Abstract:
A completion suspension valve system is described which allows a well to be suspended and desuspended remotely without a dual bore riser to the surface. This is achieved by incorporating a remotely actuatable valve into the production bore of a tubing hanger. The valve is hydraulically operable and may be controlled via the tubing hanger running tool or via the xmas tree. The valve can be closed and tested after the tubing hanger has been installed, thereby isolating the well. The dual bore riser and running tool are retrievable and the MODU type vessel is then free to continue drilling and completion operations elsewhere. The xmas tree can therefore be deployed from a workclass supply boat instead of a MODU type vessel.

Description:
FIELD OF THE INVENTION 
     The present invention relates to subsea well installations and particularly, but not exclusively, to well installations and a completion suspension valve system that facilitates the economic suspension and desuspension of a well. 
     BACKGROUND OF THE INVENTION 
     A typical subsea wellhead assembly has a high pressure wellhead housing supported in a lower pressure wellhead housing and secured to casing that extends into the well. One or more casing hangers land, i.e. are supported by the wellhead housing, and the casing hangers being located at the upper end of a string of casing that extends into the well to a deeper depth. A string of tubing extends through the casing for production fluids. A xmas or production tree is mounted to the upper end of the wellhead housing for controlling the well fluid. The production tree is typically a large, heavy assembly, having a number of valves and controls mounted thereon for controlling the passage of well fluid through the production tree. 
     One type of production tree, sometimes known as a “conventional tree” has two bores, one of which is a production bore and the other bore is the tubing annulus access bore. In this type of wellhead assembly, the tubing hanger is supported by the wellhead housing and the tubing hanger has two passages through it; one passage being the production passage and the other passage being an annulus passage which communicates with the tubing annulus surrounding the tubing. Access to the tubing annulus is necessary to circulate fluids down the production tubing and up through the tubing annulus or vice versa to either kill the well or circulate heavy fluid during completion. After the tubing hanger is installed and before the drilling riser is removed for installation of the tree, the downhole safety valve is closed and plugs are temporarily placed in the passages of tubing hanger; this is known as well suspension. 
     The conventional tree has isolation tubes that stab into engagement with passages in the tubing hanger when the tree lands in the wellhead housing. This type of tree is normally run on a completion riser that has two strings of conduit and this is known as a dual bore completion riser. In such a completion riser, one string extends from the production passage of the tree to the surface vessel, whilst the other string extends from the tubing annulus passage in the tree to the surface vessel. 
     To assemble and run such a dual bore completion riser is very time-consuming. In addition, drilling vessels may not have such a completion riser available, requiring one to be supplied on a rental basis and, furthermore, in deeper waters it is often technically difficult to configure such a dual bore riser. 
     With such conventional tubing hanger types, the tubing hanger is installed before the tree is landed on the wellhead housing and tubing is typically run on a small diameter riser through the drilling riser and blow-out preventer (BOP). Before the drilling riser is disconnected from the wellhead housing, a plug is installed in the tubing hanger as a safety barrier. This plug is normally lowered on a wireline through the small diameter riser. After the tree is installed the plug is then removed through the riser that was used to install the tree. 
     This sequence of events requires that a mobile offshore drilling unit (MODU) type of vessel is necessary to affect well desuspension because conduits must be established between the vessel and the production tree through which plugs may subsequently be pulled. It is desirable to be able to permit a well to be desuspended without the need to establish a dual bore riser to surface and thereby permit non-MODU type vessels to conduct xmas tree installation operations and desuspension operations. 
     Published international application WO 03/067017 (ABB Vetco Gray) discloses a hydraulic ram which is used to retrieve a plug from a tubing hanger. Although this arrangement allows the plug to be retrieved through a wellhead, it also requires a separate package to be run, established with the xmas tree, operated and retrieved, thus incurring substantial additional operational costs and risk. 
     SUMMARY OF THE INVENTION 
     It is a further object of the present invention to obviate the need for such a package and its associated operations. 
     It is also an object of the present invention to avoid the requirement for a separate trip needed for the valve and to permit remote actuation of the valve (for the life of the field). 
     This is achieved in the broadest aspect of the invention by incorporating a remotely actuatable valve into the production bore of a tubing hanger. The valve is hydraulically operable and may be controlled via the tubing hanger running tool or via the xmas tree. The valve can be closed and tested after the tubing hanger has been installed, thereby isolating the well. The dual bore riser and running tool are retrievable and the MODU type vessel is then free to continue drilling and completion operations elsewhere. The xmas tree can therefore be deployed from a workclass supply boat instead of a MODU type vessel. Furthermore, because desuspending the well no longer requires a dual bore riser to be established to surface, true deployment and desuspension is conducted from a suitably configured utility vessel, such as an anchor handler or supply type vessel. The xmas tree is run from the utility vessel and established with the subsea wellhead and, after completion of appropriate testing, the suspension valve is opened, thereby desuspending the well. 
     It will be understood that the suspension valve essentially replaces a plug which may be run or retrieved on wireline or by some other means. Because there is a wide variety of equipment and techniques available to retrieve obstinate stuck plugs, the valve system in accordance with the broadest aspect of the invention also incorporates contingency features which permit the valve to which control has been lost and which is in the closed condition to be overridden to the open position. This continuous override system is consistent with a supply boat/anchor handler deployment philosophy outlined above. A further inventive aspect of the contingency system is provided by the inclusion of a mechanical nipple attached to the actuation mechanism of the valve and the actuation mechanism interfaces with the hydraulic ram attached to the top of the xmas tree or safety package, such as to allow the valve to be overridden. 
     Thus, the present invention not only comprises a completion suspension valve which permits the wells to be conveniently isolated and de-isolated but incorporates an override means by which a closed valve may be overridden to the open position with the overriding means not requiring a rigid riser to surface. 
     In a preferred arrangement, the fact that the hydraulic ram has the means to deploy and manipulate the override device has certain implications. For both manufacturability and operability, the hydraulic ram requires to have a relatively short maximum length so that the reach of the ram into the well is somewhat limited. 
     It is therefore desirable that the valve override nipple is located as near to the top of the well as possible. In the interests of simplicity and reliability, the override nipple is connected directly to the valve operating mechanism and, consequently, it is advantageous that the valve itself is located as near to the top of the well as possible. In practice, the maximum length of the hydraulic ram is about 30 ft. 
     The completion suspension valve has the essential requirement that it contains pressure from below. However, the valve must also contain pressure from above, such that it may be tested prior to disconnection of the running tool and subsequent departure of the rig. Where the available envelope, i.e. the volume within or surrounding a bore is restricted, flapper and ball type valves are typically used as they offer the best combination of throughbore and pressure capacity for a given body volume. However, it should be noted that flapper valves do not typically offer a bidirectional sealing capability. Thus, apertured ball valves may fulfill the identified requirement but existing solutions require a centralised ball valve which does not fit within the established envelope restrictions of a tool. 
     It is a further object of the invention to provide a valve arrangement which is useable within existing envelope restrictions to provide a completion suspension valve, instead of a plug. 
     In accordance with one aspect of the present invention, there is provided a method of suspending the well comprising the steps of: 
     providing a dual bore tubing hanger having an annulus bore and a production bore; 
     disposing a remotely operable valve in the production bore, and 
     actuating remotely the valve moved between an open and a closed position. 
     According to another aspect of the present invention there is provided a completion suspension valve system comprising: 
     a suspension valve housing, said valve housing having a production bore; 
     a valve element disposed in said suspension valve housing; 
     said valve being remotely actuatable between an open position and a closed position. 
     According to a further aspect of the present invention there is also described a method of remotely suspending a well as claimed in claim  14 , a ball element for use in a completion suspension valve as claimed in claim  19 , a ball valve seat for use with the ball element as claimed in claim  21 , a ball valve actuating mechanism as claimed in claim  23 , a method of opening a closed ball valve and locking it in the open position as claimed in claim  24 , a completion valve override system as claimed in claim  25 , applications of the valve as claimed in claims  26 ,  27  and  28  a ball actuation mechanism for moving an apertured ball tube using a single actuatable rod as claimed in claim  34 , and a method of opening a closed ball valve and retaining it in an open position using a sealing override plug as claimed in claim  35 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention will become apparent from the following description when taken in combination with the accompanying drawings in which: 
         FIG. 1  is a diagrammatic longitudinal section of a wellhead system, BOP and marine riser for use with a completion suspension valve according to a first embodiment of the invention; 
         FIG. 2  is a similar diagrammatic view to  FIG. 1  but of a completion string to be inserted into the BOP and wellhead system, the string including a completion valve sub, a tubing hanger, a tubing hanger running tool and a dual bore sub sea test tree; 
         FIG. 3  shows the completion string of  FIG. 2  inserted into the wellhead system of  FIG. 1  with the tubing hanger locked into the wellhead; 
         FIG. 4  is a similar view to  FIG. 3  but with more detail depicting the hydraulic lines coupled to the completion valve when in installation mode, the valve being shown in the closed position; 
         FIGS. 5   a, b  and  c  depict a central ball valve element mounted in a housing and shown in an open, intermediate and closed condition to facilitate explanation of operation of  FIG. 4  but with bar pockets also shown.  FIG. 5   d  shows a side elevation of the arrangement; 
         FIGS. 6   a, b  and  c  are top, front and perspective views of an offset ball valve element used in the completion sub of  FIG. 4  in accordance with a preferred embodiment of the present invention; 
         FIG. 6   d  is a sectional view taken on A-A of  FIG. 6   a;    
         FIGS. 7   a, b, c  and  d  depict respective top, side and front views of an offset bore seat for engaging with the ball element of  FIGS. 6   a - 6   d;    
         FIG. 7   d  is a sectional view taken on the lines B-B of  FIG. 7   a;    
         FIGS. 8   a, b  and  c  depict longitudinal partly sectioned and partly cut-away views respectively of a completion suspension valve sub with a ball element seat pocket for receiving an offset ball element as shown in  FIGS. 6   a  to  d  and an offset bore seat as shown in  FIGS. 7   a  to  d;    
         FIG. 9   a  is a plan view of the completion suspension valve showing the offset production bore; 
         FIG. 9   b  is a longitudinal sectional view taken on the lines C-C of  FIG. 9   a  and depicting various completion suspension valve components; 
         FIG. 10  is an enlarged detailed view of the top of the suspension valve housing; 
         FIGS. 11   a, b  and  c  are respective longitudinal sectional views of the completion suspension valve assembly showing the valve in the normally closed, normally open and overridden open positions respectively; 
         FIG. 12   a  is a plan view of a completion suspension valve with the ball element in the closed position; 
         FIG. 12   b  is a longitudinal sectional computer aided design (CAD) of a suspension valve in the normally closed position taken on the line A-A of  FIG. 12   a;    
         FIGS. 12   c  and  d  are cross-sectional views taken on the lines B-B and C-C respectively of  FIG. 12   b;    
         FIGS. 13   a, b, c  and  d  are views similar to  FIGS. 12   a, b, c  and  d  respectively with the ball element in the open position; 
         FIG. 14  is a view similar to  FIG. 4  but depicting a completion suspension valve in production mode with a Christmas tree coupled to the wellhead and other production control elements coupled thereto; 
         FIGS. 15   a  and  15   b  depict a production control system similar to that shown in  FIG. 14  but with an axially movable ram shown retracted in  FIG. 15   a  and extended in  FIG. 15   b  for interfacing with an override mechanism of the completion suspension valve; 
         FIGS. 16   a, b  and  c  are longitudinal cross-sectional views through the lower mandrel portion of the hydraulic ram for engaging with an override nipple,  FIG. 16   a  showing the mandrel prior to engagement with the nipple,  FIG. 16   b  depicting the spring loaded dogs engaged with the override nipple and  FIG. 19   c  showing the extended mandrel for valve override actuation; 
         FIG. 17  depicts a side-sectional view, drawn to an enlarged scale, of the override nipple in the completion suspension valve bore prior to engagement by the mandrel as in  FIG. 16   a;    
         FIG. 18   a  shows a top view of the completion suspension valve in the valve override open position; 
         FIG. 18   b  is a longitudinal sectional view taken on the lines A-A of  FIG. 18   a;    
         FIGS. 18   c  and  d  are respective cross-sectional views taken on the lines B-B and C-C of  FIG. 18   b;    
         FIG. 19  is a view similar to  FIG. 17  but with the nipple in the overridden position and engaged with a detent finger to lock the valve in the open position; 
         FIG. 20  is a perspective view with the main body removed showing the completion suspension valve in the overridden position with the ball element held open and abutting the offset valve seat and the override nipple shown abutting shoulders on the suspension valve guide shafts; 
         FIG. 21  is a diagrammatic view of an alternative application of the completion suspension valve in an in-line tree where the valve system is disposed within the wellhead; 
         FIG. 22  is a further diagrammatic view of a further alternative application of the completion suspension valve incorporated in a sub-sea test tree; 
         FIG. 23  depicts a further application of a completion suspension valve in accordance with the invention incorporated into an insert tree with the completion suspension valve being shown coupled to the wellhead; 
         FIG. 24  depicts part of a completion suspension valve in accordance with the invention which is used to manufacture the completion suspension valve, view depicting a main body and a lower body which are welded together to permit assembly; 
         FIGS. 25   a, b  and  c  depict longitudinal sectional and cross-sectional views on lines A-A and B-B respectively of a two-piece main body which is assembled together to form a completion suspension valve in accordance with an alternative embodiment of manufacture in accordance with the present invention; 
         FIG. 26  is an enlarged sectional side view of part of the ball element and bearing element used for mounting the ball within the completion housing to permit the ball to be held unloaded from the seat during rotation and which is then allowed to float on the seat for sealing; 
         FIG. 27  is a longitudinal sectional view through a completion suspension valve housing with the valve closed in accordance with an alternative embodiment with a xmas tree attached to the wellhead; 
         FIG. 28  is a view similar to  FIG. 27  but with the valve in the open condition; 
         FIG. 29  is a view of the suspension valve assembly shown in  FIG. 1  shown with a complete xmas tree and lower user package connected to the wellhead; 
         FIG. 30  is an enlarged view of an override plug shown in  FIG. 1  for maintaining the suspension valve in a fully open position; 
         FIGS. 31   a,b  are longitudinal sectional views ( 31   b  being an enlarged detail) of the plug of  FIG. 29  shown disposed in the annulus bore above the actuator rod; 
         FIGS. 32   a,b  are similar views to  FIGS. 31   a,b  with the override plug and actuation rods displaced downwardly in the annulus bore by pressure; 
         FIGS. 33   a,b  are views similar to  FIGS. 32   a,b  and depicting further downward displacement of the upper end of the override plug to secure the plug in the annulus bore and to retain both annulus and production bores in the open position. 
         FIGS. 34   a,b,c  depict an alternative embodiment of a completion suspension valve tool in accordance with the present invention in which a flapper valve is used in place of an offset aperture ball valve;  FIG. 34   a  depicting the flapper valve in a closed position, i.e. during installation;  FIG. 34   b  depicting the flapper valve in a normally open position, i.e. for production and FIG.  34   c  depicting the flapper valve in the overridden, locked open position; 
         FIG. 35  is a longitudinal sectional view of an alternative flapper valve arrangement for use with the completion suspension valve tool in accordance with the present invention, with the valve in the open condition; 
         FIG. 36  is a similar view to  FIG. 35  but with the flapper valve partially actuated to the closed condition; 
         FIG. 37  is a similar sectional view of the flapper valve housing with the flapper valve in the closed condition; 
         FIG. 38  is a sectional view of the flapper valve housing with the flapper valve shown in the supported condition where the valve provides differential containment from below and above the flapper valve; 
         FIG. 39  is an enlarged detailed sectional view of the flapper valve element and associated components, the valve being shown in the partially closed condition; 
         FIGS. 40   a  and  40   b  depict side enlarged sectional and bottom views of the flapper valve in the open position illustrating the coil windings of the torsion spring and the reaction lugs, and 
         FIGS. 40   c  and  40   d  are similar to  FIGS. 40   a  and  40   b  and depict the flapper valve in the closed position. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is first made to  FIGS. 1 to 3  of the drawings.  FIG. 1  depicts a longitudinal section of a wellhead system, BOP and marine riser for receiving a completion string with a suspension valve as shown in  FIG. 2 . The wellhead system depicts a wellhead  10  to which is coupled a blow-out preventer  12  to which in turn is coupled a marine riser  14 . Within the wellhead  12  there is shown intermediate casing  16  which is typically 13⅝″ in diameter and within the intermediate casing, inner casing  18 , which is typically 10¾″ in diameter. The foregoing structure typically forms a subsea wellhead system into which tools are run for well completion. 
     The completion string shown in  FIG. 2  consists of a suspension valve sub or housing  20  in which is located in a suspension valve  22 , which will be later described in detail, and which is coupled to completion tubing  24  which defines a production bore  26  and annulus bore  27  which runs through the completion sub  20 . The housing  20  is coupled to a tubing hanger  28  which in turn is mounted to a tubing hanger running tool  30  which is in turn coupled to a 5″×2″ subsea test tree  32 . The subsea test tree  32  is in turn coupled to a dual bore riser  34  which consists of a production riser  36  and an annulus riser  38 . 
     In use the completion string shown in  FIG. 2  is run into the wellhead system shown in  FIG. 1  to arrive at the arrangement shown in  FIG. 3  where the tubing hanger  28  is locked into the wellhead  10  with shoulders on the tubing hanger  28   a  abutting the inner faces  18   a  of the inner casing. In this position a locking profile  40  of the tubing hanger engages with a mating recess  42  in the interior surface of the wellhead to lock the completion string into the wellhead system in the position shown in  FIG. 3 . 
     Reference is now made to  FIG. 4  of the drawings which depicts a view of the wellhead part of  FIG. 3 . In this diagram, the valve  22  is shown in further detail with the valve being clearly depicted within the production bore  26  and having a valve actuation bar  44  shown coupled to a guide shaft  46 . It will be seen that the upper and lower ends of the guide shaft  46   a , 46   b  respectively are coupled to hydraulic lines  48 , 50  which pass through the completion sub housing, tubing hanger  28  and into the tubing hanger running tool  30  for connection to a source of hydraulic power (not shown) for actuating the guide shaft  46  to cause the suspension valve  22  to move between an open and a closed position, which will be later described in detail. 
     The suspension valve  22  is based on a rotatable apertured ball valve element similar to the type shown in  FIGS. 5   a  to  5   d  of the drawings.  FIGS. 5   a, b  and  c  depict a ball valve element  50  with a central aperture  51  being rotatably mounted in a conduit body  52 . This central aperture element is described first to facilitate understanding of the operating principle. It will be seen that the ball valve element has a pair of trunnions  54  (only one of which  54   a  is shown in detail) which are mounted on circular recesses  56  in the longitudinal conduit body. The ball element  50  has a pair of bar pockets  58  (one of which is shown) for receiving a pair of actuation bars  60 . The actuation bars have bar ends  62  which may be coupled to guide shafts similar to the guide shaft  46 . The guide shafts  46  are constrained to move rectilinearly and, as they move, they move the actuation bars  60 . The actuation bars are slideable relative to the centred ball valve  50  such that, as the bars move vertically downwards, the ball valve moves from the open position shown in  FIG. 5   a  through a partially closed position as shown in  FIG. 5   b  when the bars have been rotated approximately 45° to a fully closed position shown in  FIGS. 5   c  where the bars have been rotated through approximately 90°. It will be noted that in the positions  5   a ,  5   b  and  5   c  bar ends  62  have moved relative to the ball valve element  50  so as to allow the ball valve  50  to rotate between the open and closed positions shown in  FIGS. 5   a  and  5   c  respectively.  FIG. 5   d  depicts a side view of a ball valve element  50  without the actuation bars with the position of the bar pocket shown in broken outline. It will be appreciated that because the actuation bars  60  slide relative to the ball element, the locus of movement of the bar ends  62  is a straight line. 
     Reference is now made to  FIGS. 6   a  to  d  which depicts a similar apertured ball valve element but one in accordance with a preferred embodiment of the present invention. The ball valve element is an offset ball valve element as best seen in the top view of  FIGS. 6   a  to  6   d  of the drawings. The offset ball valve element  64  operates in a similar way to the centre ball valve element and it will be seen that the valve element  64  has an offset bore  66 , best seen in  FIG. 6   a , and also has two bar pockets  68 .  FIG. 6   b  shows a front view and  FIG. 6   c  is a perspective view. A sectional view taken on A-A to  FIG. 6   a  is shown in  FIG. 6   d  and it will be seen that one part of the ball element  70  has a greatly thickened section and the front section  72  is relatively thinner, this being the result of moving or offsetting the bore  66  from the centre. The offset ball element has trunnions  74  which project a short distance envelope from opposite sides. Trunnions  74  allow the ball element to be mounted within the valve housing  20 , as will be later described in detail, typically via bearings and the trunnions also further define an axis about which the ball element  64  rotates. The valve bore  66  is offset in one direction from the centre of the spherical ball element but the bore as best seen in  FIG. 6   a  is still centrally disposed between the trunnions  74 . In this particular application the production flow bore  26  is extremely close to the outside diameter of the valve element  64  so that this does not permit a ball element with a centred bore as shown in  FIGS. 5   a  to  5   c  to be accommodated. Accordingly, the offset ball element permits an offset centred ball valve to be disposed in the production bore  26  of the completion sub-structure shown in  FIGS. 1 to 4  and allows a ball valve element to be used as a basis for the completion suspension valve  22 . As will be later described in detail, a further advantage of the offset bore is revealed when the valve is viewed in the closed condition. The offsetting of the bore  66  allows less material to be present on one side of the ball to create the thin section  72  but also allows more material to be present on the other side of the ball to create the thick section  70 . As will be further described in detail, the thick portion  70  of the ball element is load-bearing under differential pressure in the closed condition and the increased thickness of portion  70  of the ball element  64  results in an increase in the differential capacity of the valve  22 . Therefore, for a given sphere and bore size differential pressure bearing capability is increased by offsetting the bore  66 . 
     Reference is now made to  FIGS. 7   a, b  and  c  of the drawings which depicts various views of an offset bore valve seat  76  for engaging with the ball element shown in  FIG. 6 . In particular,  FIG. 7   a  is the top view of the seat,  FIG. 7   b  is a side view and  FIG. 7   c  is a front view.  FIG. 7   d  is a sectional view taken on lines B-B shown in  FIG. 7   a.    
     It will be understood that the seat  76  acts as an intermediate seal element between the ball  64  and the valve housing  20 . In traditional apertured ball valves, such as the centred apertured valve shown in  FIGS. 5   a - d , the valve seat is a cylindrical element where the outside diameter of the cylinder interfaces with a bore in the body called a seat pocket. The bore of the cylinder is equivalent to, and axially aligned with, the bore of the valve. A partially concave hemispherical surface is disposed at one end of the cylinder. This surface interfaces with the spherical surface of the ball element. In traditional arrangements of centred ball valves, the bore, the outside ball diameter and the partially concave hemispherical end surfaces all share a common centre line and all these features may be considered to be concentric to one another. In the embodiment shown in  FIGS. 6 and 7 , it will be understood that the offset bore of seat  76  and the outside diameter or surface  78  of the seat  76  do not share the same general position as the axial centre lines are offset from one another. This offsetting results in a thin seat wall  80  occurring at one side of the valve seat  76  and the relatively thicker or heavier wall  82  occurring at the other side of the valve seat. This is best seen in  FIG. 7   d . In applications where the valve flow bores  66 , 67  are extremely close to the outside diameter of the valve body or housing at a particular point, there is insufficient room to accommodate a valve seat with a concentrically disposed bore because there would be too much material in the seat wall for the limited space available. When the valve seat  76  with the offset bore  67  is used, then the valve seat  76  is aligned such that the thinnest portion of the seat wall  80  is coincident with the space-constrained portion of the body. A further advantage of offsetting the bore  67  is that a larger outside diameter of seat  76  may be accommodated and the larger the outside diameter of the valve seat  76 , the greater is the area of contact that may be offered to the ball element  64  via the partially hemispherical face. This contact area is important as the bearing stresses that develop during differential loading of the valve are transmitted through this surface and offsetting the bore  66 , 67  permits the larger outside diameter which permits a larger pressure differential capacity to be used for a given bore and given offset ball valve body size. 
     A further function of the offset ball valve seat  76  is to engage sealingly with the valve element  64 . This seal is normally achieved by the incorporation of a resilient seal such as an elastomer O-ring between the valve seat and the seat pocket of the body. This elastomeric seal becomes fully effective when the valve is closed and the differential pressure is present across the valve. In traditional designs the valve seat is of the concentric type as described above and an elastomeric seal sits in a groove parallel to the end face of the seat and normal to the cylindrical axis of the seat pocket. However, because the offset valve seat has a portion with a thin wall  80 , the thinness of the wall may become a limiting factor in the ability of such a valve seat to contain such a differential pressure. Accordingly, the applicant, which involves providing a seal groove at an inclination such that at its lowest point the seal groove is orientated to be coincident with the thinnest portion of the offset valve seat  80  so that the length of the thin portion of the seat, which is exposed to the differential pressure, is minimised, presents a further inventive feature. This is best seen in  FIGS. 8   a ,  8   b  and  8   c  of the drawings which show respectively longitudinally partly sectioned views and partial cut-away views of a completion valve sub with a ball element seat pocket for receiving the offset ball element  64  shown in  FIGS. 6   a  to  6   d  and the offset ball valve seat  7   a  to  7   d . As best seen in  FIG. 8   b , the completion valve sub  20  defines the production bore  26  including trunnion receiving recesses  75 , only one of which is shown in the interests of clarity, for receiving the trunnion  74  of the offset ball valve element  64 . The offset ball valve is shown coupled to the offset ball seat  76  via an inclined or helical groove  84 . It will be best seen in  FIG. 8   b  that the lowermost point of the groove  86  is adjacent to the valve seat  76  at the point where there is a thin section  80 . In this position, when the ball  64  contacts the seat  76 , i.e. at the bottom of the seat, then the thin portion  80  of the seat gains support from the presence of the ball section  72  and the unsupported thin section or portion of the seat  76  is protected from differential pressure by the presence of the inclined helical seal  86 . Thus, it will be understood that the inclined seal groove  84  described above, when used in conjunction with the eccentric bore seat, maximises the bore size and differential pressure capacity of the suspension valve for a given bore offset and valve body diameter. 
     Reference is now made to  FIGS. 9   a  and  9   b  of the accompanying drawings.  FIG. 9   a  depicts a top view of a completion sub housing  20  with the ball element  64  in the closed position and  FIG. 9   b  is a longitudinal sectional view taken on the lines C-C of  FIG. 9   a , again showing the ball valve element in a closed position. It will be seen that the completion valve sub  20  has a housing which defines an elongate production tubing bore  26  in which is disposed the offset ball valve element  64 . The offset ball valve element is shown in the closed position with the thicker section  70  uppermost and the thinner section  72  lowermost. One actuation bar  60  is shown with the bar end  62  coupled to a guide shaft  46  which is moveable in response to hydraulic pressure within elongate guide shaft recess  47 . The shaft  46  is supported for movement by three chevron-bearing seals  49 . The top of the recess  47  is coupled to a hydraulic line  48  which when pressurised forces the guide shaft  46  downwards such that the bar end  62  is moved down with the shaft  46 , thus causing the ball element  64  to rotate to an open position, as will be later described in detail. The offset ball seat  76  is shown disposed just above the thickened ball section  70  and also the lower portion of the groove  86  is shown for receiving the inclined helical seal. 
     As will be later described in detail, the bore  26  contains a nipple  88  normally held in place to the housing  20  by a shear pin  90  which is engageable by a mandrel (not shown) for moving the nipple  88  when the ball valve has in the closed position and when shifted this nipple engages with a detent to retain the ball valve open; in this position it is known as the overridden open position. 
     Reference is now made to  FIG. 10  of the drawings which depicts the top part of the completion valve sub  20  depicting the main bore  26  and the guide shaft bore  47  in which is disposed the elongate guide shaft  46 . Also shown are the hydraulic lines  48 ,  50  which are coupled to opposite ends of the guide shaft recess  47  for actuating the guide shaft to move between the recess and open and close the ball valve. 
     Reference is now made to  FIGS. 11   a ,  11   b  and  11   c  which are respective longitudinal section views of the completion suspension valve housing showing the ball valve  22  normally closed, the valve normally opened and the valve in the overridden open position respectively. In the interests of clarity, some parts previously described have been omitted such as the hydraulic lines. 
     Reference is first made to  FIG. 11   a  of the drawings showing the ball valve in the closed position. There it will be seen that the thick section  72  is uppermost and abuts the offset ball seat  76 . In this position, the nipple  88  is in the uppermost position. Reference is now made to  FIG. 11   b  which depicts the ball element in the normally closed position. This has arisen by virtue of actuating the guide shaft  46  to move downwards within guide shaft bore  47  such that the bar ends  62  are disposed beneath the ball element  64 . In this position, the bore  66  of the offset ball is aligned with the production bore  26 , so there is communication through the entire production bore within the completion valve sub housing  20 . It will be appreciated that the diameter of the bore  66  is the same as the main production bore diameter  26  and this is due to the fact that the offset ball valve element  64  is used. 
     A hydraulic piston is formed by the inclusion of a seal  49  between the shaft and the valve body near the upper end of the shaft bore  47 . In the embodiment shown, the seal  49   a  is of a chevron or v-type packing and is made of non-elastomeric material, in this case Teflon, as a long service life is required. This type of seal is available from Greene Tweed although there are other suitable oilfield seals. A chamber  92  is formed by the inclusion of the seal  49   a  at the upper end of the shaft  46  and the hydraulic port  48  is provided in the upper surface of the housing and the chamber  92 . For convenience, chamber  92  is generally identified as the valve open chamber. 
     A further hydraulic piston is formed by the inclusion of a seal  49   c  between the shaft  46  and valve body  20  near the lower end  47   a  of the shaft bore  47 . Again, in this embodiment, the seal  49   c  is of the chevron or v-type packing. A chamber  94  is formed by the inclusion of this seal  49   c  at the lower end of the shaft  46  and the hydraulic port  50  is provided between the housing and this chamber  94  which is identified as the valve closed chamber. 
     When hydraulic control fluid is introduced to the valve open chamber  92 , any fluid in the valve closed chamber  96  is permitted to be displaced as the actuation shaft  46  is moved downwards. The ends  62  of the bars  60  connected to the shaft  46  move sympathetically from the shaft via the pin joint connection. It will be understood that the bar position is constrained such that it must always project its axial centre line through the centre of rotation of the ball by virtue of engagement with the bar pockets  68 . The bars  62  rotate about the ball centre and bear upon the inside faces of the bar pockets  68  into which they are engaged thereby causing rotation of the ball element  64  within the completion sub housing  20 . As the guide shaft actuation stroke proceeds the distance between the shaft/bar connection point and the ball centre is reduced. In addition to rotating the ball element  64 , the bar also engages further into the bar pockets to compensate for this diminishing distance. This situation prevails until the ball valve is rotated halfway in the actuation cycle at which point the reverse situation occurs and the actuation bars are retracted from the bar pockets and, as shown in  FIG. 11   b , full opening is achieved when the bottom end  46   b  of the guide shaft contacts the bottom surface  47   b  of the shaft bore  47 . 
       FIG. 11   c  depicts the overridden position where it will be seen that the nipple  88  has been moved down towards the ball valve element  64  to lock the element in the open position as will be later described in detail. 
     Reference is now made to  FIGS. 12   a, b, c  and  d  which are diagrammatic views which better illustrate the completion valve assembly within the valves in the closed position, and to  FIGS. 13   a, b, c  and  d  which better illustrate the same valve assembly but with the valve in the open position. 
     For convenience,  FIGS. 12   a, b, c  and  d  should be read with  FIG. 11   a  and likewise  FIGS. 13   a, b, c  and  d  should be read with  FIG. 11   b.    
     Turning first to  FIG. 12 ,  FIG. 12   a  shows a top view with the ball element in the closed position, and  FIG. 12   b  is a section taken on the lines A-A of  FIG. 12   a .  FIG. 12   c  is a section taken on lines B-B through the completion sub-housing at the level of the nipple, and  FIG. 12   d  is a section taken through the completion sub housing and through the bar ends on lines C-C. 
     Like parts refer to like numerals already described and it will be noted that in  FIG. 12   c  the nipple  88  has a general U-shape which surrounds the production bore  26  and also has legs  88   a  and  88   b  which surround respective guide shafts  46 . Referring to  FIG. 12   b , it will be seen that the legs  88   a , 88   b  rest on an annular land  96  of the guide shafts  56  for forcing the guide shafts  46  into the open position as will be later described in detail. In the position shown in  FIG. 12   b , it will be seen that the nipple  88  is secured to the completion sub-housing  20  by virtue of shear pin  90 , as, best seen in  FIG. 12   c.    
     Reference is also made to  FIG. 13  which depicts the valve in the open position with the ball bore  66  aligned with the production bore. In the position shown in  FIG. 13   b  it will be seen that the guide shaft  46  has been moved within the guide shaft bore  47  as previously described with the amount of travel being limited by the abutment of the annular land  96  on shoulders  98  within the shaft bore  47 . Sectional views shown in  FIGS. 13   c  and  13   d  are taken at the same level as those in  FIGS. 12   c  and  d  for ease of comparison. 
     Thus, it will be understood that in response to hydraulic pressure applied via hydraulic lines  48 , 50  to the guide shaft  46 , the shaft being coupled to the ball element  64  causes the ball element to move between closed positions shown in  FIG. 11   a  and  FIGS. 12   a - d  and the open position shown in  FIG. 11   b  and on  FIGS. 13   a - d.    
     Reference is now made to  FIG. 14  of the drawings which depicts a wellhead with a completion suspension valve in accordance with an embodiment hereinbefore described disposed within the wellhead similar to that shown in  FIG. 4  but the system is also shown in production mode with a dual bore production xmas tree  100  shown coupled to the wellhead, and the hydraulic control system  102  shown coupled to an umbilical  104  and the production xmas tree  100  for controlling hydraulic operation of the completion suspension valve  22 , as well valves within the production xmas tree  100 . At the top of the xmas tree  100  is a tree cap. 
     Reference is now made to  FIGS. 15   a  and  15   b  of the drawings which are similar to  FIG. 14  but which depict an isolation valve override piston  108  carrying a mandrel  110  shown coupled to the top of the lower riser package with the piston  108  being shown in the retracted position in  FIG. 15   a  and being shown in the extended position in  FIG. 15   b . It will be seen that in  FIG. 15   b  the piston  108  has a shaft which is sufficiently long to allow it to extend through the lower riser package  107 , the xmas tree  100 , the tubing hanger  28  and the top part of the suspension valve sub housing  20  for engaging with the override nipple  88  to lock the ball element in the open position as will now be described. 
     The hydraulic piston  108  is part of the valve override tool package which is extendable to deploy a tool which interfaces with the override nipple  88  of the valve  22 . The piston  108  may be a multi-stage telescopic device and is extended and retracted by the supply of hydraulic chambers to fluid within the ram housing (not shown in the interests of clarity). As will be appreciated by a person of ordinary skill in the art, such an arrangement is consistent with double-acting hydraulic rams which are widely used throughout many areas of industry. The piston housing  112  is itself mounted to a hydraulic connector which, in turn, is connected to a profile  114  at the upper end of the subsea safety package and this connection allows both a structural and pressure type connection between these elements. A safety package consists of one or more valve or piston/ram elements disposed in the production bore and annulus bores which are capable of cutting obstructions and which may straddle them and thereafter sealing such that the well is isolated. The safety package is in turn connected to the top of the xmas tree  100  via a hydraulic connector (not shown in the interest of clarity) which allows a structural and pressure type connection to be established between the lower riser safety package  107  and the xmas tree  100 . 
     Reference is now made to  FIGS. 16   a ,  16   b  and  16   c  of the drawings of diagrammatic cross-sectional views through part of the suspension valve sub  20  and, in particular, depicting a cross-sectional view through the nipple  88  and valve override tool/mandrel  110  at the lower end of the piston  108 . 
     Reference is first made to  FIG. 16   a  where it will be seen that the ram override tool or mandrel  110  consists of a lower mandrel portion  114  which is mounted and moveable within a mandrel sleeve  116 . A pair of spring-loaded dogs  120  are located within windows  122  and are compressed between the surface of the mandrel  124  and the wall  25  of the production bore  20 . The dogs have springs  126  which are shown compressed in  FIG. 16   a  which is also the position for mandrel deployment so that the dogs are shown in the retracted position. 
     The overriding operation begins by establishing the tool package  110  on top of the xmas tree  100 . This can occur subsea by establishing the package onto an already present xmas tree as described above or, alternatively, the xmas tree and override package may be run simultaneously and it will be appreciated that in the latter scenario the override package also incorporates the functionality necessary to run the xmas tree. As shown in  FIGS. 15   a  and  15   b , once the override package xmas tree and wellhead system are established, the xmas tree valves are opened and the override tool is extended into the position best seen in  FIG. 15   b.    
     Reference is now made to  FIG. 17   b  which depicts the override mandrel  114  moved down relative to the nipple  88  such that the dogs  120  are disposed adjacent the circumferential nipple groove  89 . In this position the springs  126  bias the dogs  120  into the groove  89  as shown in  FIG. 16   b , so that the body of the override tool is engaged with the override nipple  88 . Further downward movement of the override tool causes the mandrel  114  to move downwards and causes the mandrel surface  124  to compress the springs  126  and support the dogs  120  in the position shown in  FIG. 16   c . In this case, the override tool  110  is securely engaged with the override nipple  88 . 
     Reference is now made to  FIG. 17  of the drawings which is an enlarged view of part of the completion valve sub housing as shown in  FIG. 12   c  and showing the override nipple  88  connected to the suspension valve housing by virtue of the shear pin  90 . It will also be clearly seen in  FIG. 17  that the valve nipple and leg  88   a  have a lower portion  88   d  which abut an annular land on the guide shaft  46 . Also, as shown in  FIG. 16   c , shoulders  130  of the mandrel engage with inner shoulders  132  of the mandrel sleeve. Continued pressure on the piston and mandrel forces against the override nipple  88  so the pressure is sufficient to shear the shear pin  90 . When the pin  90  is sheared the nipple  88  is moved downwards to the position shown in  FIG. 11   c . Reference is also made to  FIGS. 18   a - d  which are similar to  FIGS. 12 and 13 , with sectional views  18   c  and  18   d  taken at the same level as in the aforementioned diagrams. 
     It will be seen that the nipple legs  88   a , 88   b  contact the annular land around the guide shafts and consequently the guide shafts are also moved downwards to the position shown in  FIGS. 11   c  and  18   d.    
     Reference is also made to  FIG. 19  of the drawings which is a similar view to  FIG. 17 . 
     As best seen in  FIG. 17  it will be seen that the outside surface of the nipple  88  has a notch  134  for receiving a detent finger  136  ( FIG. 19 ) which is located in the valve housing  20  at the position shown when the nipple  88  is at its lowermost position. In this position, as shown in  FIG. 19 , the notch  134  is engaged with an upper angled face  138  of the detent finger  136 . The detent finger is resiliently biased so that it exerts force to retain it in the position shown in  FIG. 19 . It will be appreciated that the upper end of the detent finger and its corresponding groove are shaped like a barb and once the finger  136  engages with the nipple  88  and is resiliently retained in this lowermost position, the ball valve element is maintained in the open position and this position is known as the overridden open position. 
     The override tool  110  then retracts the piston  108  and this will initially retract the mandrel  114  and desupport the spring-loaded dogs  120 . Further retraction of the piston  108  develops sufficient force to cause the dogs  120  to collapse into the windows  122  due to the angled mandrel surface  124  at the top of the nipple groove. Once the dogs  120  are collapsed, the override tool  110  is free to disengage with the nipple  88  which is then retained in the downward position shown in  FIG. 20  by the detent finger  136 . The piston  108  may be retracted and the appropriate xmas tree functions performed and the override package  108  may then be retrieved. 
       FIG. 20  is a partly broken away and perspective view showing the completion suspension valve  22  in the overridden position with the offset ball element  64  held open and abutting the offset valve seat  76  with the override nipple  88  shown abutting the annular shoulders on the suspension valve guide shafts  46  and the detent finger  136  showing engaged with the notch  134  in the override nipple  88 . 
     It will be understood by those of ordinary skill in the art that an efficiently packaged valve arrangement such as that described above with reference to the completion suspension valve has further applications. For example,  FIGS. 21 to 23  illustrate such applications. 
     Firstly with reference to  FIG. 21  this shows the application of the completion suspension valve to an in line tree. In this particular application, a valved tubing hanger  140  is provided within a wellhead  10 . Like numerals refer to like parts as described above with reference to  FIGS. 1 to 20 . Thus, it will be appreciated that the valves and actuators within the valve tubing hanger located inside the envelope of the wellhead bore and this arrangement is enabled via the use of a contact arrangement of components such as the completion suspension valve. 
     Reference is now made to  FIG. 22  which depicts the application of a completion suspension valve in a subsea installation tree, generally indicated by reference numeral  152 . It will be appreciated that the valves may be suitable for use in a 5″×2″ dual bore subsea installation tree (Expro North Sea Limited). The dual bore subsea tree provides both 2″ annulus valves and 5″ production bore valves in the annulus bore  27  and production bore  20  respectively and actuators within the envelope defined by the bore of the marine riser  158  and the BOP  160 . 
     A further application of the completion suspension valve described above is depicted in  FIG. 23  where it is used for a hybrid tree insert. In a conventional or dual bore system, the tubing hanger is landed and locked to the wellhead. The xmas tree is subsequently landed on top of the hanger which implies that the tree must be removed prior to the retrieval of the tubing hanger. 
     In contrast, in a horizontal system the xmas tree is established onto the wellhead and the tubing hanger subsequently landed on a shoulder inside the tree. This implies that the hanger and tubing must be retrieved prior to retrieving the tree. 
     In a further arrangement, as best seen in  FIG. 23 , the hanger  28  is run through the wellhead  10  and locked thereto. A tree  162  with a bore  164  large enough to allow through passage of the hanger  28  is then run and established onto the wellhead  10 . A valved insert  166  is located within the bore  164  and the insert  166  with the tubing hanger  28  serves to divert flow from the hanger  28  into the tree outlet  170 . It is convenient if the insert  166  utilises valves  172  to divert the flow and these valves  172  occur in the restricted envelope defined by the tree bore  164  and the valve function is fulfilled by the valve arrangement outlined as described above. 
     Reference is now made to  FIG. 24  of the drawings which is a longitudinal sectional view through part of the main body of a completion suspension valve housing in accordance with a preferred embodiment of the invention and depicting how the completion suspension valve sub may be assembled in accordance with the preferred embodiment. In this arrangement, the main body  20  provides a large axis bore  174  between its bottom face  176  and its ball cavity  178 . The seat seal, ball seat, apertured ball element and actuation valve may be inserted through the large axis bore. The lower body engages with trunnion mechanisms which support and locate the ball element. The main body and the lower body include weld preparations which allow a circumferential weld to be performed. The weld is about ⅙th of the distance from the bottom, at about 5-10 cms before the body widens to its full diameter, thereby unitising the main and lower bodies permanently. It will be understood by a person of ordinary skill in the art that a low heat process such as electro beam welding is preferred to avoid risk of damage to heat sensitive components. Referring also to  FIG. 10 , it will be seen that the guide shafts are assembled within the valve housing by removing access cap  176  which sealingly engages with the valve body  20 . A thread  178  and the outside diameter of the cap  176  engages with a thread  180  at the top of the body  20 . The guide bar  46  is installed into the guide bore  47  within the body  20 . A closed chamber is thereby formed around the guide bar. The hydraulic control port  48  communicates with the upper end of the body as described before such that hydraulic fluid is supplied into the closed chamber and, as also described above, a similar arrangement occurs at the lower end of the body to enable the valve closed chamber to be formed. 
     An alternative arrangement of assembling a completion suspension valve is hereinbefore described with reference to  FIGS. 1 to 20  is depicted in  FIG. 25   a, b  and  c  of the drawings.  FIGS. 26   a, b  and  c  illustrate a two-piece main body  182 , 184  with the body being split down a vertical plane with two large access windows  186 , 188  machined into one half. The offset ball element and offset ball seat are installed through the lower window  188  and the override nipple  88  is installed through the upper window  186 . The guide rods  46  are installed in the remaining body half and a gasket or seal  190  is fitted round the periphery of each window. The two body halves  182 , 184  are then brought together and the windows  186 , 188  are then covered and sealed. An array of cap screws  196  are installed around each window and at the top and bottom of the body providing a closure with sufficient strength to resist the separate forces developed by fluid pressures within the completion suspension valve. 
     Reference is now made to  FIG. 26  of the drawings which depicts an enlarged sectional side view of part of the ball element and bearing element used for mounting the ball within the completion suspension valve housing to permit the ball to be held unloaded from the seat during rotation and which is then allowed to float on the valve seat for sealing. 
     It will be understood that the increased torque delivered by the bar rotation mechanism is desirable as it increases operating reliability. Similarly, reliability can be enhanced by reducing friction losses encountered during rotation of the ball. This is achieveable by ensuring that the ball rotates by virtue of its trunnions engaging with bearings and not by virtue of the sphere of the ball engaging with the partial hemispherical surface of the valve seat. Ensuring that constant rotational constraints are caused at the smallest radius possible, ensures that such frictional forces or losses are minimised. 
     During rotation of the ball it is desirable that its position is fixed and determined by the bearing position. Accordingly, the valve seat may be tentatively pushed on top of the ball by a small spring to maintain contact and prevent ingress of debris between the sealing surfaces of the ball and seat. Frictional losses arising from such contact are always in proportion to the very small force exerted by the spring and are constantly considered to be negligible. 
     However, in the closed condition, the contact between the ball and valve seat is only sufficient to contain a very small differential across the valve element. It is desirable therefore that the contact force between the ball and valve seat increase in response to an increase in differential pressure to maintain a contact force in proportion to the prevailing differential pressure and resulting in higher sealing reliability of the valve. 
     The arrangement shown in  FIG. 26  permits this to be achieved when in the closed condition, so that bearings on which the ball is located are allowed to move upwards either in the presence of a differential or when the ball is fully in the closed position. This solution is achieved by allowing the ball to float upwards by machining the trunnion  74  so that it is no longer a complete cylindrical extrusion emanating from each side of the ball. As shown in  FIG. 26 , areas of the trunnion  190  have bee machined away from either side leaving only a central portion  192 . Both trunnions  74  are machined in this way and effectively this leaves each trunnion with its circular surface divided into two curved parts  194   a,b . Surfaces  194   a,b  engage with the bore  196  of a plane bearing  198 . Plane bearing  198  is mounted on a pocket  200  cut into the inside surface  25  of the valve body  200  and thus when the circular portions  194   a,b  of the ball trunnion  192  are engaged with the bore  196  of the bearing, the position of the ball element relative to the valve body is fixed. 
     It will be understood that this relationship is only operational as long as the ball is not in the closed position. When the ball is rotated to the fully closed position, the trunnion bearing upper surface  194   a  is adjacent to a rebate  200  in the bore  196  of the plane bearing. A differential pressure applied from below across the valve results in ball  64  following the seat  76 . The ability of the ball to move allows the contact force between the ball and seat to intensify in proportion to the prevailing differential pressure, thus ensuring that high sealing integrity is achieved. Axial seat travel is limited by a shoulder  201  which contact the top of a pocket  203  in the body bore. The amount of ball float always exceeds the available seat travel to ensure that a compressive load is maintained. 
     As differential pressure is removed, the corresponding pressure force it exerts on ball and seat system decreases. When this force decreases to a value less than that exerted by a seat spring  202 , the spring  202  pushes the seat  76  and ball  64  downwards until the trunnion load bearing face  194   b  contacts the bore  196  of the plane bearing. In this position the ball is once again ready to be rotated to the open condition and the position of the ball is once more fixed relative to the valve body. 
     Embodiments of the invention also permit the valve to be overridden to the open position and furthermore the overriding means do not require a rigid riser to the surface. The use of the offset bore allows the provision of a ball valve within a confined space and differential thickness on either side of the valve allows the ball to accommodate an increase in the differential capacity of the valve for a given sphere and bore size. 
     Furthermore, offsetting the bore allows a larger outside diameter of seat to be accommodated so that a greater area of contact is offered to the ball via the partial hemispherical face. In addition, the use of a seat seal groove when used in conjunction with the eccentric bore seat maximises the bore size and differential capacity for a given bore offset and body diameter and the use of the incline bore allows the thin portion of the seat to be supported from the presence of the ball. 
     In the case of the apertured ball valve embodiment, the use of the sliding actuation bars permits relative rotation of the movement between the mechanism and the bars with the result that a torque can be developed which is further from the ball centre resulting in higher torques and higher reliability of movement. 
     Further reliability is enhanced by further reducing frictional losses encountered during rotation of the ball by using a floating ball element to maintain a contact force in proportion to the prevailing differential pressure which results in higher sealing reliability of the valve by ensuring that a compressive load is always maintained with the amount of ball float exceeding the available seat travel. 
     In the foregoing description it will be understood by those of skill in the art that an annulus bore and annulus valve is provided on each of the embodiments and that operation of the annulus valve is performed using existing well known annular valve control techniques. 
     A further modification to the embodiments of the invention described above is shown in  FIGS. 27 to 33 . It has been described above how the provision of a valve in the production bore of the tubing hanger is beneficial. It will be understood by those of ordinary skill in the art that many wellhead equipment manufacturers already posses concepts and solutions for the provision of a remotely operable barrier in the annulus bore of the tubing hanger. The completion suspension valve invention already described could be used in conjunction with such an “annulus valved hanger”. This would provide a system in which remotely controllable barriers occurred in both the production and annulus bores and is a configuration for facilitating the trees-on-wire deployment philosophy, previously outlined. 
     One implication of the configuration outlined above is that each valve is manipulated by a dedicated actuator each of which, in turn, is served by both open and close lines. It will be understood that the space available to accommodate these actuators, ports and interfaces is limited and it may be extremely difficult to include all the necessary features within the given envelope. Further the provision of multiple actuators with their associated control lines creates an increasing quantity of penetrations through the hanger body. It is generally accepted that in the interests of simplicity and reliability that the number of penetrations through the hanger should be kept to an absolute minimum because each penetration is perceived as a potential leak path. 
     With a further reconfiguration of the completion suspension valve actuator described, a system is provided whereby a single actuator provides simultaneous control to both the production valve and an annulus valve. This minimises the quantity of actuators required to one and also minimises the control line requirements to two (one open and one close). With this approach it becomes significantly easier to provide both annulus and production bore valves within the confined envelope already described. By adopting this approach a further benefit is enabled which will now be described. 
     The importance of providing a means to override the suspension valve  22  has already been described above. In the embodiment previously described, a nipple  88  attached to the actuator rod  46  was provided which was manipulated by a hydraulic ram  110 . Whilst this is an adequate solution, an alternative, simpler method of override is described in this embodiment which will be described in more detail later. 
     There now follows a description of the valve with reference to  FIGS. 27-33 . The production bore valve closure elements, their location and method of rotation are all as previously described and like numerals refer to like parts but with the suffix ‘a’ added. The main difference arises in relation to the actuator rod. 
     Firstly, with reference to  FIG. 27 , the lowermost portion of a single actuator rod  46   a  connects to a yoke  200  which is in turn connected to the two rotation bars  201 , one of which is shown. This actuator rod  46   a  is sealed at its lower end to the valve body via a v-type packing  202 . Above this the rod  46   a  provides two piston portions  204 , 206 . The lower of these  204  is the actuation portion, comprising the close chamber  208  and open chamber  210  to which control fluid is supplied via lines  48   a , 50   a  respectively and vented to cause cycling of the valve  22   a . The upper piston  206  is an equalising piston with a lower chamber  212  ported to the annulus  27   a  and the upper chamber  214  ported to the production bore  26   a . These upper and lower piston systems are separated by a seal  216 , again of the v-packing type. The actuator stem  46   a  exits the main body  20   a  of the production valve assembly via a further v-packing seal  218 . The actuation rod continues upwards to interface with the annulus bore  27   a  in the tubing hanger  28   a  via a final v-packing seal  220 . 
     A side port  222  which communicates with the well annulus  27   a  intersects with the annulus bore in the tubing hanger  28   a . The position of the latter, uppermost v-packing  220  relative to this side port  222  indicates whether the annulus bore  27   a  is closed or open. When the actuator rod  46   a  is in its uppermost position, the v-packing  220  sealingly interfaces the annulus bore  27   a  above the side port  222 . This effectively closes the annulus bore  27   a . When the actuator rod  46   a  is in its lowermost position the v-packing  222  sealingly engages the annulus bore  27   a  below the side port  220  ( FIG. 32   b ). This means that the annulus may be considered open. Under normal operation of the actuator rod  46   a  therefore the v-packing  220  may be considered to be acting as a “valve” and may be referred to as such in the following text. 
       FIG. 28  shows the valve  22   a  in the open position. This position is achieved when normal actuation is performed via the hydraulic lines  48   a , 50   a  as outlined previously i.e. to open, pressure is applied into the open line  50   a  and pressure is vented from the close line  48   a . This causes the actuation rod  46   a  to move downwards. The production bore valve  22   a  is rotated as previously described to the closed position shown. In the annulus bore  27   a  the uppermost v-packing  220  is translated from a position above a side outlet ( FIG. 27 ) to a position below said side outlet  222 . In this lowermost position a fluid path is established between the well annulus  27   a  and its corresponding outlet  224  on the top face of the hanger  28   a . Closure of both the production and annulus bores  24   a , 27   a  is the reverse of the aforementioned process. 
     The presence of the actuator rod  46   a  in the annulus bore  27   a  now conveniently accommodates an alternative method of override. Inspection of  FIGS. 27 and 28  reveals that the override nipple  88  present in the initial embodiment has been omitted in this embodiment. Override is instead performed by dropping a sealing override plug  226  down the annulus bore onto the top of the actuator rod and pressuring it downwards. There now follows an illustrative sequence of events best described with reference to  FIG. 29 . 
     The tubing hanger  28   a  is installed and locked and tested. The production and annulus valves are closed. The xmas tree  12   a  has been deployed and locked onto the wellhead  10   a  and the appropriate testing conducted. The production, annulus and control stabs have been established between the xmas tree  12   a  and the hanger  28   a . An unsuccessful attempt is now made to open the tubing hanger valves  22   a . The valve  22   a  now requires opening by another means or, in other words, overriding. 
     Override operations commence with an ROV (remotely operated vehicle) (not shown in the interest of clarity) pulling a selector handle  228  at the top end of the xmas tree running tool  230 . This releases the override plug  226  which falls down the annulus bore  27   a  until it contacts the upper end of the actuator rod, as shown in  FIGS. 31   a , 31   b . The override plug is shown in  FIG. 30 ; it is generally elongate and has an upper tubular housing  232  coupled to a lower plug pin  234  by a shear pin  236 . Spring-loaded detent fingers  238  are retained against the housing wall by a retaining ring  240 . At the top of the housing a V seal stack  242  is located. Pressure is now applied into the annulus bore  22   a  above the plug  226  which results in downwards movement both of the plug  226  and actuator rod  46   a . The pressure can be supplied either from an installation umbilical which terminates in the tree running tool or the power may be provided by the ROV which may dock with the tree running tool. Pressure continues to be applied until the actuation rod  46   a  is displaced to its lowermost condition, as shown in FIGS.  32   a  and  b , at which point both valves are open and the actuation rod  46   a  endstops. 
     In  FIG. 32   a,b  the seal stack  242  of the override plug  226  remains above the side outlet  222  of the annulus bore  27   a , keeping said bore effectively sealed. Pressure is further increased above the override plug seal stack  242  with the lower end of plug pin  234  abutting the endstoppped actuation rod  46   a  and this pressure develops a force across the shear pin  236  which subsequently breaks. This allows the upper housing  232  of the plug to travel downwards and he upper end  232  of the plug  226  travels past the end of the annulus bore side outlet  222 , thereby rendering the annulus open, as best seen in  FIGS. 33   a,b . As the upper end  232  of the plug  226  engages with the lower plug pin  236  the detent ring  240  is lifted. The spring-loaded detent fingers  238  are released which engage in a mating groove  244  in the annulus bore  27   a . Both bores are now fully open and the actuator rod  46   a  is locked in the fully open condition. 
     It will be understood that the embodiment shown in  FIGS. 27 to 33  can be used instead of the embodiments described with reference to  FIGS. 1 to 26  and can also be used in all of the aforementioned applications. 
     It will also be understood that a single actuation rod may be used in the embodiments described with reference to  FIGS. 1 to 26  as for the embodiments in  FIGS. 27 to 33 , i.e. a single actuation rod with a T-stem or yoke to couple to the actuation bars. Also for all embodiments, two actuation bars may be disposed in parallel on each side of the ball valve element; one bar above and the other bar below the centre of rotation. 
     Various modifications may be made to the embodiments hereinbefore described without departing from the scope of the invention. For example, although the completion suspension valve is described with reference to use of an apertured offset ball valve element, a different type of valve structure may be used to achieve the same function. In  FIGS. 34   a, b  and  c , an alternative completion suspension valve is shown in which the valve element is provided by a hinged flapper valve  300 . In  FIG. 34   a , the flapper valve is shown in the normally closed position in which the valve is biased closed to block the production bore  302 .  FIG. 34   b  depicts the valve in the normally open position in which an internal sleeve  304  is moved down to abut the valve and force it into the open position where it lies parallel to the valve bore. Movement of this sleeve is achieved hydraulically using hydraulic lines in a similar manner to previously described as will be understood by a person of ordinary skill in the art. In this position, the function of the valve is similar to the system with the apertured offset ball valve  64 .  FIG. 34   c  shows the valve open but in the overridden position and it will be seen that the valve sleeve  304  has been forced down such that it is further towards the lowermost portion of the valve housing; in this position the valve is maintained fully open for reasons set forth above. It will also be appreciated that the valve bore in this arrangement is, like the apertured ball valve, is offset, i.e. it is not centred on the valve housing. 
     It will be understood that flapper valves are widely used in the oil and gas industry as down hole safety valves. These valves are incorporated in the completion tubing of a producing well at a location typically 200 meters approximately below the wellhead. These valves are operated by a single hydraulic line which conveys control fluid from the lower end of the tubing hanger down through the primary annulus and into the actuator of the valve. These flapper valves are typically failsafe-closed valves and rely on a torsion spring to deliver the flapper to the closed condition. The actuator is typically imbalanced to well bore pressure. In the open condition control pressure must be maintained on the valve control line to hold an actuation sleeve in its lowermost position. In this position, the actuation sleeve displaces the flapper element, rotating it via a pivot pin to a position outside the system bore. When the well bore pressure is present, venting the control pressure allows the actuation piston to travel upwards. As the piston travels upwards the flapper valve element is encouraged to rotate by the torsion spring. Once the piston has reached its uppermost position the flapper valve element engages on to a seat whereby the bore is occluded and a seal is established. Increasing pressure from beneath the valve increases the intensity of the force and hence the integrity of the seal. 
     These flapper valves are designed to isolate the formation from the surface equipment. Consequently the ability of such valves to provide only differential containment from below is perfectly adequate for the intended purpose. It would, however, be advantageous if a similar valve existed for containing differential pressure from both directions. Such a valve would have many applications such as, but not limited to; landing string lubricator valves; landing string retainer valves and lightweight intervention system lubricator valves. 
     It is also an object of the present invention to provide a flapper valve assembly with bi-directional sealing performance and so permit the use of the flapper valve in the aforementioned applications. 
     The structure shown in  FIGS. 35-40  achieves this and a detailed description of this flapper valve housing assembly will now be given. 
     Reference is first made to  FIG. 35  of the drawings which depicts a flapper valve housing or sub generally indicated by reference numeral  320 . The housing  320  has a threaded connection  322  for connection to a tubing hanger (not shown) as described above. Disposed within the housing  320  are an upper piston  324  and a lower piston  326 , the upper and lower pistons being movable within a bore  328  of the housing  320 . The upper end of piston  324  is engaged with the bore  328  of the main body by a seal  330  and the middle portion of the upper piston has annular shoulders  332  which also engage with the main housing  320  via a seal  334 . The diameter of seal  330  is less than the diameter of seal  334  and a hydraulic chamber generally indicated by reference numeral  336  is formed between these seals. Chamber  336  is also known as is also known as the upper piston top chamber. A hydraulic control port  338  conveys hydraulic fluid from the top of the main body  320  via a hydraulic line  340  to the hydraulic chamber  336 . 
     In  FIG. 35  there is shown an upper seal ring generally indicated by reference numeral  342  which is connected by a threaded connection  344  to the housing  320  so that the seal ring  342  is rigidly engaged to the housing. It will be seen that seal ring  344  engages with the outer diameter of the lower part  348  of the piston  324  and to the inner diameter of the main body  320 , via seals  350  and  352  respectively. 
     A further hydraulic chamber is formed between the seals  334 ,  350  and  352 , this further hydraulic chamber generally indicated by reference numeral  354  and is known as the upper piston bottom chamber. Chamber  354  is best seen in  FIG. 36  when the upper piston has been moved upwardly. A hydraulic control port  358  connected via hydraulic line  360  to the upper piston bottom chamber  354 . 
     Similarly the lower piston  326  is sealed to the main housing body  320  via seal  362 . The upper part of piston  326  is sealed to a lower seal ring  364  via seal  366  which is located on the outside diameter of part of the piston  326 . This also seals to the inside diameter of the main body. A hydraulic chamber  368  is formed between seals  362  and  366  and is the lower piston top chamber  368 . The hydraulic control port  370  conveys hydraulic fluid from the top of the main body  320  to the lower piston top chamber  368  via hydraulic line  372 . As with the upper seal ring  342  the lower seal ring  364  is threadedly engaged via connection  374  to the housing body  320  and a lower end cap  376  is coupled via threaded connection  378  to the main body  320 . The lower end cap  376  seals the lower portion  380  of the piston via seals  382 ,  383  which are connected between the external diameter of the lower portion of piston  326  and the internal diameter of the end cap  376 . A further hydraulic chamber  384  (best seen in  FIG. 38 ) is formed between seals  362  and  382 . This chamber is known as the lower piston bottom chamber  384 . The hydraulic control port  386  conveys hydraulic fluid via hydraulic control line  388  to the chamber  384 . 
     The lower end cap  376  offers a downward facing thread  390  for subsequent connection to a tubular member. 
     As best seen in  FIG. 39  a seat ring  392  is connected to the upper seal ring  342  by threaded connection  394  and accordingly the seat ring  392  is rigidly connected to the upper seal ring  342 . Still referring to  FIGS. 35 and 39  it will be seen that the seat ring engages with a pivot pin  394  on which a flapper valve element  396  is mounted. 
     As now described with reference to  FIGS. 40   a ,  40   b ,  40   c  and  40   d  a torsion spring  398  is disposed around the pivot pin  394 . The torsion spring  398  has reaction lugs  400 ,  402  which engage respectively with the outside of the seat ring  392  and the reaction spigot which bears on the rear side  404  of the flapper element  396 . The torsion spring  398  is configured such that the coils  398   a  of the torsion spring (best seen in  FIG. 40   b ) bias the flapper valve element  396  to move to the closed position shown in  FIGS. 40   c  and  40   d  when the upper piston  324  is actuated upwardly. 
     The operational sequence of the flapper valve assembly  320  will now be described with reference to  FIGS. 35-40 . 
     Firstly with reference to  FIG. 35  will be seen that this Figure depicts the flapper valve element  396  in the fully open position with both the upper and lower pistons  324 ,  326  being disposed in their lowermost positions. The flapper element  396  is displaced into an annular cavity  406  between the upper piston  324  and the main body housing  320  and advantageously the flapper element  396  is protected from flow in this position by the presence of the upper piston  324 . 
     The upper top piston chamber  336  is vented by operating a valve (not shown) at the control system which permits the fluid trapped in the chamber  336  to return to a tank (not shown) in the control system, via a control line, allowing hydraulic fluid in the chamber  336  to be discharged. The venting of the hydraulic chamber is achieved using control lines, a tank and a venting arrangement of a type that is well known in the art. Hydraulic pressure is then applied via line  360  to the upper piston bottom chamber  354  and as a result of this pressure differential the upper piston  326  is moved upwards to a position best seen in  FIG. 36 . The flapper valve element  396  which had been pushed to the outside diameter of the piston and retained in annular space  406  is now rotated about pivot  394  into the bore  328  under the action of the torsion spring  398 . The rotation of the flapper valve element  396  is controlled by the position of the upper piston  324 . The upward travel of the upper piston  324  continues until the shoulder  332  of the upper piston  324  abuts the shoulder  408  of the main body as best seen in  FIG. 37  at which point the upper piston  324  is considered to be in its uppermost position. At this point the flapper valve element  396  has fully rotated as shown in  FIG. 37  to be engaged with valve seat  405  of the seat ring  342  at which position the valve is considered to be fully closed. Hydraulic pressure into the upper piston bottom chamber  356  may now be vented in a similar manner to that described above. 
     In this condition the flapper valve arrangement is capable of providing differential pressure containment from below the flapper valve element  396 . However it will be understood that, if a differential pressure is applied from above the flapper valve element  396 , this would cause the flapper element  396  to move off the valve seat  342  and allow the pressure to pump through the bore  328 . 
     The lower top piston chamber  368  is now vented in a similar manner to that described above allowing hydraulic fluid therein to be discharged. Hydraulic pressure is then applied to the lower piston bottom chamber  384  via hydraulic line  388  and as a result of the pressure differential the lower piston  326  is moved upwards as best seen in  FIG. 38 . The lower piston  326  travel continues until the upward facing conical face  410  at the top end of the lower piston  326  engages with a similarly shaped conical surface  412  on the underside of the flapper valve element  396 . In this position the lower piston  326  effectively pushes the valve element  396  on to the valve seat  405  best seen in  FIG. 38 . In this condition the flapper valve element is locked and the assembly is now capable of providing differential containment both from below and above the flapper valve element  396 . The magnitude of the containment from above is related to the pressure prevailing in the lower piston bottom chamber  384 . In the arrangement illustrated in order to contain a given differential pressure from above then a similar pressure is required to be applied to the lower piston bottom chamber. 
     Opening of the flapper valve element  396  is the reverse of the previously described sequence. The lower piston  326  is first moved back to its lowermost position shown in  FIG. 1  by applying hydraulic pressure via port  370  and hydraulic line  372  followed by applying hydraulic pressure to the upper hydraulic chamber  366  via port  338  and hydraulic line  340  to force the upper hydraulic piston  324  back to the position shown in  FIG. 35  which in turn would displace the flapper valve element  396  back to its annular recess  406 . 
     Reference is now made to  FIGS. 40   a,b,c  and  d  of the drawings which depicts the torsion spring  398  disposed about the pivot  394 . The torsion spring  398  has a first spring reaction lugs  400  which engages with the outside of the seat ring  392  and second reaction lugs  402  which engages with a recess  404  on the back of the flapper valve element  396 , as best seen in  FIGS. 40   a ,  40   c . The torsion spring  398  is configured such that the coils  398   a  encourage the flapper valve element  396  to move to the closed position as shown in  FIGS. 36 ,  37  and  39 ,  40   c  and  40   d . As best seen in  FIGS. 39 and 40   c  the upper conical surface  414  of the valve element  396  engages with the valve seat  405  of the upper seal ring  342 . 
     It will be appreciated that the flapper valve assembly described with reference to  FIGS. 35-40  may be used with the completion suspension valve system described with reference to  FIG. 1-33  as an alternative to the flapper valve arrangement described in  FIG. 34 . 
     The foregoing embodiments provide a number of inventive solutions and advantages which have not been hitherto present in the art. The principal advantage is that the completion suspension valves allow the well to be desuspended without the need to establish a dual bore riser to surface. This allows non-MODU type vessels to conduct xmas tree installation operations and desuspension operations. Such vessels are readily available and are chartered for a fraction of the cost of an MODU. 
     It will be seen that the completion suspension valve has a variety of applications, such as an in-line tree, a subsea installation tree and a hybrid tree insert and the completion suspension valve has the advantage that the valves can be located within the restricted envelope defined by the tree bore, thus facilitating installation and removal.