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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/272,418 filed Mar. 1, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to casing hanger systems and other systems in which a tubular member is landed atop another. The invention also relates more broadly to devices and methods to relieve the problems generally related to stacked tubular members that are placed under high axial load conditions. 
     2. Description of the Related Art 
     In many conventional subsea wellheads, the upper end of the casing hanger engages a running tool by a seating arrangement wherein the casing hanger presents an annular upwardly and inwardly directed seating surface. One standard casing hanger top surface is a flat surface, angled at 15 degrees from the horizontal, facing upward and radially inward. The slope is intended to allow “trash,” such as bits of rock and mud impurities, to fall off the top surface. The slope also aids in radially guiding and landing the running tool atop the casing hanger or in placing a second casing hanger atop a lower casing hanger. The running tool that engages the upper end of the casing hanger provides a complimentary downwardly and outwardly-directed annular engagement face. Sometimes, locking dogs are also used to aid in securing the two members together. 
     Unfortunately, this form of engagement has been found to be disadvantageous. The running tool exerts great weight loads onto the seating surface. Also, multiple casing hangers are often stacked upon one another. This is done when multiple casings are run. The additional casing hangers would also load the seating surface. The loads imparted to the casing hanger are directly related to the weight of the casing string sections being carried by the casing hanger and running tool. The loads are extremely high and may be in excess of a million pounds. The axial force applied to the upper end of the casing hanger by the running tool is damaging to the casing hanger and, as a result the upper end of the casing hanger may deflect radially outwardly reducing the engagement of the running tool and casing hanger. Radial deflection at the upper end of the casing hanger reduces the fatigue life of the casing hanger equipment. 
     SUMMARY OF THE INVENTION 
     A stacking arrangement for wellhead tubular members is described wherein the adjoining ends of the tubular members are specially shaped to prevent or limit outward radial deflection and the damage associated therewith. In an exemplary described embodiment, a casing hanger is provided with an upper longitudinal end having an inwardly sloped inner bearing surface and an inwardly sloped outer bearing surface. An outwardly sloped engagement surface adjoins the two bearing surfaces and is designed to be captured radially within a complimentary shaped surface on a running tool or other engaging member landed atop the casing hanger. A grooved section is provided to assist removal of trash from the upper end of the casing hanger. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional cut-away view of a wellhead housing and casing hanger constructed in accordance with the present invention. A running tool is located atop the casing hanger. 
     FIG. 2 is an enlarged view of the portions of the casing hanger that are illustrated in FIG.  1 . 
     FIG. 3 is a cross-sectional cut-away view of a wellhead housing and primary casing hanger constructed in accordance with the present invention. A second casing hanger is installed atop the primary casing hanger. 
     FIG. 4 is an enlarged view of portions of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention provides an improvement that is generally applicable to instances in which one tubular member is stacked atop another, particularly in a bore. In an exemplary embodiment described herein, the subject matter of the present invention has particular applicability to subsea casing hanger systems. 
     FIGS. 1 and 2 depict, in cross-section, portions of a subsea wellhead  10  that incorporates a casing hanger system. Because the general aspects of construction and operation of subsea wellheads and casing hanger systems are well understood by those of skill in the art, those aspects will not be described here in significant detail except for purposes of providing a background for illustrating the invention. 
     The wellhead, schematically shown at  10 , has a radially outer wellhead housing  12  that encloses a central bore  14 . An inwardly and upwardly directed casing hanger seat  16  is located proximate the lower end of the bore  14 . 
     A casing hanger  18  is disposed within the bore  14  and seated upon the casing hanger seat  16 . The casing hanger  18  has a reduced diameter lower portion  20  and a larger diameter upper portion  22 . The intersection between the two portions forms a downwardly facing shoulder  24 . A bore  26  is defined centrally within the casing hanger  18 . 
     The upper end of the casing hanger  18  is best shown in FIG.  2  and features two substantially parallel inner and outer walls  28  and  30 , respectively. A shaped, annular upper landing surface  32  interconnects the inner and outer walls  28 ,  30 . The upper surface  32  includes an inwardly sloped inner bearing surface  34  and an inwardly sloped outer bearing surface  36 . An outwardly sloped engagement surface  38  adjoins the inner and outer bearing surfaces  34 ,  36 . The engagement surface  38  is designed to be captured radially within a complimentary shaped surface on a running tool or other engaging member above the casing hanger  18 . Preferably, the engagement surface  38  is normal to the upper surface  32 . 
     There is preferably a grooved section  40  (illustrated here in FIG. 4) within a radial portion of the upper surface  32  to help remove trash from the upper surface  32  of the casing hanger  18 . The grooved section includes at least one radial groove  40  that is formed by cutting away a portion of the inner bearing surface  34  so that the outer bearing surface  36  becomes substantially continuous to the bore  26 . Trash and impurities may then be flowed through the groove  40  into the bore  26  rather than becoming hung up against the engagement surface  38 . There may be multiple grooved sections located at angular intervals about the circumference of the casing hanger  18 . 
     A casing hanger packoff  42 , such as the MS-i seal, is shown located on inside of wellhead housing  12  in FIG.  1 . The packoff  42  is adapted to provide a fluid tight seal between the bore  14  of the wellhead housing  12  and the upper portion  22  of the casing hanger  18 . The packoff  42  is best shown in FIG.  4  and includes a U-shaped seal member  44  with a pair of legs  46 ,  48  that extend upwardly. The packoff  42  also includes a separate compression ring  50  that is selectively insertable between the legs  46 ,  48 . The packoff  42  is moveable by a running tool between an unset position above the casing hanger  18  (shown in FIG. 1) and a set position that is shown in FIG.  4 . In its conventional role, the packoff  42  has also helped to resist outward radial forces and outward deformation of the upper end of the casing hanger  18  when set. 
     A running tool  52  is shown in FIGS. 1 and 2 to be radially disposed within the casing hanger  18  and is useful for setting the casing hanger  18  within the wellhead housing  12  and setting the packoff  42 . The running tool  52  may be a known running tool assembly suitable for these purposes, such as the DPRT (drill pipe running tool) available commercially from ABB Vetco, Inc. The DPRT running tool is ideal since it is capable of performing multiple operations including setting the packoff seal  42  and running casing sections. A standard DPRT, or other running tool, will of course have to be modified to operably interact with the capture feature portions of the upper end of the casing hanger  18 . These modifications will be described shortly. The upper end  54  of the running tool  52  is affixed to drill pipe  56 , which is used to lower the casing hanger  18  from a drilling vessel. The running tool  52  supports that casing hanger  18 , packoff  42 , and the string of casing when it is run into the well. The running tool  52  features an upper collar  58  that is secured to a central mandrel  60 . 
     The mandrel  60  provides a lower extension portion  62  having a radially inner cylindrical portion  64  (see FIG. 2) and a radially protruding, surrounding annular gripping portion  66 . The gripping portion  66  is shaped and sized to mate with and engage the upper surface  32  of the casing hanger  18 . Thus, the gripping portion  66  provides a downwardly facing recessed annular bearing face  68  and a non-recessed annular bearing face  70  that lies radially outside of the recessed bearing face  68 . An inwardly directed engagement face  72  interconnects the two bearing faces  68 ,  70 . When the running tool  52  is engaged with the upper end  22  of the casing hanger  18 , the recessed annular bearing face  68  of the running tool  52  adjoins the inner bearing face  34  of the casing hanger  18 . The non-recessed face  70  of the running tool  52  adjoins the outer bearing face  36  of the casing hanger  18 , and the engagement face  72  of the running tool  52  adjoins the engagement surface  38  of the casing hanger  18 . The bearing faces  34 ,  36  of the upper surface  32  of the casing hanger  18  receive and absorb the major downward or axial loads upon the casing hanger  18  that are imposed by the running tool  52 . Because the inwardly-directed engagement surface  38  of the gripping portion  66  matingly adjoins the inwardly-directed engagement face  72  of the running tool  52 , radial outward deflection of the upper end of the casing hanger  18  is prevented. Thus, the upper end of the casing hanger  18  is considered to be radially captured by the running tool  52 . 
     Referring again to FIGS. 1 and 2, a radially outer seal setting portion  76  is shown extending downwardly from the collar  58  of the running tool  52 . The seal setting portion is shaped and sized to engage the compression ring  50  of the packoff seal  42  and urge it into the seal member  44  so that the seal  42  becomes energized. In addition, the seal  42  is moved downwardly by the seal setting portion  76  to a location wherein it is disposed between the casing hanger  18  and the wellhead housing  12 . 
     During an operation in which the casing hanger  18  is landed, the running tool  52  is affixed to the casing hanger  18  at its lower end and to a string of drill pipe  56  at its upper end. The running tool  52  is lowered on the drill pipe string until the casing hanger  18  is seated within the wellhead housing  12 . Casing (not shown) is then cemented into place, in a manner that is known in the art. The running tool  52  is then rotated and run in further to set the seal  42 . 
     Although a running tool  52  is illustrated in FIGS. 1 and 2 as contacting and engaging the upper end of the casing hanger  18 , it is pointed out that other tools or devices would do so in a similar manner. These alternative tools or devices include a second casing hanger, a wear bushing, plug type test tool or BOP isolation test tool. FIGS. 3 and 4 depict a second, upper casing hanger  80  that has been stacked atop the primary, lower casing hanger  18  so that a second casing string may be coaxially disposed within the first casing string. The second casing hanger  80  has a radially reduced tubular lower section  82  and an enlarged diameter upper section  84  that is threadedly affixed thereto. 
     The lower end of the enlarged upper section  84  of the second casing hanger  80  is modified so that it can sit atop the lower, or primary casing hanger  18 . The modification is that the upper section  84  is provided with a downwardly facing seating surface  86  that is shaped to be complimentary to the upper seating surface  32  of the lower casing hanger  18 . The downwardly facing seating surface  86  includes inner and outer bearing faces  88 ,  90  and an engagement face  92  that are similar or identical to the faces  68 ,  70  and  72  described earlier with respect to the running tool  52 . 
     If the primary casing hanger  18  is landed in a subsea high pressure housing, one could test above it or put another casing hanger above it (see FIGS. 3 and 4) and not worry about the radial deflection because the profile prevents the casing hanger from deflecting outward even if the seal is not there. The capture feature provided by the present invention is beneficial since it prevents, to a great degree, outward radial deflection of the upper portions of the casing hanger in response to axial loading. The design, therefore, increases the bearing capacity of a casing hanger. The presence of a pair of sloped surfaces also performs a guidance function. 
     While the invention has been shown in only some 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 stacking arrangement for wellhead tubular members wherein the adjoining ends of the tubular members are specially shaped to prevent or limit outward radial deflection and the damage associated therewith. A casing hanger is provided with an upper longitudinal end having an inwardly sloped inner bearing surface and an inwardly sloped outer bearing surface. An outwardly sloped engagement surface adjoins the two bearing surfaces and is designed to be captured radially within a complimentary shaped surface on a running tool or other engaging member landed atop the casing hanger. A grooved section is provided to assist removal of trash from the upper end of the casing hanger.