A wellhead connector for connecting a riser or production tree to a wellhead of a subsea well utilizes a singular annular piston to lock the connector onto the wellhead. The wellhead connector includes a housing that contains dogs for engagement with the exterior of the wellhead housing. A cam ring is also included, which has an inner side for engaging the dogs and moving them inward into a locked position with the wellhead housing. The cam ring is of a reduced proportion relative to prior art. As such, the cam ring outer side is dimensioned to contact the inner side of the connector housing under load. Connecting rods connect the piston to the cam rings. As the piston moves downward, the cam ring also moves downward, forcing the dogs inward into a locked position. As the piston moves upward, the cam ring also moves upward, thereby unlocking the connector. A secondary annular piston is also provided to guarantee unlocking. 
     At preload, a profile on the lower portion of the connector body engages a stepped profile on the outer diameter of the wellhead thereby creating a secondary load path for reacting to the applied bending moment.

RELATED APPLICATIONS 
     This application is related to and claims priority and benefit of U.S. patent application Ser. No. 11/776,171, filed originally as a utility application and converted to a provisional application. 
     BACKGROUND 
     1. Field of the Invention 
     This invention relates in general to subsea wells, and in particular to a connector for connecting a riser to a subsea wellhead housing. 
     2. Description of the Prior Art 
     In a subsea well of the type concerned herein, a tubular wellhead is located on the sea floor. During drilling operations, a riser extends from a vessel at the surface down to the wellhead. A wellhead connector connects the lower end of the riser to the wellhead. After the riser is disconnected, a similar wellhead connector may be used to connect a subsea production tree to the wellhead. The wellhead connector has a housing which slides over the wellhead. In one type, a plurality of dogs are carried by the wellhead connector. The dogs include grooves on their interior sides. A cam ring moves the dogs inwardly into engaging contact with grooves formed on the exterior of the wellhead. 
     A plurality of pistons are spaced apart from each other circumferentially around the wellhead housing to move the cam ring axially between a locked and unlocked position. Because of the large cam ring cross-section and number of pistons, the connectors are large, heavy, and expensive to manufacture. Therefore, what is needed is a wellhead connector that is lighter, more efficient, and less expensive to manufacture. 
     SUMMARY OF THE INVENTION 
     The wellhead connector of the present invention utilizes a singular annular piston to lock the connector onto the wellhead. The connector includes a housing that contains a plurality of dogs having a set of grooves formed on their inner sides for engagement with a set of grooves on the exterior of the wellhead housing. A cam ring is also included, which has an inner side for engaging the dogs and moving them inward into a locked position with the wellhead housing. The cam ring is of a reduced proportion relative to prior art. As such, the cam ring outer side is dimensioned to contact the inner side of the connector housing under load. A plurality of connecting rods connect the annular piston to the annular cam ring. At preload, a profile on the lower portion of the connector body engages a stepped profile on the outer diameter of the wellhead thereby creating a secondary load path for reacting to the applied bending moment. As the piston moves downward, the cam ring also moves downward, forcing the dogs inward into the locked position. As the piston moves upward, the cam ring also moves upward, thereby unlocking the connector. A secondary annular piston is also included to guarantee unlocking. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial sectional view illustrating a wellhead connector according to an embodiment of the present invention, with the left side shown unlocked and the right side shown locked. 
         FIG. 2  is a partial sectional view illustrating an upper connecting rod and nut connection to the cam ring according to an embodiment of the present invention, with the cam ring bearing surface to nut bottom bearing surface shown. 
         FIG. 3  is a partial sectional view illustrating the primary piston, secondary piston and cap ring in the connector lock position according to an embodiment of the present invention, with secondary piston and cap ring hydraulic conduits shown. 
         FIG. 4  is an enlarged view of the interface between a raised profile on the lower outer diameter of the wellhead housing and the tapered shoulder of the lower inside diameter of the connector housing. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an exemplary embodiment is disclosed that illustrates a wellhead  20 , which is a tubular member located vertically on the sea floor. A plurality of circumferential grooves  22  are formed on the exterior of wellhead  20  to provide a locking profile with a plurality of circumferential grooves  26  formed on the inside surfaces of dogs  24 . Dogs  24  comprise part of a wellhead connector  28 , which may be connected to a subsea production tree  29  by threads  31 . Alternately, wellhead connector  28  could be secured to the lower end of a string of riser (not shown) which extends from a vessel at the surface. 
     The wellhead connector  28  includes a tubular housing  30 . Housing  30  has an inner diameter that is slightly greater than the outer diameter of the wellhead  20 . The housing  30  will slide over the wellhead  20  as the wellhead connector  28  is lowered into place. Dogs  24  are carried in aperture  32  spaced apart from each other around an inner circumference of wellhead connector  28 . The dogs  24  will move between the retracted (i.e., unlocked) position shown on the left side in  FIG. 1  to a locked position shown on the right side in  FIG. 1 . 
     Each dog  24  has an outer side  34  that is inclined. In this embodiment, the outer side  34  is a toriodal surface for optimized mechanical efficiency and load distribution. It inclines radially outward in a downward direction. A beveled edge  36  is located at the upper end of the outer side  34  of each dog  24 . The inclination of each outer side  34  may be about three degrees relative to vertical. 
     A cam ring  38  is reciprocally carried by the housing  30  within an annular cam ring cavity  37 . Aperture  32  is located between the cam ring cavity  37  and the inner wall of housing  30 . The cam ring  38  is a solid annular member that moves vertically within annular cavity  37  in housing  30 . Cam ring  38  has an inner side  39  that is inclined and which mates with the outer side  34  of dog  24 . In this embodiment, the inner side  39  is a straight conical surface with a wider base at the bottom than that of the upper end. It inclines radially outward in a downward direction. A beveled edge  43  is located at the lower end of the inner side  39  of cam ring  38 . The inclination of inner side  39  may be about three degrees relative to vertical. When cam ring  38  is in an upper position as shown on the left side of  FIG. 1 , cam ring outer diameter  45  has nominal running clearance with the outer diameter  49  of annular cavity  37 . During connector lock on wellhead  20 , cam ring outer diameter  45  contacts the outer diameter  49  of annular cavity  37  during downward travel of cam ring  38 , connecting rods  44  and primary piston  42 . Outer diameter  45  of cam ring  38  and outer diameter  49  of annular cavity  37  have a low coefficient of friction coating applied to significantly reduce hydraulic force required for connector  28  lock and unlock on wellhead  20 . 
     A single, annular hydraulic chamber  40  is located in the wellhead connector housing  30  below cam ring cavity  37  and separated from cam ring cavity  37  by a bulkhead  41 . Bulkhead  41  comprises downward facing surfaces  81  and upward facing surfaces  54  and is a solid annular disk shaped region of housing  30 , except where penetrated by passages  46 . Hydraulic chamber  40  extends around the circumference of wellhead  20  and has an axis coaxial with the axis of wellhead  20 . Hydraulic chamber  40  has an inner cylindrical wall  40   a  and an outer cylindrical wall  40   b . Inner and outer walls  40   a  and  40   b  are concentric relative to each other. A cap ring  51  is bolted to the bottom of connector housing  30  and is the bottom closure for hydraulic chamber  40 . 
     The hydraulic chamber  40  contains an annular primary piston  42  that moves vertically within hydraulic chamber  40 . Primary piston  42  has an inner diameter with a bidirectional seal  53  that slidingly engages hydraulic chamber inner wall  40   a . Primary piston  42  has an outer diameter with a bidirectional seal  56  that slidingly engages hydraulic chamber outer wall  40   b.    
     Primary piston  42  is connected to a plurality of connecting rods  44  (only two shown). Each connecting rod  44  extends through a passage  46  extending through bulkhead  41  of the housing  30  and further connects up to the cam ring  38 . A bidirectional seal  47  in each passage  46  seals around one of the connecting rods  44  to seal the pressure in hydraulic chamber  40  from cam cavity  37 . Each connecting rod  44  is cylindrical and has an outer diameter less than the distance between the inner and outer walls  40   a ,  40   b  of hydraulic chamber  40 . Referring to 
       FIG. 2  and  FIG. 3 , the ends of connecting rods  44  are threaded for securing into nuts  58  in cam ring  38  and threaded holes  79  in primary piston  42 . The bottom surface of nut  58  and cam ring bearing surface  60  are spherical to allow connecting rods  44  to angularly deflect under load conditions. Bottom surface of nut  58  and cam ring bearing surface  60  have low coefficient of friction coatings applied to facilitate relative angular deflection of connecting rods  44  and nuts  58  to cam ring  38  under load conditions. Connecting rods  44  cause cam ring  38  to move up and down relative to dogs  24  in unison with primary piston  42 , as can be seen by comparing the left and right sides of FIG. I. In an exemplary embodiment, primary piston  42  is connected to cam ring  38  via twelve connecting rods  44 , however, other numbers of connecting rods can be used. 
     Referring to  FIG. 3 , primary piston  42  has a lower side with an annular band  83  extending downward and concentric with a longitudinal axis of housing  30  ( FIG. 1 ). Annular band  83  has a bottom surface  85  that is flat and located in a plane perpendiclar to the longitudinal axis of housing  30 . Annular band  83  has inner and outer side walls  87  that are inclined and converge toward each other in a downward direction. The inner side wall  87  joins band bottom surface  85  with an annular inner border surface  89 , which extends inward to an inner side  91  of primary piston  42 . The outer side wall  87  joins band bottom surface  85  with an an outer border surface  93 , which extends outward to an outer side  95  of primary piston  42 . Border surfaces  89 ,  93  are flat and located in a single plane parallel to and elevated above band bottom surface  85 . When viewed in a transverse cross-section, as in  FIG. 3 , annular band  83 , side walls  87 , and border surfaces  89 ,  93  are symmetrical about a center line  97  equidistant between inner side  91  and outer side  95 . 
     A secondary piston  52  is also provided to assure unlocking in the event primary piston  42  fails. Secondary piston  52  is an annular member carried in annular hydraulic chamber  40  below primary piston  42 . Secondary piston  52  has an inner diameter with a bidirectional seal  55  that slidingly engages hydraulic chamber inner wall  40   a . Secondary piston  52  has an outer diameter with a bidirectional seal  57  that slidingly engages hydraulic chamber outer wall  40   b . Referring to  FIG. 3 , secondary piston  52  includes an bidirectional upper seal  62  that slidingly engages hydraulic chamber outer wall  40   b . Upper seal  62  allows secondary piston  52  to travel past hydraulic port  64  without leakage of hydraulic pressure from hydraulic chamber  40  on the lower side of secondary piston  52  into hydraulic chamber  40  between the top side of secondary piston  52  and the bottom side of primary piston  42 . Secondary piston  52  is not physically connected to primary piston  42  nor to connecting rods  44 . When at its lower position, secondary piston  52  rests on top of the upper horizontal surface of cap ring  51 . 
     Referring to  FIG. 3 , secondary piston  52  has an upper side with an annular recess  101  having a mating configuration to annular band  83  to receive and mate with annular band  83  while primary piston  42  and secondary piston  52  are in abutment with each other. Secondary piston  2  has a lower side with an annular band  103 . Annular band  103  has a flat bottom surface  105  that is perpendicular to a longitudinal axis of housing  30  ( FIG. 1 ). Side walls  107  join annular band  103  with the inner and outer border surfaces  109  on the lower side of secondary piston  52 . Side walls  107  converge toward each other in a downward direction. Inner and outer border surfaces  109  are flat and located in a single plane parallel to and elevated above bottom surface  105 . Annular band  103 , side walls  107  and border surfaces  109  are symmetrical about center line  97  when viewed in a transverse sectional plane. 
     Cap ring  51  is bolted to the bottom face of connector housing  30  and is the bottom closure of hydraulic chamber  40 . Referring to  FIG. 3 , cap ring  51  has an inner diameter with a bidirectional seal  66  that statically engages hydraulic chamber inner wall  40   a . Cap ring  51  has an outer diameter with a bidirectional seal  68  that statically engages hydraulic chamber outer wall  40   b.    
     Cap ring  51  has an upper side having an annular recess  111  with a mating configuration for secondary piston annular band  103  for receiving annular band  103  while secondary piston  52  is in abutment with cap ring  51 . 
     Two upper ports  48  extend through housing  30  to hydraulic chamber  40  above primary piston  42 . Upper ports  48  provide hydraulic fluid pressure to the upper side of primary piston  42  to force it downward. Two lower ports  64  extend through housing  30  to hydraulic chamber  40  below primary piston  42  and above secondary piston  52  when secondary piston  52  is in its lower position, shown on both sides of  FIG. 1 . Lower ports  64  provide hydraulic fluid pressure to the lower side of primary piston  42  to force primary piston  42  upward to unlock connector  28 . 
     Two secondary lower ports  50  extend through housing  30  to hydraulic chamber  40  below secondary piston  52 . Secondary lower ports  50  provide hydraulic fluid pressure to the lower side of secondary piston  52  to force secondary piston  52  and primary piston  42  upward to unlock connector  28  in the event of unsuccessful connector  28  unlock using lower ports  64  to unlock connector  28 . 
     Referring to  FIG. 3 , four upper hydraulic conduits or grooves  70  machined radially in the horizontal direction in recess  101 , on top surface of secondary piston  52  allow hydraulic pressure from lower hydraulic ports  64  to communicate to inner half of piston chamber  40  below primary piston  42  and above secondary piston  52  when primary piston  42  is in a lower position contacting secondary piston  52 . 
     Four lower hydraulic conduits or grooves  72  machined radially in the horizontal direction in recess  111  on top surface of cap ring  51  allow hydraulic pressure from secondary lower hydraulic ports  50  to communicate to inner half of piston chamber  40  below secondary piston  52  and above cap ring  51  when secondary piston  52  is in its lower position contacting cap ring  51 . 
     In operation, the wellhead connector  28  will be lowered over the wellhead  20  until reaching the position shown in  FIG. 1 . Initially, dogs  24  will be in the retracted position, shown on the left side of  FIG. 1 . The cam ring  38  and primary piston  42  will be in an upper position because of the position of dogs  24 . Secondary piston  52  would be staged in the lower position shown. Hydraulic fluid is then supplied to an upper port  48 , which forces primary piston  42  to move downward bringing with it cam ring  38 . This will initially start the dogs  24  moving inward by the engagement with the beveled edge  43  of cam ring  38 . The cam ring  38  and connecting rods  44  will continue downward with the primary piston  42  until the inner side  39  of cam ring  38  engages the outer toroidal surface  34  of dogs  24  until dogs  24  have fully engaged wellhead housing  20  and a selected hydraulic pressure is reached. At that point, cam ring  38  will be spaced slightly above the top surface  54  of bulkhead  41  of tubular housing  30  as shown in the right side of  FIG. 1 . When dogs  24  are in the fully locked position, a control mechanism (not shown) will release the hydraulic fluid flow through the upper port  48 . Primary piston  42  will be closely spaced to from the top of secondary piston  52 . 
     A raised profile  74  is formed on the lower outer diameter of wellhead  20  proximate the lower inner profile of housing  30 . Referring to  FIG. 4 , raised profile  74  is engaged by a tapered shoulder  76  of the lower inside diameter of housing  30 . Raised profile  74  is spaced below wellhead profile  22  at as great a distance as possible without increasing the overall length of the wellhead connector. Raised profile  74  is also provided with a tapered shoulder  78 . 
     In operation, before preload and after landing the wellhead connector  28  on the wellhead  20 , a slight clearance exists between tapered shoulder  76  and tapered shoulder  78 . At preload, housing  30  deflects downward, engaging shoulders  78  and  76  creating a secondary load path for the applied bending moment. The secondary load path increases the bending capacity of the connector and wellhead. 
     When it is desired to release the wellhead connector, hydraulic fluid pressure is supplied to a lower port  64 . This causes the primary piston  42  to push upward. As the primary piston  42  moves upward, cam ring  38  moves upward out of engagement with dogs  24 . Because of the angle of the downward facing shoulders of grooves  26 , an upward pull on housing  30  after cam ring  38  has released dogs  24  causes dogs  24  to slide out of engagement with grooves  22 . If primary piston  42  leaks, the hydraulic fluid pressure can be directed through a secondary lower port  50  causing secondary piston  52  to move upward engaging primary piston  42  to unlock the wellhead connector. 
     The invention has significant advantages. The reduced cross-section cam ring and single annular piston results in a smaller, lighter, more efficient, and less expensive wellhead connector than the prior art types. The use of a separate primary and secondary pistons enables the connector to be released even if the primary piston leaks. 
     While this invention has been shown in only one 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 spirit and scope of the invention.