a second tube. This further enhances the fault tolerance. In addition, the local profibus fieldbus loop in a DEM can be made redundant, which also further increases the fault tolerance.

It should be noted that although the invention has been illustrated by its application to the control of downhole chokes, it can be equally applicable to the control of other downliole devices.

The combination of the described power and communication architecture substantially improves fault tolerance in the electrical control of subsea wells.

Claims

1. Apparatus for controlling a fluid well comprising a control device (D) for location downhole and operable selectively by first and second drive means (2, 3), there being first and second power supply means (A, B) for powering the first and second drive means respectively, the arrangement being such that if one of the power supplies fails, the respective drive means is operable via the other power supply.

2. Apparatus for controlling a fluid well, comprising: a) a control device (D) for location downhole; b) first drive means (2) for o erating the control device; c) second drive means (3) for operating the control device, the control device being operable selectively by the first and second drive means; d) first power supply means (A); e) second power supply eans (B); f) first switching means (13), for switching power to the first drive means; and g) second switching means (11) for switching power to the second drive means; wherein i) the first and second power supply means (A, B) are connected to the first switching means (13) and also to the second switching means (11), the arrangement being such that, in normal operation, power from the first power supply means (A) powers the first drive means (2) via the first switching means (13) and power from the second power supply means (B) powers the second drive means (3) via the second switching means (11), and in the event of a fault, power from the first power supply means (A) powers the second drive means (3) or power from the second power supply means (B) powers the first drive means (2).

Apparatus according to Claim 2, wherein, in normal operation, power from the first power supply means (A) is provided to the first switching means (13) from the second switching means (11) and power from the second power supply means (B) is provided to the second switching means (11) from the first switching means (13).

4. Apparatus according to any of Claims 1 to 3, comprising first and second control channels (Ch A, C B) for control signals for the first and second drive means (2, 3), and first and second means (DEM 1A, DEM IB) for routing control signals from the first and second channels respectively to the first and second drive means, the routing- eans being cross-connected so that, in the event of a fault, control signals from the second channel (Ch B) are routed via the first routing means (DEM 1A) to the second drive means (3) and/or control signals from the first channel (Ch A) are routed via the second routing means (DEM IB) to the first drive means (2).

5. Apparatus for controlling a fluid well comprising a control device (D) for location downhole and operable selectively by first and second drive means (2, 3), there being first and second control channels (Ch A, Ch B) for control signals for the first and second drive means, the apparatus further comprising -first and second means (DEM 1A, DEM IB) for routing control signals from the first and second chamiels 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 (Ch B) are routed via the first routing means (DEM 1A) to the second drive means (3) and/or control signals from the first channel (Ch A) are routed via the second routing means (DEM IB) to the first drive means (2).

6. Apparatus for controlling a fluid well, comprising: a) a control device (D) for location downhole; b) first drive means (2) for operating the control device; c) second drive means (3) for operating the control device, the control device being operable selectively by the first and second drive means; d) a first control channel (Ch A), for control signals for the first drive means; e) a second control channel (Ch B), for control signals for the second drive means; f). first switching means (13), for sλvitching control signals to the first drive means; and g) second, switching means (11), for switching control signals to the second drive means; wherein i) the first and second control chamiels (Ch A, C B) are connected to the first switching means (13) and also to the second switching means (11);^' and ii) the first switching means (13) includes means (DEM 1A) for routing control signals from the first control channel (Ch A) to control the first drive means (2) and the second switching means (11) includes means (DEM IB) for routing control signals from the second control channel (Ch B) to control the second drive means (3), the first and second routing means being cross-connected so that, in the event of a fault, control signals from the second channel (Ch B) are routed via the first routing means (DEM 1 A) to the second drive means (3) and/or control signals from the first channel (Ch A) are routed via the second routing means (DEM IB) to the first drive means (2).

7. Apparatus according to either of Claims 5 and 6, comprising first and second power supply means (A, B)^'for the first and second drive means respectively, the arrangement being such that if one of the power supplies fails, the respective drive eans is operable via. the other power supply.-

8. Apparatus for controlling a fluid well comprising, a control device (D) for location downhole- and operable selectively by first and second drive means (2,3), there being first and second power supply means (A,B) and first and second control channels (Ch A, Ch B) 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 5 and second means (DEM 1A, DEM IB) 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 (Ch B) are routed via the first routing means (DEM 1A) to the second drive means (3) and/or control signals from the first channel (Ch A) are routed via the 10 second routing means (DEM IB) to the first drive means (2).

9. Apparatus for controlling a fluid well, comprising: a) a control device (D) for location downhole;

] 5 b) first drive means (2) for operating the control device; c) second drive means (3) for operating the control device, the control device being operable selectively by the first and second drive means; d) first power supply means (A); e) second power supply means (B); 0 f) a first control channel (C A), for control signals for the first drive means; g) a second control channel (ChB), for control signals for the second drive means; h) first switching means (13), for switching power and control signals to5 the first drive means; and i) second switching means (11), for switching power and control signals to the second drive means; wherein i) the first and second power supply means (A, B) and the first and second control channels (Ch A, Ch B) are connected to0 the first switching means (13) and also to the second switching means (11), the arrangement being such that, in normal operation, power from the first power supply means (A) powers the first drive means (2) via the first switching means (13) and power from the second power supply means (B) powers the second drive means (3) via the second switching means (11), and in the event of a fault, power from the first power supply means (A) powers the second drive means (3) or power from the second power supply means (B) powers the first drive means (2); and ii) the first switching means (13) includes means (DEM 1A) for routing control signals from the first control channel (Ch A) to control the first drive means (2) and the second switching means (11) includes means (DEM IB) for routing control signals from the second control channel (Ch B) to control the second drive means (3), the first and second routing means being cross-connected so that, hi the event of a fault, control signals from the second channel (Ch B) are routed via the first routing means (DEM 1A) to the second drive means (3) and/or control signals from the first channel (Ch A) are routed via the second routing means (DEM IB) to the first drive means (2).

10. Apparatus according to Claim 9, wherein, in normal operation, power from the first power supply means (A) is provided to the first switching means (13) from the second switching means (11) and power from the second power supply means (B) is provided to the second switching means (11) from the first switching means (13).