Patent Publication Number: US-6988553-B2

Title: Horizontal spool tree wellhead system and method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. patent application Ser. No. 10/222,259 filed Aug. 16, 2002, now U.S. Pat. No. 6,805,200, which claims the benefit of the provisional application U.S. patent application Ser. No. 60/313,612 filed Aug. 20, 2001. 

   FIELD OF THE INVENTION 
   This invention relates generally to a subsea wellhead system for use in the drilling and completion of oil or gas wells at substantial depths beneath the water surface and, more particularly, to a wellhead system with a horizontal spool tree. 
   BACKGROUND OF THE INVENTION 
   Conventional wellhead system includes a wellhead housing mounted on the upper end of a subsurface casing string extending into the well bore. A riser and blowout preventer (BOP) are then installed. During the drilling procedure, the BOP is installed above a wellhead housing (casing head) to provide pressure control as casing is installed, with each casing string having a hanger on its upper end for landing on a shoulder within the wellhead housing. Upon completion of this process, the BOP is replaced by a Christmas tree installed above the wellhead housing, with the tree having a valve to enable the oil or gas to be produced and directed into flow lines for transportation to a desired facility. 
   In accordance with a relatively recent development in this field, the conventional casing and tubing heads making up the Christmas tree are replaced by a horizontal tree which comprises a spool with a generally horizontal through port mounted above and in axial alignment with a horizontal through port in the wellhead housing. In this application, the hangers for the casing strings are supported one above the other within the bore of the wellhead housing, and the tubing hanger for the production or tubing string is supported in the bore of the spool to suspend the production string within the casing strings. 
   The vertical bore through the tubing hanger of a horizontal tree may be closed by a wire line tool to direct production fluid through aligned side ports (generally horizontal through ports) in the hanger and spool for recovery and delivery of production fluid to a suitable location. A redundant seal may be provided by a well cap installed in the tree above the tubing hanger, with the vertical bore aligned with that of the tubing hanger closed by a wire line plug to permit vertical access to the production tubing string upon removal of the plug. 
   The completion of a well with a horizontal tree conventionally includes an isolation/orientation sleeve which is installed within the tree bore in order to isolate and thus allow testing from the exterior of a metal seal between the adjacent ends of the wellhead housing and spool. A portion of the sleeve also carries a guide tube with an upwardly contoured guide surface positioned to cooperate with a lug on the tubing hanger to rotate the tubing hanger into a desired position as it is lowered onto a shoulder in the bore of the spool. This technique automatically aligns the side port in the hanger with the side port in the tree bore. 
   A BOP stack then is lowered onto the upper end of the spool with a bore in alignment with the spool bore, and a tubing hanger then lowered through the BOP and into the spool bore. A lug on the lower end of the hanger cooperates with the guide surface to orient the hanger into a proper landed position on a seat in the bore of the spool. A tree cap is then lowered into a landed position, following which the BOP may be removed to permit installation of a cover on the top of the tree. 
   More particularly, the complete drilling and completion operation involves lowering a wellhead housing at the upper end of a production casing string onto an outer housing at the upper end of a conductor casing surrounding the production casing string. The conductor casing and housing at its upper end conventionally have been lowered onto the ocean floor to suspend the production casing string within a well bore by means of a permanent base having guide posts, using cables extending to the surface. 
   The wellhead housing is lowered with the casing by a running tool on a drill pipe into the upper end of a BOP whose lower end is connected to the wellhead housing by means of a releasable connector, with the bores of the wellhead housing, the BOP and drill pipe in axial alignment. The BOP is normally of the ram type well known in the art and having a stack including at least one pipe ram and at least one blind ram. The upper outer diameter of the wellhead housing may have grooves thereabout to receive teeth of the releasable latch of the connector. 
   An outer casing hanger on the upper end of the casing may be lowered for landing onto a shoulder in the bore of the wellhead housing to suspend it within the conductor casing. An inner casing hanger in turn may be lowered into a landed position on the outer casing hanger to suspend an inner production casing within the casing. The hangers and the strings on which they are mounted may thus be lowered into the well on running strings through the riser and BOP bore. 
   A wear bushing may then be lowered through the riser and BOP for landing with its bore aligned with that of the casing hanger and extending upwardly within the bore of the wellhead housing so as to prevent wear on the seal at the upper bore of the wellhead housing during drilling of the production well. The lower portion of this bore is aligned with the bore of the upper casing hanger, while the upper end is enlarged to provide a tapered seat at its lower end. Each of the casing hangers and the wear bushings are releasably locked in place and the outer diameters of the hangers are sealed with respect to the bore of the wellhead housing. 
   At this stage, the wear bushing may be raised by a suitable running tool, and the BOP and connector at its lower end are released from the upper end of the wellhead housing and raised to the surface to enable the spool of a horizontal tree to be lowered on a running tool releasably connected to its upper end. The tree may then be guidably lowered onto the upper end of the wellhead housing by means of sleeves lowered over the guide posts. The tree may be releasably connected at its lower end to the upper end of the wellhead housing. Thus, the connector has latches on its lower end which move over the upper end of the housing for locking thereto with their bores aligned. 
   Upon landing of the horizontal tree on the wellhead housing, a lower end of an orientation sleeve in the tree moves into the upper enlarged end of the casing hanger to form a continuation of the bore therethrough. This lower end of the orientation sleeve carries a seal assembly for sealing between it and the upper enlarged bore of the casing hanger. A seal which was lowered with the upper casing hanger seals off the annulus between the casing hanger and well bore housing. A metal seal ring is carried by the tree to land upon and form a seal with a tapered seat on the inner diameter of the upper end of the bore of the housing so as to be energized as the tree is connected to the housing by a connector. 
   The upper portion of the bore through the horizontal tree may be enlarged to receive an elongated wear bushing which is lowered with the tree to land on an intermediate shoulder in the upper enlarged bore of the tree. The wear bushing has an inner diameter somewhat less than that of the bore of the tree so as to prevent wear on the seal surfaces of the tree bore as tools are lowered into and raised from the well bore during drilling of the hole to receive the tubing. 
   In accordance with one of its functions, the sleeve isolates the metal seal ring to permit the ring to be tested from the outside of the tree. In addition, the sleeve has an upwardly extending guide surface which cooperates with a lower guide surface on a tubing hanger to orient the tubing hanger into a desired rotational position as it is lowered into the tree bore. Thus, for horizontal trees of this type, a side port in the tubing hanger is aligned with side port in the tree to direct production fluid to a flow line. 
   In accordance with horizontal tree practices, a first plug is lowered on a wire line and locked within the upper end of the hanger bore above its side port, and a tree cap that may contain another wire line plug is lowered into and locked within the bore of the tree. The BOP is then removed and replaced by a protective cover which is axially aligned and rotationally oriented into a position over the upper end of the tree. 
   The conventional procedure for installing a horizontal tree on a subsea wellhead involves the installation and the removal, and the subsequent reinstallation, of the BOP stack. U.S. Pat. Nos. 5,465,794; 5,544,707; 5,555,935; 6,062,314 and 6,039,119 disclose different equipment for horizontal tree applications. Each such installation is time consuming and expensive. 
   The present invention provides an improved horizontal spool tree wellhead system and method in which only one installation of the BOP stack is required. The disadvantages of the prior art are overcome by the present invention, and improved horizontal spool tree wellhead system and method are hereinafter disclosed which does not require repeated installation and removal of the BOP stack. 
   SUMMARY OF THE INVENTION 
   According to the method of invention, a subsea well is drilled and completed with a casing installed in the well bore and a wellhead housing having a wellhead housing bore installed on the upper end of the casing. The method includes lowering a spool of a horizontal tree for connection with an upper end of the wellhead housing, with the spool having a spool bore and at least one spool side port extending radially from the spool bore. A BOP may then be installed on the upper end of the tree, with the BOP having a BOP bore aligned with a spool bore and the wellhead housing bore. After a large diameter hole has been drilled with the tree and BOP in place, a casing string carrying a casing hanger may be lowered through the bores of the BOP, the spool tree and the wellhead housing and into the large diameter hole. At least one production hole with a smaller diameter may then be drilled with the drill string extending through the tree and the BOP on the wellhead. An orientation/isolation sleeve may then be installed in the tree spool, with the sleeve having a predetermined rotational orientation with the spool tree. Thereafter, a tubing hanger having a side port extending radially from the tubing hanger central bore may be lowered for landing on the orientation sleeve and rotationally oriented by the sleeve to align the tubing hanger side port with the spool side port. 
   According to the system of the present invention for drilling and completing a subsea well, a horizontal spool tree includes a spool bore and at least one side port extending radially from the spool bore. A BOP bore is aligned with the spool bore and the wellhead housing bore, and a casing string is supported on a casing hanger from the bore within the wellhead housing. An orientation sleeve may then be installed with a predetermined rotational orientation with respect to the spool tree, with the orientation sleeve having an internal diameter less than the cutting diameter of the drill bit used to drill the production hole. A tubing hanger is then landed on the orientation sleeve, as discussed above, so that its side port is oriented properly with respect to the spool side port. 
   The significant feature of the invention is that the size of the drill bit for drilling the large diameter hole is not restricted by the tree bore, which may be the full bore of the wellhead. A drill bit at the end of a drill string passes through the BOP through the tree and wellhead housing, and into the formation to drill the casing hole. Also, the size of the bit used to drill the production hole need not be restricted by the orientation sleeve. The orientation sleeve may have an internal diameter substantially less than the cutting diameter of the drill bit used to drill the production hole, since the orientation sleeve is installed after drilling the production well. A sleeve-shaped subsea tree bore protector may be provided within the bore of the tree while drilling the well, and may be removed prior to installing the orientation/isolation sleeve. 
   A further feature of the invention is that the orientation/isolation sleeve seals at its lower end to a casing hanger and its upper end to the tree. This sleeve preferably includes an upper guide surface for engagement with the tubing hanger to orient the tubing hanger with respect to the tree. The orientation sleeve also is preferably fixed to the tree to resist vertical and rotational forces applied to the orientation/isolation sleeve. 
   The tubing hanger preferably includes a fluid production port which extends laterally from the central tubing hanger bore for producing fluids from production tubing extending from the tubing hanger into the production well. During completion, an internal tree cap may be positioned above the production tubing hanger and secured to the tree. One plug may be positioned within a central bore of the tubing hanger, and another plug positioned within a central bore of the tree cap. 
   The casing hanger is preferably landed on a shoulder within the wellhead housing. The orientation sleeve preferably extends from within the wellhead housing into the spool bore of the horizontal tree, while the tubing hanger is positioned within the spool bore of the tree and is supported on the orientation sleeve. A conventional BOP stack may thus be used above the tree. 
   These and further objects, features, and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1–4  illustrate the improved procedure in accordance with this invention for drilling and completing a well. In the figures, the steps of the procedure are numbered sequentially. 
       FIG. 1  illustrates a horizontal spool tree wellhead system according to the present invention, with a tree as shown in dashed lines installed on a wellhead housing, and a BOP stack shown in dashed lines positioned above the tree. Conventional tree components and the BOP stack are thus illustrated in dashed lines, with a drill-through tree bore protector lining an interior of the bore through the tree. 
       FIG. 2  illustrates an outer casing hanger landed in the wellhead housing, and an inner casing hanger landed on the outer casing hanger, with both hangers being positioned within the wellhead housing. Each hanger supports a respective casing which extends downward into the well. 
       FIG. 3  illustrates an orientation/isolation sleeve installed on the inner casing hanger, with the isolation sleeve including a tapered upper guide surface. 
       FIG. 4  shows a tubing hanger landed on the isolation sleeve and oriented with respect to the tree by the upper guide surface on the orientation/isolation sleeve. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  illustrates the first stage of operation with the improved system in accordance with the present invention, wherein a horizontal tree  22  has been guidably lowered on a running tool (not shown) onto the upper end of the wellhead housing  20 . When so lowered, the tree  22  is releasably connected with the upper end of the wellhead housing  20  by a connector  26 . A metal seal ring  24  carried in the lower end of the bore lands upon a taper on the upper end of the housing  20 , and is caused to seal with the tree  22  upon makeup through the subsea connector  26 . As shown in  FIG. 1 , an outer casing  16  may be supported on an outer casing housing  11 , which in turn is fixed to the guide base  14  configured for landing the tree  22  onto the guide base. The inner casing  12  is supported on the inner casing housing  20  discussed above. Conventional tether lines  18  may be used for lowering and landing the tree  22  onto the guide base  14 . 
   Referring still to  FIG. 1 , the running tool has been removed and a BOP stack  28  has been lowered onto the upper end of the tree  22  for releasable connection thereto by means of another releasable connector. At this time, the system is prepared for drilling therethrough, with the outer casing  10  and the inner casing  12  suspended within the well bore. For this purpose, the bore at the upper end of the tree is preferably protected by a protector sleeve  40  landed and held down on a shoulder of the tree. The large diameter hole which subsequently may receive the casing strings  18  and  19 , as shown in  FIG. 2 , may thus be drilled into a subsea formation with a large diameter bit  90  as shown in  FIG. 1  at the lower end of drill string  92 . The bit  90  may have a cutting diameter which is only slightly less than the internal diameter of the housing  20 , since the bore through the tree  22  preferably is at least as large as the most restrictive bore through wellhead  20 . 
   After drilling of the large diameter hole, a casing hanger  38 , as shown in  FIG. 2  for supporting the casing string  18  may be landed on a shoulder  39  in the bore of the housing  20 , and a hanger  42  for the inner casing string  19  may be landed on the hanger  38 , or alternatively the hanger  42  may land on the hanger  38  and/or a shoulder on housing  20 . With the BOP and drilling riser still in place, a wear bushing (not shown) having a diameter less than the sleeve  40  may be lowered on a running tool into the bore of the tree and landed on the upper end of the hanger  42  for axial extension through the bore of the tree and into the bore of the housing  20 . The lower end of the wear bushing may fit closely within and seal with the enlarged upper end of the upper casing hanger  42 . In this position, the wear bushing, like the protector sleeve  40 , covers the production port  50  in the left side of the spool  23 , and the upper and lower ports  52 ,  54  in the right side of the tree spool. 
   Following drilling of another hole to receive a production tubing string, the wear bushing and the protector sleeve  40  are removed, as shown in  FIG. 3 . With the BOP and drilling riser still in place, an orientation/isolation sleeve  60  is then lowered into the bore of the tree  22  and oriented, landed and locked down to the tree above the upper casing hanger  42 , as shown in  FIG. 3 . Various mechanisms may be used to orient the sleeve  60  with respect to the tree  22 , including an orientation key on the sleeve that fits within a groove in the casing hanger  42 . A lower extension  62  of the sleeve  60  extends into and is locked to the enlarged upper end of the upper casing hanger  42 , while the upper end  64  of the sleeve seals with the tree spool  23 . In this position, the orientation sleeve  60  thus seals with the bores of the tree and wellhead housing to isolate the metal seal ring  24  and thus provide two pressure barriers. One or more lock down mechanisms  66  may be used to axially secure the orientation sleeve to the spool  23 . 
   The upper end  64  of the orientation sleeve  60  does not block access to the side ports  52 ,  54  in the tree bore, so that a production tubing hanger  70 , as shown in  FIG. 4 , may be lowered through the drilling riser and the BOP  28  to land on the upper end of the orientation/isolation sleeve  60 , as shown in  FIG. 4 , in which position a key or lug on the upper tubing hanger engages the tapered guide surface  72  on the orientation sleeve  60 , so that the key moves into a vertical locking slot in the sleeve  60 . In this position, a shoulder on the tubing hanger lands on a shoulder of the sleeve  60  and is held down in the bore of the tree  22 . 
   A production tubing will thus be suspended from the tubing hanger  70  and passed through the orientation/isolation sleeve  60  and into the production well bore. More particularly, the drilling and completion system has reached the same stage as discussed above in connection with the prior art without having to remove the BOP and drilling riser. 
   The well may thus be completed in essentially the same way as described above. A tree cap  74 , as shown in  FIG. 4 , has been lowered through the riser and BOP and installed in the tree bore above the tubing hanger, with its bore aligned with that of the tubing hanger of the subsea tree. Plugs  76  and  78  are installed in the tree cap and bore of the upper tubing hanger. The completed assembly  10  is thus shown in  FIG. 4 . The riser and BOP may then be removed, and the upper end of the bore of the subsea horizontal tree  22  closed by the protective cover. 
   The tree  22  and spool  23  may include one or more side ports leading to various conduits and controlled by valves to permit various workover tests and other functions to be performed, as shown and described in U.S. Pat. No. 5,544,707. Alternatively, the tree may be constructed in accordance with one or more of those shown and described in provisional application Ser. No. 60/295,857, entitled “HORIZONTAL TREE,” filed May 25, 2001, and in utility application Ser. No. 10/155,482 filed May 24, 2002, entitled “HORIZONTAL SPOOL TREE ASSEMBLY.” Thus, the tree  22  may have different conduit connections with its side port or ports. From the descriptions of the present invention as discussed above, its advantages are attained regardless of the type of horizontal tree. 
   A significant advantage of this invention is that the bore through the horizontal spool of the tree may be at least as large as the most restrictive diameter of the bore through the wellhead. Accordingly, the size of the large diameter well is only restricted by the bore size of the wellhead, even though the tree is already in place on the wellhead when drilling this large diameter well. The maximum bore of the tree is thus significantly greater than the bore through the one or more casing hangers, which is unlike the prior art operation wherein the BOP was removed and the tree first installed after the casing hangers were hung in the well. 
   Another significant advantage of the present invention is that the orientation/isolation sleeve need not be installed until just prior to running the tubing string in the well, and after drilling the portion of the hole intended to receive this tubing string. Accordingly, the bore through the tree is not restricted by an orientation/isolation sleeve while the production tubing well is being drilled. By running the sleeve in after the drilling operation is complete, a highly reliable orientation/isolation sleeve may be used. The present invention thus makes possible the drilling of a production well for receiving the production tubing string wherein the bit diameter used to drill the production hole is not restricted, and may be only slightly less than the diameter of the casing hanger, and only slightly less than the diameter of a protector sleeve provided within the spool  23 . Referring to  FIGS. 2 and 3 , for example,  FIG. 2  depicts a bit  94  suspended on a drill string  96  which may be passed through the tree  22  and through the casing hangers  38  and  42  to drill a production hole. The cutting diameter or bit diameter BD of the bit  94  as shown in  FIG. 2  may thus be greater than the bore diameter SD of the orientation sleeve  60  as shown in  FIG. 3 , which again is not possible in the prior art technique since the orientation sleeve was installed with the tree before drilling the production hole. 
   The orientation/isolation sleeve as disclosed herein may be a single sleeve that achieves three purposes: (1) sealing between the casing hanger (or the wellhead) and the tree, (2) providing orientation for the tubing hanger so that the side port of the tubing hanger is properly oriented with respect to the side port in the spool, and (3) securing the orientation sleeve to the tree to resist vertical forces, either upward or downward, that may be imparted to the orientation sleeve. If desired, separate components could be used to perform each of these functions. Orientation of the tubing hanger could be achieved with a device separate from the function of sealing between the casing hanger and the tree. In many applications, it will be desirable to lock the sleeve to the tree, although in other applications the sleeve could be locked to the wellhead housing, or could be axially fixed between the casing hanger locked to the wellhead housing and the tubing hanger locked to the tree. In most applications, other functions of the orientation/isolation sleeve are to support the weight of the tubing hanger and thus the tubing string, and to safely withstand the fluid pressure levels in the tree. 
   While preferred embodiments of the present invention have been illustrated in detail, it is apparent that modifications and adaptations of the preferred embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention as set forth in the following claims.