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FIELD OF THE INVENTION 
     This invention concerns subsea completions and more particularly relates to a completion arrangement that may be used to provide a large bore, high pressure, conventional (i.e. non-horizontal), concentric Christmas tree and tubing hanger system. 
     BACKGROUND OF THE INVENTION 
     For deep water developments it is now considered that conventional trees have advantages over horizontal trees. The horizontal tree concept has proven to be less advantageous than originally forecast in terms of installation times and design complexity. A demand has therefore arisen for a large bore conventional tree. 
     As the size of the production bore in a conventional Christmas tree and tubing hanger arrangement is increased, a large offset is often provided between the wellhead centerline and the fluid conducting bores at the tubing hanger/tree interface, primarily to avoid excessive enlargement of the tree block. In the case of parallel bore tubing hangers, this offset can arise in each of the production and tubing annulus bores. For concentric tubing hangers, there need be no offset in the production bore, but the tubing annulus offset is correspondingly larger. A very large offset in either the production bore or the tubing annulus bore will prevent wireline access. 
     During installation of parallel bore tubing hangers it is necessary to set wireline plugs in both bores. This requirement therefore restricts the permissible bore offsets. Tubing hangers are available which have a hydraulically operated annulus isolation valve rather than a plug. However it has usually been the practice to provide wireline access to this valve, for emergency operation in case of hydraulic actuator failure. Provision of such access and the consequent need to avoid dog legs at the tubing hanger/tree interface makes the tubing hanger and tree design relatively bulky and incapable of accommodating large numbers of downhole service lines. 
     Providing full wireline accessibility in a large bore conventional completion therefore leads to a large and heavy Christmas tree and tubing hanger installation. The upper weight limit for the lifting gear used to transfer equipment between supply and installation vessels is approximately 35 tonnes. This limit is reached for a conventional Christmas tree for use with 5½ inch (140 mm) tubing. We have realized that improved space utilization and various consequential design improvements are possible, both in the tubing hanger and in the tree, if wireline accessibility for the annulus isolation valve is abandoned. By this means the tubing size can be increased to seven inches (178 mm) or more while maintaining the tree weight within the 35 tonne limit and the tree dimensions likewise within acceptable limits. 
     SUMMARY OF THE INVENTION 
     Against this background, in accordance with a first aspect of the invention, we provide a tubing hanger forming a substantially centrally located production bore and a radially offset tubing annulus passage selectively closeable by a remotely operable valve; the tubing hanger upper end in use co-operating with a Christmas tree to define a void in which couplers for service lines running from the tree through the tubing hanger are accommodated; the valve communicating with the void whereby the couplers are bathed in fluid flowing to or from the annulus passage. 
     For large diameter production bores, this arrangement makes effective use of the space available across the horizontal section of the tubing hanger, with plenty of room around the periphery of the production bore for accommodation of service lines. For example, a hanger for seven inch (178 mm) tubing according to the present invention may accommodate up to 8 service lines; whereas the maximum number of service lines that can be accommodated in a comparable prior art parallel bore tubing hanger is 4. Currently tubing hangers for horizontal trees can only accommodate a maximum of 7 service lines. We have found that it is unnecessary to include sealing arrangements for isolating this fluid from the couplers, contrary to established practice with some prior hanger designs. The present invention may therefore provide a particularly simple and compact tubing hanger annulus passage to Christmas tree seal, capable of accommodating a relatively large number of service line couplers. 
     For most efficient space utilization, the valve is preferably located at the upper end of a tubing annulus passage in the tubing hanger. The valve is preferably pressure balanced, thereby requiring actuators of relatively small size and the same time being intrinsically reliable. Fluid communication between the void and the Christmas tree is preferably by means of one or more drillings extending from the void into the Christmas tree. 
     In a further aspect, the invention also provides means for orienting a tubing hanger and a tubing hanger running tool. Prior methods of aligning a tubing hanger and a tubing hanger running tool have included the use of an orientation joint above the tubing hanger running tool. This usually has an orientation helix and a keyway or the like, which interact with a pin or key projecting into the BOP interior. Alternatively the pin, helix and keyway may be provided between the tubing hanger running tool and the BOP. Both these arrangements involve complex manufacturing and care in use. Unless detailed records of the BOP used during the original installation of the tubing hanger are still available, there may be difficulties in setting up the orientation equipment correctly. A further orientation method involves actively rotating the completion riser at the surface to bring a spring loaded key on the running tool into alignment with a corresponding keyway in the tubing hanger. This method is impractical for deep water applications. 
     This further aspect of the invention provides a tubing hanger forming a substantially centrally located production bore, and a tubing hanger running tool; the running tool including a recess longitudinally engageable with a projection extending from the tubing hanger production bore, whereby the running tool may be orientated for engagement with the tubing hanger. High tolerance alignment of the running tool to the tubing hanger may thus be achieved directly and independently of any BOP; such alignment being passive, i.e. arising automatically as the running tool is landed on the tubing hanger, and effective even in deep water. The tool may also include an orientation helix to guide the projection into the recess. As it is associated with the tool, this helix does not restrict the tubing hanger production bore or require additional space within the tubing hanger. The tubing hanger production bore may also include a recess or projection for longitudinal engagement with a complementary projection or recess on a Christmas tree to provide alignment between the tree and tubing hanger. 
     Abandonment of wireline access to the tubing hanger annulus passage also allows a simplified, more compact and hence lighter Christmas tree to be used. Accordingly, in another aspect, the invention provides a Christmas tree having a body within which is formed a production flow bore having a lower end for connection to a tubing hanger production bore, and a tubing annulus conduit having a lower end for connection to a tubing hanger tubing annulus passage; the production flow bore and the tubing annulus conduit being interconnected by a crossover conduit formed within the tree body; the tubing annulus conduit including a deviation within the tree body, allowing room for a valve in the crossover conduit. The need for an external, separately formed, crossover conduit is thus avoided. Preferably the tree production flow bore has an upper end at the top of the tree body and is sufficiently aligned with the tubing hanger production bore to allow wireline access to the tubing hanger production bore through the tree production flow bore upper end. More preferably, the tree production flow bore is coaxial with the tubing hanger production bore which in turn is substantially centrally located within the tubing hanger. 
     As the production bore of a conventional tree increases in diameter, the Christmas tree height and weight also increase, partly for the reasons discussed above, concerning wireline accessibility, and partly due to the need to use larger valves. In accordance with a yet further aspect of the invention, a compact, relatively lightweight Christmas tree forms a production flow bore having a lower end for connection to a tubing hanger production bore, and a tubing annulus conduit having a lower end for connection to a tubing hanger tubing annulus passage; at least two removable plugs being provided in series in the tree production flow bore to act as pressure barriers. In conventional trees, at least one of these barriers, and more usually both, are provided by means of gate valves having large and heavy actuators. Substitution of the gate valves by plugs therefore saves considerable bulk and weight. The plugs are preferably wireline installed crown plugs. 
     The various aspects and preferred features of the invention are described below with reference to illustrative embodiments shown in the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the interface between a tubing hanger embodying the invention and a Christmas tree; 
     FIG. 2 shows a tubing hanger tubing annulus passage valve in more detail, in a closed position; 
     FIG. 3 shows the valve of FIG. 2 in an open position; 
     FIG. 4 shows a tubing hanger running tool engaged with the tubing hanger of FIG. 1; 
     FIG. 5 shows a first embodiment of the Christmas tree of the invention; 
     FIG. 6 is a fluid circuit diagram of the tree of FIG. 5; 
     FIG. 7 shows a second embodiment of the Christmas tree of the invention; 
     FIG. 7 a  shows a modification of the second embodiment; and 
     FIG. 8 is a fluid circuit diagram of the tree of FIGS. 7 and 7 a.   
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the bottom of a Christmas tree  10  attached to a tubing hanger  12  landed in a wellhead housing  14 . The tubing hanger  12  is supported by casing hangers  16  and held down by a lock down ring  18 . It includes a concentric production flow bore  20  and a highly radially offset annulus flow passage  22  formed by a pair of intersecting drillings  24 ,  26 . The upper end of the annulus flow passage  22  leads to an inlet port  28  of a pressure balanced integral shuttle valve  32 , closeable for retaining annulus fluids below the tubing hanger. This valve has outlet ports  30  communicating with a void  34  formed between the top of the tubing hanger  12  and a seal stab assembly  36  of the Christmas tree  10 . A circumferentially spaced series of drillings  38  in the seal stab assembly  36  (only one drilling  38  shown) communicate with an annulus flow conduit  40  in the tree  10  via an annular gap  42  formed between the seal stab assembly  36  and the tree body  44 . The lower ends of the drillings  38  communicate with the void  34  thereby linking it with the Christmas tree annulus flow conduit  40 . 
     As shown in FIG. 1, there is ample space in the seal stab assembly  36  and tubing hanger  12  for service lines, circumferentially spaced about the production flow bore  20  at a similar radius from the hanger centerline in comparison to the annulus flow conduit/passage  22 ,  38 ,  40 . One such line  46  is schematically indicated in dotted lines. With seven inch (178 mm) production tubing  48 , there is enough room for up to 8 circumferentially distributed service lines. 
     The void  34  provides space for service line couplers  35 , which are bathed in the annulus fluid. The void  34  is sealed by an annular sealing ring  37  between the tubing hanger  12  and lock down ring  18 , a further annular sealing ring  39  between the lock down ring  18  and the tree seal stab assembly  36  and a third annular sealing ring  41  between the tree seal stab assembly  36  and the tubing hanger  12 . 
     The shuttle valve  32  is of similar construction to an annulus access valve disclosed in U.S. Pat. No. 5,769,162, except that it is inverted so that instead of being provided at a lower end of the tubing hanger annulus flow passage, it is situated at the upper end of the flow passage  22 , in the broadest part of the tubing hanger  12 . This maximizing the space available for the valve  32 , besides maximizing space in the lower part of the tubing hanger for the large diameter production flow bore  20 . This results in a very compact tubing hanger design. 
     As shown in FIG. 2, the valve  32  comprises an open-ended tubular shuttle  50  contained partly in a bore  52  formed in the tubing hanger  12 , and partly in a housing  54  screw threaded into a counter bore  56  and sealed to the counter bore  56  by O-rings  58 . A lower end of the shuttle  50  carriers a pair of sealing rings  60  which make a sliding seal between the shuttle and the bore  52 . An upper end of the shuttle  50  carriers two pairs of sealing rings  62 ,  64  which similarly make a sliding seal with the housing  54 . The shuttle  50  has an external circumferential collar  66  carrying a pair of O-rings  68  which make a sliding seal with a portion of the counter bore  56  between the lower end of the housing  54  and the bore  52 . This portion of the counter bore thus forms a chamber  70  within which the collar  66  slides as a piston. Hydraulic fluid is supplied to and vented from the chamber  70  through ports  72 ,  74 . In the position shown in FIG. 2, the collar  66  lies at the upper end of the chamber  70 , with the sealing rings  62 ,  64  lying to either side of the ports  30  to close the valve  32 . In this position, supplying hydraulic fluid to the port  72  will cause the collar  66  and shuttle  50  to move downwardly, bringing the upper end of the shuttle  50  and the seals  64  below the ports  30 , thus opening the valve  32 . This position is shown in FIG. 3, in which position supplying hydraulic fluid to the port  74  will cause the shuttle to move upwardly, returning to the closed condition shown in FIG.  2 . With the valve closed, neither annulus pressure at the port  28  nor pressure in the void  34  will tend to cause movement of the shuttle  50 . The valve  32  is therefore pressure balanced and reliable in operation. The size of the collar  66  and chamber  70  required for actuation of the shuttle is therefore small. 
     FIG. 4 shows a running tool  76  engaged with the tubing hanger  12 . A production flow bore seal stab  78  of the running tool  76  includes an orientation slot or keyway  80  longitudinally engageable over a key  82  projecting radially into the tubing hanger production bore  84 . An optional orientation helix  86  is provided on the bottom of the seal stab  78 , for coarse alignment of the running tool  76  to the tubing hanger  12 . Provision of an orientation key and keyway at the interior surface of a concentric tubing hanger production bore provides simple and direct passive orientation between the tubing hanger and the running tool, without reliance on orientation components carried by a BOP. 
     FIGS. 5 and 6 show one possible layout of a Christmas tree  100  according to the invention. The Christmas tree production bore  88  is closed at its upper end by an internal tree cap  90 , below which are positioned two crown plugs  92 ,  94  in series. Plug  94  replaces the conventional production master valve and plug  92  replaces the conventional production swab valve, thereby eliminating the bulk and weight of the associated valve actuators. The upper crown plug  92  together with the tree cap  90  maintains a permanent dual pressure barrier in the tree production bore. 
     A production outlet branch  96  is connected to the production bore  88  between the two plugs  92 ,  94 . A 6⅜ inch (162 mm) production wing valve  98 , which may be a conventional gate valve, is provided in the outlet branch  96 . 
     An annulus flow conduit  102  is provided in the tree, connected to the conduit  40  and void  34 , FIG.  1 . This conduit  102  contains an annulus master valve  104  and annulus outlet valve  106 . An annulus line  108  is branched off the annulus flow conduit  102  from between the valves  104 ,  106  and contains an annulus wing valve  110 . The valves  104 ,  106 ,  110  may be conventional gate valves and together maintain the dual pressure barrier philosophy for the annulus conduit. 
     As shown more particularly in FIG. 5, the annulus flow conduit  102  contains a deviated portion  112  which provides space in the tree block  114  for a crossover valve  116 , which again may be an otherwise conventional gate valve. Crossover valve  116  is provided in a crossover conduit  118  formed in the tree block and extending between the deviated portion  112  of the annulus flow conduit  102  and the production flow bore  88 . This eliminates the need for a separately formed external crossover conduit. All of the tree valves, except the annulus wing valve  110 , are integrally formed with the tree block  114 . Annulus wing valve  110  is located within a separate manifold  109 , bolted and sealed to the tree block  114 . 
     FIGS. 7 and 8 show an alternative tree layout that is similar to the one of FIGS. 5 and 6, but which differs in that a production master valve  120  is provided upstream of the production wing valve  98  in the production outlet branch  96 , replacing the crown plug  94 . A second crown plug  122  is provided in the production flow bore  88  above the production outlet branch  96 , in addition to the crown plug  92 , to maintain a dual pressure barrier. As shown in FIG. 7, the production wing valve  98  is situated in a separate valve block  124 . A separately fabricated external flow loop  126  forming the crossover conduit connects the production outlet branch  96  in the valve block  124  with the annulus line  108  in the manifold  109 , between the annulus wing valve  110  and the annulus line  108 /annulus flow conduit  102  junction. A crossover valve  116  (not visible in FIG. 7) is provided in the external flow loop  126 . 
     The two crown plugs  92 ,  122  maintain the permanent dual pressure barrier in the tree production bore and the tree cap  90  is therefore optional in this embodiment. Where the tree cap  90  is not used, a debris plate (not shown) can be located above the upper plug  92  if desired, to ensure that stray objects falling onto the tree do not block access to the crown plugs. 
     FIG. 7 a  shows a modification of FIG. 7, in which the flow loop  126  is replaced by a crossover conduit formed in the tree block  114  by a pair of drillings  128 ,  130 . These extend behind the plane of the drawing and intersect each other behind the production bore  88  as shown. Drilling  130  intersects the production outlet branch  96  at the junction between the production wing valve block  124  and the tree block  114 . A crossover valve  116  is provided in a position accommodated by the deviated portion  112  of the annulus flow conduit  102 , in like manner to FIG.  5 . The crossover conduit and crossover valve  116  of FIG. 7 a  is represented in FIG. 8 in dotted lines. However, because this internal crossover conduit is relatively difficult to manufacture, the external flow loop and crossover valve of FIG. 7 may be more practical. 
     The invention in its preferred forms provides a large bore concentric tubing hanger with an integral, offset, pressure balanced annulus shuttle valve. The shuttle valve is located in such a way that it has minimal impact on the functionality and size of the tubing hanger and provides the primary means of retaining annulus fluids. The invention may be utilized in order to maximize the diameter of the production tubing and the number of downhole service lines. The invention advantageously provides a conventional concentric bore subsea Christmas tree system that can accommodate the largest possible diameter production bore. With such a system there need be no dog leg at the interface of the Christmas tree and tubing hanger production bores. Tree height and weight may be minimized by re-configuring the associated valves and adopting a tree pressure barrier philosophy similar to that of a horizontal subsea Christmas tree, using two plugs in the production bore, rather than valves. The system is of relatively simple configuration and may employ a riser and tooling similar to those used with horizontal tree systems. This offers some potential for standardization between tree types. Compatibility with existing concentric subsea test trees and monobore riser technology is also provided, with the subsea test tree stackup achievable below the BOP shear rams. The tubing hanger and Christmas tree may be designed to retain 10,000 psi (68.9 MNm −2 ) working pressure. Continuous monitoring of downhole electrical and hydraulic equipment is possible while the completion is being run or pulled, by virtue of the tubing hanger/running tool orientation system, which allows connection of downhole service lines to appropriate service couplers in the running tool. It also allows passive re-engagement of the tubing hanger running tool to the tubing hanger during retrieval or intervention operations.

Summary:
A concentric tubing hanger having a radially offset tubing annulus passage closeable by a remotely operable valve, preferably a pressure balanced, hydraulically-operated shuttle valve positioned at an upper end of the tubing annulus passage. The tubing hanger is of relatively compact design, accommodating a large diameter production bore and a large number of downhole service lines. Service line couplers and outlet ports of the valve are housed in a void defined between the tubing hanger and a seal stab assembly of a subsea Christmas tree. The couplers are bathed in an annulus fluid. A tubing hanger running tool has a slot and an orientation helix which cooperate with a key projecting into the production bore to provide passive orientation between the tubing hanger rubbing tool and tubing hanger. A subsea Christmas tree has an annulus flow conduit having a deviated portion, allowing room in a tree block for a bypass conduit and a valve. Two or more valves in the production flow path may be substituted with crown plugs to save tree bulk and weight.