A well installation includes a metallic structure having downhole metallic tubing for conveying product towards the surface, a sub surface safety valve disposed in the downhole metallic tubing for interrupting flow of product towards the surface when closed and allowing flow of product towards the surface when open. Also, valve control means are disposed downhole and arranged for detecting electrical signals in the downhole metallic tubing and for holding the valve open while receiving an electrical signal, and surface signaling means for applying an electrical signal to metallic structure of the well for transmission via the downhole metallic tubing towards the valve control means so as to hold the valve open. The system can be made more robust by ensuring that the control means will only hold the valve open while receiving a signal of given characteristics to help prevent stray currents erroneously holding the valve open.

CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage of PCT International Application No. PCT/GB2009/001499, filed on Jun. 16, 2009, and published in English on Dec. 23, 2009, as WO 2009/153551 A1, and which claims priority to GB 0811223.7 filed on Jun. 18, 2008 and GB 0811224.5 filed on Jun. 18, 2008, the entire disclosures of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     This invention relates to the control of sub surface safety valves. 
     BACKGROUND OF THE INVENTION 
     Sub surface safety valves are provided in oil and gas wells to shut off the flow of product from the formation towards the surface in situations where continuing flow of product could be dangerous. It is now universal practice that all producing wells should include a functioning sub surface safety valve. This means that if a sub surface safety valve fails and it is uneconomic to work over the well installation and provide a new valve, the whole well will have to be shut down. 
     Typically sub surface safety valves are operated via hydraulic control lines from the surface of the well. They are generally arranged to be fail safe such that if the supply of hydraulic fluid is interrupted to the valve, the valve will close. It is of course possible for sub surface safety valves to fail in use or for there to be some damage to the control line. 
     At least some sub surface safety valves are installed as tubing retreivable safety valves such that if the valve itself fails it may be withdrawn from the well without decommissioning the whole well and replaced by a new valve. In such circumstances the new valve will automatically connect into the control line ports to allow control of the new valve. Even with such a system in place, if there is some problem with the control line, a situation can be reached where it is not possible to conventionally provide a functioning sub surface safety valve and the whole well may need to be shut down at least for work over. 
     The present invention is directed at providing alternative solutions. 
     BRIEF SUMMARY OF THE INVENTION 
     It would of course be desirable to have a system where it was possible to avoid the decommissioning of a well due to failure of a sub surface safety valve. Such a system might be one where the chance of this happening is minimised as far as possible from day one, or a solution where it is possible to retrofit a valve in a well where an existing hydraulically controlled sub surface safety valve has failed. 
     Of course any system provided should preferably be very robust so that almost whatever occurs, the sub surface safety valve will be closable at appropriate times and there should preferably be a way to effect such closing of the valve from the surface when this is desired. 
     According to one aspect of the present invention there is provided a well installation comprising metallic structure including downhole metallic tubing for conveying product towards the surface, a sub surface safety valve disposed in the downhole metallic tubing for interrupting flow of product towards the surface when closed and allowing flow of product towards the surface when open, valve control means disposed downhole and arranged for detecting electrical signals in the downhole metallic tubing and for holding the valve open whilst receiving an electrical signal, and surface signalling means for applying an electrical signal to metallic structure of the well for transmission via the downhole metallic tubing towards the valve control means so as to hold the valve open. 
     According to another aspect of the present invention there is provided a method of controlling a sub surface safety valve in a well installation which comprises metallic structure including downhole metallic tubing for conveying product towards the surface, the sub surface safety valve being disposed in the downhole metallic tubing for interrupting flow of product towards the surface when closed and allowing flow of product towards the surface when open, the method comprising the steps of: 
     applying an electrical signal to metallic structure of the well for transmission via the downhole metallic tubing towards the valve; detecting electrical signals in the downhole metallic tubing in the region of the valve; and holding the valve open whilst an electrical signal is detected. 
     This can allow remote control of the valve, and more particularly allow remote powering of the control from the surface using electrical energy. That is to say, the transmission of signals downhole into the well can in effect be the transmission of power downhole to control the valve. 
     The installation/method can be such that there is no power source downhole for controlling operation of the valve—the power being supplied from the surface. 
     The electrical signal may be a dc signal. The surface signalling means may be arranged to apply a dc signal. The surface signalling means may be arranged to apply a signal which is substantially constant. 
     The electrical signal may be an ac signal. The surface signalling means may be arranged to apply an ac signal. The valve control means may be arranged for detecting a dc signal and/or an ac signal. 
     The valve control means may be arranged for holding the valve open whilst receiving an electrical signal having a predetermined characteristic. The method may comprise the step of holding the valve open whilst receiving an electrical signal having a predetermined characteristic. 
     The predetermined characteristic may be frequency. The valve control means may comprise a filter for filtering out signals having unwanted frequencies. The method may comprise the step of filtering out signals having unwanted frequencies. 
     The valve can be a latch valve in which the valve is held open by a latch which is controlled by the valve control means. The control means may be arranged to hold the latch in a latching position whilst the electrical signal is received and to cause or allow the latch to move to a release position in the absence of such a signal to allow the valve to close. 
     The valve may be biased towards a closed position. The valve may be arranged for being returned to an open position from a closed position by the application of pressurised fluid from the surface of the well down the tubing. Thus, for example, the power delivered downhole to control the valve may be insufficient to allow the electrical power to be used to move the valve from a closed to an open state but in many circumstances this does not matter. The fluid might be gas, liquid or a mixture of gas and liquid. 
     The valve control means may have a spaced pair of contacts for contacting with the downhole metallic tubing to facilitate detection of signals. 
     Each contact may comprise a plurality of teeth for biting into the metal of the metallic tubing to improve electrical connection. Each contact may comprise a setting means which is such as to set the control means into position in the metallic tubing and hold it against movement relative to the tubing. 
     The valve control means may comprise a solenoid for holding the valve open. The solenoid may control the latch. 
     The solenoid may be fed with current directly tapped from the metallic tubing. The solenoid may be connected between the spaced contacts. 
     Impedance generation means may be provided around the tubing in the region of the valve for generating a local electrical impedance in the tubing across which signals can be detected. The impedance generation means may be located so that the local electrical impedance is generated between the spaced contacts. In effect the local impedance can force more of the available current into the control means for use in holding open the valve. 
     The impedance generation means may be arranged to electromagnetically generate the local electrical impedance. The impedance generation means may be arranged so as to not impair the structural integrity of the tubing with which it is used. The impedance generation means may be structurally distinct from the tubing with which it is used. The impedance generation means may be arranged to generate the local electrical impedance in the tubing without modifying the dimensions or materials of the tubing in that region. 
     The impedance generation means may be arranged to be mounted on the tubing portion. The impedance generation means may be arranged to be mounted around the tubing portion. The impedance generation means may be arranged so that tubing may run uninterruptedly through or past the impedance generation means when the apparatus is installed and in use. 
     The impedance generation means may be arranged so that the value of the local electrical impedance is dependent on frequency. The impedance generation means may be arranged so that the value of the local electrical impedance is dependent on frequency and will exhibit a maximum within a predetermined range of frequencies. This means that the impedance generation means may be constructed so as to generate a relatively high impedance to signals within a chosen frequency range and a lower impedance to signals outside this range. 
     Preferably the impedance generation means is arranged to generate an impedance which is tuned or tuneable to a chosen frequency of signals to be seen by the local impedance. 
     This arrangement can force a significant portion of the electrical power flowing in the tubing into the valve control means for control of the valve. Of course if the situation allows the introduction of an insulation joint in the tubing in the region of the valve, signals may be extracted across that—with almost 100% of the power, in the tubing concerned, at that location being extractable. 
     The impedance generation means may comprise a generally toroidal portion of magnetic material for surrounding the tubing portion. A winding may be provided on the toroidal portion of magnetic material. This can allow the tubing portion to act as a single turn winding in a transformer also comprising the toroidal portion of magnetic material and said winding provided on the toroidal portion of magnetic material. 
     Here it is to be understood that word toroidal is used in a broad way to refer to any ring like shape that can encircle a length of tubing—it is not relevant what shape the ring adopts nor is it relevant what shape a cross-section through the material of the ring has. 
     The winding may be connected to at least one impedance component. The at least one impedance component may be chosen so that the impedance seen in a tubing portion passing through the toroidal portion of magnetic material varies with frequency. The impedance component may comprise a capacitor connected in series with the winding. 
     The provision of impedance generation means is more likely to be useful where the present equipment can be completion conveyed than where there is a requirement to retrofit the equipment. 
     The valve control means may comprise a solenoid connected between the spaced contacts for controlling the valve. A capacitor may be connected in parallel with the solenoid. 
     The valve control means may comprise a transformer with a first winding for receiving signals tapped from the metallic tubing and a second winding for feeding signals to a component for controlling the valve, such as a solenoid. 
     The first winding may be connected between the spaced contacts. An impedance component may be connected to the first winding to give a desired detection frequency characteristic. A capacitor may be connected in parallel with the first winding. The capacitor and first and second winding may be chosen so as to be tuned to the frequency of the signals to be extracted from the tubing. The number of turns on the windings can be chosen to help give the desired performance. Generally the first winding will have more turns than the second to help achieve impedance machining with the component used to control the valve. 
     Such arrangements may be used together with impedance generation means. 
     According to another aspect of the present invention there is provided a valve arrangement comprising a subsurface safety valve for location in downhole metallic tubing for interrupting flow of product towards the surface when closed and allowing flow of product towards the surface when open and valve control means for detecting electrical signals in the downhole metallic tubing and for holding the valve open whilst receiving an electrical signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  schematically shows a well installation including a sub surface safety valve and associated control unit; 
         FIG. 2  schematically shows in more detail the control unit of the installation shown in  FIG. 1 ; 
         FIG. 3  schematically shows an alternative sub surface safety valve control arrangement; and 
         FIGS. 4A to 4C  schematically show a valve, of a type which may be used in the set ups shown in  FIGS. 1 to 3 , in a closed position, a reset position and an open position. 
         FIG. 5  schematically shows an embodiment of a spaced pair of contacts relative to a production tubing. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically shows a well installation comprising downhole metallic tubing  1  comprising casing  11  and within this, production tubing  12 . A sub surface safety valve  2  is provided downhole in the well within the production tubing  12 . This sub surface safety valve  2  is provided for permitting the flow of product, that is oil and/or gas, towards the surface during normal circumstances but obstructing such flow when circumstances necessitate this. A sub surface safety valve control unit  3  is provided downhole in the region of the sub surface safety valve  2 . 
     A power signal transmitter  4  is provided at the surface and is connected to the downhole metallic tubing  1 . In practical terms it makes little difference whether the transmitter  4  is connected to the casing  11  or production tubing  12 , or even the well head, because in general terms the casing  11  and production tubing  12  will be in contact with one and other at many locations within the well and thus they tend to act as a single conduction path. 
     Another terminal of the transmitter  4  is connected to ground and thus valve controlling signals may be applied to the metallic tubing  1  for transmission downhole towards the valve control module  3 . In some implementations the connections to ground might be via another, nearby, well. 
     The valve control module  3  has a spaced pair of contacts  31  which contact with the downhole metallic tubing  1 , in particular, with the production tubing  12  in this embodiment. Although not shown in detail in the drawings, in a set up of the type shown in  FIG. 1 , the valve control unit  3  may be completion conveyed with the production tubing  12  for example as a mandrel tool which fits around the production tubing  12 . As such very good electrical connection can be easily achieved between the valve control module  3  and the production tubing  12 . 
     In operation as the transmitter  4  transmits signals into the metallic tubing  1 , a resulting current i will flow in the tubing  11 ,  12 . Where this current i meets the contacts  31   a  proportion of the current .DELTA.i will flow through the valve control module  3 . This proportion of the current .DELTA.i can be used to control operation of the valve  2 . 
     In the present embodiment no power source is provided downhole for control of the valve  2 . Rather power is extracted directly from the signals i applied to the metallic tubing  11 ,  12  by the transmitter  4 . Thus there is direct power supply from the surface for controlling the valve  2 . The valve control module  3  is arranged so that provided signals having an appropriate frequency are received from the metallic structure  11 ,  12 , the valve  2  will be held open. 
     On the other hand if such signals cease, the valve will be allowed to close. 
     Of course such ceasing of the signals i may be as the result of a deliberate act of stopping transmission from the surface transmitter  4  or may be due to more unexpected circumstances such as a failure of the transmitter  4  or some larger scale failure of the well installation as a whole. 
     The valve control module  3  includes some capability to discriminate between signals having the correct characteristic showing them to be from the transmitter  4  and other signals which might be in the metallic structure of the well e.g. noise. To put this another way, the valve control module  3  has filtering capabilities. Thus if signals are picked up having the incorrect frequency these will not be sufficient to hold the valve  2  open in the absence of signals i from the surface transmitter  4 . 
       FIG. 2  shows more detail of the valve control module  3  of the well installation shown in  FIG. 1 . In this embodiment the valve control module  3  comprises impedance generation means  32  disposed in the region of the valve  2 . The impedance generation means  32  is arranged for generating a local electrical impedance in the production tubing  12  in the region of the valve  2 . The impedance generation means comprises a generally toroidal piece of magnetic material  32   a  around which is wound a winding  32   b  which is connected in series with a capacitor  32   c . The winding  32   b , magnetic material  32   a  and the production tubing  12  passing through the toroid of magnetic material  32   a  act as a transformer with the production tubing  12  acting as a single turn winding. The magnetic material  32   a , winding  32   b  and capacitor  32   c  are chosen so that a significant electrical impedance is seen by electrical signals flowing in the production tubing as it passes through the toroid  32   a . More particularly the value of the capacitor and number of turns of the winding  32   b  are chosen so that, in effect, the impedance generation means  32  is tuned to the frequency of the signals i transmitted by the transmitter  4 . That is to say, the impedance generation means  32  is arranged so that there is resonance at the signal frequency of the transmitter  4  generating a high electrical impedance in the tubing  12  at that location to signals having the signal frequency. 
     In order to get good transmission characteristics through the metallic structure  11 ,  12  of the well, the transmitter  4  will be arranged to transmit at very low frequencies for example at 10 Hz or below. Thus the capacitor must be chosen to have a value which is appropriate for generating the high impedance in the tubing  12  at such very low frequencies. The provision of a multi-turn winding around the toroidal core  32   a  whilst the production tubing  12  is a single turn winding helps to decrease the capacitance value required to produce a usefully increased impedance in the production tubing in the region of the impedance generation means  32 . 
     It has been found by the applicant that it is possible to generate an impedance of in the order of 50 m.OMEGA. and, whilst in absolute terms this is not a high electrical impedance, compared with the impedance of a section of production tubing without the presence of such impedance generation means there is vast difference. Thus such an arrangement helps to drive more current .DELTA.i through the valve control module  3  so that this current i is available for control of the valve  2 . 
     Of course if circumstances were to allow, an insulation joint might be provided in the tubing  12  in the region of the control module  3 , but in many circumstances the provision of such an insulation joint is impossible or highly undesirable. 
     The control module  3  also comprises a solenoid  33  which is connected in series between the spaced contacts  31  and receives the current .DELTA.i which is caused to flow through the valve control module  3 . This solenoid  33  is used to directly control the operation of the valve  2 , in particular to hold the valve to open whilst there is sufficient current .DELTA.i flowing through the solenoid  33 . 
     It will be appreciated that the impedance generation means  32  provides a filtering function in that the impendance generated in the production tubing  12 , is tuned to a particular frequency and thus it is this frequency which will be driven through the valve control module  3 . Other frequencies which are substantially unaffected by the impedance generation means  32  will continue to flow almost entirely through the production tubing  12  and thus in normal circumstances would be insufficient to provide sufficient current in the solenoid  3  to hold the valve  2  to open. Of course in alternatives further filtering means may be provided in the valve control module  3  to block out frequencies outside of a desired range. 
     Insulation  34  may be provided on the outer surface of the production tubing  12  between the spaced contacts  31  to insulate against electrical contact between the production tubing  12  and casing  11  in that region. 
       FIG. 3  shows in schematic form an alternative form of valve control module  3 . This form of valve control module  3  is one which is more suited to a retro fitting operation where the provision of an insulation joint or impedance generation means  32  to affect the production string  12  is not possible. 
     Such a control module  3  can be used as part of a replacement valve arrangement comprising a valve  2  along with the control module  3 . The valve arrangement can be introduced into the production tubing of the well in place of a tubing retrievable valve unit that has ceased to function and been removed. 
     Thus, in this case the valve control module  3  is provided in a tool which is housed within the production tubing  12 . It again has a spaced pair of contacts  31  which in the embodiment shown in  FIG. 5  will take the form of a spaced pair setting devices each of which comprises a plurality of teeth which are arranged to bite into the metal of the production tubing  12  in order to make good mechanical and electrical contact. Here again the objective is to try to encourage as much current .DELTA.i from the signaling current i in the metallic structure  11 ,  12  to flow through the valve control module  3 . 
     Here a solenoid winding  33  is connected in parallel with a capacitor  35  and these two components are connected in series between the spaced contacts  31 . The solenoid winding  33  is provided with a magnetic core  36  and arranged to control the valve  2 . That is to say, the solenoid  33 ,  36  is arranged to hold the valve to open whilst sufficient current .DELTA.i flows through the solenoid  33 . Again here the capacitor  35  is chosen in combination with the characteristics of the solenoid winding  33  in order to promote maximum current flow through the solenoid  33  at the signalling frequency of the applied signals i. 
     Again of course additional filtering may be provided if necessary. 
     In an alternative implementation a substantially non-varying dc signal may be applied at the surface by a transmitter corresponding to that  4  shown in  FIG. 1 . In such a case a resulting dc current will flow in the metallic structure  11 ,  12  which can be “scavenged” for use in holding open the valve  2 . With a dc implementation the valve control module  3  will be similar to that shown in  FIG. 3  but with the capacitor  35  omitted as ‘tuning’ will not be relevant. The scavenged current will flow in the solenoid winding  33  to hold open the valve  2 . Using dc gives good signal transmission characteristics but has the disadvantage of not being as helpful in applying differentiation between bona fide and fault/random signals. 
       FIGS. 4A to 4C  show more detail of one type of valve  2  which may be used in the arrangements such as those shown in  FIGS. 1 to 3 . This is a latching poppet valve which can be moved from the closed state to the open state by the application of pressure from the surface. 
     The valve  2  is shown in the closed state in  FIG. 4A , in a transition state in  FIG. 4B  and in the open state in  FIG. 4C . In the open state product P can flow through an opening  21  and into the main body  22  of the valve  2  and through and out of the valve  2  for onwards transmission towards the surface. 
     The valve comprises a main piston  23  which is arranged for axial movement within the main body  22  to selectively obscure the opening  22  and thus block the path of product through the valve  2 . The main piston  23  is shown in the obstructing position in  FIG. 4A . The main piston  23  is biased towards this obstructing position by a spring  24  but is held open against the action of this spring  24  by a latch  25  when the valve is in the open position as shown in  FIG. 4C . 
     Thus in the open position as shown in  FIG. 4C  the latch  25  is holding the piston  23  against the action of the spring  24 . This means that to hold the valve in the open position shown in  FIG. 4C  no great amount of energy is required. Thus it will be seen how a relatively low energy solenoid  33  in the arrangement shown in  FIGS. 2 and 3  can be used to hold this latch  25  in the latching position and thus hold the valve  2  open during normal use. 
     It will also be appreciated that the valve control modules  3  are arranged so that when power to the solenoid  33  is ceased the latch  25  can move out of engagement with the main piston  23  such that the main piston  23  may be forced by the spring  24  into the closed position shown in  FIG. 4A . 
     The valve  2  also comprises an auxiliary piston  26  which is shown in a rest position in  FIGS. 4A and 4C . It is held in this rest position by a spring  27 . However if the valve  2  is overpressured from the surface then, as shown in  FIG. 4B , this pressure acts on the auxiliary piston  26  driving it downwards against the main piston  23 . Together, the auxiliary piston  26  and main piston  23  move downwards from the position shown in  FIG. 4A  to the position shown in  FIG. 4B . This means that the main piston  23  may be relatched with the latch  25  to return it to the open position as shown in  FIG. 4C . Once the overpressure from the surface is removed, the auxiliary piston  26  can be driven back to its rest position by the spring  27  so that the valve returns so its open state as shown in  FIG. 4C . 
     The details of this valve  2  as shown in  FIGS. 4A to 4C  are included just by way of example to illustrate the type of valve which might be used with the direct surface powered control systems as shown in  FIGS. 1 to 3 . Useful aspects of this valve  2  are that very low power is required to hold the valve  2  in the open position and no downhole power is required to reopen the valve from a closed position as this may be achieved by applying overpressure from the surface. The present control systems may be used with other similar type valves  2  which are commercially available for use in the oil and gas industry.