Patent Publication Number: US-2011061854-A1

Title: Subsea assembly

Description:
FIELD OF THE INVENTION 
     The present invention relates to a subsea assembly, and in particular, but not exclusively, to a subsea tool deployment assembly 
     BACKGROUND TO THE INVENTION 
     In the oil and gas exploration and production industry, the use of subsea production systems has become the method of choice for exploiting oil and gas fields. Such systems employ Christmas trees, which are formed of valves, spools and the like, mounted directly on the wellhead at seabed level. Production from such Christmas trees may be directly communicated to a production platform or vessel, or may be collected via a subsea manifold such that production fluid from a number of wellbores may be collectively flowed to a single production platform or vessel. 
     Many of the fields developed with subsea production trees are moving into the second phase of production, known as the intervention phase. An intervention operation typically involves extensive production logging programmes followed by the appropriate remedial operations, such as re-perforating and water shut-off. Accordingly, an intervention operation may involve a significant number of trips into and out of the wellbore with many different tooling strings and arrangements. 
     The conventional approach to performing intervention operations on subsea production arrangements is to do so from a drilling rig or ship utilising a workover system incorporating a marine riser extending between the Christmas tree and the surface vessel. In such conventional approaches, isolation of wellbore fluids within the wellbore and marine riser is achieved via a BOP stack located on the surface vessel, wherein intervention tooling and the like is selected and made up at surface level and then run into the marine riser through the BOP. The BOP incorporates a number of valves, such as ram valves, shear valves, gate valves and the like which are actuated in response to, for example, pressure imbalances or fluctuations within the wellbore, in order to prevent leakage of wellbore fluids into the environment. The BOP valves are designed to sever any object, such as wireline, passing through the BOP so as to ensure complete fluid isolation. Accordingly, actuation of a BOP valve may result in the loss of a tooling string into the wellbore, necessitating an expensive and time-consuming fishing operation to retrieve the tool. However, the requirement to ensure that leakage is prevented from the BOP stack outweighs the desire to prevent loss of a tool into the well. 
     The valves within a BOP stack are typically hydraulically actuated and in the event of loss of hydraulic power the valves typically respond, as a precautionary measure, by closing until remedial action can be carried out in order to minimise the risk of leakage of wellbore fluids into the environment. In many known arrangements, the valves may be closed by a differential pressure acting against a valve member to urge said member towards a closed position. In this case the differential pressure may be established by wellbore pressure acting against a sealing surface of the valve member. As noted above, closure of a BOP valve may result in damage to wellbore equipment, and even the loss of equipment into the wellbore. 
     Furthermore, any requirement for fluid injection into the wellbore, such as well-kill fluid, is achieved directly from surface level and communicated through the marine riser. 
     There are many problems associated with conventional intervention systems related to, for example, the existence of the riser, the cost and availability of a suitable surface vessel, the time required to carry out an intervention operation, among others. In an attempt to alleviate such problems, the present applicant has proposed a self contained well intervention system which is adapted to be mounted directly on the Christmas tree, thus eliminating the requirement for a marine riser and associated specialist surface vessel and equipment. Such a subsea intervention system is disclosed in the applicant&#39;s international application WO 2004/065757. In a preferred system, the appropriate intervention equipment, such as wireline tools, wireline, winches and the like, are provided within a subsea vessel mounted on the wellhead, wherein the subsea vessel is exposed to and contains wellbore fluid and pressures. Accordingly, by providing all necessary components within the subsea vessel, the requirement for external access, and the concomitant risk of leakage of wellbore fluids into the surrounding water, is eliminated, or at least minimised. Consequently, the requirement for valve arrangements which automatically close upon failure may not be required given that fluid containment is achieved by the subsea vessel. 
     Additionally, it should be noted that the elimination of the marine riser thus also eliminates a flow path directly into the wellbore for fluid injection, such as injection of well-kill fluids. 
     The present invention seeks to provide fluid control apparatus which has particular application in a subsea intervention system. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a subsea assembly comprising: 
     a valve assembly comprising:
         a valve block defining a throughbore;   a valve member mounted within the valve block and adapted to be moved between open and closed positions to selectively provide a fluid barrier within the throughbore;   first and second valve stems extending from respective sides of the valve member and through a side wall of the valve block, each valve stem including a sealing region in fluid communication with the throughbore, wherein a cross sectional area of the sealing region of the first valve stem is substantially equal to the cross-sectional area of the sealing region of the second valve stem; and       

     a winch assembly mounted relative to the valve assembly and comprising a winch chamber in fluid communication with the throughbore of the valve block, wherein the winch assembly comprises a winch drum carrying a spoolable member adapted to extend through the throughbore of the valve block. 
     In use, the subsea assembly may be adapted to be mounted, either directly or indirectly, on a subsea wellhead, such that the valve assembly may provide fluid control between the valve assembly and a well bore through the wellhead. Additionally, the winch assembly may deliver and retrieve the spoolable member to and from the well bore. 
     As each valve stem is in fluid communication with the throughbore of the valve block, each sealing region will therefore be exposed to substantially the same fluid pressure. The pressure acting on each sealing region will therefore apply substantially equal forces on the first and second valve stems and as such a force equilibrium will be established. Accordingly, the valve stems will not be biased in any particular direction by fluid pressure within the throughbore to move the valve member. In this respect, in the event of a loss of valve control, such as the failure or loss of actuator hydraulic, pneumatic, or electric power or the like, the valve member will remain in the same position. The valve assembly may be conveniently termed “fail-as-is”. This arrangement prevents or substantially minimises the possibility of the valve member inadvertently closing on a spoolable member extending through the valve assembly. 
     In one embodiment the spoolable member is adapted to provide support for a downhole tool such that said downhole tool may be deployed and retrieved utilising the spoolable member. 
     The subsea assembly may further comprise a tool storage assembly mounted relative to the valve assembly and comprising a tool storage chamber in fluid communication with the throughbore of the valve block. The tool storage chamber may contain at least one downhole tool adapted to be secured to the spoolable member to subsequently be deployed into the well bore. The at least one tool may comprise an intervention tool, for example, or any tool adapted to perform a required in-well operation. The at least one tool may be adapted to travel through at least a portion of the throughbore of the valve block. 
     The first and second valve stems may extend from opposing sides of the valve member. The first and second valve stems may be arranged to be colinear. 
     The sealing region of each valve stem may engage respective stem sealing arrangements adapted to prevent fluid leakage from the throughbore past each valve stem. 
     The valve block may define first and second lateral bores extending through an outer wall portion of the valve block and opening into the longitudinal bore, wherein the first and second valve stems extend through the first and second lateral bores respectively. The valve stems are preferably adapted to be translated relative to the respective lateral bores. 
     A valve stem sealing arrangement may be disposed between the sealing regions of each valve stem, respectively, and the first and second lateral bores, respectively. This arrangement therefore establishes the valve stems to be in sealing engagement with a respective lateral bore. 
     Alternatively, the lateral bores may define a larger cross-sectional area than the valve stems such that first and second fluid cavities are defined between an outer surface of a respective valve stem and an inner surface of a respective lateral bore. The fluid cavities may be annular. In a preferred embodiment, each fluid cavity is in fluid communication with the longitudinal bore of the valve block such that each fluid cavity and valve stem is, in use, exposed to fluid pressure within said longitudinal throughbore. 
     The valve assembly may further comprise first and second cap members adapted to be secured to the outer surface of the valve block to cover and seal a respective lateral throughbore. At least one of the cap members may be formed separately from the valve block and subsequently secured thereto, for example by bolting, welding, screwing or the like. A sealing arrangement such as a metal-to-metal seal, gasket or the like is preferably provided between each cap member and the outer surface of the valve block. At least one of the cap members may be integrally formed with the valve block. 
     Each cap member may be adapted to receive a portion of a respective valve stem. Preferably, each cap member defines a bore adapted to receive a portion of a respective valve stem, wherein the valve stem is slidably engaged within the bore. A valve stem sealing arrangement may be disposed between the sealing regions of each valve stem and a respective bore of each cap member. This arrangement therefore establishes the valve stems to be in sealing engagement with a respective bore of the cap members. 
     In one embodiment, at least one, and preferably both of the cap members defines a stepped bore, with a first bore section of a first diameter, and a second bore section of a second, larger diameter. Advantageously, a valve stem sealing arrangement may be disposed between respective valve stems and respective first bore sections, wherein a fluid cavity may be defined between the valve stem and the second bore section. In one embodiment, this fluid cavity within the cap member may advantageously combine with or open into the fluid cavity defined between the valve stem and respective lateral bore in the valve block. It should be understood that terms such as diameter are used herein for clarity and convenience and as such are not intended to limit the scope of the invention, or features of the invention to being round in cross-section. Accordingly, non-round geometries are also envisaged. 
     The valve member may be moveable between open and closed positions via at least one of the first and second valve stems. 
     The valve assembly may further comprise a valve actuator. One or both of the first and second valve stems may be coupled, either directly or indirectly, to a valve actuator. In one embodiment, at least a portion of a valve actuator may be mounted on a side wall of the valve block, and more preferably on one, or alternatively both of the cap members. The valve assembly preferably comprises a hydraulic actuator. The hydraulic actuator may comprise an actuator piston arrangement. In one embodiment, one of the first and second valve stems may form part of a hydraulic actuator, such as an actuator piston arrangement. 
     Alternatively, the valve actuator may comprise an electrical actuator, such as a solenoid arrangement, pneumatic actuator or the like. 
     The valve actuator may be adapted to be manually operated, for example via a submersible Remotely Operated Vehicle (ROV). Accordingly, in the event of failure of the valve actuator, for example as a result of loss of power, hydraulic or otherwise, the valve may still be operated as desired. Advantageously, the valve actuator may comprise an ROV interface permitting engagement and operation by an ROV. 
     The valve may comprise a gate valve. 
     The subsea assembly may be adapted to be mounted on a wellhead Christmas tree, such as a horizontal or vertical tree. In one embodiment the valve assembly of the subsea assembly is adapted to be mounted on a Christmas tree. Alternatively, the winch assembly, or any other component or part of the subsea assembly is adapted to be mounted on a Christmas tree. 
     The valve assembly may be adapted to be in fluid communication with a production bore of the Christmas tree, and preferably also an annulus bore of the tree. In a preferred arrangement, the valve assembly permits fluid communication from both the production and annulus bores of a conventional Christmas tree through and/or past said valve assembly. The longitudinal bore of the valve block is preferably in fluid communication with a tree production bore. Fluid communication may be achieved by an isolation sleeve extending between the throughbore of the valve block and a production bore of a Christmas tree. The isolation sleeve may isolate the production bore from the annulus bore of the Christmas tree. 
     The valve assembly of the present invention may be adapted to be mounted on a Christmas tree via a suitable tree connector, which is known in the art for conventional Christmas trees. 
     In one arrangement the valve assembly may be adapted to be mounted on a single type of Christmas tree, such as a horizontal tree or a vertical tree or the like. Preferably, the valve assembly is adapted for use on different types of Christmas tree, such as both a horizontal and vertical type tree. 
     It is well known in the art that horizontal trees incorporate a central production bore and an annulus bore passage extending axially through a wall portion of the tree. In a preferred embodiment, the throughbore of the valve block is aligned with a central axis of said valve block, such that, in use, the production bore of a horizontal Christmas tree may be axially aligned with the throughbore of the valve block. 
     In contrast, a conventional vertical or dual bore Christmas tree incorporates parallel production and annulus bores, arranged side-by-side and both off-set from the central axis of the Christmas tree. In embodiments where the throughbore of the valve block is centrally aligned, said bore will be off-set from the production bore of a vertical Christmas tree. The valve assembly may be provided in combination with an adaptor, conveniently termed herein a cross-over adaptor, for permitting use of the valve assembly on various types of Christmas trees, preferably a vertical Christmas tree. 
     The cross-over adaptor may, in use, align the production bore of a Christmas tree, for example a vertical Christmas tree, with the throughbore of the valve block. The cross-over adaptor may comprise a first throughbore adapted to communicate with both the production bore of a Christmas tree and the throughbore of the valve block. The adaptor may further comprise a second throughbore adapted to communicate with the annulus bore of a Christmas tree. In embodiments where the adaptor is for use with a vertical Christmas tree, the first and second throughbores may be off-set from the central axis of the adaptor. 
     The first throughbore may be obliquely aligned relative to the central axis of the adaptor. In this arrangement, the obliquely aligned bore may centralise the production bore of the Christmas tree with the throughbore of the valve block. 
     Alternatively, and in a preferred embodiment, the first throughbore may be aligned parallel with the central axis of the adaptor. This arrangement facilitates easier manufacture of the adaptor. The adaptor may comprise a body portion and a flange adapted to be secured to the valve block. The flange may be off-set from the body portion such that the first bore is aligned with a central axis of the flange. Accordingly, the off-set flange permits the first bore to become aligned with the throughbore of the valve block when said valve block is secured to the adaptor via the flange. 
     The valve assembly may be secured to a Christmas tree via a conventional tree connector, such as an H4 connector, dual bore connector or the like. A cross-over adaptor, such as the one described above, may be interposed between the valve block and the tree connector. The cross-over adaptor may be separately formed and subsequently secured to the tree connector. Alternatively, the cross-over adaptor may be integrally formed with the tree connector. 
     In one embodiment the valve assembly comprises a fluid passage, hereinafter an annulus passage for convenience, which, in use, may be in fluid communication with an annulus bore of a Christmas tree. At least a portion of the annulus passage may be formed within the valve block. Alternatively, or additionally, at least a portion of the annulus passage may be formed externally of the valve block. In this arrangement at least a portion of the annulus passage may comprise a conduit, such as a pipe or the like. 
     The annulus passage may by-pass the valve member. The annulus passage may extend from a region below the valve member to a region above the valve member. The annulus passage may be coupled to the valve block below and/or above the valve member. In one embodiment, the annulus passage may be coupled to the valve block at a location below the valve member and coupled to a further component mounted on the valve assembly above the valve member. 
     The provision of an annulus passage permits communication through the annulus bore to be achieved, even with the valve assembly in place coupled to a Christmas tree. This arrangement permits a conventional riser to be coupled in fluid communication with a tree without requiring the valve assembly to be removed. 
     The subsea assembly may form part of a tool deployment system and the valve assembly may be adapted to support components of the tool deployment system, such as the winch assembly, a tool storage package, a downhole tool, a plug-pulling tool or the like, or any suitable combination thereof. 
     The valve block of the valve assembly may comprise at least one port in a side wall thereof. The at least one port may permit fluid access into the throughbore of the valve block. In one embodiment, the at least one side port may comprise a well-kill fluid port adapted to permit a well-kill fluid to be introduced into the throughbore of the valve block and ultimately into the wellbore. This arrangement is particularly advantageous in subsea tool deployment apparatus in that conventional access from a drilling vessel through a BOP is not permitted due to the absence of a marine riser and the access restrictions imposed by the component of the subsea apparatus. 
     The valve assembly may comprise a quick connector arrangement adapted to permit a robust connection between a well-kill fluid conduit extending from a well-kill fluid source and the well-kill side port. The well-kill fluid source may be located subsea or alternatively at surface level. The quick connector arrangement may also be adapted for quick release, which may be beneficial in circumstances where a source of well-kill fluid is provided on a surface vessel. Accordingly, deviation of the vessel from its intended location may result in a quick disconnect of the well-kill fluid conduit to prevent damage to the conduit and leakage of fluids into the environment. 
     The at least one side port may comprise a purging fluid port adapted to permit purging fluid to be introduced into the throughbore of the valve block to purge fluids, such as well bore fluids, from the subsea assembly, such as subsea intervention apparatus. 
     The at least one side port may comprise a methanol injection port. Alternatively, or additionally, the at least one port may comprise a pressure testing port adapted to permit pressurised fluid to pressure text gasket connections and the like, for example between the valve block and a tree connector. 
     In view of conventional Christmas tree arrangements, the subsea assembly may be adapted to be mounted above a Christmas tree. However, in alternative arrangements the subsea assembly may be adapted to be mounted to the side of a Christmas tree. 
     The subsea assembly may comprise a connector which replicates a Christmas tree connector, such as a landing string latch. In this arrangement, when the subsea assembly is secured to a Christmas tree, additional components or apparatus adapted to be coupled to a Christmas tree may be coupled to the tree via the subsea assembly, without the requirement for connection adaptors. The connector of the subsea assembly may be integrally formed with a component of said assembly, such as the valve block. Alternatively, the connector may be separately formed and subsequently secured to the valve block. In one embodiment the connector may be formed on an apparatus mounted on the valve assembly, such as a plug pulling tool. The provision of a conventional Christmas tree connector provided on or in conjunction with the valve assembly advantageously permits a conventional workover riser to be secured to the Christmas tree without requiring the valve assembly to be removed. 
     The valve assembly may further comprise a shearing arrangement adapted to shear a solid object, such as the spoolable member or a tool or the like extending through the valve assembly. The shearing arrangement may comprise a shear/seal ram (SSR) which is known in the art. Preferably, the SSR is positioned below the valve member. 
     The subsea assembly may further comprise a sensor adapted to sense the passage of an object therethrough. In a preferred embodiment, the sensor is mounted within the throughbore of the valve block, preferably, but not exclusively, below the valve member. Accordingly, the sensor may sense an object, such as a tool string, tractor or the like, which has passed the valve member while being run into a wellbore on the spoolable member. Accordingly, the object may be run into the wellbore at a relatively slow rate of advancement until the object is detected by the sensor, indicating that the valve member has been cleared, following which the rate of advancement may be increased. Furthermore, when an object is being retrieved from a wellbore, detection by the sensor will indicate that the object is approaching the valve member and as such the rate of retrieval may be reduced to prevent or substantially minimise snagging or the like of the object on the valve member. 
     The sensor may comprise an inductive sensor or antenna and may comprise a wound coil. In one embodiment, the sensor may be adapted to sense a target on a passing object. For example, the target may comprise a passive, inductively resonant target. The target may comprise a coil, which coil may be wound in series with a capacitor. The target may comprise a unique frequency or other identifiable characteristic such that the target may be uniquely identified. This arrangement may therefore be utilised to permit sensing of different objects or different regions of a single object. In use, the sensor forms an excitation and sensing filed which is arranged to excite and detect the presence/absence of the target as well as the target&#39;s frequency or identifiable characteristic. 
     The spoolable member may comprise wireline. Alternatively, or additionally, the spoolable member may comprise coiled tubing. 
     According to a second aspect of the present invention, there is provided a valve assembly comprising: 
     a valve block defining a throughbore; 
     a valve member mounted within the valve block and adapted to be moved between open and closed positions to selectively provide a fluid barrier within the throughbore; and 
     first and second valve stems extending from respective sides of the valve member and through a side wall of the valve block, each valve stem including a sealing region in fluid communication with the throughbore, wherein a cross sectional area of the sealing region of the first valve stem is substantially equal to the cross-sectional area of the sealing region of the second valve stem. 
     According to a third aspect of the present invention, there is provided a subsea assembly comprising: 
     a valve assembly comprising a valve block defining a longitudinal throughbore adapted to be in communication with a wellbore and a lateral bore extending through a wall portion of the and opening into the longitudinal bore to be in fluid communication therewith and with the wellbore; and 
     a winch assembly mounted above the valve assembly and comprising a winch chamber in fluid communication with the throughbore of the valve block, wherein the winch assembly comprises a winch drum carrying a spoolable member adapted to extend through the throughbore of the valve block. 
     The bore may be or define a well-kill fluid bore adapted to permit well-kill fluid to be communicated into the subsea fluid control assembly and ultimately into a wellbore. This arrangement is particularly advantageous in subsea intervention apparatus in that conventional access from a drilling vessel through a BOP is not permitted due to the absence of a marine riser and the access restrictions imposed by the winch assembly mounted above the valve assembly. 
     The valve assembly may comprise a valve assembly defined according to the first and second aspects. 
     According to a fourth aspect of the present invention, there is provided a cross-over adapted to be coupled between a dual bore Christmas tree and a subsea assembly, said cross-over comprising: 
     a body portion adapted to be secured to a Christmas tree; 
     a flange portion formed with the body portion and adapted to be secured to a subsea assembly; and 
     first and second throughbores extending through the body and flange portions wherein said throughbores are off-set from a central axis of the body portion; 
     wherein the flange is off-set from the body portion such that the first throughbore bore is aligned with a central axis of the flange. 
     The subsea assembly may comprise an assembly according to the first aspect. 
     The first and second throughbores of the cross-over may be adapted to be in fluid communication with a respective bore of a dual bore Christmas tree. It is well known in the art that a dual bore Christmas tree, also known as a vertical tree, incorporates a production bore and an annulus bore. Accordingly, in use, the cross-over may be utilised to align the production bore of a Christmas tree, preferably a vertical Christmas tree, with a throughbore of, for example, a throughbore of the subsea assembly. 
     According to a fifth aspect of the present invention, there is provided a subsea tool deployment assembly comprising: 
     a subsea assembly according to the first aspect; and 
     a tool storage assembly mounted relative to the subsea assembly and comprising a tool storage chamber in fluid communication with the throughbore of the valve block, wherein the tool storage chamber contains at least one downhole tool adapted to be secured to the spoolable member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic representation of a subsea assembly in accordance with an embodiment of an aspect of the present invention; 
         FIG. 2  is a perspective view of a portion of the subsea assembly shown in  FIG. 1 ; 
         FIG. 3  is a longitudinal cross-sectional view of the portion of the subsea assembly shown in  FIG. 2 ; 
         FIG. 4  is a longitudinal cross-sectional view of the subsea assembly shown in  FIG. 2 , taken through line  4 - 4  of  FIG. 3 ; and 
         FIG. 5  is a longitudinal cross-sectional view of a portion of alternative subsea assembly incorporating a valve assembly shown in  FIGS. 2 ,  3  and  4  and a connector adaptor according to an embodiment of an aspect of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Reference is first made to  FIG. 1  of the drawings in which there is shown a subsea assembly, generally identified by reference numeral  11 , in accordance with an embodiment of an aspect of the present invention. The subsea assembly  11  in the embodiment shown is a self contained subsea tool deployment assembly and is shown mounted on a wellhead  2  of a well bore  13 , via a Christmas tree  4 . As will be described in detail below, the tool deployment assembly  11  is adapted to deploy and retrieve tools into and from the well bore  13  to perform in-well operations, such as intervention operations or the like. 
     The exemplary tool deployment assembly  11  of the invention comprises a valve assembly  12  which incorporates means for controlling fluid flow to and from the well bore  13 . The valve assembly  12 , which will be described in detail below, is mounted on the Christmas tree  4  via a conventional connector  14 . Mounted immediately above the valve assembly  12  is a plug pulling tool  18  which, in use, is adapted to remove and set plugs within the Christmas tree  4 . In the exemplary embodiment shown, the valve assembly  12  and plug pulling tool  18  together define a well control package  10 . 
     A tool storage assembly  5  is mounted above the plug pulling tool  18  and includes a tool storage chamber  6  which contains a plurality of downhole tools  7 . The tool storage chamber  6  is in fluid communication with the well bore  13  via the well control package  10 , Christmas tree  4  and well head  2  and as such the stored tools  7  are exposed to bore fluids. 
     A winch assembly  8  is mounted above the tool storage assembly  5  and comprises a winch chamber  9  within which is located a vertically mounted winch drum  15  which carries a spool of wireline  17 . In use, the wireline  17  extends upwardly from the winch chamber  9  through a first lubricator tube  19 , over a sheave  21  and down through a second lubricator tube  23 . The winch chamber  9  is in fluid communication with the tool storage chamber  6  via the lubricator tubes  17 ,  23  and sheave  21 . The wireline  17  then extends through a central axis of the winch drum  15 , and into the chamber  6  of the tool storage assembly  5 . In use, a tool  7  is selected from the tool chamber  6  and secured to an end of the wireline  17  and subsequently run into the well bore  13 , as shown in  FIG. 1 , through the well control package  10 , Christmas tree  4  and well head  2 . 
     As all components of the assembly  11  are in fluid communication with each other and thus with the well bore  13 , the assembly  11  is therefore fully self-contained. 
     The well control package  10  will now be described in further detail, initially with reference to  FIG. 2 . As noted above, the package  10  incorporates a valve or fluid control assembly  12  which is mounted on a Christmas tree connector  14  via a flanged connection  16 , such that the valve assembly  12  may be secured to the Christmas tree (not shown) via said connector  14 . The valve assembly  12  may be secured to either a horizontal or vertical Christmas tree, as will be discussed in detail hereinafter. The plug pulling tool  18  is mounted on the valve assembly  12  via a flanged connection  20 . 
     Cross-sectional views through the well control package  10  are shown in  FIGS. 3 and 4  which will be discussed in detail below. However, the relative locations of external components of the well control package  10  can be readily identified in  FIG. 2  and as such reference to  FIG. 2  may be made during the following discussion. 
     Reference is now additionally made to  FIG. 3  of the drawings in which there is shown a longitudinal cross-sectional view of the well control package  10  of  FIG. 2 . The cross-section in  FIG. 3  is viewed in the direction of arrow A of  FIG. 2 . The valve assembly  12  comprises a valve block  22  defining a longitudinal throughbore  24  within which is slidably mounted a valve member  26  in the form of a gate valve member. The longitudinal bore  24  is adapted to be in fluid communication with a production bore of the Christmas tree (not shown), wherein the valve assembly  12  is adapted to provide a degree of fluid control between the well control package  10  and the Christmas tree production bore. A pair of valve stems  28 ,  30  extend from opposite sides of the valve member  26  and extend through respective lateral bores  32 ,  34  in the valve block  22 . Each valve stem  28 ,  30  includes a respective sealing region  29 ,  31 , wherein the lateral cross-sectional area of the sealing regions  29 ,  31  are substantially equal which permits fluid pressure applied against each valve stem  28 , to establish equal and opposite forces, which therefore prevents any movement of the valve member  26  by virtue of bore pressure, as will be discussed in further detail below. 
     First and second cap members or bonnets  36 ,  38  are secured, by bolting, to the side of the valve block  22  to cover and seal respective bores  32 ,  34 . The cap members  36 ,  38  and respective lateral bores  32 ,  34 , in combination with the valve stems  28 ,  30 , collectively define first and second annular chambers  40 ,  42 . Each annular chamber  40 ,  42  is in fluid communication with the longitudinal bore  24  such that the valve stems  28 ,  30  are exposed to bore fluid pressure. 
     Each cap member  36 ,  38  defines a throughbore  44 ,  46  within which a respective valve stem  28 ,  30  is slidably mounted. Specifically, the sealing regions  29 ,  31  of the respective valve stems  28 ,  30  are mounted within a respective throughbore  44 ,  46 . A sealing arrangement is defined between each valve stem  28 ,  30  and respective bores  44 ,  46 , wherein the cross-sectional sealing area defined by the sealing arrangement for each valve stem  28 ,  30  and bore  44 ,  46  is substantially equal. Accordingly, when each sealing arrangement between the valve stems  28 ,  30  and respective cap members  36 ,  38  is exposed to fluid pressure within the longitudinal bore  24 , no pressure force differential will be established. Accordingly, no movement of the valve stems  28 ,  30  and thus valve member  26  will occur by virtue of bore pressure alone. In this respect, in the event of a loss of valve control, such as the failure or loss of actuator hydraulic, pneumatic, or electric power or the like, the valve member  26  will remain in the same position. In this arrangement, the valve assembly  12  may be conveniently termed “fail-as-is”. The ability to provide such a “fail-as-is” valve assembly within a well control package  10  is permitted, at least in terms of safety, as fluid communication upwardly past the valve member  26  will be contained within the entire subsea assembly  11  ( FIG. 1 ). This is in contrast with conventional topside BOPs where any valve assembly is required to have a failure mode resulting in the closure of the valve to prevent a blow-out and leakage of fluids into the environment. 
     Additionally, the arrangement provided by the present invention prevents or substantially minimises the possibility of the valve member  26  unnecessarily closing on the wireline  17  ( FIG. 1 ) or on a downhole tool  7  ( FIG. 1 ) which would otherwise result in loss of the tool  7  into the wellbore  13  which is extremely undesirable. 
     The valve assembly  12  further comprises a hydraulic valve actuator  48  adapted to displace the valve member  26  between open and closed configurations. The valve actuator  48  comprises a hydraulic piston arrangement mounted on the cap member  36 , wherein an end portion of valve stem  28  defines a piston  50  slidably mounted within a cylinder  52 . In use, hydraulic pressure from an external source may act on the piston  50  to displace the valve stem  28  and thus valve member  26 . 
     The valve assembly  12  further comprises respective ROV interfaces  54 ,  56  mounted on each cap member  36 ,  38 . Accordingly, in the event of failure of the valve actuator  48 , for example as a result of loss of hydraulic power, the valve assembly  12  may still be operated as desired. 
     In the embodiment shown in  FIG. 2  the valve assembly  12  is configured for use with a horizontal Christmas tree (not shown). It is well known in the art that horizontal trees incorporate a production bore which is aligned with the central axis of the tree. As noted above, the longitudinal bore  24  of the valve assembly is in fluid communication with the production bore of a Christmas tree which is achieved via an isolation sleeve  58  which in use extends between a lower portion of the longitudinal bore  24  and an upper portion of a tree production bore (not shown). The isolation sleeve  58  isolates a production bore flow path  60  from an annulus bore flow path  62 . 
     The valve assembly  12  further comprises an annulus fluid passage  64  extending generally axially upwardly through a wall portion of the valve block  22 . A lower end of the annulus fluid passage  64  opens into the annulus bore flow path  62  and as such in use is in fluid communication with the annulus bore of the Christmas tree. The annulus fluid passage  64  extends outwardly through the side wall of the valve block  22  via a port  66  positioned below the valve member  26 . A dual gate valve assembly block  68  is mounted on the valve assembly  12  at the location of the port  66  and in use selectively controls fluid communication to and from the annulus bore of the Christmas tree. A conduit  70 , shown in  FIG. 2 , extends between the dual gate valve block  68  and the plug pulling tool  18  such that fluid communication to and from the annulus bore of the Christmas tree may be achieved passed the valve member  26 . The provision of an annulus passage permits communication through the annulus bore to be achieved, even with the valve assembly  12  in place coupled to a Christmas tree. This arrangement permits a conventional riser and/or other conventional components or equipment to be coupled in fluid communication with a tree without requiring the valve assembly  12  to be removed. 
     The valve assembly comprises additional side ports to permit communication of various fluids to and from the valve assembly  12  and well control package  10 . For example, side port  72  is provided to permit purge fluid to be communicated to and from the well control package  10 . Additionally, side port  74  is provided to permit methanol to be injected into a lower portion of the valve block  22  of the valve assembly  12 . Further ports which cannot be seen in the cross-sectional plane of  FIG. 2  are also present, some of which will be discussed hereinafter. 
     Reference is now made to  FIG. 4  of the drawings in which there is shown a further cross-sectional view of the well control package  10  of  FIG. 2 , in this case taken through line  4 - 4  of  FIG. 3  and generally viewed in the direction of arrow B in  FIG. 2 . The valve assembly  12  further comprise a shearing arrangement incorporating a shear/seal ram (SSR)  76  adapted to shear a solid object, such as wireline or a tool or the like extending through the valve assembly  12 . SSRs are known in the art and as such no further description will be given. 
     Positioned below the SSR  76  is a further port  78  which extends between the longitudinal bore  24  and an outer surface of the valve block  22 . Port  78  is adapted to permit a well-kill fluid to be introduced into the longitudinal throughbore  24  and ultimately into the wellbore. This arrangement is particularly advantageous in subsea tool deployment apparatus in that conventional access from a drilling vessel through a BOP is not permitted due to the absence of a marine riser and the access restrictions imposed by the intervention vessel which is mounted above the valve assembly. 
     Fluid communication of well-kill fluid is controlled by a dual gate valve assembly block  80  which is mounted on the valve block  22  of the valve assembly  12 . A conduit  82  extends from the gate valve assembly block  80 . A portion of the conduit  82  is shown in  FIG. 4 . However, a complete view of the conduit  82  is provided in  FIG. 2 . Mounted on an upper end of the conduit is a quick connect/release mechanism  84  which permits a supply conduit  86  to be readily connected and disconnected from conduit  82 . For example, in embodiments where the supply conduit  86  extends from a surface vessel, the supply conduit  86  may be quickly released in circumstances where deviation of the vessel from its intended location occurs to thus prevent fracture of one or both of the conduits  82 ,  86 . 
     Referring again to  FIG. 4 , the valve block  22  further comprises an additional port  88  extending through the wall thereof at a location above the valve member  26 . The port  88  is adapted to permit methanol to be injected into the throughbore  24  above the valve member  26  to thus dissolve any hydrates which may have been deposited around the valve member  26 . 
     Referring still to  FIG. 4 , the valve assembly  12  further comprises a sensor  90  mounted within the longitudinal throughbore  24  of the valve block  22 , below both the valve member  26  and the SSR  76 . The sensor  90  is adapted to sense the passage of an object, such as a tool  7  shown in  FIG. 1 , or alternatively, or additionally, a tool string, tractor or the like. Thus, the sensor  90  may sense the tool  7 , which has passed the valve member  26  while being run into the well bore  13 . Accordingly, the tool may be run towards the well bore  13  at a relatively slow rate of advancement until the tool  7  is detected by the sensor  90 , indicating that the valve member  24  and SSR  76  have been cleared, following which the rate of advancement may be increased to run the tool into the well bore  13 . Furthermore, when a tool  7  is being retrieved from the well bore  3  back through the valve assembly  12 , detection by the sensor  90  will indicate that the tool  7  is approaching the valve member  26  and SSR  76  and as such the rate of retrieval may be reduced to prevent or substantially minimise snagging or the like of the tool  7  within the valve assembly  12 . 
     In the embodiment shown, the sensor  90  is an inductive sensor adapted to sense a target on a passing tool. For example, the target may comprise a passive, inductively resonant target which may comprise a unique frequency such that the target may be uniquely identified. This arrangement may therefore be utilised to permit sensing of different tools or different regions of a single tool. In use, the sensor  90  forms an excitation and sensing field which is arranged to excite and detect the presence/absence of the target as well as the target&#39;s frequency or identifiable characteristic. 
     It should be noted that the upper end of the plug pulling tool  18  defines a connector  92  which is arranged to replicate the upper connector of the Christmas tree  4  ( FIG. 1 ) upon which the well control package  10  is mounted. In the arrangement shown in  FIGS. 2 ,  3  and  4 , the well control package  10  is shown adapted for use on a horizontal Christmas tree and as such the connector  92  includes a central production bore  94  and a side annulus passage  96  ( FIG. 3 ). As shown in  FIG. 4 , the side annulus passage  96  is in communication with the fluid conduit  70  which extends externally of the valve assembly  12  to bypass the valve member  26 . Providing a connector  92  in this manner advantageously permits additional components or apparatus which are intended to be coupled to the Christmas tree  4  to still be coupled to the tree, albeit via the valve assembly  12 , without the requirement for connection adaptors. Additionally, the provision of the connector  92  advantageously permits a conventional workover riser to be secured to the Christmas tree  4  without requiring the valve assembly  12  to be removed. 
     Reference is now made to  FIG. 5  of the drawings in which there is shown a cross-sectional view of a well control package in accordance with an alternative embodiment of the present invention. The well control package is generally identified by reference numeral  110  and is similar to the well control package  10  first shown in  FIG. 2 . For convenience, therefore, like features share like reference numerals, incremented by 100. Accordingly, the well control package  110  includes a valve assembly  112 , the lower end of which assembly  112  is secured to a Christmas tree connector  114  and the upper end of the assembly  112  is secured to and supports a plug pulling tool  118 . It should be noted that the valve assembly  112  is identical to that assembly  12  first shown in  FIG. 2  and as such no further description shall be given. 
     The well control package  110  in this embodiment is adapted to be mounted on a vertical or dual bore Christmas tree (not shown). Vertical Christmas trees are well known in the art and incorporate parallel production and annulus bores, arranged side-by-side. Accordingly, both the production and annulus bores are off-set from the central axis of the Christmas tree. Thus, as the longitudinal bore  124  of the valve block  122  is centrally aligned, said bore  124  will be off-set from the production bore of a vertical Christmas tree. The valve assembly  112  is therefore provided in combination with a cross-over adaptor  100 . The cross-over  100 , which will be described in detail below, permits the identical valve assemblies  12 ,  112  of the present invention to be used on various types of Christmas trees. 
     The cross-over adaptor  100  incorporates a first throughbore  101  adapted to communicate with both the production bore of a Christmas tree and the longitudinal throughbore  124  of the valve block  122 . An isolation sleeve  158  extends between the longitudinal throughbore  124  of the valve block  122  and the first bore  101  of the adaptor  100 . The adaptor  100  further incorporates a second throughbore  102  adapted to communicate with the annulus bore of a Christmas tree. 
     The adaptor  100  incorporates an off-set or eccentric flange connector  103  such that the first bore  101  may extend from an off-set lower position to a centrally aligned upper position. Accordingly, the off-set flange  103  permits the first bore  101  to become aligned with the longitudinal throughbore  124  of the valve block  122  when said valve block  122  is secured to the adaptor  100  via the off-set flange  103 . 
     In the embodiment shown in  FIG. 5 , the upper end of the plug-pulling tool  118  incorporates a connector  192  which replicates the upper connector of a vertical or dual bore Christmas tree to thus permit any necessary conventional connection to the tree to be achieved without removing the well control package  110 . 
     It should be understood that the embodiments described are merely exemplary and that various modifications may be made thereto without departing from the scope of the present invention. For example, the plug pulling tool may be located below the valve assembly. Additionally, the upper end of the valve assembly may incorporate a suitable connector which may replicate a Christmas tree connector. Furthermore, the location of the ports extending through the side wall of the valve block may be altered.