You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
This application is based on U.S. Provisional Patent Application No. 60/284,307, which was filed on Apr. 17, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to subsea wellheads for oil and gas wells, and in particular to a nested stack-down casing hanger configuration which allows the pressure in the intermediate casing annuli to be monitored without penetrating the outer pressure containing housing or casing walls which separate the annuli from the external environment. Although the present invention has particular utility with respect to subsea wells, the invention is also applicable to land and offshore surface drilled wells. 
     In order to conform to various regulations and to protect life, property, and the environment, it is common practice on surface drilled wells to monitor the pressure in the various casing annuli for sustained casing head pressure (SCP). Pressure containing side outlets are provided in the casing and tubing heads, through which the annulus pressure can be measured. However, because such side outlets themselves create potential leak points, and because of the difficulty in detecting leaks, side penetrations in subsea wellhead housings are usually avoided. Exceptions are made in the regulations for high pressure subsea wells, such that it is required only to monitor pressure in the production annulus. In fact, such body penetrations are actually prohibited by some regulations. In any event, body penetrations in subsea wellheads could create potential hazards greater than those originally addressed by annulus monitoring. 
     Despite the difficulties inherent in monitoring annulus pressure in subsea wells, regulations have been proposed which would require that the pressure be monitored in every annulus in the well. Thus there is a need for a method of monitoring annulus pressure which does not require penetration of the pressure containing casings or housings. Even in the absence of such regulations, such a method would be most useful and desirable. Several prior art methods for monitoring annulus pressure in subsea wells are described in U.S. Pat. Nos. 5,544,707 and 4,887,672. A more complete discussion of the various regulations and the state of the prior art with respect to annulus pressure monitoring is presented in copending U.S. patent application Ser. No. 09/776,065, which is commonly owned herewith and the entirety of which is hereby incorporated by reference for all purposes. 
     Typical prior art wellhead systems have utilized a “stack-up” casing hanger configuration. In this type of system, the hanger for each successively smaller diameter casing string is landed on top of the hanger for the next larger casing string. Each hanger is locked and sealed to the wellhead housing bore above the next lower hanger. Thus, as each hanger is installed in the wellhead housing, the next lower hanger (and the associated annulus) becomes inaccessible. 
     For the purposes of illustration, a typical stack-up subsea wellhead system is shown in FIG.  1 . The wellhead system comprises a conductor housing  12  attached atop conductor pipe  18  and locked into permanent guide base  10 . The wellhead housing  14  is landed in the conductor housing  12  and includes wellhead bore  16 . Second intermediate casing hanger  32  is landed in the wellhead housing  14  and supports second intermediate casing string  42 . Hanger  32  is provided with annulus access port  36 , which allows for fluid communication between the wellhead bore  16  and the “C” annulus  50  after installation of hanger  32 . After the hanger  32  is landed in the wellhead housing  14 , pack-off  34  is installed between hanger  32  and the wellhead housing  14 , preventing further communication with access port  36 . 
     First intermediate casing hanger  26  is then landed atop second intermediate casing hanger  32  and supports first intermediate casing string  40 . Hanger  26  is provided with annulus access port  30 , which allows for fluid communication between the wellhead bore  16  and the “B” annulus  48  after installation of hanger  26 . After the hanger  26  is landed on hanger  32 , pack-off  28  is installed between hanger  26  and the wellhead housing  14 , preventing further communication with access port  30 . 
     Production casing hanger  20  is then landed atop first intermediate casing hanger  26  and supports production casing string  38 . Hanger  20  is provided with annulus access port  24 , which allows for fluid communication between the wellhead bore  16  and the production or “A” annulus  46  after installation of hanger  20 . The “A” annulus is located between the production casing string  38  and the production tubing, shown in phantom at  44 . After the hanger  20  is landed on hanger  26 , pack-off  22  is installed between hanger  20  and the wellhead housing  14 , preventing further communication with access port  24 . As is apparent from the figure, once all the casing hangers have been installed in the wellhead housing  14 , access to the “B” and “C” annuli is prevented. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, these and other disadvantages in the prior art are overcome by providing a wellhead system which comprises a wellhead housing and a plurality of concentric casing strings, each of which is suspended from a corresponding casing hanger. The casing hanger for the radially outermost casing string is supported in said wellhead housing and the casing hanger for each successively smaller casing string is supported in the casing hanger for the next radially larger casing string. Each casing string defines a corresponding annulus which surrounds said casing string and is located below the casing hanger for said casing string. Furthermore, at least one casing hanger comprises a bypass port or similar means for providing fluid communication between the annulus below said casing hanger and an area above said casing hanger. 
     Thus, the wellhead system of the present invention comprises a “stack-down” casing hanger configuration. In this type of system, the hanger for each successively smaller diameter casing string is landed or “nested” within the hanger for the next larger casing string. This approach allows the pack-off for each casing hanger to be retrieved independently, thus allowing fluid communication to be established with any of the casing annuli after all of the casing strings and hangers have been installed. Thus the pressure in each annulus may be monitored while the well is in production mode. 
     These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a prior art wellhead system having a stack-up casing hanger configuration. 
     FIG. 2 is a cross-sectional view of the preferred embodiment subsea wellhead housing landed and locked in the stack-down wellhead, with the low-pressure drilling riser connected to the housing. 
     FIG. 3 is a cross-sectional view of the preferred embodiment subsea wellhead system with the intermediate casing hanger landed and locked in the wellhead housing. 
     FIG. 4 is a close-up cross-sectional view of the expandable load shoulder mechanism for the intermediate casing hanger. 
     FIG. 5 is a cross-sectional view of the preferred embodiment subsea wellhead system with the production casing hanger landed and locked in the intermediate casing hanger. 
     FIG. 6 is a close-up cross-sectional view of the expandable load shoulder mechanism for the production casing hanger. 
     FIG. 7 is a cross-sectional view of the preferred embodiment subsea wellhead system with the casing hanger pack-offs retrieved. 
     FIG. 8 is a cross-sectional view of the preferred embodiment subsea wellhead system with a horizontal Christmas tree connected to the top of the wellhead housing. 
     FIG. 9 is a close-up cross-sectional view of the lower portion of the Christmas tree shown in FIG.  8 . 
     FIG. 10 is a cross-sectional view of an alternative embodiment surface drilled wellhead housing landed and locked in the stack-down wellhead, with the low-pressure drilling riser connected to the housing. 
     FIG. 11 is a cross-sectional view of the alternative embodiment surface drilled wellhead system with the intermediate casing hanger landed and locked in the wellhead housing, and the high pressure drilling riser engaging the intermediate casing hanger. 
     FIG. 12 is a cross-sectional view of the alternative embodiment surface drilled wellhead system with the production casing hanger landed and locked in the intermediate casing hanger. 
     FIG. 13 is a cross-sectional view of the alternative embodiment surface drilled wellhead system with the production casing hanger pack-off retrieved. 
     FIG. 14 is a cross-sectional view of the alternative embodiment surface drilled wellhead system with both casing hanger pack-offs retrieved. 
     FIG. 15 is a cross-sectional view of the alternative embodiment surface drilled wellhead system with the external production tieback connector engaging the intermediate casing hanger. 
     FIG. 16 is a cross-sectional view of the alternative embodiment surface drilled wellhead system with the internal production tieback connector engaging the production casing hanger. 
     FIG. 17 is a close-up cross-sectional view of the internal production tieback connector engaging the production casing hanger. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2, the wellhead system of the present invention comprises a wellhead housing  54  which is landed in a stack-down wellhead  52 . The lower end of wellhead housing  54  is welded or otherwise rigidly attached to an outer casing  55 . Wellhead housing  54  is sealed and locked to stack-down wellhead  52  by a seal and lock assembly  60 . Wellhead housing  54  further comprises a wellhead bore  56 . A low pressure drilling riser connector  58  is locked and sealed to the upper end of wellhead housing  54 . 
     Referring to FIG. 3, an intermediate casing hanger  62  is supported and locked within wellhead housing  54  by an expandable load shoulder  64 . Suspended from hanger  62 , via an adapter  70 , is an intermediate casing string  72  which cooperates with outer casing  55  to define a “C” annulus  74 . An annular space  67  is defined between hanger  62  and wellhead housing  54 . A pack-off  66  isolates space  67  from wellhead bore  56 . Intermediate casing hanger  62  further comprises a second expandable load shoulder  68 , the purpose of which is described below. 
     Referring to FIG. 4, expandable load shoulder  64  comprises an internally toothed ring  80 , which resides in an internal groove  82  formed in wellhead housing  54 . Load shoulder  64  further comprises a drive ring  84 , an externally toothed ring  90 , and a stepped insert  92 , all of which are carried on intermediate casing hanger  62 . Before hanger  62  is landed in wellhead housing  54 , drive ring  84  and toothed ring  90  rest upon a support ring  86 . As hanger  62  is landed, an external shoulder  88  on drive ring  84  impinges on a lower shoulder  94  of groove  82 . As hanger  62  descends, drive ring  84  drives toothed ring  90  upward against stepped insert  92 . Toothed ring  90  is thus cammed outward into locking engagement with internally toothed ring  80 , and the weight of intermediate casing hanger  62  and intermediate casing string  72  are thus supported. Hanger  62  further comprises an annulus access port  76  which communicates with a groove  78 . Port  76  and groove  78  provide for fluid communication between annular space  67  and “C” annulus  74 , and thereby provide a fluid bypass around expandable load shoulder  64 . 
     Referring to FIG. 5, a production casing hanger  96  is supported and locked within intermediate casing hanger  62  by expandable load shoulder  68 . Suspended from hanger  96  is a production casing string  102 , which cooperates with intermediate casing string  72  to define a “B” annulus  104 . An annular space  100  is defined between production casing hanger  96  and intermediate casing hanger  62 . A pack-off  98  isolates space  100  from wellhead bore  56 . 
     Referring to FIG. 6, expandable load shoulder  68  comprises a retainer ring  108 , which is carried by intermediate casing hanger  62  and includes an internal lower lip  110 . Load shoulder  68  further comprises a lock ring  120  and an energizing mandrel  112 , which includes an external upper lip  114 . A locking mandrel  122  is threadedly connected to hanger  62 . Before production casing hanger  96  is landed in intermediate casing hanger  62 , energizing mandrel  112  is suspended from retainer ring  108  via engagement of lips  114  and  110 . Lock ring  120 , which is outwardly biased, is disposed below mandrel  112 . As production casing hanger  96  descends, an external shoulder  118  on hanger  96  impinges upon an internal shoulder  116  on energizing mandrel  112 . Lips  114  and  110  disengage, and mandrel  112  drives lock ring  120  downward. As lock ring  120  contacts locking mandrel  122 , lock ring  120  is cammed inward into a groove  126  in hanger  96 , and the weight of hanger  96  and production casing string  102  are thus supported. Adjacent to expandable load shoulder  68 , intermediate casing hanger  62  is provided with an internal slot  106 . Slot  106  provides for fluid communication between annular space  100  and the “B” annulus  104 , and thereby provides a fluid bypass around expandable load shoulder  68 . 
     FIG. 7 shows the wellhead system of the present invention with both of the pack-offs retrieved in preparation for the production mode. Referring to FIG. 8, a subsea Christmas tree  128  is connected to the upper end of wellhead housing  54  via a connector  130 . A stab  136  extends from tree  128  into the wellhead housing  54  and engages intermediate casing hanger  62 . Christmas tree  128  further comprises a tree bore  138  and an annulus port  132 . When the production tubing and tubing hanger (not shown) are installed in the tree  128 , the annulus port  132  provides access to the production or “A” annulus between the production tubing and the production casing  102 . Thus the pressure in the production annulus may be monitored during production. 
     Referring to FIG. 9, the pressure in the “B” annulus  104  may be monitored via a fluid path  166 . Path  166  comprises legs  146  and  148  in the tree  128 . Leg  146  exits the OD of tree  128  and may be connected to an external gage or other means for monitoring pressure. A leg  150  passes from the tree  128  into the stab  136 . A leg  152  continues longitudinally through stab  136  and intersects a leg  154 , which then passes into a lower section  140  of stab  136 . Leg  154  intersects a leg  156 , which continues longitudinally through lower section  140  and exits into a space  158 . Space  158  is defined below a seal assembly  142 , which seals between hanger  62  and lower portion  140 . Space  158  is in fluid communication with annular space  100 , which has already been shown to communicate with the “B” annulus  104 . Thus path  166  is in fluid communication with the “B” annulus  104  and can be used to monitor the pressure therein. 
     Pressure in the “C” annulus  74  may be measured via a fluid path  168 . Path  168  comprises legs  160  and  162  in tree  128 . Leg  162  is in fluid communication with a space  164  which is defined between stab  136  and wellhead housing  54 . Space  164 , in turn, is in fluid communication with space  67 , which has already been shown to communicate with the “C” annulus  74 . Thus path  168  is in fluid communication with the “C” annulus  74  and can be used to monitor the pressure therein. 
     Alternative Embodiments 
     The present invention may also be utilized in a surface drilled well. Referring to FIG. 10, prior to completion the surface drilled system is essentially identical the subsea case (compare with FIG.  2 ). Referring to FIG. 11, an intermediate casing hanger  182  is landed in the wellhead housing  54  and locked therein via expandable load shoulder  64 , in a manner similar to the subsea case. A low pressure drilling riser  59  is attached to wellhead housing  54  via low pressure drilling riser tieback  58 . A high pressure drilling riser  172  is connected to hanger  182  via a high pressure drilling riser tieback  170 . An annular space  178  is defined between tieback  170  and wellhead housing  54 . An annular space  180  is defined between hanger  182  and wellhead housing  54 . A riser annulus  176  is defined between high pressure drilling riser  172  and low pressure drilling riser  59 . It should be understood that in the configuration shown in FIG. 11, annulus  176  is in fluid communication with both the tree at the surface and the “C” annulus  74  via space  180 . Thus the pressure in the “C” annulus  74  may be monitored from the surface. 
     Referring to FIG. 12, a production casing hanger  184  is landed within intermediate casing hanger  182  and is locked therein via expandable load shoulder  68 . Pack-off  98  seals between hanger  182  and hanger  184 . FIG. 13 shows the wellhead system with pack-off  98  retrieved. FIG. 14 shows the wellhead system with both pack-offs retrieved and the low pressure drilling riser tieback disengaged. 
     Referring to FIG. 15, an external production riser  188  is connected to wellhead housing  54  via an external production tieback connector  185 . An external production tieback  186  is attached to intermediate casing hanger  182  via a lock down nose  190  and is sealed thereto via a seal  196 . An annular space  192  is defined between wellhead housing  54  and tieback  186 . An annulus monitoring port  194  provides fluid communication between annular space  192  and the exterior of tieback  186  and may be connected to a gauge or other pressure monitoring means. 
     Referring to FIG. 16, an internal production riser  198  is connected to external production tieback  186  via an internal production tieback connector  196  and a ratch-latch mechanism  202 . Connector  196  is sealed to production casing hanger  184  via a seal  204 . An annular space  200  is defined between internal production riser  198  and external production tieback  186 . It should be understood that in the configuration shown in a FIG. 16, annulus  200  is in fluid communication both with the tree at the surface and the “B” annulus  104 . 
     Referring to FIG. 17, the communication path between annulus  200  and annulus  104  can be seen to bypass ratch-latch  202  and lock down nose  190  and continue on to the “B” annulus  104  in a manner similar to the subsea case. A communication path can also be traced between annulus  192  and the “C” annulus  74  via an annulus access port  206  in hanger  182 . Since annulus  192  communicates with monitor port  194 , the pressure in the “C” annulus  74  may be monitored during production. 
     The embodiments here presented are at present considered to be the best modes for carrying out the invention. However, it should be understood that variations in the shape, number, and arrangement of the various elements may be made without parting from the true spirit and scope of the invention. Therefore, it is the applicant&#39;s intent to claim all such variations as fall within the scope of the invention.

Summary:
A wellhead system for petroleum producing wells comprises a “stack-down” casing hanger configuration. In this stack-down system, the hanger for each successively smaller diameter casing string is landed or “nested” within the hanger for the next larger casing string. This approach allows the pack-off for each casing hanger to be retrieved independently, thus allowing fluid communication to be established with any of the casing annuli after all of the casing strings and hangers have been installed. Thus the pressure in each annulus may be monitored while the well is in production mode.