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BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to wellhead assemblies, more specifically to assemblies and methods for connecting a horizontal tree to a wellhead housing. 
   2. Background of the Invention 
   Well fluid from a subsea well typically flows up a string of production tubing to a subsea wellhead. Sometimes well fluid is transmitted through a production riser to a Christmas tree on a vessel at the surface of the sea. It is often desirous however to transport the well fluid through a subsea Christmas tree to a collection facility or processing site. In either situation, the production riser or the subsea tree mounts to the wellhead housing. Typically, the production riser or subsea tree has a connector that receives an upper portion of the wellhead housing and then engages a grooved profile on the wellhead housing with a plurality of dogs. 
   Previously, the connector included a fluted receiving portion that receives and lands on the upper portion of the wellhead housing. The plurality of dogs align with the grooved profile on the outer surface of the wellhead housing when the connector lands on the wellhead housing. A piston housed with the connector slides axially up and down. The piston typically engages the dogs to cam the dogs, which engage the grooved profile of the wellhead housing. When the dogs are cammed radially inward, the connector and the dogs are in their locked positions. The dogs can be biased radially outward, or the dogs can be actuated radially outward upon upward movement of the piston in order to cam the dogs radially outward, or unlocked position. 
   The piston is actuated with hydraulic fluid that is injected into annular chambers formed around upper and lower portions of the piston with seals. When the seals function properly, hydraulic fluid is injected into an upper chamber to actuated the piston axially downward and thereby move the dogs into their locked position to engage the wellhead housing. Hydraulic fluid is injected into the lower chamber to actuate the piston axially upward to unlock the dogs from the wellhead housing. 
   Previously, when the seals failed so that hydraulic fluid could not build enough pressure to unlock the connector from the wellhead housing, a secondary piston assembly that was typically located external to the connector was used to lift the piston and unlock the connector. External piston assemblies are typically cumbersome and heavy, which causes the operator to take up valuable space around the subsea tree and wellhead assembly. Additionally, the parts of the connector that were engaged by the external piston had to be capable of sustaining the additional force of being pulled or pushed externally by the external piston. 
   SUMMARY OF THE INVENTION 
   In the subsea wellhead of this invention, the subsea wellhead assembly includes a connector that receives an axially upper portion of the tubular wellhead member which can be unlocked without an external piston. The connector has a plurality of locking dogs that actuate radially inward and outward between locked and unlocked positions, and a main piston housed within the connector for actuating the plurality of locking dogs with the wellhead member. The piston of the connector includes a chamber located in a lower portion of the main piston when the main piston is in its axially downward position. Hydraulic fluid can be injected into the chamber to exert an axially upward force on the main piston. The upward force on the piston from the hydraulic fluid causes the piston to move axially upward for actuating the locking dogs to the unlocked position. 
   The chamber can be formed with a second piston that is sealingly housed within the lower portion of the main piston when in its axially lower position. The chamber can also be formed with a bellow that is located within the axially lower portion of the piston. In either version, the hydraulic fluid increases pressure on the piston and increases the size of the chamber by forcing the piston axially upward. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an overall sectional view of an upper portion of a wellhead assembly and a connector for a horizontal tree assembly, each being constructed in accordance with the prior art. 
       FIG. 2  is an overall sectional view of a connector assembly for a horizontal tree, being constructed in accordance with this invention. 
       FIG. 3  is an alternative embodiment of the connector assembly shown in  FIG. 2 . 
       FIG. 4  is another alternative embodiment of the connector assembly shown in  FIG. 2 . 
       FIG. 5  is another alternative embodiment of the wellhead connector assembly shown in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a connector assembly  11  of a horizontal tree is connected to the upper portion of a wellhead housing  13 . Wellhead housing  13  has a grooved profile  15  located toward an upper portion of wellhead housing  13  for engaging connector  11 . Connector  11  has a tubular receiving portion  17  located toward its lower end. Receiving portion  17  receives grooved profile  15  and the upper portion of wellhead housing  13 . A plurality of dogs  19  are located axially above receiving portion  17  so that dogs align with grooved profile  15  on wellhead housing  13 . Each dog  19  has a grooved profile  21  that matingly engages grooved profile  15  on wellhead housing  13 . 
   As receiving portion  17  and dogs  19  receive the upper portion of wellhead housing  13 , dogs  19  are in an unlocked position so that dogs  19  are radially outward of group profile  15 . Dogs  19  cannot move axially relative to connector  11 , but can move radially inward and outward for engaging and disengaging group profile  15 . Connector  11  includes a piston cam  23  located radially outward from receiving portion  17  and dogs  19 . An inclined face  25  is formed on an inner surface of piston  23  for engaging an outer surface of dogs  19 , to thereby actuate dogs  19  between radially inward and outward positions. Piston  23  reciprocates axially upward and downward, thereby sliding inclined face  25  relative to the outer surface of dogs  19 . As piston  23  and inclined face  25  slide axially downward relative to outer surface of dogs  19 , inclined face  25  exerts a radially inward force on dogs  19  thereby pushing dogs  19  radially inward. When dogs  19  are pushed radially inward by piston  23 , group profile  21  of dogs  19  engages grooved profile  15  of wellhead housing  13 . 
   Hydraulic fluid actuates piston  23  axially upward and downward for engaging and disengaging connector  11  with wellhead housing  13 . Piston  23  is located radially between dogs  19  and an outer casing  27  of connector  11 . Outer casing  27  connects to receiving portion  17  with fasteners  29 , thereby defining a piston chamber for piston  23  to slide axially upward and downward while locking and unlocking connector  11 . An upper seal  31  surrounds the outer circumference of an upper portion of piston  23  and engages the inner surface of outer casing  27 . A lower seal  33  located axially below upper seal  31 , surrounds the outer circumference of a lower portion of piston  23 . Upper seal  31  and lower seal  33  thereby defining an upper piston chamber  35 . Hydraulic fluid is transmitted into upper piston chamber  35  to actuate piston  23  in an axially downward direction to cam dogs  19  into engagement with grooved profile  15  on wellhead housing  13 . 
   Another lower seal  37 , located on a radially interior surface of piston  23  engages an outer surface of receiving portion  17 . Lower seals  33  and  37  defining a lower piston chamber  39 , which receives hydraulic fluid to actuate piston  23  in an axially upward direction to unlock connector  11  from wellhead assembly  13 . 
   It is essential that seals  31 ,  33 ,  37  maintain engagement with the surfaces of outer casing  27  and receiving portion  17  for upper and lower piston chambers  35  and  39  to maintain proper pressure in order to actuate piston  23  between upward and downward positions. Should one of seals  31 ,  33 ,  37  fail, it is difficult for the pressure in pressure chambers  35  and  39  to reach a preselected amount necessary to actuate piston  23  between upper and lower positions. Therefore, override rods  41  extend from a radially upward position to engage in upper portion of piston  23 . Override rods  41  extend from piston  23  to an override plate  43  located above piston  23  and outer casing  27 . In the event of a failure of one of seals  31 ,  33 ,  37 , override plate is moved in an axially upward position to thereby pull piston  23  in an upward direction with override rods  41 . Typically, override plate  43  is moved in an axially upward position with a secondary piston assembly (not shown) that is external to connector  11 . Such external secondary piston assemblies in the prior art have typically been cumbersome and heavy. 
   Referring to  FIG. 2 , an improved connector  11 ′ is shown attached to the upper portion of wellhead housing  13 . Connector  11 ′ includes a receiving portion  17 ′, outer casing  27 ′, dogs  19 ′, and a piston  23 ′. While locking and unlocking under normal operating conditions, connector  11 ′ functions essentially the same as the prior art connecter assembly  11  shown in  FIG. 1 . Improved connector assembly  11 ′ includes an override rod  45 . Override rod  45  extends between override plate  43 ′ and piston  23 ′ in substantially the same manner as override rod  41  in prior art  FIG. 1 . Override rod  45  preferably includes a rod passage  47  extending axially therethrough. A piping connection  49  is located toward an axially upper portion of rod passage  47 . Hydraulic fluid is transmitted into rod passage  47  through piping connection  49 . A piston passage  51  is ported through piston  23 ′. Piston passage  51  has an end that is in fluid communication with rod passage  47 . A secondary piston  53  is located toward the lower end of piston  23 ′. 
   Piston passage  51  has another end that is in fluid communication with secondary piston  53  so that rod passage  47  is in fluid communication with secondary piston  53 . Seals  55  located around the inner and outer circumferences of secondary piston  53  engage interior surfaces of piston  23 ′. Seals  55  define an override chamber  57  above secondary piston  53 . Secondary piston  53  slides axially relative to piston  23 ′ as hydraulic fluid is transmitted through rod passage  47  and piston passage  51  into override chamber  57 . A lip  59  is formed on a radial exterior surface of secondary piston  53 . In this embodiment lip  59  engages a shoulder  63  protruding radially inward from a mechanical guide  61  on piston  23 ′. Shoulder  63  prevents secondary piston  53  from sliding beyond a preselected piston stroke by engaging lip  59  when secondary piston  53  slides axially downward relative to piston  23 ′. 
   In operation connector  11 ′ lands on wellhead housing  13  with receiving portion  17 ′ sliding over and receiving an upper portion of wellhead housing  13  including group profile  15 . Upon landing on wellhead housing  13 , dogs  19  are at an axial elevation substantially equal to grooved profile  15 . A hydraulic fluid is transmitted into upper piston chamber  35 ′ in a manner known in the art to actuate piston  23 ′ from an axial upward position to an axial downward position. While moving from the axial upward position to the downward position inclined face  25 ′ engages the outer surface of dogs  19 ′, thereby camming dogs  19 ′ into engagement with grooved profile  15  on wellhead housing  13 . Connector  11 ′ of the horizontal tree is now connected to wellhead housing  13  of the subsea well. 
   Under normal operating conditions the hydraulic fluid is transmitted into lower piston chamber  39 ′ in order to build enough pressure to move piston  23 ′ axially upward relative to dogs  19 ′. As piston  23 ′ moves axially upward, inclined face  25  slides out of engagement with the outer surface of dogs  19 ′ thereby allowing dogs  19 ′ to expand radially outward out of engagement with grooved profile  15 , thereby unlocking connector  11 ′ from wellhead housing  13 . In the event that one of lower seals  33  or  37  becomes damaged or inoperable, piston  23 ′ must be actuated from its axial downward position to its axial upward position through a secondary means. In the prior art, override plate  43  would be actuated upward in order to pull piston  23  from its lower to its upper position. In this embodiment as shown in  FIG. 2 , an operator transmits a hydraulic fluid through piping connector  49  and rod passage  47  into piston passage  51  to override chamber  57 . Hydraulic pressure increases in override chamber  57  to create a force that moves piston  23 ′ relative to secondary piston  53 . Because secondary piston  53  and piston  23 ′ are initially in their lowered position, the lower surface of secondary piston  53  is preferably engaging an upper surface of receiving portion  17 ′, thereby forcing piston  23 ′ to move axially upward as the pressure inside override chamber  57  increases. As piston  23 ′ moves axially upward dogs  19 ′ are moved out of engagement from grooved profile  15  in the manner described above. 
   Referring to  FIG. 3 , in an alternative embodiment, seals  55  shown in  FIG. 2  located on secondary piston  53  are removed. In the embodiment shown in  FIG. 3  an inner seal  63  is located along a radially inward surface that engages secondary piston  53 ′. An outer seal  65  is located along a radially outward surface that also engages secondary piston  53 ′. Inner seal and outer seal  63  and  65  define an override chamber  67 . Override chamber  67  performs substantially the same function as override chamber  57  shown in  FIG. 2 . 
   In operation, connector assembly  11 ′ operates substantially the same in  FIG. 3  as the embodiment shown in  FIG. 2  with the exception that inner seal  63  and  65  are positioned on piston  23 ′ rather than on secondary piston  53 ′. 
   Referring to  FIG. 4 , another alternative embodiment is shown for a secondary piston  53 ″. In this embodiment secondary piston  53 ″ engages seal  69  located on radially inward and radially outward surfaces of piston  23 ′. Seal  69  and piston  53 ″ define an override chamber  71  that receives hydraulic fluid to actuate secondary piston  53 ″ in substantially the same manner as the embodiments discussed above. Piston  53 ″ also preferably includes a second stage piston  73  located radially between the inner and outer surfaces of secondary piston  53 ″. Second stage piston  73  moves axially upward and downward relative to the remaining portion of secondary piston  53 ″. 
   In this embodiment, a pair of lips  75  are preferably formed toward an upper portion of second stage piston  73  for limiting the axial movement of second stage piston  73  relative to secondary piston  53 ″. In this embodiment, a pair of shoulders  77  formed on the interior surfaces of second piston  53 ″ engage second stage piston  73  to provide a mechanical barrier to second stage piston  73  sliding relative to secondary piston  53 ″ beyond an axial elevation when lips  75  engage shoulders  77 . In this embodiment, seals  79  are located on the interior surfaces of secondary piston  53 ″ that engage second stage piston  73 . In this embodiment seals  79  are axially located below shoulders  77 , thereby defining an annulus  81  between lips  75  and shoulders  77 . In this embodiment, a second stage passage  83  extends from the axial upper surface of second stage piston  73  to annulus  81 . Second stage passage  83  preferably transmits hydraulic fluid from override chamber  71  to annulus  81  during operation. 
   In operation, connector assembly  11 ′ and piston  23  engage wellhead housing  13  as described above. In the event that seals  33 ′ and  37 ′ fail, operator uses secondary piston  53 ″ to slide piston  23 ′ axially upward and out of engagement with dogs  19 ′ to unlock connector  11 ′ from wellhead housing  13 . In some situations, piston  23 ′ needs to be forced axially upward along a greater stroke length than the stroke of secondary piston  53 ,  53 ′ in  FIGS. 2 and 3  allows. Accordingly, when a longer stroke is desired, operator utilizes secondary piston  53 ″ shown in  FIG. 4 . The hydraulic fluid is transmitted through rod passage  47  of override rod  45  through piping connector  49  and piston passage  51  to override chamber  71 . As fluid pressure inside override chamber  71  increases piston  23 ′ is forced axially upward relative to secondary piston  53 ″ as described above. Second stage passage  83  transmits hydraulic fluid to annulus  81  surrounding second stage piston  73  to balance the hydraulic pressure in annulus  81  with override chamber  71 , so that second stage piston  73  does not initially move relative to the remaining portion of secondary piston  53 ″. A shoulder  70  is located along the inner surface of piston  23 ′ that engages the outer surface of secondary piston  53 ″. After piston  23 ′ slides axially upward a preselected distance so that the outer portion of secondary piston engages shoulder  70  of piston  23 ′, the hydraulic pressure inside override chamber  71  continues to force piston  23 ′ to move relative to second stage piston  73 . Second stage passage  83  allows any hydraulic fluid in annulus  81  to flow back into override chamber  71  as second stage piston  73  slides axially downward relative to the remaining portion of secondary piston  53 ″. As piston  23 ′ and secondary piston  53 ′ slide axially upward relative to second stage piston  73 , piston  23 ′ continues to slide out of engagement with the outer surface of dogs  19 ′ so that dogs  19 ′ can disengage from wellhead housing  13 . 
   Referring to  FIG. 5 , in another alternative embodiment secondary pistons  53 ,  53 ′,  53 ″ are replaced by a bellow  85 . In this alternative embodiment, bellow  85  can be positioned within piston  23 ′ so that it moves with piston  23 ′ as connector assembly  11 ′ locks with wellhead housing  13 . Alternatively, bellow  85  can be positioned on receiving portion  17 ′ and is received by piston  23 ′ as connector  11 ′ locks into engagement with wellhead housing  13 . In operation, an operator needing to unlock connector  11 ′ from wellhead housing  13  after a seal failure, injects hydraulic fluid through rod passage  41  and piston passage  51  into bellow  85 . Upon receiving hydraulic fluid inside of bellow  85 , bellow  85  expands a preselected length to force piston  23 ′ axially upward. Upon expanding the preselected length, piston  23 ′ has moved so that dogs  19 ′ expand radially outward and out of engagement from wellhead housing  13 , and connector assembly  11 ′ is disengaged. 
   In all of the embodiments discussed above, the backup unlocking system (the secondary piston or the bellow) is housed within the connector. This reduces the amount of space previously required for backup unlocking assemblies like external pistons. Additionally, unlike the external pistons, the connector rods do not experience the forces associated with unlocking the connector through the backup system. Rather, the main piston, the upper surface of the receiving portion, and either the secondary piston or bellow experience the additional forces. This reduces the need for stronger or larger connection rods. 
   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. For example, in all the embodiments shown, secondary piston  53  has been shown as a member that moves with piston  23 ′ as main piston  23  moves into its locked position. Alternatively, secondary piston  53  could be rigidly attached to the outer surface of receiving portion  17  so that main piston  23  receives secondary piston  53  upon moving into its axially lower position.

Summary:
In a subsea wellhead assembly, a connector attaches a production riser or subsea tree to a wellhead housing. The connector includes a piston that cams a plurality of dogs into and out of engagement with the wellhead housing while sliding axially upward and downward relative to the wellhead housing. The piston is typically actuated by injecting hydraulic fluid into either an upper chamber or lower chamber to move the piston axially up or down. While in the axially downward, or locked position, the piston houses an override chamber. Hydraulic fluid is injected into the override chamber to move the piston axially upward and unlock the connector when the seals of the lower chamber fail.