Abstract:
A subsea wellhead assembly includes an outer wellhead member with a bore containing at least one conical generally upward facing load shoulder that inclines relative to an axis of the bore. The outer wellhead member also contains an overpull recess having a generally downward facing overpull shoulder positioned below the at least one generally upward facing load shoulder. An inner wellhead member is lowered into the housing, the inner wellhead member having at least one conical downward facing load shoulder that inclines relative to an axis of the bore. A load ring has an inner profile that slidingly engages the downward facing load shoulder and an outer profile that slidingly engages the upward facing load shoulder.

Description:
FIELD OF THE INVENTION 
     This technique relates in general to centralizers for centering well pipe strings in wellbores, and, in particular, a well pipe hanger centralizer and overpull indicator. 
     BACKGROUND OF THE INVENTION 
     Centralizers have been in use for a long time. As their name suggests they have been used to center a tubular in a borehole. Centralizers are used on well pipe hangers to centralize the hanger, allowing the hanger to land on the corresponding tag shoulder properly, thereby preventing premature actuation of setting resistance devices, such as hanger load rings. The centralizers prevent the casing from side loading the hanger against the wellhead, which could result in premature actuation of setting resistance devices. The centralizers have been made of metal and non-metallic materials such as thermoplastic polyamides, glass, and mineral filled nylons and poly-tetra fluoro ethylene, also known as Teflon and injection molded polyurethane. These centralizers were made in hinged segments that could be clamped onto a tubular and in some applications the centralizers were formed right on to or slipped over the rod or tubular. Many centralizers use slotted bodies which may potentially scratch sealing bores. 
     A need exists for a technique that ensures that a well pipe hanger is properly centralized and set. The following technique may solve one or more of these problems. 
     SUMMARY OF THE INVENTION 
     In an embodiment of the present technique, an outer wellhead member has a bore containing at least one conical generally upward facing load shoulder that inclines relative to an axis of the bore. The outer wellhead member also contains an overpull recess having a generally downward facing overpull shoulder positioned below the at least one generally upward facing load shoulder. An inner wellhead member is landed in the outer wellhead member. The inner wellhead member has at least one conical downward facing load shoulder that inclines relative to an axis of the bore. A load ring is carried by the inner wellhead member for supporting the inner wellhead member on the upward facing load shoulder. The load ring has an inner profile that slidingly engages the downward facing load shoulder of the inner wellhead member and an outer profile that slidingly engages the upward facing load shoulder of the outer wellhead member. The load ring is carried by the inner wellhead member for movement between a retracted position, wherein the outer profile is spaced radially inward from the upward facing load shoulder, and an expanded position wherein the outer profile is in engagement with the upward facing load shoulder. 
     A centralizer/overpull assembly is carried by the inner wellhead member, below the load ring, for centralizing the inner wellhead member as it is lowered into the outer wellhead member, thereby preventing the load ring from prematurely setting. The centralizer/overpull assembly comprises a spring element and a centralizer/overpull ring positioned within a recess on the outer surface of the inner wellhead member. The centralizer/overpull ring has a generally upward facing overpull shoulder that is geometrically complimentary to the generally downward facing overpull shoulder of the outer wellhead member. The centralizer/overpull assembly is carried by the inner wellhead member for movement between a retracted position and an expanded position. When the inner wellhead member is in the landing position, the centralizer/overpull assembly is forced inward into the retracted position within the recess in the inner wellhead member. When the inner wellhead has been properly set and the load ring fully engaged, the centralizer/overpull assembly springs radially outward into the overpull recess in the outer wellhead member. In order to determine if the inner wellhead member and load ring are in the proper position, an overpull is made on the well string connected to the inner wellhead member. The generally downward facing overpull shoulder of the recess in the outer wellhead member abuttingly contacts the generally upward facing overpull shoulder of the centralizer/overpull ring, preventing upward movement of the inner wellhead member and indicating that the inner wellhead member and load ring are in the proper position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical sectional view of an inner wellhead member, load ring, and centralizer/overpull assembly shown in the set position within an outer wellhead member and constructed in accordance with this invention. 
         FIG. 2  is an isolated sectional view of the inner wellhead member of  FIG. 1 , shown prior to setting. 
         FIG. 3  is an enlarged view of a portion of the inner wellhead member of  FIG. 2 . 
         FIG. 4  is an enlarged view of a portion of the inner wellhead member and the centralizer/overpull assembly shown in the landing position within the outer wellhead member. 
         FIG. 5  is an enlarged view of a portion of the inner wellhead member and the centralizer/overpull assembly shown in the set position within the outer wellhead member of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an embodiment of an outer wellhead member  11  is illustrated. In this embodiment, outer wellhead member  11  is a landing sub connected to well pipe positioned below a high pressure wellhead housing. However, outer wellhead member  11  may be a high pressure housing or other housing having an axial bore  13 . 
     As best illustrated in  FIG. 1 , bore  13  has a tag/load shoulder  15  located within it. Tag/load shoulder  15  is a circular ledge located at a junction between a larger diameter upper portion in bore  13  and a slightly smaller diameter lower portion of bore  13  to define one embodiment of a profile portion of bore  13 . As best illustrated in  FIG. 5 , bore  13  also has an annular overpull recess  17  located within it, a select distance below tag/load shoulder  15 . Overpull recess  17  is formed by a generally downward facing overpull shoulder  18  and a generally upward facing shoulder  19 , with a surface  20  that is generally cylindrical and concentric to the axis of outer wellhead member  11  extending therebetween. 
     Referring again to  FIG. 1 , an inner wellhead member  21  lands within outer wellhead member  11  in the illustrated embodiment. For example, inner wellhead member  21  may be a well pipe hanger, such as a casing hanger. Inner wellhead member  21  has a profile made up of a load shoulder  25 , which may be geometrically complimentary to tag/load shoulder  15  of outer wellhead member  11 . In this embodiment, there is one inner wellhead member load shoulder  25  and one outer wellhead member tag/load shoulder  15 . The number of load shoulders  15 ,  25  can vary. An enlarged portion  27  of inner wellhead member  21  directly above load shoulder  25  has a diameter only slightly less than the inner diameter of bore  13 . 
     Referring to  FIGS. 2 and 3 , in the illustrated embodiment, a split load ring  29  is mounted on inner wellhead member  21  to support inner wellhead member  21  on outer wellhead member tag/load shoulder  15  ( FIG. 1 ). In this particular embodiment, load ring  29  is a ring having an upper portion  30  and a lower portion  31  connected to one another by a thin band portion  35  that acts as a designed failure point. Ring  29  is positioned on a flange  32  that extends radially outward from inner wellhead member  21 . However, load ring  29  is not limited to this particular embodiment. Load ring  29  has an external load shoulder  33  for mating with outer wellhead member tag/load shoulder  15  ( FIG. 1 ). Load ring  29  has an internal shoulder  34  for mating with inner wellhead member load shoulder  25 . Internal load shoulder  34  faces upward and inward at the same angle as inner wellhead member load shoulder  25 . Load ring external shoulder  33  faces downward and outward at the same angle as outer wellhead member tag/load shoulder  15  ( FIG. 1 ). External load shoulder  33  of load ring  29  is dimensioned to land on tag/load shoulder  15  in outer wellhead member bore  13  ( FIG. 1 ). 
     Referring still to  FIG. 3 , a centralizer/overpull assembly  46  is mounted to the inner wellhead member  21 , below load ring  29 . Centralizer/overpull assembly  46  comprises a spring element  47  and a split centralizer/overpull ring  49  positioned within an annular recess  51  on the outer surface of inner wellhead member  21 . In this particular embodiment, spring element  47  comprises an elastomeric ring. The size and stiffness of spring element  47  can be varied depending upon the desired centralizing force. In this particular embodiment, recess  51  contains a detent  53  that is geometrically complimentary to elastomeric ring  47 . In this particular embodiment, centralizer/overpull ring  49  has an inner surface  55  with a detent  57  that is geometrically complimentary to elastomeric ring  47 . The outer surface of centralizer/overpull ring  49  comprises a generally upward facing shoulder  59 , geometrically complimentary to generally downward facing overpull shoulder  18  of outer wellhead member  11  ( FIG. 5 ). The outer surface of centralizer/overpull ring  49  also comprises a generally downward facing shoulder  61 , geometrically complimentary to generally upward facing shoulder  19  of outer wellhead member  11  ( FIG. 5 ). Surface  63  is generally cylindrical and concentric to the axis of inner wellhead member  21  and extends between generally upward facing shoulder  59  and generally downward facing shoulder  61 . Surface  63  is geometrically complimentary to surface  20  of outer wellhead member  11  ( FIG. 5 ). Elastomeric ring  47  rides in recess  51 , between the outer surface of inner wellhead member  21  and centralizer/overpull ring  49 . 
     In the initial orientation of inner wellhead member  21  of  FIGS. 2 and 3 , prior to tripping inner wellhead member  21  into outer wellhead member  11 , centralizer/overpull ring  49  is fully expanded, with the outer diameter of centralizer/overpull ring  49  being greater than or equal to the outer diameter of load ring  29 . In the trip-in position of  FIG. 4 , as inner wellhead member  21  is tripped into the well, centralizer/overpull ring  49  is compressed into recess  51  by outer wellhead member  11  as centralizer/overpull ring  49  centralizes inner wellhead member  21  in outer wellhead member  11 . In the landed position of  FIGS. 1 and 5 , centralizer/overpull ring  49  is expanded radially outward into the overpull recess  17  in the surface of bore  13  of outer wellhead member  11 . 
     In the trip-in operation of inner wellhead member  21 , load ring  29  will be in the retracted position shown in  FIGS. 2 and 3 . The outer diameter of load ring  29  in this position is no greater than the outer diameter of inner wellhead member  21  at enlarged diameter portion  27 . Prior to entering bore  13  of outer wellhead member  21 , centralizer/overpull assembly  46  is expanded, with the outer diameter of centralizer/overpull ring  49  being greater than or equal to the outer diameter of load ring  29 . As inner wellhead member  21  is lowered into bore  13 , centralizer/overpull ring  49  and spring element  47  are compressed by bore  13  to centralize inner wellhead member  21  in outer wellhead member  11  and prevent premature side loading of load ring  29 . As inner wellhead member  21  is further lowered into bore  13 , centralizer/overpull ring  49  lands on outer wellhead member tag/load shoulder  15 . The generally downward facing shoulder  61  of centralizer/overpull ring  49  will contact tag/load shoulder  15 , pushing ring  49  radially inward into recess  51 , further compressing spring element  47 , allowing centralizer/overpull ring  49  to move below tag/load shoulder  15 . 
     As inner wellhead member  21  further enters bore  13 , load ring  29 , and in particular, external load shoulder  33  ( FIG. 3 ) will land on outer wellhead member tag/load shoulder  15 . As inner wellhead member  21  continues to move downward in the landing position, the force on load ring  29 , causes the upper portion  30  of load ring  29  to separate from the lower portion  31  of ring  29  at the designed failure point, which can be thin band  35 , effectively pushing and moving the upper portion  30  of load ring  29  radially outward just above tag/load shoulder  15 . Therefore, load ring  29  can include a shearable lock mechanism, thin band  35 , that holds upper portion  30  in a run-in position until landing on upward facing tag/load shoulder  15 . 
     Referring to  FIGS. 1 and 5 , from this point, inner wellhead member  21  moves downward relative to load ring  29 , until load ring  29  is fully engaged with shoulders  15  of outer wellhead member  11  and shoulder  25  of inner wellhead member  21  and inner wellhead member  21  is in a set position within outer wellhead member  11 . In addition, as inner wellhead member  21  moves downward relative to load ring  29 , centralizer/overpull assembly  46  moves downward with inner wellhead member  21  until centralizer/overpull assembly  46  reaches overpull recess  17  ( FIG. 5 ). When inner wellhead member  21  reaches the set position, centralizer/overpull assembly  46  simultaneously reaches recess  17 , spring element  47  expands, pushing centralizer/overpull ring  49  radially outward and into overpull recess  17 , and moving centralizer/overpull assembly  46  to an expanded position. 
     Once fully engaged, generally downward facing load shoulder  25  of inner wellhead member  21  is in contact with internal load shoulder  34  of load ring  29 , and tag/load shoulder  15  of outer wellhead member  21  is in contact with external load shoulder  34  of load ring  29 . Downward load on inner wellhead member  21  transfers from inner wellhead member  21  load shoulder  25  through load ring  29  to outer wellhead member tag/load shoulder  15 . 
     A reduction in the indication of the weight of the running string is an indication that load ring  29  has set. As a further assurance, the operator can apply a selected overpull. An overpull is a test where a lifting force is applied by the running string to inner wellhead member  21 . Referring to  FIGS. 1 and 5 , when in the set position, centralizer/overpull ring  49  rests within recess  17  on the outer wellhead member  21 . As previously discussed, the outer surfaces of centralizer/overpull ring  49  are geometrically complimentary to the surfaces of recess  17 . As a result, generally upward facing overpull shoulder  59  of centralizer/overpull ring  49  abuts against generally downward facing overpull shoulder  18  of recess  17 . For a given centralizer/overpull assembly  46 , a defined overpull, for example, 200,000 pounds, is necessary to cause centralizer/overpull ring  49  to move radially inward into recess  51  on inner wellhead member  21 , thereby moving centralizer/overpull assembly  46  to a retracted position. Therefore, the operator will pull upward a fraction of that amount, for example, 100,000 pounds, to determine if generally upward facing overpull shoulder  59  of centralizer/overpull ring  49  is abuttingly contacting generally downward facing overpull shoulder  18  of recess  17 . If the pull upward does not result in the upward movement of the inner wellhead member  21 , this indicates that centralizer/overpull assembly  46  is located correctly and, therefore, load ring  29  is located correctly and has properly set. The operator can then slack off the weight. 
     The technique has significant advantages. The centralizer/overpull ring maintains full circumferential contact with the inner surface of the outer wellhead member, preventing sealing bores from being scratched. The centralizing force of the centralizer/overpull assembly can be custom tailored by switching out different size and stiffness spring elements. Additionally, the centralizer/overpull assembly acts as a centralizer and also allows an overpull to be made to ensure that the load ring is properly set. 
     While the technique 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 technique.