Patent Publication Number: US-7909103-B2

Title: Retrievable tubing hanger installed below tree

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to provisional application 60/793,467, filed Apr. 20, 2006. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to a subsea wellhead assemblies, and more particularly to a configuration with a tubing hanger landed in a subsea wellhead housing, a spool with a lateral flow outlet and control valves landed on the wellhead housing, and wherein the tubing hanger is retrievable without removing the spool. 
     BACKGROUND OF THE INVENTION 
     A subsea well typically has a wellhead housing located on the sea floor. One or more casing hangers are supported in the wellhead housing, each located at the upper end of a string of casing. In one type of wellhead assembly, a tubing hanger located at the upper end of a string of tubing is installed in the wellhead housing. The operator may perforate the well at that point and install a wire line plug in the production passage of the tubing hanger. The operator then lands a production tree on the wellhead housing, the tree having a number of valves for controlling the well fluid. The tree has a production flow passage and an isolation sub that stabs into the production passage of the tubing hanger. The operator then removes the wire line plug by lowering a tool through the production flow passage of the tree. For a workover operation involving pulling of the tubing hanger, the tree must be disconnected from the wellhead housing. If the tree needed to be retrieved for repair work, this can be done without pulling the tubing. 
     In another type of wellhead assembly, the tree is installed on the wellhead housing before running the tubing. The operator connects the drilling riser to the tree, lowers the tubing hanger through the drilling riser and lands the tubing hanger in the tree. The tubing hanger has a lateral flow outlet that registers with a lateral flow outlet in the tree. The operator installs a wire line plug in the tubing hanger vertical bore above the flow outlet. The tree does not need to be disconnected from the wellhead housing for pulling the tubing for a workover operation. If the tree needed to be retrieved for repair, the tubing would have to be pulled. 
     U.S. Pat. No. 5,372,199 discloses a configuration with a lower tubing hanger landed in the wellhead housing and supporting a string of tubing. A tree having a lateral flow outlet lands on the wellhead housing. An upper tubing hanger is landed in the bore of the tree. The upper tubing hanger has an isolation tube on its lower end that stabs into engagement with the production passage in the lower tubing hanger. The upper tubing hanger has a lateral flow outlet that registers with the lateral flow outlet of the tree. An internal tree cap is installed within the bore of the tree above the upper tubing hanger. The tubing can be pulled without disconnecting the tree from the wellhead housing by first retrieving the tree cap, then the upper tubing hanger, and then the lower tubing hanger and tubing. Similarly, the tree can be retrieved without pulling the tubing. 
     In the various configurations described above, the tree is a large, heavy and complex assembly that is run on a string of drill pipe. The running procedure requires a vessel with a derrick. It may not be economical to utilize the same vessel that drilled the well to complete the well and install the tree. Designs for trees that can be run on a lift line are known, but these systems typically do not have the ability to pull the tubing without disturbing the connection between the tree and the wellhead housing. 
     SUMMARY OF THE INVENTION 
     The wellhead assembly of this invention has a tubing hanger that lands in the wellhead housing and has features that enable the tubing to be pulled without disturbing the connection between the tree and the wellhead housing. A spool, which may be considered to be at least part of a production tree, lands on the wellhead housing. The spool has a bore and a laterally extending production flow outlet. A tree cap lands within the bore of the spool, the tree cap having an axially extending flow passage and a laterally extending a production flow outlet that aligns with the production flow outlet of the spool. Upper and lower seals on the tree cap seal between the tree cap and the bore of the spool above and below the production flow outlet of the spool. The upper seal is the uppermost pressure barrier in the bore of the spool. An isolation tube on a lower end of the tree cap sealingly engages the production flow passage of the tubing hanger. 
     In the first embodiment, the tubing hanger is installed and the well completed before running the spool. The tree cap is installed in the spool at the surface and the assembly is lowered together onto the wellhead housing. In the second embodiment, the operator installs the spool before drilling out through the wellhead housing. The drilling riser is coupled to the spool and the well is drilled to total depth through the spool. After reaching total depth, the operator runs the tubing through the spool and completes the well. Then the tree cap is installed. 
     In both embodiments, the tree cap may be retrieved from the spool for pulling the tubing through the tree for workover operations. Also, in the preferred embodiment, the tree cap has a flange that lands on the rim of the spool and a mandrel that protrudes above the spool. The mandrel has an external profile for attaching workover pressure control equipment to the tree cap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is schematic sectional view of a subsea wellhead assembly constructed in accordance with the present invention. 
         FIG. 2  is a schematic sectional view of the tubing hanger being installed in the subsea wellhead housing of  FIG. 1  in accordance with a first method of the invention. 
         FIG. 3  is a schematic sectional view of the spool and tree cap being installed on the wellhead housing of  FIG. 1  in accordance with the first method of the invention. 
         FIG. 4  is a schematic sectional view of the tubing hanger being lowered through the previously installed spool in accordance with a second method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a completed subsea wellhead assembly in accordance with both methods. A subsea well  11  has a wellhead housing  13  with a conductor casing  15  extending therefrom to a predetermined depth within the subsea well. A casing hanger  17  is landed within wellhead housing  13  with a string of casing  19  extending therefrom to another predetermined depth within subsea well  11 . 
     A tubing hanger  21  is landed within wellhead housing  13 , with a string of tubing  23  extending therefrom within string of casing  19 . In the preferred embodiment, tubing  23  extends to a production depth such that tubing  23  receives well fluid from within subsea well  11 . Tubing hanger  21  has an axially extending production flow passage  22 . A tubing annulus  25  is defined between the interior surface of string of casing  19  and the exterior surface of string of tubing  23 , Tubing hanger  21  optionally may have a tubing annulus passage  24  extending axially through it offset from and parallel to production flow passage  22 . In addition, a tubing annulus valve  26  may be located within tubing annulus passage  24  for opening and closing passage  24 . In one embodiment, tubing annulus valve  26  is biased by a spring to a closed position. Tubing hanger  21  is rotated or oriented to a desired orientation relative to wellhead housing  13 . Orientation may be accomplished in a variety of known ways. 
     A production tree or spool  27  lands on and connects to an upper end portion of wellhead housing  13  with an external connector  28 , shown schematically. Spool  27  has a bore  29  extending axially therethrough that has a diameter at least equal to the diameter of tubing hanger  21  so that tubing hanger  21  can be retrieved through spool  27 . Optionally, bore  29  may be as at least as large as the portion of the bore of wellhead housing  13  above casing hanger  17  to allow casing hanger  17  to be installed through spool  27  in accordance with the second method of this invention. Spool  27  has an outlet port  31  extending through a side wall of spool  27  for the flow of production fluids from tubing  23 . At least one outlet valve  30  is mounted to the exterior of spool  27  to control the flow of well fluids exiting spool  27  through outlet port  31 . Well fluids flowing through outlet valve  30  are delivered by methods known to those skilled in the art to a subsea collection manifold or to a platform located at the surface. Spool  27  will have additional equipment associated with subsea trees, such as cross-over piping and valves. 
     The subsea wellhead assembly also preferably includes a tree cap  33  having a lower cylindrical portion that is closely received within bore  29  of spool  27 . Tree cap  33  may either connect to spool  27  internally or externally as shown. In this embodiment, tree cap  33  has an external flange  32  that lands on the rim or upper end of spool  27 . An external connector  34  connects tree cap  33  to a profile formed on the upper portion of spool  27 . 
     Tree cap  33  has an axially extending production passage  36 . An isolation tube  35  is secured to the lower end of tree cap  33 . Isolation tube  35  extends downward and stabs into sealing engagement with production passage  22  in tubing hanger  21  to receive well fluids from tubing  23 . An outlet opening  37  extends laterally from production passage  36  through a sidewall of tree cap  33  to allow fluid flow to spool outlet port  31 . Upper and lower seals  38 A,  38 B extend around tree cap  33  and sealing engage spool bore  29  above and below outlet port  31 . In this embodiment, upper seal  38 A is the uppermost pressure barrier that seals to bore  29 . 
     A tubing annulus access port  39  extends through a sidewall of spool  27  below lower seal  38 B for registering with and monitoring annulus  25 . Tubing annulus access port  39  is in communication with spool bore  29  below lower seal  38 B. A valve  41  is mounted to the exterior of tubing annulus access port  39  for opening and closing port  39 . 
     Tree cap  33  has a valve  43  above lateral flow outlet  37  for opening and closing access to its production passage  36 . If desired, a wire line plug profile could be formed in production passage  36  above flow outlet  37  for installing a wire line (or ROV tool installable) plug as a second pressure barrier within production passage  37 . Tree cap  33  optionally has a cylindrical mandrel portion above its flange  32  that has a grooved profile  45  for coupling to pressure control equipment, such as a riser or blowout preventer, during wire line or similar workover operations. Tree cap  33  may have an actuator  47  extending downward from its lower end for engaging and opening tubing annulus valve  26 . Actuator  47  could be a fixed probe that compresses the spring within tubing annulus valve  26  to cause it to open. Alternately, actuator  47  could be hydraulically extended and retracted. 
     In this embodiment, tubing hanger  21  has a number of auxiliary passages  49  (only one shown) extending from its lower end to its upper end. Auxiliary passages  49  are used to control downhole safety valves (not shown), to communicate with downhole sensors, and for other functions, such as supplying power to a downhole electrical submersible pump. Auxiliary passage  49  is shown schematically connected to a downhole auxiliary line  50  that extends alongside tubing  23  for supplying hydraulic fluid pressure or electrical or optical signals. Each auxiliary passage  49  has a coupling receptacle on the upper end of tubing hanger  21 . 
     Preferably tree cap  33  has mating auxiliary passages  51  extending through it. A coupling  52  associated with each auxiliary passage  51  depends downward from tree cap  33  and stabs into sealing engagement with one of the auxiliary passages  49  in tubing hanger  21 . In this embodiment, the upper ends of at least some of the tree cap auxiliary passages  51  extend to a side of tree cap  33  above spool  27 . A controls module  53  having electrical and hydraulic control circuitry mounts to tree cap  33  for supplying hydraulic fluid pressure and electrical power to downhole safety valves and sensors. Controls module  53  may optionally be retrievable from tree cap  33  as well as retrievable along with tree cap  31  controls module  53  may also control tree cap valve  43 , if one is utilized. A separate controls module  55  may be mounted to a side of spool  27  for controlling valves  30 . If so, preferably controls module  55  is retrievable from spool  27 . 
     In the first method of operation, subsea wellhead housing  13  and conductor casing  15  are landed within subsea well  11 . As shown in  FIG. 2 , a blowout preventer assembly (“BOP”)  57  is attached to an upper end portion of wellhead housing  13 . BOP  57  is a lower part of a string of drilling riser  59  that extends to a drilling vessel. Drilling operations are conventionally conducted through BOP  57  and wellhead housing  13 . When at total depth, casing hanger  17  and string of casing  19  are lowered through drilling riser  59  and BOP  57 , landed within wellhead housing  13  and cemented into place within the well in a manner known in the art. More than one string of casing may be installed. 
     Tubing hanger  21  and a string of tubing  23  are then lowered on a running tool  61  and drill string through drilling riser  59  and BOP  57 . Tubing hanger  21  is oriented, landed, sealed, and latched conventionally in the bore of wellhead housing  13 . For example, the orientation may be with a pin and slot arrangement associated with BOP  57 , or a separate orientation spool might be employed. When tubing hanger  21  lands, tubing  23  will extend into the subsea well to a production depth. Normally, the operator will circulate the drilling mud from casing  19  by pumping down tubing annulus  25  and returning fluid up tubing  23 , or vice-versa. Running tool  61  will open tubing annulus valve  26  and the downhole safety valve to allow circulation to occur. The operator may also perforate and test the well in a conventional manner at this point. 
     After perforating and testing the well, the operator lowers a temporary plug  63  ( FIG. 3 ) on a wire line through the drill string and running tool  61  and latches it within production passage  22  of tubing hanger  21  to seal subsea well  11 . The drilling riser and blowout preventer assembly  57 ,  59  are then removed from connection with wellhead housing  13 . The drilling vessel may also leave the vicinity to drill another well. At this point, the operator can install additional equipment, such as piping on flow lines to a subsea manifold or the surface without BOP  57  and drilling riser  59  being in the way. 
     At the surface, the operator assembles tree cap  33  to spool  27  with the desired orientation. The operator subsequently lowers the pre-unitized assembly of tree cap  33  and spool  27 , as illustrated in  FIG. 3 , preferably on a lift line. It is not necessary for the vessel used to lower the assembly to have a derrick or the capability of running drill pipe. The operator orients and lands flow spool  27  complete and pre-unitized with tree cap  33  on an upper end portion of wellhead housing  13 . The orientation of spool  27  to wellhead housing  13  may be handled conventionally, such as with the assistance of an ROV (remote operated vehicle) and video cameras. Upon landing, isolation spool  35  stabs into engagement with production passage  22  of tubing hanger  21 , thereby defining an axial passage extending from a production depth of subsea well  11  to outlet opening  37  of tree cap  33 . Outlet opening  37  aligns with outlet port  31  so that well fluids can flow directly from outlet opening  37  through outlet port  31 . 
     Also, upon landing of spool  27 , auxiliary couplings  52  connect auxiliary lines  50  to control module  53 . In addition, tubing annulus valve actuator  47  stabs into tubing annulus valve  26  and opens it, which places annulus access port  39  in fluid communication with tubing annulus  25 . The operator plugs control modules  53 ,  55  into a subsea umbilical that delivers electrical and hydraulic power and control signals. The operator can then remove plug  63  through tree assembly  33  to initiate well fluid production from subsea well assembly  11 . This may be handled with a subsea plug removal tool (such as shown in U.S. Pat. No. 6,719,059) that is lowered on a lift line and attached to tree cap profile  45  with the assistance of an ROV. Upon removing plug  63 , the operator opens valve  30  to communicate well fluids from string of tubing  23  to a subsea manifold or to a collection facility located on a surface. 
     For workover operations through tubing  23 , the operator may attach a riser to tree cap  33  and perform operations through tubing  23 , such as wire line operations. For a workover operation requiring the retrieval of tubing  23 , the operator can install wire line plug  63  back in tubing hanger  21  using a subsea plug retrieval tool, then retrieve tree cap  33  on a lift line. The operator would then attach a workover or drilling riser to spool  27  and pull tubing hanger  21  and tubing  23  in a conventional manner through the workover riser. Prior to pulling tubing hanger  21 , the operator would typically render the well safe by “killing” in a routine manner. Well circulation would be in the same manner as during completion, which is via running tool  61 , tubing annulus passage  24  in tubing hanger  21  and tubing  23 . 
     If desired, the workover operation may include further drilling, such as drilling a sidetracked portion of the well to a more productive zone. In one method, after pulling tubing  23 , the operator pulls casing hanger  17  along with production casing  19  through spool  27  and the workover or drilling riser. The operator would then lower a drill string through the riser and spool  27  and drill a sidetracked portion of the well. The operator would run casing or a liner through the riser and spool  27  into the sidetracked portion and install a string of tubing in the sidetracked portion. The operator would complete the sidetracked portion of the well in the same manner as described above. 
       FIG. 4  illustrates an alternative embodiment, which involves drilling the well through spool  27 . Wellhead housing  13  and conductor casing  15  are installed in a conventional manner as in the first method. After installing wellhead housing  13  and outer casing  15 , the operator then orients, lands and connects spool  27  to an upper end portion of wellhead housing  13 . Typically spool  27  is installed off via a lift line, but it could also be run on a drill string. The operator then lowers the drilling riser  59  and connects BOP  57  with the profile on an upper end portion of spool  27 . The operator then continues drilling through BOP  57  and spool  27 . Such an operation is also known as “drill through” operations. 
     Upon drilling subsea well  11  to a desired depth, operator then lowers casing hanger  17  with string of casing  19  attached thereto through drilling riser  59  and BOP  57  and lands, sets and seals casing hanger  17  within wellhead housing  13 . The operator then lowers tubing  23  to the production depth of subsea well  11  and lands tubing hanger  21  in wellhead housing  13 . The operator completes and tests the well in a conventional manner through the drilling riser and BOP  57 . Using a wire line, the operator then lowers plug  63  ( FIG. 2 ) through BOP  57  to sealingly close subsea well  11 . The operator then removes drilling riser  59  and BOP  57 . 
     The operator then lowers tree cap  33  via a lift line to land within spool  27 . As before, isolation tube  35  is attached to tree cap  33  and stabs into sealing engagement with production passage  22  in tubing hanger  21 . Tree cap auxiliary passages  51  mate with auxiliary passages  49  in tubing hanger  21 . Upon landing tree cap  33  within spool  27 , the operator can remove plug  63  from tubing hanger  21  to allow well fluids to flow from a lower end portion of string of tubing  23  to outlet opening  37 . The operator then opens valve  30  to allow flow of well fluids from subsea well  11  to a subsea manifold collection manifold or to the surface. 
     The invention has significant advantages. In addition to serving as a pressure barrier, the tree cap provides a communication flow path for the production fluid from the tubing hanger to the production flow outlet in the spool. Completing the well before running the spool, as in the first embodiment, allows the drilling rig to be moved, if desired, before installing the spool. The spool and tree cap can be assembled as a unit and lowered on a lift line on a vessel that may lack a derrick. In the second embodiment, the well may be drilled to total depth and casing installed through the spool. In both embodiments, for workover operations requiring retrieval of tubing, the tree cap can be pulled without disturbing the spool. Auxiliary lines, such as for downhole sensors and safety valves, may be lead through the tree cap to the exterior of the tree cap above the spool. The control module associated with these functions may be mounted to the tree cap and retrievable along with the tree cap. The controls for the valves of the spool may be in a separate module, if desired, and attached to the spool. Landing the tree cap on the rim of the spool avoids the need for a landing shoulder within the bore of the spool. 
     While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.