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CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of PCT Application PCT/GB2011/001508 filed Oct. 20, 2011, which claims priority to Great Britain application 1017914.1 filed Oct. 20, 2010, each of which are incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    Wellbores for accessing oil and gas reserves are typically provided with one or more valves or pairs of valves for restricting the wellbore. These valves are used to resist the flow of fluids in the bore and may ultimately be used to close the wellbore, such as BOPs. The valves are used to prevent undesirable exposure of pressurised fluids during the drilling or operation of a well. For example, in a downhole intervention operation, wireline valves allow the well operator to insert and remove tools deployed on coiled tubing or wireline within a wellbore while maintaining pressure in the well. 
         [0003]    Wireline valves are intended to stop the flow of a fluid through a tubular or to seal an annular space between two tubulars. Different types of wireline valves, such as annular or blind, are available. For example, blind valves crush or shear tubulars then seal the wellbore. 
         [0004]    The valve often comprises a pair of seals that are pressed against each other to prevent fluid flowing through the bore. A valve sometimes has a cutter for shearing equipment such as piping or wireline that may be located in the bore to allow the valve to close to seal off the wellbore. 
         [0005]    Wireline valves generally require movement of parts into a pressurised fluid in the wellbore. The valves are usually hydraulically activated, although some valves are mechanically activated. 
         [0006]    In order to seal the bore, valve actuators must move parts that are exposed to the wellbore fluid pressure. Fluid pressure in wells can easily exceed 50 MPa and the resultant forces on parts of the valve are typically several tons. 
       SUMMARY 
       [0007]    According to an aspect of the invention there is provided an apparatus for restricting a flow of fluid through a bore, the apparatus comprising: 
         [0008]    a bore sealing member; and 
         [0009]    an actuation assembly for moving the bore sealing member between a first configuration and a second configuration, the actuation assembly comprising an activation member with a first end portion and a second end portion, wherein the actuation assembly is configured to isolate the first and second end portions of the activation member from a bore fluid. 
         [0010]    Isolating the first end portion and the second end portion of the activation member from a bore fluid enables the activation member to move between a first position corresponding to the first configuration and a second position corresponding the second configuration, without pressure associated with a bore fluid acting on an end portion of the activation member; such as to resist movement of the activation member between the first position and the second position. 
         [0011]    The first configuration may be a wellbore open configuration. 
         [0012]    The second configuration may be a wellbore closed configuration. 
         [0013]    The apparatus may be configured to displace substantially the same volume of fluid in the first configuration and in the second configuration. The total volume of fluid displaced by the apparatus may be the same in the first configuration and in the second configuration. The displaced volume of fluid may be a static volume of fluid. Movement of the apparatus between the first and second configurations may not affect the displaced volume of fluid. Maintaining the same volume of fluid displaced by the apparatus in the first configuration and in the second configuration ensures that force is not required to displace additional fluid when the apparatus is moved between the first and second configurations. 
         [0014]    Alternatively, the apparatus may be configured to displace a different volume of fluid in the second configuration than in the first configuration. For example, the apparatus may be configured to displace a greater volume of fluid in the second configuration. Displacing a greater volume of fluid in the second configuration may bias the apparatus towards the first configuration. Alternatively, the apparatus may be configured to displace a greater volume of fluid in the first configuration; for example to bias the apparatus towards the second configuration. 
         [0015]    The apparatus may be biased towards the first configuration. Alternatively, the apparatus may be biased towards the second configuration. 
         [0016]    The apparatus may be configured to occupy substantially the same volume within a wellbore fluid envelope in the first configuration and in the second configuration. 
         [0017]    The first end portion of the activation member may be configured to be proximal to the bore in the first configuration and the second end portion of the activation member may be configured to be distal to the bore in the first configuration. 
         [0018]    The actuation assembly may be configured to move the activation member in a direction substantially perpendicular to the bore. 
         [0019]    The actuation assembly may further comprise an isolator comprising a first side configured for receiving the first end portion of the activation member. The actuation assembly may be configured to define an isolation chamber between the first end portion of the activation member and a portion of the isolator. 
         [0020]    The actuation assembly may be configured to separate the isolator first side from the bore fluid and to position an isolator second side in fluid communication with the bore fluid. The isolator may be configured to form a boundary between a first portion of the apparatus and a second portion of the apparatus, the first portion of the apparatus configured to be exposed to wellbore fluid pressure and the second portion of the apparatus configured to be exposed to a second fluid pressure. The second fluid pressure may be lower than the wellbore fluid pressure, for example the second fluid pressure may be atmospheric. 
         [0021]    The actuation assembly may further comprise a first end portion seal, the first end portion seal configured to prevent the first end portion contacting the bore fluid. For example, to prevent bore fluid entering the isolation chamber. 
         [0022]    The first end portion seal may be an annular seal between the activation member and the isolator. 
         [0023]    The first end portion and the second end portion of the activation member may be configured to be in fluid communication. For example, the first end portion of the activation member may be linked to the second end portion of the activation member via a fluid passage, such as a conduit. Alternatively, the first end portion and the second end portion may be isolated such that the first end portion and second end portions may be subjected to different fluid pressures. 
         [0024]    The isolator may be configured to accommodate a stroke of the activation member. For example, the isolator may be a cylinder. 
         [0025]    The activation member may be a piston. 
         [0026]    The isolator may be configured to maintain substantially the same position relative to the bore during movement of the activation member from the first position to the second position. For example the isolator may be fixed. 
         [0027]    The activation member may further comprise a bore sealing member interface. The bore sealing member interface may be located between the first end portion and the second end portion of the activation member. 
         [0028]    The activation member may comprise a central portion located between the first and second end portions. The central portion may be configured to be in fluid communication with the bore fluid. Alternatively, the central portion may be configured to be isolated from the bore fluid. 
         [0029]    The actuation assembly may further comprise an activation member housing configured for receiving the central portion. 
         [0030]    The isolator may be attached to the activation member housing. For example, the activation member housing may comprise the isolator. 
         [0031]    The isolator may be configured to control the bore fluid pressure acting axially on the activation member. 
         [0032]    The activation member housing may comprise a first portion and a second portion, the first portion configured to be in fluid communication with the bore fluid and the second portion configured to be isolated from the bore fluid. 
         [0033]    The actuation assembly may further comprise a proximal chamber, the proximal chamber located between the isolator and the activation member housing. The actuation assembly may be configured to enable fluid communication between the proximal chamber and the bore. Additionally or alternatively, the actuation assembly may be configured to isolate the proximal chamber from fluid in the bore. 
         [0034]    The actuation assembly may further comprise an intermediate chamber. The intermediate chamber may be separated from the proximal chamber by the activation member housing. The intermediate chamber may be located between the proximal chamber and the second end portion of the activation member. 
         [0035]    The actuation assembly may further comprise a second end portion seal. The second end portion seal may be configured to prevent the second end portion contacting the bore fluid. For example, the intermediate chamber may be fluidly isolated from the proximal chamber. 
         [0036]    The first end portion seal may be configured to seal a first cross-sectional area of the activation member perpendicular to the direction of extension. The second end portion seal may be configured to seal a second cross-sectional area of the activation member perpendicular to the direction of extension. The first and the second cross-sectional areas may be substantially the same. Alternatively, the second cross-sectional area may be greater than the first cross-sectional area. For example, the second end portion seal may comprise an opening for receiving a larger activation member diameter than an opening of the first end portion seal. Alternatively, the first cross-sectional area may be greater than the second cross-sectional area. 
         [0037]    The first and second cross-sectional areas may be selected according to a wellbore fluid characteristic/s and/or a wellbore characteristic/s and/or a desired force/s required to move the bore sealing member between the first and second configurations. For example, where the second end portion seal diameter is greater than a first end portion seal diameter, the difference between the diameters may be less for a larger fluid pressure. 
         [0038]    The second end portion seal may be an annular seal between the central portion and the activation member housing. 
         [0039]    The activation member may be configured to move axially within the activation member housing. 
         [0040]    The actuation assembly may further comprise an activation member housing seal between the intermediate chamber and the proximal chamber. 
         [0041]    The actuation assembly may be housed in an actuation assembly casing. The activation member housing seal may be located between the activation member housing and the casing. 
         [0042]    The intermediate chamber may be in fluid communication with the isolation chamber. For example, the second intermediate chamber may be connected to the isolation chamber via a fluid conduit. 
         [0043]    The isolation chamber may be in atmospheric fluid communication. For example, the isolation chamber may be fluidly connected, such as via a venting conduit, to outside of the apparatus. The activation member may comprise at least a portion of the venting conduit. For example, the activation member may comprise an axial passage. 
         [0044]    The actuation assembly may further comprise a distal chamber. The distal chamber may be located between the intermediate chamber and the second end portion. 
         [0045]    The apparatus may be configured to be hydraulically activated. For example, the distal chamber may be may be an activation chamber configured to be in fluid communication with a hydraulic source. Additionally, or alternatively, the apparatus may comprise a mechanical actuator. For example, the distal chamber may comprise a first threaded member. The first threaded member may be configured to receive a second threaded member. 
         [0046]    The mechanical actuator may be configured to maintain the bore sealing member in the second configuration. For example, the mechanical actuator may be a manual lock. 
         [0047]    The first threaded member may be configured to be located at a fixed distance from the bore. The first threaded member may be configured to rotate about a rotation axis substantially perpend portionicular to the bore. 
         [0048]    Axial movement of the first threaded member with respect to the second threaded member may be restricted, such that rotational movement of the first threaded member with respect to the second threaded member results in axial movement of the first threaded member with respect to the second threaded member. 
         [0049]    The second threaded member may be configured to move the activation member. For example a proximal end portion of the second threaded member may contact a portion of the activation member proximal to the second end portion of the activation member. 
         [0050]    The apparatus may be configured to define the rotational movement of the activation member about an axis parallel to the direction of extension of the activation member. The actuation assembly may be configured to prevent rotation of the activation member about the axis parallel to the direction of extension of the activation member. For example, the activation member may comprise a linear element. The linear element may be a slot. Additionally, or alternatively the linear element may be a radial protrusion. 
         [0051]    The bore sealing member interface may be configured to connect the central portion to a bore sealing member support. 
         [0052]    The bore sealing member support may be configured to move coaxially with the activation member. The bore sealing member support may be configured to move simultaneously with the activation member. 
         [0053]    The bore sealing member interface may be configured to restrict movement of the bore sealing member support relative to the activation member. For example, the bore sealing member interface may be configured to restrict rotation of the bore sealing member support. Additionally, or alternatively, the bore sealing member interface. 
         [0054]    At least a portion of the bore sealing member support may be configured to substantially surround the isolator. For example, the bore sealing member support may be a sleeve. 
         [0055]    The bore sealing member support may be configured to define the rotational movement of the bore seal about the axis parallel to the direction of extension of the activation member. The bore sealing member support may be configured to prevent rotation of the bore seal about an axis parallel to the direction of extension of the activation member. For example, the bore sealing member may comprise a profiled portion, the profiled portion configured to restrict circumferential movement of the bore sealing member. 
         [0056]    The apparatus may further comprise a locking member. The locking member may be configured to maintain the bore sealing member in the second configuration. For example, the locking member may be configured to engage the activation member in the second position such that axial movement of the activation member is restricted. 
         [0057]    The apparatus may comprise a wireline valve. 
         [0058]    The apparatus may comprise a BOP. 
         [0059]    The apparatus may comprise a gate valve. 
         [0060]    The apparatus may be configured to expose only an intermediate portion of the activation member to the bore fluid pressure, the intermediate portion located between the first and second end portions. 
         [0061]    According to an aspect of the invention there is provided a method of restricting fluid flow in a bore, the method comprising: 
         [0062]    moving a bore sealing member from a first configuration to a second configuration by moving an activation member from a first position to a second position, the activation member comprising a first end portion and a second end portion, wherein the activation member is moved from the first position to the second position with the first end portion and the second end portion isolated from a bore fluid. 
         [0063]    According to an aspect of the invention, there is provided an apparatus for restricting a flow of fluid through a bore, the apparatus comprising: 
         [0064]    a bore sealing member; and 
         [0065]    an actuation assembly; 
         [0066]    wherein the bore sealing member is connected to the actuation assembly by a profiled head, the profiled head being rotationally asymmetrical about a central longitudinal axis of the bore sealing member. 
         [0067]    Providing such a profiled head prevents rotation of the bore sealing member about its longitudinal axis such that an orientation of the bore sealing member may be maintained, such as an upright orientation relative to a wellbore. 
         [0068]    The invention includes one or more corresponding aspects, embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. For example, it will readily be appreciated that features recited as optional with respect to one aspect may be additionally applicable with respect to any of the other aspects, without the need to explicitly and unnecessarily list those various combinations and permutations here. 
         [0069]    The above summary is intended to be merely exemplary and non-limiting. 
     
    
     
       BRIEF DESCRIPTION 
         [0070]    These and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0071]      FIG. 1  is a sectional view of an existing wireline valve; 
           [0072]      FIG. 2  is a sectional view of a part of a wireline valve in accordance with an embodiment of the present invention, the sectional view corresponding to line B-B of  FIG. 5 ; 
           [0073]      FIG. 3  is a sectional view of the part of a wireline valve of  FIG. 2  showing the wireline valve in a first open configuration; 
           [0074]      FIG. 4  is a sectional view of the part of a wireline valve of  FIG. 2  showing the wireline valve in a first closed configuration; and 
           [0075]      FIG. 5  is a sectional view of the part of a wireline valve of  FIG. 2  showing the wireline valve in a second closed configuration. 
       
    
    
     DETAILED DESCRIPTION 
       [0076]    Reference is first made to  FIG. 1  of the drawings, which is a sectional view of a conventional wireline valve, such as supplied by the applicant. The wireline valve  10  shown is a ram wireline valve comprising a pair of sealing heads  12   a,    12   b  for sealing a wellbore  14  to restrict the passage of fluid  16  through the wellbore. The wireline valve  10  is shown in a partially closed configuration, with a first sealing head  12   a  in an open position and a second sealing head  12   b  in a closed position. Each sealing head  12   a,    12   b  is attached to an activation rod  18   a,    18   b.  A first activation rod  18   a  is shown in a retracted position, with the first sealing head  12   a  located proximal to a first rod housing  20   a;  whilst a second activation rod  18   b  is shown in an extended position, with the second sealing head  12   b  distal to a second rod housing  20   b.    
         [0077]    Annular activation rod seals  22   a,    22   b  are located between the respective first and second activation rods  18   a,    18   b  and their respective rod housing  20   a,    20   b.  The annular activation rod seals  22   a,    22   b  isolate sealing head chambers  24   a,    24   b  from rod housing chambers  26   a,    26   b  such that the sealing head chambers  24   a,    24   b  are in fluid communication with the wellbore fluid  16  whilst the rod housing chambers  26   a,    26   b  are isolated from the wellbore fluid  16  and therefore not subject to a wellbore fluid  16  pressure. 
         [0078]    Each activation rod  18   a,    18   b  is attached to a respective threaded sleeve  28   a,    28   b,  which in turn is connected to a respective screw  30   a,    30   b.  Each screw  30   a,    30   b  is housed in a screw casing  32   a,    32   b  such that the axial position of each screw  30   a,    30   b  is fixed with respect to the bore  14 . Each screw  30   a,    30   b  is operable by a handle  34   a,    34   b  such that each screw  30   a,    30   b  rotates in the screw casing  32   a,    32   b.  The screwthread interface between each screw  30   a,    30   b  and the respective threaded sleeve  28   a,    28   b  and a rotational restriction on each sleeve  28   a,    28   b  results in axial movement of each sleeve  28   a,    28   b  when the respective screw  30   a,    30   b  is rotated. Axial movement of each sleeve  28   a,    28   b  results in axial movement of the respective sealing head  12   a,    12   b  such that the wellbore  14  can effectively be selectively opened or closed to the passage of fluid  16  through the wellbore  14 . 
         [0079]    Movement of an open sealing head  12   a  to the position of a closed sealing head  12   b  requires the displacement of a volume of fluid  16  in the wellbore corresponding to the additional volume of the rod  18   a,    18   b  that enters the respective sealing head chamber  24   a,    24   b.  The displacement of fluid  16  under wellbore pressure requires work. The pressure of the wellbore fluid  16  acting on the cross-sectional area of the rod  18   a,    18   b  perpendicular to the direction of extension requires a force, which may be several tons depending on the particular wellbore pressure and the diameter of the rod  18   a,    18   b.    
         [0080]    Each sealing head  12   a,    12   b  comprises an aperture  36   a,    36   b  for receiving a pin  38   a,    38   b;  each pin  38   a,    38   b  attached to the respective rod housing  20   a,    20   b.  Each aperture  36   a,    36   b  and corresponding pin  38   a,    38   b  is offset from a central axis of extension  39   a,    39   b  of each activation member  18   a,    18   b  such that rotation of each sealing head  12   a,    12   b  about each axis of extension  39   a,    39   b  is prevented. 
         [0081]    Reference is now made to  FIGS. 2 ,  3 ,  4  and  5  of the drawings, which illustrate a part of a wireline valve  40  in accordance with an embodiment of the present invention. As will be described, the wireline valve  40  is configured to be moved between an open configuration and a closed configuration in a wellbore  42  by an activation stem  44  with an activation stem first end portion  46  and an activation stem second end portion  48  isolated from a bore fluid  50 , such that a wellbore fluid  50  pressure does not act against the movement between the first and second configurations. 
         [0082]    The illustrated wireline valve  40  comprises a sealing head  52  attached to a seal sleeve  54 . The seal sleeve  54  is cylindrical and comprises a first and a second profiled portion  56   a,    56   b  as can best be seen in  FIG. 2 . The two profiled portions  56   a,    56   b  abut corresponding first and second sealing head profiled portions  58   a,    58   b.  In the embodiment shown the profiled portion  56   a,    56   b  is a flat portion. The profiled portions  56   a,    56   b,    58   a,    58   b  ensure that the sealing head  52  cannot rotate relative to the seal sleeve  54 , about a longitudinal axis  60 . 
         [0083]    The seal sleeve  54  is connected to the activation stem  44  by a set of keys  62   a.  The wireline valve  40  is configured to locate the seal sleeve  54  fully in the wellbore fluid  50  such that no static fluid pressure difference acts across the seal sleeve. The set of keys  62   a  are attached to the activation stem  44  at a central portion  64 . In the embodiment shown the keys  62   a  have axial apertures, allowing the passage of fluid in a proximal chamber  66  that houses the seal sleeve  54 . 
         [0084]    In the embodiment shown, the central portion  64  comprises a shoulder joining two cylindrical portions of activation stem  44  of different diameters. 
         [0085]    The activation stem  44  is received in a cylinder  68 , with a cylinder seal  70  separating a cylinder chamber  72  from the proximal chamber  66  such that the cylinder chamber  72  is isolated from the wellbore fluid  50  pressure. The activation stem first end portion  46  is located in the proximal chamber  66  in the open configuration as shown in  FIG. 3  and also located in the proximal chamber  66  in the closed configurations of  FIGS. 4 and 5 . The activation stem first end portion  46  is thus always separated from the wellbore fluid  50  by the cylinder seal  70  such that the activation stem first end portion  46  is never exposed to the wellbore fluid  50  pressure. 
         [0086]    The cylinder  68  is connected to an activation stem housing  74  via supports comprising axial slots  76  to allow the passage of the keys  62   a  from the first configuration of  FIG. 3  to the second configurations of  FIGS. 4 and 5 . The activation stem housing  74  comprises an activation stem seal  78  separating an intermediate chamber  80  from the proximal chamber  66  such that the intermediate chamber  80  is isolated from the wellbore fluid  50  pressure. The activation stem second end portion  48  is also separated from the proximal chamber  66  by the activation stem seal  78 , such that the activation stem second end portion  48  is isolated from the wellbore fluid  50  pressure. The activation stem second end portion  48  is thus always separated from the wellbore fluid  50  by the activation stem seal  78  such that the activation stem second end portion  48  is never exposed to the wellbore fluid  50  pressure, either in the positions of the open configuration as shown in  FIG. 3  or the closed configurations of  FIGS. 4 and 5 , or positions therebetween. 
         [0087]    In the embodiment shown, the diameter of the activation stem  44  at the cylinder seal  70  is less than the diameter of the activation stem  44  at the activation stem seal  78  in both the open and the closed configurations. The central portion  64  comprises a transition from a first end portion  46  diameter to a second end portion  48  diameter, such that the cylinder seal  70  receives a first cylindrical portion of activation stem  44  of lesser diameter than a second cylindrical portion received by the activation stem seal  78 . 
         [0088]    In the embodiment shown, the activation stem housing  74  is attached to a wireline valve casing  82  with a housing seal  84  preventing the passage of the wellbore fluid  50  in the annulus between the housing  74  and the casing  82 . The housing  74  further comprises vents  86   a  fluidly connecting the cylinder chamber  72  through an annular passage  88  to the exterior of the casing  82 , via the intermediate chamber  80 , radial passages  89   a  and an axial passage  90  in the activation stem  74 . 
         [0089]    The intermediate chamber  80  is separated from a retraction chamber  92  via an intermediate seal  94 . The retraction chamber  92  is connected via a retraction port  96  to a first hydraulic source. The retraction chamber  92  is separated from an extension chamber  98  by a hydraulic seal  100 . The extension chamber  98  is connected to a second hydraulic source via an extension port  102 . 
         [0090]    In the open configuration of  FIG. 3 , the activation stem  44  is in a retracted position. To move the wireline valve  40  from the open configuration of  FIG. 3  to the closed configuration of  FIG. 4 , pressure is applied to the extension chamber  98  by the supply of hydraulic fluid through the extension port  102 , such that a pressure is applied on an outer axial surface  91  of the second end portion  48  of the activation stem  44 . The axial force acting inwardly on the second end portion  48  exceeds the axial force acting outwardly on the first end portion  46  and frictional resistances such that the activation stem  44  moves towards the bore  42 . To move the wireline valve  40  from the closed configuration of  FIG. 4  to the open configuration of  FIG. 3 , pressure is applied to the retraction chamber  92  by the supply of hydraulic fluid through the retraction port  96 , such that a pressure is applied on an inner axial surface  93  of the second end portion  48  of the activation stem  44 . Hydraulic fluid is also extracted from the extension chamber  98  through the extension port  102 . 
         [0091]      FIG. 5  shows an alternative closed configuration of the wireline valve  40  of  FIG. 2 . To move the activation stem  44  from the open position of  FIG. 3  to the closed position of  FIG. 5 , mechanical force is applied to the second end portion  48  by rotating a handle  104 . Rotation of the handle  104  causes a screw  106  to rotate, the axial position of the screw  106  relative to the bore  42  being restricted by the casing  82  such that the screw  106  maintains the same axial position during rotation. The screw  106  is connected by screwthread to a threaded sleeve  108 , the rotational movement of the threaded sleeve restricted by the activation stem  44  such that rotation of the screw  106  results in axial movement of the threaded sleeve  108 . The axial movement of the threaded sleeve  108  thus causes axial movement of the activation stem  44  such that the wireline valve  40  is moved to the closed configuration of  FIG. 5 . Mechanical movement of the activation stem  44  by the handle  104  may be aided by a pressure in the activation chamber  98 . 
         [0092]    The closed configuration of  FIG. 5  may also be used subsequent to the configuration of  FIG. 4 . For example, hydraulic fluid may be used to rapidly move the valve  40  to the closed configuration and thereafter the screw  106  may be rotated to position the threaded sleeve  108  to act as a mechanical lock to prevent movement of the valve  40  to the first configuration under wellbore fluid pressure. Supply of hydraulic fluid to the extension chamber  98  may be stopped, the sleeve  108  maintaining the valve in the closed configuration of  FIG. 5 . In the embodiment shown, as the diameter of the activation stem seal  78  is greater than the diameter of the cylinder seal  70 , rotating the screw  106  in an opposite direction to return the threaded sleeve  108  to the position of  FIG. 4  is sufficient to move the valve  40  to the open configuration of  FIG. 3 ; provided that there is no significant pressure difference between the extension  98  and retraction  92  chambers. 
         [0093]      FIG. 5  further shows the section line B-B indicating the sectional view depicted in  FIG. 2 . 
         [0094]    In an alternative embodiment the retraction port may be connected to the cylinder chamber  72 . For example, where there is no intermediate chamber  80  and the axial passage  90  extends to the retraction chamber. 
         [0095]    It will be apparent to those of skill in the art that the above described embodiment is merely exemplary of the present invention, and that various modifications and improvements may be made thereto, without departing from the scope of the invention.

Summary:
Apparatus ( 10 ) for restricting a flow of fluid through a bore ( 14 ) comprises a bore sealing member ( 12   a ) and an actuation assembly for moving the bore sealing member ( 12   a ) between first and second configurations. The actuation assembly is configured to isolate first and second end portions ( 46, 48 ) of the activation member ( 12   a ) from a bore fluid ( 50 ). The apparatus ( 10 ) provides for moving the bore sealing member ( 12   a ) between the first and second configurations without a bore fluid ( 50 ) pressure acting on an end portion ( 46, 48 ) of the activation member ( 12   a ).