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FIELD OF THE INVENTION 
     The present invention relates to an improved control system in a subterranean well. Particularly, but not exclusively the present invention relates to improved control system for controlling a plurality of tools, equipment and apparatus which are positioned in a subterranean well. 
     BACKGROUND TO THE INVENTION 
     Directional drilling has made the extraction of hydrocarbons from small reservoirs economically viable because the borehole can be directed in three dimensions through a number of pockets of hydrocarbons. 
     The hydrocarbons contained in each of these reservoirs flows through a production tube to the surface. Balanced fluid or optimised flow regimes are designed to intend to get the flow from the reservoirs to the surface as quickly as possible and maximise the amount of hydrocarbons extracted from each reservoir. These flow regimes may dictate that the different reservoirs be emptied at different times. The flow of hydrocarbons from a reservoir into the production tube is controlled using downhole tools such as valves. Downhole valves are, generally speaking, hydraulically controlled. 
     Hydraulic systems are used to control the operation of tools positioned in the well and can comprise surface equipment such as a hydraulic tank, pump etc and control lines for connecting the surface equipment to the downhole tools. The control lines can be connected to one or more downhole tools. 
     Several basic arrangements of hydraulic control lines are used in a well. In a direct hydraulic arrangement, each tool that is to be controlled will have two dedicated hydraulic lines. The “open” line extends from the surface equipment to the tool and is used for transporting hydraulic fluid to the downhole control valve to operated the tool, while the “close” line extends from the tool to the surface equipment and provides a path for returning hydraulic fluid to the surface. The practical limit to the number of tools that can be controlled using the direct hydraulic arrangement is three, that is six separate hydraulic lines, due to the physical restraints in positioning hydraulic lines in a well. The tubing hanger through which the hydraulic lines run also has to accommodate lines for a gauge system, at least one safety valve and often a chemical injection line, which limits the number of hydraulic lines the hanger can accommodate. 
     When it is desirable to control more than three tools in a well, a common close arrangement can be employed in which an open line is run to each tool to be controlled and a common close line is connected to each tool to return hydraulic fluid to the surface. The common close system has a practical limit of controlling five tools through the six separate hydraulic lines. 
     In another arrangement, a single hydraulic line is dedicated to each tool and is connected to each tool via a separate, dedicated controller for each tool. To open the tool, the hydraulic fluid in the dedicated line is pressurised to a first level. Thereafter, the hydraulic fluid in the dedicated line is pressurised to a higher level so as to close the tool. 
     In a digital hydraulics system, two hydraulic lines are run from the surface equipment to a downhole controller that is connected to each of the tools to be controlled. Each controller is programmed to operate upon receiving a distinct sequence of pressure pulses received through these two hydraulic lines. Each tool has another hydraulic line is connected thereto as a common return for hydraulic fluid to the surface. The controllers employed in the single line and the digital hydraulics arrangements are complex devices incorporating numerous elastomeric seals and springs, which are subject to failure. In addition, these controllers used small, inline filters to remove particles from the hydraulic fluid that might otherwise contaminate the controllers. These filters are prone to clogging and collapsing. Further, the complex nature of the pressure sequences requires a computer operated pump and valve manifold, which is expensive. 
     An alternative, simpler arrangement which can be used to operate a large number of tools has been proposed utilising RFID tags to activate downhole tools. The RFID tags are programmed with a message for a specific downhole tool. The tag is sent down a control line which runs adjacent the tools. The control line includes a tag reader for each downhole tool, each reader reading the message on the tag as it passes. When the reader associated with the tool the message is intended for reads the tag, the message is relayed to the tool control and the instruction is carried out. The instruction may be to open a valve to allow hydrocarbons to flow into the production tube. Such a system requires a common open line running to all tools, a common close line running to all tools and a tag line down which the RFID tags can be flowed down. 
     The drawback of such a system is the requirement for power to be continuously supplied to the readers to detect the presence of a tag and then to provide power to the control system to actuate the specific tool. The power is generally provided by batteries. As these batteries are continually supplying power the downhole readers, they can be drained over a period of 2 to 3 weeks and require replacement which can be an extremely expensive and time consuming process. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided an improved control system for use in a subterranean well, the system comprising: 
     at least one apparatus positioned within the subterranean well; 
     at least one power generation device positioned within the subterranean well, the at least one power generation device adapted to supply electrical power to the at least one apparatus; and 
     at least one control line positioned in the subterranean well, the at least one control line adapted to supply a hydraulic pressure applied from surface to the at least one power generation device from which the at least one power generation device generates electrical power. 
     In one embodiment, the present invention provides a control system for use in a subterranean well which includes a power generation device, which generates electrical power in response to the application of hydraulic pressure from surface. As electrical power can be generated by the power generation device as and when required, the downhole life of such a system is extended. 
     The/each power generation device may be adapted to supply electrical power to more than one downhole apparatus. In one embodiment a power generation device may power an RFID tag reader and a downhole tool such as a valve. 
     The/each power generation device may be adapted to supply electrical power to an energy storage device such as a battery, a capacitor, a spring, a compressed fluid device such as a gas spring or the like. 
     In an alternative embodiment, the/each power generation device may be adapted to supply electrical power to a drive means to raise a weight against gravity. Energy would be stored in such a device, which can be harnessed by allowing the weight to fall under the influence of gravity. 
     In one embodiment, the power generation device converts the applied hydraulic pressure in to linear motion. 
     Preferably, the power generation device comprises a piston to convert the applied hydraulic pressure in to linear motion. 
     In one embodiment, the power generation device is further adapted to convert the linear motion into rotary motion. The power generation device may include a ball screw or rack and pinion for this purpose. 
     In an alternative embodiment, the power generation device is adapted to convert the applied hydraulic pressure in to rotary motion. 
     Preferably, the power generation device is adapted to convert rotary motion to electrical power. The power generation device may include a generator for this purpose. The generator may be a dynamo. A dynamo can generate AC or DC power. 
     In one embodiment, in which the power generation device produces AC power, the control system further comprises a rectifier or switch mode regulator. A rectifier or switch mode regulator converts an AC input into a DC output. 
     The power generation device may include a biasing means adapted to resist the application of hydraulic pressure. 
     In one embodiment in which the power generation device converts the applied hydraulic pressure in to linear motion using a piston, the piston is moveable between a first position and a second position and comprises a biasing means to bias the piston to the first position. In this embodiment, the hydraulic pressure moves the piston against the biasing means to the second position, generating linear motion. Once the applied hydraulic pressure is removed the biasing means returns the piston to the first position generating further linear motion which is, in turn, converted into electrical power. 
     The biasing means may comprise a compression spring, a wind up spring, a coil spring, a leaf spring, a gas spring, well pressure, a suspended weight or the like. 
     Alternatively, downhole pressure could be utilised to provide the biasing means or to return the piston to the first position. 
     In a further alternative, a second control line may be provided in the well to provide the biasing means or to return the piston to the first position. 
     According to a second aspect of the present invention there is provided a method of controlling at least one apparatus positioned within a subterranean well, the method comprising the steps of: 
     applying a hydraulic pressure from surface to a power generation device, the power generation device adapted to convert the applied force into electrical energy, the electrical energy being used to control at least one apparatus positioned within the subterranean well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described with reference to the accompanying drawings in which: 
         FIG. 1  is a section view through a subterranean well showing a control system according to a first embodiment of the present invention; 
         FIG. 2  is a schematic of the control system of  FIG. 1 ; 
         FIG. 3  is a schematic of the power generation device of the system of  FIG. 1 ; 
         FIG. 4  is a schematic of a control system according to a second embodiment of the present invention; 
         FIG. 5  is a schematic of a control system according to a third embodiment of the present invention; and 
         FIG. 6  is a schematic of the power generation device of the system of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is made to  FIG. 1 , a schematic of a control system, generally indicated by reference numeral  10 , according to a first embodiment of the invention. 
     The control system  10  controls the flow of hydrocarbons from each of four hydrocarbon reservoirs  12   a - d  into a production tube  14  which is disposed within a subterranean well  16 , the production tube  14  extending from the reservoirs  12   a - d  up to an oil rig  18 . Specifically, the control system  10  controls four downhole tools  20   a - d  which permit the hydrocarbons from reservoirs  12   a - d  respectively to flow into the production tube  14 . 
     Referring now to  FIG. 2 , a schematic of the control system  10  of  FIG. 1  is shown. The control system  10  controls each of the four downhole tools by selectively allowing each tool  20   a - d  to be exposed to hydraulic pressure applied through a first hydraulic line  22  and/or a second hydraulic line  24 . 
     The control system  10  comprises four control system units  26   a - d . Each control system unit  26   a - d  comprises a corresponding power generation device  28   a - d , each power generation device  28   a - d  adapted to supply electrical power to two apparatus; a corresponding needle valve  30   a - d  and a corresponding RFID tag reader  32   a - d.    
     The control system  10  further comprises a control line  34  which supplies hydraulic pressure from the rig  18  to each of the power generation devices  28   a - d . The third control line  34  includes a valve  33  which can be closed from surface to allow for hydraulic pressure to be built up in the third control line  34 . As will be discussed, each power generation device  28   a - d  is adapted to generate power from the applied hydraulic pressure, the generated power being used to operate the corresponding needle valve  30   a - d  and/or the corresponding RFID tag reader  32   a - d.    
     The power generation device  28  shown in  FIG. 3  may represent any one of the power generation devices  28   a - d . The power generation device  28  comprises a piston  40  in a housing  42 . The piston  40  is shown in  FIG. 3  located in a first position to which it is biased by a compression spring  44 . 
     The piston  40  is connected to a ball screw device  46  for converting linear motion of the piston  40  into rotary motion. The rotary motion is transferred by a transfer rod  48  to a generator  50 . The generator  50  is connected to a rectifier  52  which produces a direct current, which is supplied to the needle valve (not shown) by a first wire  54  and to the RFID tag reader (not shown) by a second wire  56 . 
     To operate the power generation device  28 , the third control line valve  33  is closed and hydraulic pressure is applied through the third control line  34 , to the piston  40 . The application of pressure moves the piston  40  towards the ballscrew  46 , against the bias of the compression spring  44  generating electrical power through the generator  50  and rectifier  52  for supply to the needle valve (not shown) and RFID tag reader (not shown). 
     Once the piston  40  has reached the extent of its travel the hydraulic pressure in the third control line  34  is released by opening the third control line valve  33 , allowing the piston  40  to travel back to the first position. During this return travel more electrical power is generated which the rectifier  52  converts to direct current for supply to the needle valve (not shown) and the RFID tag reader (not shown). 
     Referring back to  FIG. 2 , the operation of the control system  10  will now be described. The objective of the control system  10  is to allow one of the tools  20   a - d  to be operated by exposure to hydraulic pressure through one of the first or second control lines  22 ,  24 . 
     In this example, an RFID tag (not shown) is to be sent from the rig  18  with an instruction to operate the third tool  20   c . The third tool  20   c  is to be operated by opening the third needle valve  30   c  permitting a hydraulic pressure applied by the first control line  22  to be released by activating the tool  20   c.    
     The first step of this operation is to apply a hydraulic pressure to the third control line  34  to generate power, through the power generation devices  28   a - d  to, initially, operate the RFID tag readers  32   a - d , and apply a hydraulic pressure through the first hydraulic line  22  to operate the tool  20   c . The tool  20   c  is prevented from operating by the needle valve  30   c  which is closed and is containing the pressure. 
     Once the pistons  40  have reached the extent of their travel the pressure in the third control line  34  is reduced by opening the third control line valve  33 , permitting the pistons  40  to return to their start positions and generate further power. Once the readers  32   a - d  are operational and the third control line valve  33  is open, RFID tags containing the message to operate the third tool  20   c  are sent down the third control line  34 . 
     The tag flows down the third control line  34  passing through the four tag readers  32   a - d . The first, second and fourth readers  32   a,b,d  will ignore the message on the tag but the third reader  32   c  will transfer the message to the needle valve  30   c . Using power generated by the third power generation device  28   c , the needle valve  30   c  opens, releasing the hydraulic pressure in the first hydraulic line  22  permitting the tool  20   c  to operate. 
     Reference is now made to  FIG. 4 , a schematic of a control system  110  according to a second embodiment of the present invention. This system  110  includes first and second control lines  122 ,  124  and is largely similar to the system  10  of the first embodiment, the difference being that each power generation device  128   a - d  is operated by the application of hydraulic pressure through the second control line  124 . The operation of the system  110  is otherwise the same. 
     Reference is now made to  FIG. 5 , a schematic of a control system  210  according to a third embodiment of the present invention. This system is largely similar to the system  110  of the second embodiment, the difference being that the power generation devices  228   a - d  are connected to both the first and second control lines  222 ,  224 . to the power generation device  228  shown in  FIG. 6  may represent any one of the power generation devices  228   a - d . From  FIG. 6 , it can be seen that the first and second control lines  222 ,  224  are fed to either side of the piston  240 . As can be seen from  FIG. 6 , there is no biasing spring in the housing  242 , the piston  240  being moved to the left by application of hydraulic pressure through second line  224 , and returned to the start position by the application of pressure through the first hydraulic line  222 . 
     Various modifications and improvements may be made to the above described embodiments without departing from the scope of the invention. For example, each power generation device may supply power to a battery or other energy storage device for storage until required.

Summary:
A control system for use in a subterranean well comprises at least one power generation device positioned within the subterranean well, the at least one power generation device adapted to supply electrical power to at least one apparatus positioned within the subterranean well and at least one control line positioned in the subterranean well. The at least one control line connects each power generation device to surface and is adapted to supply a hydraulic pressure applied from surface to the at least one power generation device from which the at least one power generation device generates the electrical power to be supplied to the at least one apparatus.