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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
     This invention relates in general to wellhead equipment for oil and gas wells, and in particular to a casing hanger centralizing mechanism. 
     DESCRIPTION OF RELATED ART 
     A typical subsea wellhead assembly includes a wellhead housing that supports one or more casing hangers. One type of wellhead housing has a conical load shoulder machined within its bore. The casing hanger lands on and is supported by the load shoulder. In this type, the diameter of the housing bore below the load shoulder is less than the diameter of the housing above the load shoulder by a dimension equal to a radial width of the load shoulder. 
     In another type, referred to as “full bore”, the wellhead housing has a groove with substantially the same diameter above and below the groove. The load shoulder is a split ring that is installed subsequently in the groove. The casing hanger is supported by the load shoulder. This procedure allows a larger diameter bore to be employed during drilling operations. The load shoulder may be installed on a special running tool or it may be run with the casing hanger. 
     Active casing hangers may be used to transfer the casing load to the wellhead housing via a loading mechanism that includes an activation ring, shear pins that prevent premature movement of the activation ring, and a load ring on the casing hanger. This mechanism is typically designed to be activated by the weight of the string when a reaction point, such as a shoulder, formed on the interior of the wellhead housing is reached during lowering of the hanger. At this point, the shear pins on the activation ring break to allow it to slide relative to the downward movement of the hanger, thereby allowing the load ring on the hanger to align with the housing to transfer casing load to the housing. However, if the hanger snags or the pins load up unevenly and break prematurely, the activation ring may be activated prematurely. This is costly and time consuming as the hanger and casing would have to be pulled out and re-tripped. 
     A technique is thus desirable that correctly and reliably activates the loading mechanism on an active casing hanger to prevent premature activation. 
     SUMMARY OF THE INVENTION 
     In an example embodiment a wellhead housing has a bore containing at least one conical generally upward facing load shoulder that inclines relative to an axis of the bore. A housing load ring with a corresponding downward facing shoulder is supported by the load shoulder on the wellhead housing. The housing load ring has a generally upward facing shoulder that inclines relative to the bore axis. A tag shoulder is formed on an interior surface of the housing load ring. A casing hanger landed in the housing has a at least one conical downward facing load shoulder that inclines relative to the axis of the bore. A split load ring is carried by the hanger for supporting the hanger on the upward facing load shoulder. The load ring has an inner profile that slidingly engages the downward facing load shoulder of the hanger and an outer profile that slidingly engages the upward facing load shoulder of the housing load ring. The load ring is carried by the hanger for movement between a retracted position, wherein the outer profile is spaced radially inward from the upward facing shoulder of the housing load ring, and an expanded position wherein the outer profile is in engagement with the upward facing shoulder of the housing load ring. When set, the load rings and shoulders provide a path for the casing load to be transferred to the wellhead housing. 
     Further, the casing hanger has a centralizer ring comprised of segments or fingers. The fingers are fastened to the casing hanger at one end and have a hook at another end for engagement with a hook formed on a lower end of an activation ring. The fingers have a pivot or fulcrum point on an interior portion that contacts an exterior surface of the hanger. When fastened, the fulcrum point creates an outward spring effect that acts to centralize the hanger as it is lowered into the wellhead housing when it contacts the bore of the housing. A shear pin maintains the activation ring in place during installation until a corresponding tag shoulder on the activation ring comes into contact with the tag shoulder on the housing load ring. Additional downward force at this point causes the hook on the fingers to disengage from the hook on the activation ring and the pins to shear. Once the pins are sheared, the hanger and centralizing ring move downward independently of the activation ring, allowing the load ring carried by the hanger to slide radially outward into an expanded position. Once set in this position, casing load can be transferred through the casing hanger load ring, to the housing load ring, and to the wellhead housing via the load shoulder. The fingers of the centralizing ring advantageously prevent activation of the casing hanger until the correct location within the housing is reached. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side sectional view of a casing hanger and load ring carried on the casing hanger and shown within a wellhead housing in an unset position, and constructed in accordance with this invention. 
         FIG. 2  is a perspective view of a centralizing ring comprising finger segments, in accordance with the invention. 
         FIG. 3  is a side sectional view of a hook mechanism of a finger, in accordance with the invention. 
         FIGS. 4A-4E  are side sectional views of an activation sequence for setting the casing hanger of  FIG. 1  within the wellhead housing. 
         FIGS. 5A-4E  are side sectional views of a deactivation sequence for retrieving the casing hanger of  FIG. 1  from within the wellhead housing. 
         FIG. 6  is a perspective view of a centralizing ring with fingers extending from a base, in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The apparatus and method of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. This subject of the present disclosure may, however, be embodied 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 the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. For the convenience in referring to the accompanying figures, directional terms are used for reference and illustration only. For example, the directional terms such as “upper”, “lower”, “above”, “below”, and the like are being used to illustrate a relational location. 
     It is to be understood that the subject 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 of the subject disclosure and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the subject disclosure is therefore to be limited only by the scope of the appended claims. 
     Referring to  FIG. 1 , a casing hanger  10  is shown within a wellhead housing  12  having an axial bore  14 . Bore  14  has an upward facing tapered shoulder  16  located formed within. In this embodiment, the shoulder  16  inclines relative to an axis of the bore  14 . Shown resting on the shoulder  16  is an annular housing load ring  18  whose lower surface defines a downward facing shoulder  19  with a slope corresponding to the load shoulder  16 . The upper surface of the housing load ring  18  has a generally upward facing shoulder  20  that inclines relative to the bore axis A X . A tag shoulder  22  is formed on an interior surface of the housing load ring  18  that interacts with a corresponding downward facing shoulder  24  formed on an activation ring  26  carried by the hanger  10  by means of a shear pin  28 . In this embodiment, tag shoulder  22  and downward facing shoulder  24  are correspondingly tapered and the activation ring  26  is cylindrical. The activation ring  26  may have an opening to receive the shear pin  28 . When the tag shoulders  22 ,  24  contact each other, the activation sequence is initiated. The activation sequence will be discussed in further detail in a subsequent section. 
     Continuing to refer to  FIG. 1 , the casing hanger  10  has a at least one conical downward facing load shoulder  30  that inclines relative to the axis A X  of the housing bore  14 . A hanger load ring  32 , which may be a split load ring, is carried on the outer circumference of the hanger  10  for supporting the hanger  10  on the housing load ring  18 . The hanger load ring  32  has an upward facing, tapered shoulder  34 , that as will be described in more detail below, slidingly engages the downward facing load shoulder  30  of the hanger  10 . The hanger load ring  32  further includes a downward facing, tapered shoulder  36  that slidingly engages the upward facing load shoulder  20  of the housing load ring  18  when the housing load ring  18  is in an expanded position. The hanger load ring  32  in this embodiment is carried by the hanger  10  for movement between a retracted position, wherein an outer profile is  38  spaced radially inward from the upward facing shoulder  20  of the housing load ring  18 , and the expanded position wherein the outer profile  38  is in engagement with the upward facing shoulder  20  of the housing load ring  18 . When set, contact between the hanger  10  and housing load rings  32 ,  18  and shoulders  30 ,  20  transfer a load from the casing to the load shoulder  16  of the wellhead housing  12 . 
     Continuing to refer to  FIG. 1 , in this embodiment, a centralizer ring  50  having a plurality of segments or fingers  52  is shown. The centralizer ring  50  centralizes the casing hanger  10  and minimizes impact when it is lowered into the housing  12  A perspective view of the centralizer ring  50  is shown in  FIG. 2  for clarity. The fingers  52  are fastened to the casing hanger  10  at a lower end  54  of the finger  52  with bolts  56  inserted through passages  58  ( FIG. 2 ) formed at the lower end  54  of the finger  52  shown registering with passages  60  formed on the hanger  10 . It is not necessary that all lower ends  54  of the fingers  52  be fastened to the hanger  10 . This selective fastening allows for tuning of the amount of centralizing force provided. A lower portion  70  of the centralizing ring  50  forms a base that supports the fingers  52  in this embodiment. The lower portion  70  is fastened to the hanger  10  by bolts  72  shown inserted through passages  74  in the hanger  10  and corresponding passages  76  formed in the centralizing ring  50 . A rim  78  that protrudes upward from the lower portion  70  may interfere with a lower extension  80  of the finger  52  to prevent the lower end  54  from sliding outward if it is not fastened to the hanger  10 . 
     A top end  90  of the finger  52  opposite to lower end  54  has a hook  92  shown facing radially outward for engagement with a radially inward facing hook  94  formed on a lower end of the activation ring  26 . Referring to  FIG. 3  for more clarity, a downward facing surface  96  on the finger hook  92  interferes with an upward facing surface  98  on the hook  94  of the activation ring  26 . Thus the fingers  52  may be restrained at the lower end  54  by the bolt  56  and at the top end  90  by engagement of the hooks  92 ,  94 . Depending on the amount torque applied to fasten the bolt  56 , a spring effect can be created between the top end  90  and lower end  54  of the finger  52  due to a fulcrum  100  ( FIG. 1 ) formed on an interior surface of the finger  52  that contacts the hanger  10  and forms a cantilever. The fulcrum  100  protrudes radially inward and forms a pivot point at the point of contact with the hanger  10 . The outward spring effect on the top end  90  centralizes the hanger  10  as it is lowered into the wellhead housing  12 . Further, the spring effect causes the hook  92  at the top end  90  to remain engaged to the hook  94  on the activation ring  26  when the fingers  52  of the centralizing ring  50  contact the bore  16  as the hanger  10  is lowered into the wellhead housing  12 . As shown in  FIG. 3 , a pad or protrusion  102  formed on the upper end of the finger  52  protrudes radially outward beyond an outer surface  104  of the activation ring  26 . This allows the pad  102  to come in contact with the bore  16  ( FIG. 1 ) to centralize the hanger  10  while preventing the activation ring  26  from becoming snagged during the lowering of the hanger  10 . If a finger  52  is not bolted to the hanger  10  at the lower end  54  and is captured by the rim  78  of the lower portion  70 , the pad  102  on the exterior of the finger  52  will disengage the hooks  92 ,  94  when the pad  102  contacts the bore  14  of the housing  12 . 
     One example of installation of an embodiment of the casing hanger  10  is depicted in  FIGS. 4A-4E . During installation, the casing hanger  10  may be lowered into the wellhead housing  12  by a running tool (not shown) as shown in  FIG. 4A . As the hanger  10  is lowered, the fingers  52  of the centralizing ring  50  centralize the hanger  10  within the wellhead housing  12  by exerting an outward force against the bore  14  of the wellhead housing  12 . The force is exerted by the spring effect loaded into the fingers  52  during assembly. The outward bowing of the loaded fingers  52  thus define a centralizer. The hanger  10  is lowered until the pad  102  ( FIG. 3 ) contacts the upward facing tag shoulder  22 . In an example embodiment, the tag shoulder  22  may be upset radially outward from the bore  14  by about ⅛″. The remaining installation of the casing hanger  10  is accomplished via the weight of casing supported by the hanger  10 ; the weight of the casing supported by the hanger causes the hanger  10  to continue moving down. The hooks  92  disengage the hooks  94  as the pad  102  is urged radially inward through its contact with the bore  14  as shown in  FIG. 4B . The hooks  92 ,  94  only disengage at the proper diameter of the bore  14  and may be designed to withstand a one million pound load before shearing. At this point, the downward facing tag shoulder  24  formed on the activation ring  26  contacts the upward facing tag shoulder  22  on the housing load ring  18 . As shown in  FIG. 4C , the interference between the tag shoulders  24 ,  22  creates a reaction point that allows additional casing weight that will eventually shear the shear pin  28 . Referring to  FIG. 4D , the shear pin  28  is shown having been sheared releasing the casing hanger  10  and the centralizing ring  50  to continue to move downward while the activation ring  26  remains engaged to the housing load ring  18  via the interference between the tag shoulders  22 ,  24 . As the hanger  10  and centralizing ring  50  continue moving downward, the hanger load ring  32  is urged radially outward by the hanger  10  such that the hanger load ring  32  slides radially outward along a top surface of the activation ring  26  until the upward facing shoulder  34  slides into contact with the downward facing load shoulder  30  of the hanger  10  and the downward facing shoulder  36  of the hanger load ring  32  slides into contact with the upward facing shoulder  20  of the housing load ring  18 , as shown in  FIG. 4E . At this point, the hanger load ring  32  is fully set and is no longer in contact with the activation ring  26 . The casing load may thus be transferred from the casing hanger  10  through the hanger load shoulder  30 , to the hanger and housing load rings  32 ,  18 , to the housing load shoulder  16  and ultimately the wellhead housing  12 . 
     If needed, the casing hanger  10  may be retrieved from within the wellhead housing  12 .  FIGS. 5A-5E  illustrate a sequence for retrieving the casing hanger  10  by deactivation of the activation ring  26 . A running tool (not shown) may be used to unset the casing load ring  32  from the position shown in  FIG. 5A . As the casing hanger  10  moves upward, as shown in  FIG. 5B , the pad  102  on the exterior of the finger  52  of the centralizing ring  50  interferes with a recess  110  formed on the bore  14  of the wellhead housing  12 . This interference may be used to check the installation of the hanger  10  via overpull, which in an example embodiment may be calibrated at 100 kips. To fully disengage the pad  102  from the recess  110  and continue moving the hanger  10  upward, an upward force greater than the overpull force is applied to the hanger  10  as shown in  FIG. 5B . In an example embodiment the upward force to move the hanger  10  is about 150 kips. The force is sufficient to overcome the interference and the weight of the casing (not shown) carried by the hanger  10 . Sufficient friction exists between the activation ring  26  and the hanger  10  to cause the ring  26  to move upward sufficiently for the tag shoulder  24  on the ring  26  to lift off from the tag shoulder  22  on the housing load ring  18 . Upward movement of the ring  26  also causes the upper surface of the ring  26  to contact the hanger load ring  32  and lift it off of the tapered shoulder  20  of the housing load ring  18 . As shown in  FIG. 5C , the casing hanger  10  continues to be raised until and upward facing shoulder  114  on the casing load ring  32  contacts a corresponding, downward facing shoulder  112  formed on the housing  12  and inclined relative to the bore axis. As the hanger  10  continues to be raised upward in this embodiment, the hanger load ring  32  encounters a downward sloping surface  116 , along which the load ring  18  begins to move radially inward as it contracts to the retracted state, as shown in  FIGS. 5D and 5E . When the hanger load ring  32  is completely retracted and retained within a pocket  120  ( FIG. 5E ) formed on the exterior of the casing hanger  10 , the hook  92  aligns with hook  94 ; the casing hanger  10 , together with the centralizing ring  50 , activation ring  26 , and housing load ring  32 , may be retrieved from the wellhead housing  12 . 
     In an alternative embodiment shown in  FIG. 6 , a centralizing ring  140  may be have a collet finger arrangement, with a plurality of fingers  144  having a common lower end  142 . The lower end  142  of the centralizing ring  140  has bolt holes  146  around the circumference to allow fastening to the hanger  10  ( FIG. 1 ). In contrast to the ring  50  of  FIG. 1 , the fingers  144  of the centralizing ring  140  are segmented on an upper end  150  and not along the entire axial length of the ring  140 . Instead, the lower end  142  is a single member that is not segmented along its circumference. The spring effect in this embodiment may also be generated by a fulcrum  148  acting to cantilever the upper portion  150  of the fingers  144 . 
     The invention has significant advantages. Premature activation of the activation ring is prevented by the preventing disengagement of the hook on the fingers until the correct diameter of within the wellhead housing is reached. Costly re-installation of the casing hanger is thus limited. 
     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 assembly includes a housing with a bore. A hanger is lowered into the housing, the hanger having at least one centralizing finger with a hook for engaging a corresponding hook an activation ring carried by the hanger via a shear pin. A load ring is carried on the hanger and supported initially within a recess formed on the exterior of the hanger. At the correct depth within the housing, coinciding with a shoulder on the inner diameter of the housing, the pin is sheared under the weight of the casing string and the hooks disengage to allow the load ring on the hanger to slide outward and create a path for the casing load to be transferred from the hanger to the hanger load ring, to a housing load ring, and ultimately the housing.