WELL INSTALLATIONS AND SUBSURFACE SAFETY VALVES

Abstract

A subsurface safety valve control system including a downhole station 5 for provision in downhole metallic structure (2) of a well installation which comprises a subsurface safety valve V having an open state allowing flow through the valve and a closed state obstructing flow through the valve. The downhole station (5) is arranged for picking up electrical signals from the downhole metallic structure (2) to obtain valve control signals and for outputting control signals for controlling the subsurface safety valve V in dependence on the signals picked up from the downhole metallic structure. The downhole station (5) comprises a hardware electronics channel (5a) for handling electrical signals picked up from the downhole metallic structure and the hardware electronics channel (5a) is arranged for detecting the presence and/or absence of hold valve open signals. The downhole station (5) is arranged to cause the subsurface safety valve V to be held in the open state whilst expected hold valve open signals are received and is arranged to cause the subsurface safety valve to move to the closed state in the absence of expected hold valve open signals.

Claims

1. A subsurface safety valve control system downhole station for provision in downhole metallic structure of a well installation which comprises a subsurface safety valve having an open state allowing flow through the valve and a closed state obstructing flow through the valve, wherein the downhole station is arranged for picking up electrical signals from the downhole metallic structure to obtain valve control signals and for outputting control signals for controlling the subsurface safety valve in dependence on the signals picked up from the downhole metallic structure, wherein the downhole station comprises a hardware electronics channel for handling electrical signals picked up from the downhole metallic structure and the hardware electronics channel is arranged for detecting the presence and/or absence of hold valve open signals, and the downhole station is arranged to cause the subsurface safety valve to be held in the open state whilst expected hold valve open signals are received and is arranged to cause the subsurface safety valve to move to the closed state in the absence of expected hold valve open signals.

2. A subsurface safety valve control system downhole station according to claim 1 comprising a software controlled electronics channel for handling electrical signals picked up from the downhole metallic structure.

3. A subsurface safety valve control system downhole station according to claim 2 in which the software controlled electronics channel is provided for detecting auxiliary control signals which are distinct from hold valve open signals.

4. A subsurface safety valve control system downhole station according to claim 1 in which the hardware electronics channel comprises a carrier detector arrangement for detecting in received signals the presence of a carrier signal having a frequency within a predetermined frequency range; and the hardware electronics channel comprises a demodulator for demodulating received signals to acquire a demodulated signal and a detection arrangement for detecting a predetermined signal characteristic in the demodulated signal which predetermined signal characteristic indicates that the received signals comprise hold valve open signals, wherein the downhole station is arranged to hold the valve open in response to: i) the detection of a carrier signal having a frequency within a predetermined frequency range; and ii) the determination that the received signals comprise hold valve open signals from the detection of the predetermined signal characteristic in the demodulated signal.

5. A subsurface safety valve control system downhole station according to claim 1 in which the valve control signals comprise auxiliary control signals which are distinct from hold valve open signals.

6. A subsurface safety valve control system downhole station according to claim 5 in which the auxiliary control signals and the hold valve open signals are carried by a common carrier signal.

7. A subsurface safety valve control system downhole station according to claim 6 in which the auxiliary control signals and hold valve open signals are modulated on and carried by a common carrier signal.

8. A subsurface safety valve control system downhole station according to claim 7 in which the auxiliary control signals are applied to the respective carrier signal using an orthogonal modulation technique to a modulation technique by which the hold valve open signals are applied to the respective carrier signal.

9. A subsurface safety valve control system downhole station according to claim 7 in which the hold valve open signals are encoded by frequency modulation on to a common carrier signal and the auxiliary control signals are encoded by amplitude modulation on to the common carrier signal.

10. A subsurface safety valve control system downhole station according to claim 5 in which the auxiliary control signals comprise forced close signals for causing movement of the subsurface safety valve to the closed state irrespective of whether hold open signals are received.

11. A subsurface safety valve control system downhole station according to claim 5 in which the auxiliary control signals comprise re-open signals for causing re-opening of the subsurface safety valve when in the closed state.

12. A subsurface safety valve control system downhole station according claim 5 in which the auxiliary control signals are encoded onto the carrier as digital signals.

13. A subsurface safety valve control system downhole station according to claim 5 in which the auxiliary control signals are encoded onto the carrier using digital amplitude modulation.

14. A subsurface safety valve control system downhole station according to claim 5 in which the subsurface safety valve has an associated address, and the auxiliary control signals carry an address so as to allow determination at the downhole station as to whether a received auxiliary control signal is intended for the subsurface safety valve.

15. A subsurface safety valve control system downhole station according to claim 1 in which the downhole station is arranged for determining whether at least one hold open condition is satisfied such that the valve should be held open and for outputting a hold open control signal to the valve when it is determined that the at least one hold open condition is satisfied.

16. A subsurface safety valve control system downhole station according to claim 15 in which the downhole station is arranged such that re-open signals for causing re-opening of the subsurface safety valve when in the closed state, will only cause re-opening of the subsurface safety valve when the downhole station determines that said at least one hold open condition is satisfied.

17. A subsurface safety valve control system downhole station according to claim 15 in which the downhole station comprises a logic gate arrangement for determining whether at least one hold open condition is satisfied.

18. A subsurface safety valve control system downhole station according to claim 17 in which the downhole station comprises a battery health monitoring system for monitoring the health of at least one battery at the downhole station and outputting a signal to indicate that said at least one battery is healthy, said output of the battery health monitoring system being fed into the logic gate arrangement.

19. A subsurface safety valve arrangement for installation in downhole metallic structure in a well installation, the subsurface safety valve arrangement comprising a subsurface safety valve having an open state allowing flow through the valve and a closed state obstructing flow through the valve, and a downhole station according to claim 1.

20. A well installation comprising downhole metallic structure comprising downhole metallic pipe for directing oil and/or gas towards the surface, a subsurface safety valve having an open state allowing flow through the valve and a closed state obstructing flow through the valve and hence obstructing flow, in the downhole metallic pipe, of oil and/or gas towards the surface, and a control arrangement for controlling operation of the subsurface safety valve, wherein the control arrangement comprises an out of hole station and a downhole station according to claim 1.

21. A well installation according to claim 20 which comprises a plurality of subsurface safety valves each with a respective associated address and each having an associated downhole station.

22. A well installation according to claim 21 in which the well installation is a multi-lateral well installation with a main bore and at least one lateral bore branching off of the main bore and a first subsurface safety valve is provided in the main bore and a second subsurface safety valve is provided in a bore amongst the at least one lateral bore.

23. A well installation according to claim 21 in which the well installation comprises a number of separate boreholes each provided with its own metallic structure and each provided with a respective subsurface safety valve and associated downhole station.

24. A subsurface safety valve control method for controlling a subsurface safety valve provided in a well installation comprising downhole metallic structure comprising downhole metallic pipe for directing oil and/or gas towards the surface, wherein the subsurface safety valve has an open state allowing flow through the valve and a closed state obstructing flow through the valve and hence obstructing flow, in the downhole metallic pipe, of oil and/or gas towards the surface, and the control method comprises: using an out of hole station to transmit valve control signals to a downhole station by applying electrical signals to the downhole metallic structure; using the downhole station to pick up electrical signals from the downhole metallic structure so picking up the valve control signals; and using the downhole station to control the subsurface safety valve in dependence on the picked up valve control signals.

25. A subsurface safety valve control system downhole station for provision in downhole metallic structure of a well installation which comprises a subsurface safety valve having an open state allowing flow through the valve and a closed state obstructing flow through the valve, wherein the downhole station is arranged for picking up electrical signals from the downhole metallic structure to obtain valve control signals and for outputting control signals for controlling the subsurface safety valve in dependence on the signals picked up from the downhole metallic structure, wherein the downhole station comprises a hardware electronics channel for handling electrical signals picked up from the downhole metallic structure and the hardware electronics channel is arranged for detecting the presence and/or absence of hold valve open signals, and the downhole station is arranged to cause the subsurface safety valve to be held in the open state whilst expected hold valve open signals are received and is arranged to cause the subsurface safety valve to move to the closed state in the absence of expected hold valve open signals, wherein the downhole station is arranged to detect, from the electrical signals picked up from the downhole metallic structure, auxiliary control signals which are distinct from the hold valve open signals.

Description

[0094] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0095] FIG. 1 schematically shows a well installation including a subsurface safety valve operation system;

[0096] FIG. 2 schematically shows an alternative well installation including a subsurface safety valve operation system;

[0097] FIG. 3 shows a further alternative well installation including a subsurface safety valve operation system;

[0098] FIG. 4 schematically shows a downhole station of the subsurface safety valve operation system shown in FIG. 1 in more detail;

[0099] FIG. 5A schematically shows a signal as may be received at the downhole station of FIG. 4;

[0100] FIG. 5B schematically shows a signal as may be output from a carrier detector which forms part of the downhole station shown in FIG. 4;

[0101] FIGS. 6A and 6B show example implementations of a carrier detector of the downhole station shown in FIG. 4; and

[0102] FIG. 7 shows an example implementation of a tone detector of the downhole station shown in FIG. 4.

[0103] FIG. 1 schematically shows a well installation, that is to say, an oil and/or gas well installation. In the present case the well installation comprises a producing well.

[0104] The well installation comprises well installation metallic structure 1 which comprises downhole metallic structure 2 progressing down into a borehole B provided in the formation F. The downhole metallic structure 2 comprises production tubing 21 and an appropriate plurality of casing runs 22. The present well installation is a platform well installation and includes a platform 3 provided above the level of the borehole B. Note that whilst in the well installation as shown in FIG. 1 there is a single borehole and this is provided directly below the platform 3. In other situations there may be multiple boreholes each provided with their own downhole metallic structure 2 and each arranged for feeding product back from the respective borehole to the platform 3. In such a circumstance some of the boreholes may not be directly under the location of the platform 3 but rather spaced away from it. However in such a case, the platform 3 will still be above the level of the respective boreholes even though not directly above those boreholes which are spaced away from the platform.

[0105] In the present specification the term “well installation” is used to include situations where there a plurality of boreholes with their own downhole metallic structure connected back to the platform 3. Thus such a plurality of boreholes which might be considered to be individual “wells” are considered for this specification to all be part of the “well installation”.

[0106] The well installation of FIG. 1 comprises an electrical transmission system for use in the transmission of electrical communication signals between an out of hole station 4, in this case provided on the platform 3, and a downhole station 5 provided, in this case, within the production tubing 21. In the present embodiment, the out of hole station 4 comprises a transceiver portion 41 for applying electrical communication signals to the well installation metallic structure 1 and a control portion 42 connected to the transceiver portion 41.

[0107] In the present embodiment the downhole station 5 is provided in a downhole tool T which is retrofittable within the downhole metallic structure 2, in particular, in this case, within the production tubing 21. The downhole tool T in this embodiment comprises a subsurface safety valve V as well as the downhole station 5. In other cases the downhole station 5 might be provided separately to the subsurface safety valve. In the present embodiment this tool T is an example of a subsurface safety valve arrangement.

[0108] The downhole tool T may for example be deployed via wireline, and may, for example be located into a locator nipple above an existing (but perhaps failed) safety valve, suspended below an anchoring device, or below a wireline deployable lock.

[0109] The subsurface safety valve V includes a failsafe mechanism for driving the subsurface safety valve towards a closed position such as to obstruct the production tubing 21 and shut in the well as a default position. On the other hand the electrical transmission system is arranged for sending a hold open signal to the subsurface safety valve V to hold open the subsurface safety valve V in normal circumstances. This is achieved by the out of hole station 4 sending appropriate control signals to the downhole station 5 which are picked up by and cause the downhole station 5 to hold open the valve V whilst the signals are being received.

[0110] In the present embodiment the signals are picked up from the downhole metallic structure 2, in particular from the production tubing 21, via spaced contacts C provided on the tool T which are arranged for contacting with the inner surface of the production tubing 21.

[0111] As explained above, the out of hole station 4 is arranged for applying electrical signals to the well installation metallic structure 1, in this instance via the platform 3, so that wireless EM (electro-magnetic) signals may be transmitted into the well installation metallic structure 1, and particularly down the downhole metallic structure 2 for reception at, for example the downhole station 5.

[0112] In alternatives, electrical power may also be applied by the out of hole station 4 to the well installation metallic structure 1 for pick up and use at other locations such as at the downhole station 5. Similarly the out of hole station 4 may be arranged for picking up communication signals from the well installation metallic structure, in particular from the platform 3.

[0113] FIG. 2 shows an alternative well installation which comprises an electrical transmissions system of a similar type to that shown in FIG. 1. Here there are a plurality of bores B each having provided therein respective downhole metallic structure 2 and each having a downhole tool T of the type shown in FIG. 1 provided at a downhole location and including a downhole station 5 and subsurface safety valve V. Further again there is an out of hole station 4 comprising a transceiver portion 41 and a control portion 42. The transceiver portion 41 in this instance has an output connected to a respective well head which forms part of the metallic structure provided for each bore B. Thus the out of hole station 4 in the well installation shown in FIG. 2 is arranged for applying signals to the downhole metallic structure 2 in each of the boreholes B and hence the out of hole station 4 is able to communicate with the downhole station 5 in each of the boreholes B and correspondingly able to control operation of the respective valve V provided in each of the boreholes B.

[0114] FIG. 3 shows yet another alternative well installation which is similar to that shown in FIG. 2. However in this case, whilst there are again a plurality of bores B, each provided with its own downhole metallic structure 2 and each having a tool T of the type shown in FIG. 1 including a downhole station 5 and a subsurface safety valve V, here the out of hole station 4 has a different arrangement. In this instance one control portion 42 is provided but three separate transceiver portions 41 are provided. In this instance each transceiver portion 41 is associated with a particular one of the boreholes B and arranged for applying communication signals to the downhole structure 2 in that respective borehole B. Thus in this instance under the control of the control portion 42, signals may be applied to all of the sets of metallic structure 2 provided in the different boreholes B and all of these signals might be the same. However, in alternatives, at least sometimes signals may be applied to only some of the sets of metallic structure 2 and/or different signals may be provided to the different sets of metallic structure 2.

[0115] FIG. 4 schematically shows the downhole station 5 of FIG. 1 in more detail. As mentioned above, the downhole stations of this type may also be included in each of the downhole tools T included in the plurality of boreholes shown in the alternative well installations of FIG. 2 and FIG. 3.

[0116] The downhole station 5 is arranged for receiving signals picked up via the spaced contacts C on the downhole tool T. These signals are applied to a bandpass filter 51. After the bandpass filter 51, the path through the downhole station 5 splits into a hardware channel 5a and a software channel 5b. The hardware channel 5a is provided for handling hold valve open valve control signals and the software channel 5b is provided for handling auxiliary valve control signals.

[0117] In the present embodiment the hardware channel 5a comprises a carrier detector 52, a tone detector 53 and a logic gate arrangement 54 comprising two AND gates 54a, 54b.

[0118] The software channel 5b comprises an analogue to digital converter 55 and a digital signal processor receiver 56. Outputs of the digital signal processor receiver 56 are connected into the logic gate arrangement 54.

[0119] The downhole station 5 further comprises a battery health monitoring system 57 which again has outputs connecting into the logic gate arrangement 54. In the present embodiment the battery health monitoring system 57 is arranged for monitoring the health of a battery (not shown) used for powering the receive operation of the downhole station 5, that is to say powering the components mentioned above. Further in this embodiment the battery health monitoring system also monitors the health of a battery (not shown) used for powering operation of the valve V.

[0120] In the present embodiment the first AND gate 54a has five inputs and is arranged for outputting a hold open valve control signal to the valve V when appropriate signals are received on all of its inputs.

[0121] As mentioned above the valve V is arranged as a latch valve such that only a relatively small amount of power is required to hold the valve in the open position. As such the hold valve open control signal from the logic gate arrangement 54 need only be such as to hold this latch in position. The valve V is arranged such that if the hold valve open signal ceases, the valve V is delatched at that point allowing closure of the valve V under another source of power. For example the valve V may be spring biased towards the closed position and only held against closing by virtue of being latched open. In such a way when the latch releases, the valve V may be driven to a closed position under spring power alone.

[0122] In the present embodiment in order for the valve V to be held open signals must be received at the five inputs of the first AND gate 54a. A first of these inputs is connected to an output of the tone detector 53, the second of these inputs is connected to an output of the carrier detector 52, the third of these inputs is connected to a carrier detector output of the digital signal processor receiver 56, a fourth of the inputs is connected to an output of the battery health monitoring system 57 which indicates good health in the receive battery and a fifth of the inputs is connected to an output of the battery health monitoring system 57 which indicates the valve battery is in good condition.

[0123] Thus the valve V will only be held open where the signals are received from each of these inputting components. In practice this means that the hardware channel 5a must detect a carrier at the carrier detector 52 and must detect a tone at the tone detector 53. Further the digital signal processor receiver 56 must detect a carrier and the battery health monitoring system must detect a healthy receive battery and a healthy valve battery. If all of these conditions are met then the first AND gate 54a will output the appropriate signal for holding the valve open. On the other hand if any one of these signals is absent the output from the first AND gate 54a will cease and the valve V will be allowed to close.

[0124] There are various different ways that the hardware channel 5a might be implemented. The aim with the inclusion of the hardware channel 5a is to provide a very robust system for the detection of hold open valve signals transmitted from the surface, i.e. transmitted from the out of hole station 4. The hardware channel 5a is implemented using discrete components and non-programmable integrated circuits or “chips”. This means that the hardware channel is not prone to failures that could arise due to software bugs or malfunctions. The hardware channel 5a is arranged to look for characteristics in the received signals which indicate the presence of a hold valve open control signal and only give the requisite outputs to the logic gate arrangement 54 when these are detected.

[0125] In the present implementation as mentioned above, the hardware channel 5a comprises a carrier detector 52 (which may be implemented as a phase locked loop) and a tone detector 53. The carrier detector 52 looks for the presence of a signal carrier with particular characteristics. In particular in this implementation it looks for a carrier having a particular frequency or frequency range. If this frequency or frequency range is detected then the carrier detector 52 will output an appropriate signal via this output into the first AND gate 54a.

[0126] On the other hand the tone detector 53 is arranged to detect a tone which has been modulated onto the carrier. The carrier detector 52 in this embodiment also serves to demodulate the tone from the carrier. Here again this tone is one which has been modulated onto the carrier at the out of hole station 4 to represent a hold open valve signal. The tone detector 53 will only output its output into the first AND gate 54a when a tone having the predetermined frequency or in the predetermined range of frequencies is detected. In this way, via the hardware channel, a hold valve open instruction will only be determined to have been received if the appropriate carrier is detected and the appropriate tone is detected.

[0127] The output from the first AND gate 54a also feeds in as an input into the second AND gate 54b. The second AND gate 54b has two inputs. As well as receiving an input from the output of the first AND gate 54a it has an input connected to a second output of the digital signal processor receiver 56. The second AND gate 54b has an output connected to the valve V for causing reopening of the valve in appropriate circumstances.

[0128] Thus if the valve V has been closed at some point, or starts in a closed position, then provided all the hold open conditions are met as determined by the first AND gate 54a then if the digital signal processor receiver 56 outputs a reopen signal, the second AND gate 54b produces a reopen instruction signal which is fed to the valve V to cause reopening of the valve. After this has occurred the valve V is back in a normal held open position and from this point forwards whether it is held open will be controlled by the presence or absence of the hold open valve signal output by the first AND gate 54a.

[0129] Further functionality can be obtained through use of the software channel 5b. If it is determined that the valve V should be closed even though appropriate hold open signals may be being received and detected by the hardware channel 5a, the digital signal processor receiver 56 can cause closing of the valve V by deliberately ceasing output of the carrier detected signal from the digital signal processor receiver 56. This then means that not all of the necessary signals are received by the first AND gate 54a so that the first AND gate 54a ceases to output the hold valve open instruction and the valve V is caused to close.

[0130] Note that in some implementations the software channel may be done away with if there is no desire to provide additional functionality in the system. However the provision of a software channel has particular advantages as will be clear from the present description.

[0131] As mentioned above in the present embodiment the battery health monitoring system 57 also monitors the health of two batteries and only provides its necessary outputs into the first AND gate 54a when good health is determined in both of these batteries. In alternatives the battery health monitoring system could be omitted and thus at the very simplest level, the downhole station may include only the hardware channel 5a. Alternatively the battery health monitoring system 57 might be provided but only used for monitoring the health of one battery—say a battery used for the receive functionality of the downhole station or a battery used for controlling operation of the valve.

[0132] FIG. 5A schematically shows (in idealised form) a signal of the type which may be received at the downhole station having passed through the bandpass filter 51. In this instance the signal received includes a common carrier signal which has been frequency modulated to apply a tone to the carrier which represents a hold open valve signal for detection by the hardware channel 5a and has been subjected to digital amplitude modulation so as to encode signals for reception via the software channel 5b.

[0133] FIG. 5B shows a wave form output from the carrier detector 52 into the tone detector 53. Here by virtue of the function of the carrier detector 52 operating as a phase locked loop the amplitude modulation has been stripped out/ignored by the carrier detector 52 and the tone used to frequency modulate the carrier so as to represent a hold valve open signal has been demodulated from the received signal.

[0134] On the other hand the software channel 5b processes the received signal shown in FIG. 5A to extract the digital signal encoded onto the carrier by amplitude modulation. This occurs at the analogue to digital converter 55 and the resulting digital signal is fed into the digital signal processor receiver 56 for decoding and processing.

[0135] As will be appreciated this digital signal as received by the digital signal processor receiver 56 can carry whatever information is required for operation of the system.

[0136] So for example, as alluded to above, this digital signal may include an instruction to force closure of the valve V in appropriate circumstances or include an appropriate instruction for reopening the valve V in the appropriate circumstances.

[0137] Furthermore other data may be encoded in this signal.

[0138] As an example the digital signal may include an address which identifies a particular valve to which a command carried in the digital signal relates. Thus in a system of the type shown in FIGS. 2 and 3 where there are multiple downhole tools T, each with a respective downhole station 5 and a respective valve V, this address may indicate a particular one of these valves V. Thus the signals applied by the out of hole station 4 may include a command to force closure of a particular one of the valves V and this may be identified by the address included in the digital signal encoded onto the common carrier and detected by the respective digital signal processor receiver 56. In this this way an instruction to close one of the valves but leave the other valves open can be sent. Similarly an instruction to reopen a particular one of the valves may be sent with an appropriate address such that only the valve V of interest is caused to reopen.

[0139] Note that whilst mention is made of using addresses to apply individual commands the individual valves in a system including multiple valves, in general terms the hold open valve signals applied to the well installation and as detected by the hardware channel 5a will be common to all of the valves in the system. That is to say a single signal, for example a tone of a particular frequency applied to a common carrier may be applied to a well installation including multiple valves and this used to hold open all of those valves V. Then alongside this, auxiliary control signals to individual valves may be sent.

[0140] Whilst in theory it would be possible to apply individual hold open valve signals for the different valves, say by assigning a different frequency to each valve, this would lead to a more complex system and as such might be expected to be less robust. Therefore in preferred implementations, whilst addressing may be used for particular valves for the additional functionality which may be achieved via the software channel, a common hold valve open signal for processing via the hardware channel is preferably applied.

[0141] FIG. 6A schematically shows at block diagram level an implementation of the carrier detector 52 as a phase locked loop. Further FIG. 6B shows an example circuit diagram for implementing such a carrier detector using discrete components and non-programmable integrated circuits—in this case a phase locked loop integrated circuit 4046 which is commercially available from Texas Instruments.

[0142] Similarly FIG. 7 shows an example implementation of a tone detector 53 implemented using only discrete components and non-programmable integrated circuits—in this case a tone decoder integrated circuit NE567 which is commercially available from Phillips.

[0143] Note that, whilst in the present implementation frequency modulation is used to encode hold valve open signals onto the carrier and these are detected using a phase locked loop based carrier detector 52 and a tone detector 53 in the hardware channel and signals are modulated onto the carrier using digital amplitude modulation for pick up by the software channel, different signalling regimes may be used.

[0144] It is useful if the signalling regimes for the two channels are orthogonal to one another. That is to say, it is useful if the signals applied for pick up by the software channel are chosen so as to be independent from the signals which are to be picked up via the hardware channel. The use of frequency modulation in one case and amplitude modulation in the other case are examples of orthogonal signalling techniques. However there are other alternatives which might be used.

[0145] As an example, frequency shift keying as a particular type of frequency modulation may be used for encoding the hold valve open signals onto a carrier. In such a case the same type of carrier detector 52 and tone detector 53 arrangement described above may be used but the output signal at the VCO output of the carrier detector as fed into the tone detector 53 would be closer to a square wave than the sine wave form shown in FIG. 5B.

[0146] Amplitude modulation might be used to apply the hold valve open signals. In this case an envelope detector would be provided between a phase locked loop and a tone detector in the hardware channel.

[0147] In general M-ARY modulation methods may be used where the modulation selected for encoding the hold valve open signals is orthogonal to the modulation selected for encoding the auxiliary signals to be received via the software channel.

[0148] The out of hole station 4 may be arranged to accept an input from a platform's ESD system (emergency shutdown system) such that when an ESD trip occurs it shall result in automatic valve closure. This might be implemented by ceasing to transmit the hold open signals so that these cease to be detected by the hardware channel. Alternatively (or in addition) the instruction to close the valve V can be sent via the software channel.

[0149] While systems of the above type might be implemented in a full duplex or half duplex system such that the downhole station 5 is able to communicate back towards the out of hole station 4, in at least some implementations it is preferred to implement the system as a simplex system. That is to say where the surface transceiver 41 in fact is only a transmitter and the downhole station 5 is arranged to operate only as a receiver. This may help lead to a more robust system.

[0150] Where the current implementation is used including a carrier detector 52 implemented using a phase locked loop, the carrier detector 52 has a natural time constant determined by the values of the components used in setting up the circuit as shown in FIG. 6B. This time constant may be selected to have a certain period. This means that if an expected signal is lost there will be a predetermined period which will pass before the signal is seen as being lost. That is to say the output of the carrier detector 52 will not immediately cease as soon as the absence of the expected carrier is seen at the downhole station 5 but rather cease after a time which is determined by the time constant. This time constant may be selected to avoid unwanted and unnecessary closures of the valve V if there is a very brief absence of signal or very brief drop off in the strength of the signal received at the downhole station 5.

[0151] Because there may be this finite delay in the valve V being caused to close by the failure of reception of the hold open valve signals, in emergency situations it may be preferable to send (or also send) a close immediately signal via the signals to be picked up by the software channel in at least some circumstances. This may lead to initiation of closing of the valve more quickly than relying on ceasing of the hold open valve signals.

[0152] Note also that time constants in the hardware channel will likely be affected by temperature. Taking this into account a longer than optimal time constant (at a mean or normal temperature) may be chosen in the hardware channel to avoid unwanted valve closures at extremes of temperature.

[0153] Signals

[0154] The electrical signals refererred to in this specification may also be termed Electromagnetic (EM) (sometimes referred to as Quasi-Static (QS)) wireless signals. In at least the downhole environment, communication using such signals is normally in the frequency bands of: (selected based on propagation characteristics) sub-ELF (extremely low frequency) <3 Hz (normally above 0.01 Hz);

[0155] ELF 3 Hz to 30 Hz;

[0156] SLF(super low frequency) 30 Hz to 300 Hz;

[0157] ULF (ultra low frequency) 300 Hz to 3 kHz; and,

[0158] VLF (very low frequency) 3 kHz to 30 kHz.

[0159] Sub-ELF to SLF are particular suited to communications in the current system. Preferrably signals will be 1000 Hz or below, more preferably 100 Hz or below. The nomenclature used for these ranges is defined by the International Telecommunication Union (ITU).

[0160] To control and direct current advantageously, a number of different techniques may be used. For example one or more of: use of an insulating coating or spacers on well tubulars; selection of well control fluids or cements within or outwith tubulars to electrically conduct with or insulate tubulars; use of a toroid of high magnetic permeability to create inductance and hence an impedance; use of an insulated wire, cable or insulated elongate conductor for part of the transmission path or any antenna.

[0161] Various means for receiving a transmitted signal can be used, these may include detection of a current flow; detection of a potential difference; use of a dipole antenna; use of a coil antenna; use of a toroidal transformer; use of a Hall effect or similar magnetic field detector; use of sections of the well metalwork as part of a dipole antenna.

[0162] Where the phrase “elongate conductor” is used, for the purposes of EM communication, this could mean any elongate electrical conductor including: liner; casing; tubing or tubular; coil tubing; sucker rod; wireline; drill pipe; slickline or coiled rod.