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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND 
     1. Field of Invention 
     The present disclosure relates in general to a wellhead assembly for use in producing subterranean hydrocarbons. More specifically, the present disclosure relates to a wellhead assembly having high and lower pressure wellhead housings with sockets whose respective outer surfaces are generally cylindrical. 
     2. Description of Prior Art 
     Subsea wells typically include outer low pressure housing welded onto a conductor pipe, where the conductor pipe is installed to a first depth in the well, usually by driving or jetting the conductor pipe. A drill bit inserts through the installed conductor pipe for drilling the well deeper to a second depth so that high pressure housing can land within the low pressure housing. The high pressure housing usually has a length of pipe welded onto its lower end that extends into the wellbore past a lower end of the conductor pipe. The well is then drilled to its ultimate depth and completed, where completion includes landing a casing string in the high pressure housing that lines the wellbore, cementing between the casing string and wellbore wall, and landing production tubing within the casing. The aforementioned concentrically stacked tubulars exert a load onto the lower pressure housing that is transferred along an interface between the high and low pressure housings. Moreover, tilting the stacked tubulars generates a bending moment along the interface. 
     SUMMARY OF THE INVENTION 
     Disclosed herein a wellhead assembly, which in one embodiment includes an annular low pressure housing having a lower end set in a sea floor. In this example, an upper socket surface is formed along a portion of an inner surface of the low pressure housing; axially spaced apart from the upper socket surface is a lower socket surface formed along a portion of the inner surface of the low pressure housing. The wellhead assembly further includes an annular high pressure housing coaxially disposed within the low pressure housing, an upper socket surface formed along a portion of an outer surface of the high pressure housing that is in contact with the upper socket surface on the low pressure housing and that selectively exerts a load against the upper socket surface on the low pressure housing to define an upper loading interface. A lower socket surface is on the outer surface of the high pressure housing that is axially spaced apart from the upper socket surface on the high pressure housing and is in contact with the lower socket surface on the low pressure housing. The lower socket surface on the high pressure housing selectively exerts a load against the lower socket surface on the low pressure housing to define a lower loading interface. A latch assembly is coupled to the low pressure housing and the high pressure housing between the upper and lower loading interfaces. In an alternate example, the upper and lower loading interfaces project axially in a direction that is substantially parallel with an axis of the wellhead assembly. Optionally, the upper and lower loading interfaces are radially offset from one another. The wellhead assembly can alternatively further include a channel formed on an outer surface of the high pressure housing between the upper and lower loading interfaces and a passage axially formed through the high pressure housing having an end in communication with the channel and a lower end in communication with an annulus between the high and lower pressure housings on a side of the lower loading interface opposite the channel. Included with this example is a passage radially extending through the lower pressure housing and in communication with the channel. In an example embodiment the latch is made up of a C-ring set in a groove provided on an outer surface of the high pressure housing. The latch may include a profile on an inner surface of the low pressure housing. A downward facing shoulder can optionally be included on an outer surface of the high pressure housing that contacts an upward facing shoulder on an inner surface of the low pressure housing when the high pressure housing lands in the low pressure housing. 
     Also described herein is a wellhead assembly that includes a low pressure housing mounted in a sea floor having a high pressure housing landed within. The high pressure housing has upper and lower radially thinner portions and a radially thicker portion disposed between and adjacent to the upper and lower radially thinner portions. An upper loading surface is provided on an outer surface of the radially thicker portion that terminates at a location where the radially thicker portion transitions into the upper radially thinner portion. A lower loading surface is formed on the outer surface of the radially thicker portion that terminates at a location where the radially thicker portion transitions into the lower radially thinner portion. Upper and lower loading surfaces are included on an inner surface of the low pressure housing that respectively engage the upper and lower loading surfaces on the radially thicker portion. A latch is provided for engaging the low and high pressure housings disposed axially between the upper loading surface and lower loading surface on the high pressure housing. An optional channel can be included on an outer surface of the high pressure housing disposed between the upper loading surface and lower loading surface on the high pressure housing and a passage providing communication between the channel and an annulus between the low and high pressure housings and adjacent the location where the radially thicker portion transitions to the lower radially thinner portion. In an alternate example included is a production tree on an upper end of the high pressure housing. Optionally included is a casing hanger landed inside the high pressure housing and a tubing hanger landed inside the casing hanger. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a side sectional view of engaging together example embodiments of high and low pressure wellhead housings in accordance with the present invention. 
         FIG. 2  is a side perspective view of the high and low pressure wellhead housings of  FIG. 1  in engagement to form a portion of an embodiment of a wellhead assembly and in accordance with the present invention. 
         FIG. 3  is a side sectional view of the portion of the wellhead assembly of  FIG. 2  further including a production tree and in accordance with the present invention. 
     
    
    
     While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF INVENTION 
     The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. 
     It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. 
       FIG. 1  is a side sectional view of an example of a wellhead assembly  10  being formed by inserting a high pressure housing  12  into a low pressure housing  14 . A weld  16  on the high pressure housing  12  of  FIG. 1  attaches an upper portion  18  to a lower portion  20 , where the lower portion  20  extends downward and into a wellbore  21 . Similarly, the low pressure housing  14  includes a weld  22  attaching an upper portion  24  to lower portion  26 . In the example of  FIG. 1 , the lower portion  26  is anchored within a sea floor  27 . A transition  28  on the upper portion  18  indicates where its thickness changes. Below the transition  28  the thickness of the upper portion  18  is substantially the same as a thickness of the lower portion  20 , whereas above the transition its thickness increases to a maximum width to define a middle section of the high pressure housing  12 . An upper terminal end of the middle section is defined by an upper transition  29 , which indicates a location where the radial thickness of the high pressure housing  12  decreases. The radial thickness of the high pressure housing  12  above the transition  29  is less than along the middle section, but greater than below transition  28 . The changes in radial thickness define a thicker middle section with two radially thinner portions projecting axially away from the middle section. Further illustrated in the example of  FIG. 1  is a passage  30  in the upper portion  18  that extends axially downward from a channel  31  shown circumscribing the middle portion along its outer surface. The passage  30  communicates between the channel  31  and an annulus between the high and low pressure housings  12 ,  14 . 
     A lower socket surface  32  is shown formed on an outer periphery of the upper portion  18  and facing generally radially outward from an axis A X  of the wellhead assembly  10 ; a lower end of the lower socket surface  32  terminates adjacent the transition  28 . The low pressure housing  14  also includes a lower socket surface  34  that is formed on an inner circumferential surface of the low pressure housing  14 . In the example of  FIG. 1 , a lower end of the lower socket surface  34  terminates adjacent where the radial thickness of the low pressure housing  14  decreases to a thickness substantially the same as a thickness of the lower portion  26 . In one embodiment, a radial passage  36  is further illustrated that extends through the upper and thicker portion  24  of the low pressure housing  14 . In an example embodiment, the radial passage  36  is above an upper terminal end of the lower socket surface  34 . 
     Still referring to  FIG. 1 , upper socket surface  38  is similarly provided on the outer surface of the high pressure housing  12  shown facing generally radially outward from the axis A X , and having an upper end that terminates adjacent transition  29 . An upper socket surface  40  on the low pressure housing  14  faces radially inward towards axis A X  and has an upper terminal end proximate an upper terminal end of the low pressure housing  14 . As further discussed below, a latching system is included for coupling together the high and low pressure housings  12 ,  14  that includes s C-ring  42  disposed within a groove  44  formed on the outer surface of the radially thicker section of the upper portion  18 . The C-ring  42  and groove  44  illustrate one example of embodiment of a latching mechanism for engaging the high and low pressure housings  12 ,  14 . 
     Referring now to  FIG. 2 , an example is illustrated of the high pressure housing  12  landed within low pressure housing  14 . In this example, the upper socket surfaces  38 ,  40  are aligned and in contact with one another so that any bending moment forces exerted onto the high pressure housing  12  can be transferred onto the low pressure housing  14 . Axially distal from the upper socket surfaces  38 ,  40  are the lower socket surfaces  32 ,  34 , also in engagement and in contact with one another for effectively transferring bending moment loads from the high pressure housing  12  to low pressure housing  14 . In the embodiment illustrated, the lower socket surfaces  32 ,  34  are a maximal distance from the upper socket surfaces  38 ,  40 , thereby increasing bending moment transfer between the inner and outer wellhead housings  12 ,  14  and consequently reducing respective angular movement of the high pressure housing  12  within low pressure housing  14 . When in the landed configuration of  FIG. 2 , the passage  36  registers with channel  31 , so that passage  36  is in fluid communication with passage  30  and with the annulus between the high and low pressure housings  12 ,  14  As shown, passage  36  and channel  31  are between the lower socket surfaces  32 ,  34  and the upper socket surfaces  38 ,  40 . Further shown in the example of  FIG. 2  are load shoulders  45 ,  46  respectively formed on the high and low pressure housings  12 ,  14 , which are in axial contact with one another, thereby transferring an axial load from the high pressure housing  12  onto the low pressure housing  14  for supporting the high pressure housing  12  within low pressure housing  14 . Additionally, a profile  47  is shown formed on an inner surface of the low pressure housing  14  and strategically located so to engage an outer surface of the C-ring  42  for latching together the high and low pressure housings  12 ,  14 . Moreover, by locating the latching mechanism of the C-ring  42 , along with the channel  31 , axially between the upper socket surfaces  38 ,  40  and lower socket surfaces  32 ,  34 , the maximal distance between the socket surfaces can be achieved. As such, forgings of the upper portions  18 ,  24  need not be altered in order to achieve sufficient bending moment transfer between the housings  12 ,  14 . 
     Another advantage of the wellhead housing  10  disclosed herein is that in one embodiment, the socket surfaces  32 ,  34 ,  38 ,  40  each are generally vertical so that minimal forces are required to insert the high pressure housing  12  within low pressure housing  14 . In one example of use, axial forces required to urge the high pressure housing  12  inside low pressure housing  14  were less than about 200,000 pounds force. 
       FIG. 3  is a side sectional view of an example of the wellhead assembly  10  shown with a production tree  48  mounted on an upper end of the high pressure housing  12 . Further illustrated is a casing hanger  50  landed on an inner surface of the high pressure housing  12  and supporting a string of casing  52  shown depending downward into the wellbore  21 . Coaxially inserted within the casing  52  is a tubing hanger  54  having a corresponding string of tubing  56  that projects coaxially within the casing  52 . Thus, in this example, the low pressure housing  14  axially supports the load of the high pressure housing  12  tubing and casing hangers  50 ,  54 , casing  52 , and tubing  56 . Further in the example of  FIG. 3 , the tubing communicates with a main bore  58  that projects axially through the production tree  48 . 
     In one optional example, one of the socket surfaces can have a convex shape while an opposing or mating socket surface can still have a cylindrical or substantially vertical profile. Similarly, both the inner and outer socket surfaces may have convex shapes that deform when the high pressure housing  12  inserts and lands within the low pressure housing  14 . In another optional embodiment, one of the socket team members can be in a separate housing where the housing is welded to the member holding the other socket surface. 
     The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.

Summary:
A wellhead assembly for use subsea includes a high pressure housing landed within a low pressure housing. The low pressure housing is an annular member that mounts into the sea floor and having an inner surface engaging the high pressure housing along a loading interface. Upper and lower sockets are formed along axially spaced apart portions of the outer surface of the high pressure housing. As the high pressure housing inserts into the low pressure housing, the high pressure housing sockets engage corresponding sockets formed along axially spaced apart sockets on portions of the inner surface of the low pressure housing. The sockets each have cylindrically shaped outer surfaces, and when engaged with one another define the loading interface. The sockets are strategically located on the upper and lower portions of the housings to maximize their distance apart.