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BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to offshore drilling and production equipment, and in particular to a subsea well system for monitoring the pressure in a non-producing string of casing through the completion system. 
     2. Description of the Prior Art 
     A subsea well that is capable of producing oil or gas will have a conductor housing secured to a string of conductor pipe which extends some short depth into the well. A wellhead housing lands in the conductor housing. The wellhead housing is secured to an outer or first string of casing, which extends through the conductor to a deeper depth into the well. Depending on the particular conditions of the geological strata above the target zone (typically, either an oil or gas producing zone or a fluid injection zone), one or more additional casing strings will extend through the outer string of casing to increasing depths in the well until the well is cased to the final depth. Each string of casing is supported at the upper end by a casing hanger. The casing hanger lands in and is supported by the wellhead housing. 
     In some shallow wells and in some fluid injection wells, only one string of casing is set within the outer casing. Where only one string of casing is set within the outer casing, only one casing hanger, the production casing hanger, is landed in the wellhead housing. In this case, the space between the outer or first string of casing and the second or production string of casing is isolated by a casing hanger packoff that seals between the wellhead housing and the production casing hanger. 
     The more typical case is where multiple strings of casing are suspended within the wellhead housing to achieve the structural support for the well to the depth of the target zone. Where multiple strings of casing must be set within the outer casing, multiple casing hangers are landed in the wellhead housing, each set above the previous one in the wellhead housing. Between each casing hanger and the wellhead housing, a casing hanger packoff is set to isolate each annular space between strings of casing. The last string of casing extends into the well to the final depth, this being the production casing. The strings of casing between the outer casing and the production casing are intermediate casing strings. 
     When drilling and running strings of casing in the well, it is critical that the operator maintains pressure control of the well. This is accomplished by establishing a column of fluid with predetermined fluid density inside the well. During drilling operations, this fluid is circulated down into the well through the inside of the drillstring out the bottom of the drillstring and back to the surface. This column of density-controlled fluid balances the downhole pressure in the well. When setting casing, the casing is run into the pressure balanced well. A blowout preventer system is employed during drilling and running strings of casing in the well as a further safety system to ensure that the operator maintains pressure control of the well. The blowout preventer system is located above the wellhead housing by running it on drilling riser to the wellhead housing. 
     When each string of casing is suspended from the casing hanger in the wellhead housing, a cement slurry is flowed through the inside of the casing, out of the bottom of the casing, and back up the outside of the casing to a predetermined point. An open fluid communication passage in the casing hanger leading from the casing annulus to the casing interior would adversely affect the flow path of the cement slurry. This could also cause well pressure control problems for the operator under certain conditions. 
     In a subsea well capable of producing oil or gas, the production fluids flow through perforations made in the, production casing at the producing zone. A string of tubing extends to the producing zone within the production casing to provide a pressure-controlled conduit through which the well fluids are produced. At some point above the producing zone, a packer seals the space between the production casing and the tubing to ensure that the well fluids flow through the tubing to the surface. The tubing is supported by a tubing hanger assembly that lands and locks above the production casing hanger, either in the wellhead housing, in a tubing hanger spool or in a horizontal or spool tree (further described below). 
     Subsea wells capable of producing oil or gas can be completed with various arrangements of the production control valves in an assembly generally known as a tree. Trees with the arrangement of production control valves located vertically above and in line with the production tubing are generally called christmas trees. Trees with the arrangement of production control valves offset from the production tubing are generally called horizontal or spool trees. 
     For wells completed with a christmas tree, the tubing hanger assembly lands in the wellhead housing above the production casing hanger. Alternatively, the tubing hanger assembly lands in a tubing hanger spool, which tubing hanger spool is landed and locked to the wellhead housing. For wells completed with a horizontal or spool tree, the horizontal tree locks and seals on the wellhead housing. A tubing hanger assembly locks and seals in the horizontal tree. When either a tubing hanger spool or horizontal tree is located on the wellhead housing, the blowout preventer system is landed on the tubing hanger spool or horizontal tree, respectively. 
     The tubing hanger assembly in each of the above subsea well systems normally has a flow passage for communication with the annulus surrounding the tubing. This passage allows for monitoring pressure above the packer between the interior of the production casing and the interior of the tubing. In some cases the well can also be produced through this annulus flow passage. Virtually all producing wells monitor pressure in the annulus flow passage between the interior of the production casing and the interior of the tubing. 
     A sealed annulus locates between the production casing and the next larger string of casing. Normally there should be no pressure in the annulus between the production casing and the next larger string of casing, because the annular space between the production casing and the next larger string of casing is ordinarily cemented at its lower end and sealed with a packoff at the production casing hanger end. Pressure build up in the annulus between the production casing and the next larger string of casing could collapse a portion of the production casing, compromising the structural and pressure integrity of the well. Monitoring pressure in the annulus between the production casing and the next larger string of casing of a subsea well is shown in patents, however, it is not done commercially to applicant&#39;s knowledge. Improvements are desired. 
     SUMMARY OF THE INVENTION 
     In a subsea well with a tree assembly including either a tubing hanger spool or a horizontal tree, the annulus pressure between the production casing and the next larger string of casing is monitored through communication passages external to the tubing hanger. A communication passage extends through the production casing hanger from the exterior of the production casing hanger below the casing hanger packoff to an outlet in the interior of the production casing hanger. A port closure sleeve threads to the interior of the production casing hanger. 
     The port closure sleeve seals on both sides of the communication passage outlet in the interior of the production casing hanger. With the port closure sleeve located as described, the communication passage between the exterior of the production casing hanger and the bore of the production casing is isolated. The port closure sleeve as designed can be removed after the tree assembly is installed. After the tree assembly is installed, a lower end of a tubing hanger orientation sleeve mates in the interior of the production casing hanger. The tubing hanger orientation sleeve seals on its exterior surface with the interior of the production casing hanger at a point below the communication passage outlet in the interior of the production casing hanger. The tubing hanger orientation sleeve lands in the tree assembly. The tubing hanger orientation sleeve seals on its exterior surface with the interior of the tree assembly. A space between the tubing hanger orientation sleeve and the wellhead housing is created through which casing annulus pressure can communicate with a communication passage in the tree assembly. 
     The communication passages communicate pressure in the annulus of the production casing to the exterior of the tree assembly. A communication line extends to monitoring equipment at the surface for monitoring the pressure in the annulus of the production as described. 
     In one embodiment, the tree assembly includes as part of its assembly a horizontal tree. The horizontal tree lands on the wellhead housing. A tubing hanger orientation sleeve lands in the horizontal tree and mates to the interior of the production casing hanger. The tubing hanger orientation sleeve isolates a space between the exterior of the tubing hanger orientation sleeve and the interior of the wellhead housing to link the communication passage in the production casing hanger with the communication passage in the horizontal tree. 
     In another embodiment, the tree assembly includes as part of its assembly a tubing hanger spool. The tubing hanger spool lands on the wellhead housing, with the tree mounted to the upper end of the tubing hanger spool. The tubing hanger orientation sleeve lands in the tubing hanger spool and mates to the interior of the production casing hanger. In this configuration, the tubing hanger orientation sleeve is not necessarily oriented to the tubing hanger spool, although it may be. The tubing hanger orientation sleeve isolates a space between the exterior of the tubing hanger orientation sleeve and the interior of the wellhead housing to link the communication passage in the production casing hanger with the communication passage in the tubing hanger spool. The communication passage in the tubing hanger spool communicates pressure to the exterior of the tubing hanger spool. A communication line extends to monitoring equipment at the surface for monitoring the pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a sectional view of a subsea wellhead assembly constructed in accordance with this invention, shown prior to installation of a tree assembly. 
     FIG. 2 is an enlarged sectional view of a portion of the subsea wellhead assembly of FIG.  1 . 
     FIG. 3 is a sectional view of a first embodiment of a subsea well assembly constructed in accordance with this invention. 
     FIG. 4 is a sectional view of a second embodiment of a subsea well assembly constructed in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, one configuration for the subsea wellhead assembly includes a conductor housing  11 , which will locate at the sea floor. Conductor housing  11  is a large tubular member that is secured to a string of conductor pipe  13 . Conductor pipe  13  extends some short depth into the well and is typically 30 or 36 inches in diameter. 
     A wellhead housing  15  lands in the conductor housing  11 . Wellhead housing  15  is a high pressure tubular member having an interior surface  16  and an exterior surface  18 . Wellhead housing  15  secures to a first string of casing  17 , normally 20 inches in diameter, which extends through the conductor pipe  13  to a deeper depth into the well. Normally, first string of casing  17  is cemented in place. 
     An intermediate casing hanger  25  and intermediate casing  27  are installed in wellhead housing  15  and casing  17 . The intermediate casing hanger  25  lands on a lower shoulder in the interior surface of the wellhead housing  15  below the production casing hanger  19 . The intermediate casing hanger  25  is sealed by an intermediate casing hanger packoff  26  to the interior surface  16  of the wellhead housing  15 . Intermediate casing hanger  25  secures to a string of intermediate casing  27 , typically between 10 and 16 inches in diameter, with larger diameter than the production casing  23 , and with smaller diameter than the first string of casing  17 . Intermediate casing  27  extends between the first string of casing  17  and the production casing  23  to an intermediate depth. Normally, intermediate casing  27  is cemented in place. 
     A production casing hanger  19  having an interior surface and an exterior surface lands on a shoulder on the intermediate casing hanger  25 . Production casing hanger  19  is sealed by a production casing hanger packoff  21  to the interior surface  16  of the wellhead housing  15 . Production casing hanger  19  secures to a string of production casing  23 , typically between 7 and 16 inches in diameter. Production casing  23  extends through the intermediate string of casing  17  to a final depth of the well. Normally, production casing  23  is cemented in place. 
     A production casing annulus  29  exists in the space surrounding the production casing  23 . Production casing annulus  29  also surrounds production casing hanger  19  up to production casing hanger packoff  21 . Normally, there would be only nominal, atmospheric pressure in the production casing annulus  29 . Only a lower portion of production casing  23  is exposed to well pressure and this exposure is through perforations (not shown). A packer (not shown) will locate in production casing  23  above these perforations to seal the well pressure within the lower portion of production casing  23 . Pressure other than atmospheric exists in production casing annulus  29  only when a leak occurs. 
     A communication passage  31  extends laterally through production casing hanger  19  from exterior surface to interior surface. This passage allows fluid communication between the production casing annulus  29  and the interior surface of production casing hanger  19 . 
     While pumping cement down the casing, cement returns through flowby slots  32  should not enter the bore of casing hanger  19 . When production casing  23  is being installed, fluid communication between the interior surface and the exterior surface is not desired. As depicted in FIG. 2, communication passage  31  may be sealed from fluid communication prior to completion by using a port closure sleeve  33  with upper and lower seals  34 . Seals  34  locate above and below communication passage  31 . Port closure sleeve  33  is threadably connected to production casing hanger  19 . Port closure sleeve  33  has an interior surface and an exterior surface. A slot  39  in the interior surface of port closure sleeve  33  allows a tool (not shown) to unscrew the port closure sleeve  33  from the production casing hanger  19  and remove the port closure sleeve  33  through a tree assembly (not shown in FIG. 2) installed on the wellhead housing  15 , prior to running tubing. 
     Referring to FIG. 3, a horizontal tree assembly  41 , including a tree  43  and later a tubing hanger  44 , lands on wellhead housing  15 . Tree  43  is lowered with drillpipe. The tree assembly  41  includes a vertical bore  35  with lateral production outlet  36  connected to a valve  37 , lateral annulus outlet  38  from below the tubing hanger and connected to a valve  40 , and workover port  30  from above the tubing hanger  44 , connected to the annulus outlet  38  and connected to a valve  32 . After installing the tree  43  on the wellhead housing  15  and prior to installing the tubing hanger  44 , a retrieval tool (not shown) is lowered through the riser into engagement with port closure sleeve  33  and retrieves port closure sleeve  33  through the bore of tree  43 . A tubing hanger orientation sleeve  53  having an exterior surface lands in a shoulder  55  in tree  43 . The tubing hanger orientation sleeve  53  is also in sealing engagement with the tree  43 . A pin  57  located on the exterior surface of tubing hanger orientation sleeve  53  orients to a slot  56  in tree  43 . The tubing hanger orientation sleeve  53  has an interior helical cam  46  and slot  48  that mates with the tubing hanger pin  50  for aligning the tubing hanger  44  with the tree  43 . Tubing hanger  44 , which is connected to a string of tubing  52 , lands, locks, and seals in tree  43 . As the tubing hanger  44  lands, it rotates to proper orientation by the interaction of the pin  50  on the cam  46  and into the slot  48 . 
     Tree  43  has a lower interior surface that locates above the wellhead housing  15  and faces downward. A tree communication passage  47  extends upward from lower interior surface  45 . Tree communication passage  47  has a lateral portion  47 A that leads to an outlet (not shown) on the exterior of the tree  51 . 
     The tubing hanger orientation sleeve  53  has a lower end that sealingly mates in the interior surface or bowl of the production casing hanger  19 . Tubing hanger orientation sleeve  53  seals on the interior surface of the production casing hanger  19  at a point below communication passage  31 . Communication passage  31  is exposed to an annular space surrounding orientation sleeve  53 . Tubing hanger orientation sleeve  53  also seals in an interior surface  59  of tree  43  above the lower interior surface  45 . A fluid communication space  61  is thus created through which production casing annulus  29  can communicate with tree communication passage  47 . 
     In operation, the tree  43  lands, locks and seals on the wellhead housing  15 . A retrieval tool lowered through the riser and blowout preventer system retrieves the port closure sleeve  33 . The tubing hanger orientation sleeve  53  lands in the tree  43 , is rotated until the pin  57  locates in the slot  56 . In this position, the tubing hanger orientation sleeve  53  seals in the production casing hanger  19  below the communication passage  31  and in the tree  43 , thereby creating a pressure-isolated, fluid communication space  61  between the production casing annulus  29  and the tree communication passage  47 . The tubing hanger  44 , along with a string of tubing, lands in the tree  43 , orients with the tubing hanger orientation sleeve  53  as described above, and locks and seals to the tree  43 . In this position, the tubing hanger  44  provides pressure-isolated communication between the production bore and the production outlet in the tree  43 . Pressure in casing annulus  29  communicates through port  31 , space  61  and tree communication passage  47 . 
     In the embodiment of FIG. 4, a tree assembly  63 , including tubing hanger spool  65 , a tubing hanger  64 , and a tree  66 , lands on wellhead housing  15 . Tree  66 , unlike the first embodiment, is not a horizontal tree. Tubing hanger  64  lands, locks, seals, and orients in tubing hanger spool  65  rather than in tree  66 . Tree  66  lands on tubing hanger spool  65  after tubing hanger  64  is installed. Tubing hanger spool  65  has a lower interior surface  67  that locates above the wellhead housing  15  and faces downward. A tubing hanger spool communication passage  69  extends upward from lower interior surface  67 . Tubing hanger spool communication passage  69  has a lateral portion  69 A that leads to an outlet on the exterior of the tubing hanger spool  65 . 
     A spanner sleeve  73  having an exterior surface  75  lands in a shoulder  77  in tubing hanger spool  65 . Spanner sleeve  73  mates in the interior surface  20  of the production casing hanger  19 . Spanner sleeve  73  seals on the interior surface  20  of the production casing hanger  19  at a point below communication passage  31 . Spanner sleeve  73  also seals in an interior surface  79  of tubing hanger spool  65  above the lower interior surface  67 . A fluid communication space  81  is thus created through which production casing annulus  29  can communicate with tubing hanger spool communication passage  69 . 
     The invention has significant advantages. The communication passages enable pressure from the production casing annulus to be communicated to the exterior of the wellhead housing, without penetrating the wellhead housing and without complicating the tubing hanger with additional ports and seals. The system allows the production casing annulus pressure to be monitored when either a horizontal tree system or a tubing spool and conventional tree system are installed, without use of different production casing hanger and port closure sleeve components or tools. While the invention has been shown in only two 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.

Summary:
A subsea wellhead and completion system which provides for monitoring pressure in the annulus of the production casing through communication passages that are pressure-isolated from tubing annulus passages and production fluid passages. The communication passages route production casing annulus fluid pressure from the production casing annulus through the production casing hanger to a communication passage provided between the wellhead housing and an isolation sleeve that spans and seals between the production casing hanger and the tree and then through the tree to an outlet on external diameter of the tree and thence to monitoring equipment located typically at the rig. A removable closure member is located in the production casing hanger to isolate the communication passage during drilling operations. This closure member is removed after drilling operations are concluded and after the tree is installed, but before the isolation sleeve and tubing hanger are landed.