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This application claims the benefit of U.S. Provisional Application No. 60/207,707, filed May 26, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally subsea petroleum production. More specifically, the present invention relates to production riser tiebacks which connect a production riser to a high pressure wellhead housing. 
     2. Description of the Related Art 
     Tieback connectors are used to connect a production or drilling riser to a high pressure wellhead housing. The connector must be able to withstand very large forces to keep the riser sealed to the wellhead housing. This has required rather bulky connectors to withstand these forces. 
     One type of tieback connector connects to a grooved profile on the exterior of the high pressure wellhead housing. The tieback connector has a cylindrical housing that slides over the upper end of the wellhead housing. A cam member, piston, and a plurality of segments are carried in the housing. Applying hydraulic pressure to the piston strokes the cam member, pushing the dogs into engagement with the grooved profile. The housing of the connector has a fairly large diameter in order to accommodate the piston, cam member and dogs. Some production platforms are designed with relatively small holes or slots through which the connector must pass. This necessitates a connector with a smaller outer diameter. 
     BRIEF SUMMARY OF THE INVENTION 
     A tieback connector comprises a passive lower locking system and an active upper locking system to exert a positive locking force on the connection between a production riser and a high pressure wellhead. The tieback connector is comprised of an outer housing which carries lower locking dogs, upper locking dogs and a piston. The piston is located above the lower end of the production riser and controls the movement of the outer housing. As the piston is stroked the outer housing cams the lower dogs into grooved profile in the wellhead housing. As the piston is stroked further the upper dogs exert a force onto the production riser that locks the riser to the wellhead housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of the tieback connector of this invention showing a locked position on the right and an unlocked position on the left. 
     FIG. 2 is an enlarged view of a portion of the tieback connector in FIG.  1 . 
     FIG. 3 is an enlarged view of a portion of the tieback connector in FIG.  1 . 
     FIG. 4 is an enlarged view of a portion of the tieback connector in FIG.  1 . 
     FIG. 5 is an enlarged view of a portion of the tieback connector in FIG.  1 . 
     FIG. 6 is an alternate embodiment of the tieback connector of this invention, showing a locked position on the right and an unlocked position on the left. 
     FIG. 7 is another alternate embodiment of the tieback connector of this invention, showing a locked position on the right and an unlocked position on the left. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 in the drawings, the preferred embodiment of a small diameter external tieback connector  11  according to the present invention is illustrated. Tieback connector  11  is used to join a lower terminal end of a drilling or production riser  13  to a high pressure wellhead housing  15  in off shore drilling applications. Typically, the high pressure wellhead housing  15  is installed during drilling operations, and production riser  13  is attached to the wellhead housing  15  to facilitate completion and production of the well. Production riser  13  and tieback connector  11  are lowered through a slot at a surface platform (not shown). Production riser  13  includes an interior surface and an exterior surface with a riser shoulder  19  formed at a lower end of the production riser  13 . Wellhead housing  15  includes an interior surface and an exterior surface with a wellhead shoulder  21  formed at an upper end of the wellhead housing  15 . Upon connection, the wellhead shoulder  21  mates with the riser shoulder  19 , and the interior surfaces of the wellhead housing  15  and the production riser  13  form a common bore, in which production tubing is then located to deliver oil from the well to the ocean surface. 
     Tieback connector  11  includes a housing  25  having a generally cylindrical wall  27  with an interior surface and an exterior surface. An upper end cap  29  is rigidly attached to the housing  25  at an upper end  31  of the housing  25 , the upper end cap  29  having a passage through which production riser  13  passes. The housing  25  and upper end cap  29  slidingly engage the exterior surface of the production riser  13 . The tieback connector  11  is prevented from sliding off the lower end of the production riser  13  by several parts internal to the housing  25  that are discussed in more detail below. 
     A lower end  33  of the housing  25  is open to receive wellhead housing  15  during connection of the production riser  13  and tieback connector  11  to the wellhead housing  15 . 
     An initial connection is made by concentrically locating housing  25  relative to the wellhead housing  15  and lowering the production riser  13  until riser shoulder  19  engages wellhead shoulder  21 . A seal  41  is disposed in a groove on the interior surface of the housing  25  near its lower end  33  to prevent seawater from entering the tieback connector  11  after the initial connection is made. 
     After initial connection, the housing  25  of tieback connector  11  is still capable of axial movement relative to the production riser  13  and the wellhead housing  15 . The tieback connector  11  has an unlocked position in which the production riser  13  is not securely fastened to the wellhead housing  15 . While making the initial connection and immediately after the initial connection, the tieback connector  11  is in the unlocked position. The tieback connector  11  also has a locked position which results in a secure connection between the production riser  13  and the wellhead housing  15 . The tieback connector  11  is placed in the locked position before performing any completion or production operations. 
     The tieback connector  11  features an upper locking system  45  and a lower locking system  47  for securing the tieback connector  11  in the locked position. The lower locking system  47  is a passive locking system that provides the connection to the housing  25 . The upper locking system  45  is an active locking system that provides a locking and preloading force. The lower locking system  47  includes a locking element that may be a split ring or collet, but is preferably a plurality of lower dogs  51  and a lower dog retainer ring  53  disposed within housing  25 . Each lower dog  51  has a cylindrical curvature with a plurality of teeth  55  on an inner surface. The lower dogs  51  are arranged circumferentially around the interior of the housing  25 , with the plurality of teeth  55  adapted to mate with a plurality of grooves  59  formed on the exterior surface of the wellhead housing  15 . Typically, eight to twelve lower dogs  51  will be arranged within the housing  25 . The lower dogs  51  are held within housing  25  by the lower dog retainer ring  53  which is connected to the lower end of the production riser  13 . 
     Referring to FIGS. 2 and 3 in the drawings, a more detailed view of the lower locking system  47  is illustrated. The lower dog  51  and housing  25  are shown in the unlocked position in FIG.  2 . In FIG. 3, the lower dog  51  and housing  25  are shown in the locked position. Each lower dog  51  includes a stop shoulder  65  for mating with a landing shoulder  67  on the interior surface of the housing  25  when the tieback connector  11  is in the locked position. The stop shoulder  65  and the landing shoulder  67  are similarly inclined. 
     A plurality of outer grooves  71  are disposed on an outer surface of each lower dog  51 . A plurality of bands  73  are integrally located on the interior surface of housing  25 . Outer grooves  71  receive bands  73  when tieback connector  11  is in the unlocked position. Each outer groove  71  includes a conical cam surface  77  for engagement with a similarly inclined surface  79  on each band  73 . In the locked position, bands  73  mate with the outer surface of each lower dog  51  such that the plurality of teeth  55  on the lower dog  51  engage the plurality of grooves  59  on the wellhead housing  15 . Upward movement of housing  25  relative to riser  13  causes dogs  51  to move to the locked position. 
     Referring to FIGS. 1,  4 , and  5 , production riser  13  includes an upward facing shoulder  83  located on the exterior surface near its lower end. Upper locking system  45  includes several parts that are generally located between the upward facing shoulder  83  and upper end cap  29 . A piston  87  having an upper portion  89 , a lower portion  91 , and a pressure flange  93  is slidingly disposed in an annulus between the production riser  13  and the housing  25 . Pressure flange  93  includes an upper side  95  and a lower side  97 . Similar to the components comprising the lower locking system  47 , the piston  87  is adapted to move between a locked and an unlocked position. Seals  101  located between the production riser  13  and housing  25  and seals  103 ,  105  disposed around the piston  87  form a lower chamber  109  beneath the lower side  97  of the piston  87 . 
     Lower portion  91  of piston  87  includes an inclined locking surface  115 . An upper locking element may be a split ring or collet, but is preferably a plurality of upper dogs  119  circumferentially disposed within the lower chamber  109 . Each upper dog  119  has a lower landing surface  121 , a lower retraction surface  123 , and an interior locking surface  125 . Each upper dog  119  also has a cylindrical curvature with a plurality of teeth  127  formed on an outer surface. The upper dogs  119  are arranged circumferentially around the interior of the housing  25 , the plurality of teeth  127  mating with a plurality of grooves  129  formed on the interior surface of the housing  25  when the tieback connector  11  is in the locked position. Typically, eight to twelve upper dogs  119  will be arranged within the housing  25 . 
     A load transfer ring  135  having an upper landing surface  137  rests on a step  139  formed in the outer surface of the production riser  13 . Load transfer ring  135  is disposed below upper dog  119 , the upper landing surface  137  slidingly engaging the lower landing surface  121  of the upper dog  119 . A dog retraction ring  145  has a disengagement portion  147  with a retraction surface  149 . Disengagement portion  147  is located in an annulus between the load transfer ring  135  and the housing  25 . A retainer bolt  153  passes through a passage in the load transfer ring  135  and is rigidly connected between the dog retraction ring  145  and the piston  87 . As the piston  87  moves axially between the locked and the unlocked positions, the dog retraction ring  145  also moves. The retraction surface  149  of the dog retraction ring  145  mates with the lower retraction surface  123  of the upper dog  119  as the dog retraction ring  145  moves into an unlocked position. 
     A primary release port  157  (FIG. 5) allows fluid communication with the lower chamber  109 . Hydraulic fluid injected into the lower chamber  109  is capable of applying an upward force to the piston  87  and a downward force to a shoulder  165  formed on the interior surface of the housing  25 . 
     An inner seal sleeve  171  is located above the upper side  95  of the piston  87  between the upper portion  89  of the piston  87  and the interior surface of the housing  25 . Inner seal sleeve  171  has an upper portion  173  and a lower portion  175 , the upper portion  173  abutting the upper end cap  29 . Seals  177 ,  179  are disposed in the lower portion  175  of inner seal sleeve  171 . An intermediate chamber  183  is formed above the upper side  95  of the piston  87  between seals  177 ,  179  and seals  103 ,  105 . 
     A primary locking port  187  is disposed in the wall  27  of housing  25  for fluid communication with the intermediate chamber  183 . Hydraulic fluid supplied to the intermediate chamber  183  is capable of applying a downward force to upper side  95  of piston  87 . 
     A piston cap  191  is located in an annulus between the upper portion  173  of the inner seal sleeve  171  and the production riser  13 . The piston cap  191  is rigidly connected to the upper portion  89  of the piston  87 . Seals disposed around the piston cap  191  act in conjunction with seals  177 ,  179  to form an upper chamber  193 . A secondary release port  195  is disposed in the wall  27  of housing  25  and passes through inner seal sleeve  171  for fluid communication with the upper chamber  193 . Hydraulic fluid injected into the upper chamber  193  is capable of supplying an upward force on the piston cap  191  which is transmitted directly to the piston  87 . 
     All of the pressure ports  157 ,  187 , and  195  are connected to a series of valves and hot stab receptacles  196 . An external hydraulic pressure source  198  (schematically shown in FIG. 1) operates the connector  11  through the receptacles  196  by manipulating the valves located on top of the upper end cap  29 . 
     A retainer ring  197  is disposed circumferentially around the production riser  13  between the upper end cap  29  and the piston cap  191 . The purpose of the retainer ring  197  is two-fold. First, the retainer ring  197  provides a positive up stop for the piston  87  and piston cap  191  as the tieback connector  11  is being unlocked. Second, as the tieback connector  11  is being unlocked, the retainer ring  197  provides a positive down stop for the housing  25 . The retainer ring  197  engages a groove  199  in the upper end cap  29  when the housing  25  is in the unlocked position. 
     At least two mechanical release shafts  201  pass through the upper end cap  29  and are rigidly connected to the upper portion  89  of the piston  87 . Release shaft  201  allows the tieback connector  11  to be unlocked manually should the external hydraulic pressure source  198  fail. Release shaft  201  is adapted to be engaged by a remote operated vehicle (not shown), which would supply an upward force to the release shaft  201  in order to move the piston  87  upward. 
     Referring to FIGS. 1-5, the operation of tieback connector  11  is illustrated. In operation, housing  25  is concentrically aligned with the wellhead housing  15 , and the tieback connector  11  is stabbed onto the wellhead housing  15  such that riser shoulder  19  engages wellhead shoulder  21 . When initially lowered over the wellhead housing  15 , the tieback connector  11  is in the unlocked position. In the unlocked position, piston  87  is biased upward such that piston cap  191  engages retainer ring  197 . The housing  25  is biased downward by gravity when tieback connector  11  is in the unlocked position such that the groove  199  in upper end cap  29  engages retainer ring  197 . The downward bias of the housing  25  causes bands  73  of the housing  25  to align with the outer grooves  71  of the lower dogs  51 . This alignment allows the lower dogs  51  to be able to shift radially outward as the tieback connector  11  is lowered onto the wellhead housing  15 . 
     Tieback connector  11  is placed in the locked position by injecting hydraulic fluid through primary locking port  187  into intermediate chamber  183 . As fluid enters intermediate chamber  183 , a downward biasing force is exerted against upper side  95  of piston  87 . However, piston  87  is initially unable to move due to interferences between upper dogs  119 , housing  25 , load transfer ring  135 , and production riser  13  (see FIG.  4 ). The fluid also exerts an upward force on the lower portion  175  of inner seal sleeve  171 . Since inner seal sleeve  171  abuts upper end cap  29 , the upward force causes upper end cap  29  and housing  25  to move axially upward relative to both production riser  13  and wellhead housing  15 . 
     As housing  25  moves upward, a force is exerted from the biasing surfaces  79  of the housing  25  onto biased surfaces  77  of the lower dogs  51  (see FIG.  2 ). The force applied to the biased surfaces  77  causes the lower dogs to move radially inward so that the teeth  55  on the lower dogs  51  engages the grooves  59  on the wellhead housing  15 . After the lower dogs  51  have engaged grooves  59 , housing  25  continues moving upward until landing shoulder  67  engages stop shoulders  65  of the lower dogs  51 . The mating of stop shoulder  65  and landing shoulder  67  stops the upward movement of the housing  25 . At this point, the lower dogs  51  have been fully biased radially inward, and the bands  73  of the housing  25  engage the outer surface of the lower dogs  51  to hold the teeth  55  of the lower dogs  51  in engagement with the grooves  59  of the wellhead housing  15 . 
     With the lower dogs  51  engaging the wellhead housing  15 , a rigid link is created between the production riser  13 , the lower dog retainer ring  53 , the lower dogs  51 , and the wellhead housing  15 . This link results in a secure connection between the production riser  13  and the wellhead housing  15 . 
     With housing  25  biased upward, the teeth  127  of the upper dogs  119  align with the grooves  129  of the housing  25 , thereby allowing the upper dogs  119  to move radially outward. Because there is no longer an interference between the upper dogs  119  and the interior surface of the housing  25 , the force exerted by the hydraulic fluid on the upper side  95  of piston  87  causes the piston  87  to move downward. The lower portion  91  of the piston  87  exerts an outward force on the upper dogs  119 , causing the upper dogs  119  to move radially outward. The lower landing surface  121  of the upper dogs  119  slides on the upper landing surface  137  of the load transfer ring  135  as the upper dogs  119  move outward. The upper dogs  119  cease their outward movement when their teeth  127  engage the grooves  129  of the housing  25 . 
     Piston  87  and dog retraction ring  145  continue to move downward. Locking surface  115  of the piston  87  engages the interior locking surfaces  125  of upper dogs  119  as the piston moves downward. The relative inclines of locking surfaces  125  and locking surface  115  are such that upper dogs  119  are biased into an increasingly secure engagement with housing  25  as the piston  87  moves down. When the piston  87  is fully extended downward, the interference fit between locking surfaces  115  and  125  prevent the piston  87  from moving upward, even when hydraulic pressure in intermediate chamber  183  is relieved. 
     While the lower dogs  51  serve to connect production riser  13  to wellhead housing  15 , the strength of the connection is dependent upon eliminating movement of housing  25 . If the housing were to move downward, the lower dogs could become disengaged, thereby breaking the connection. Upper dogs  119  lock the housing  25  and prevent it from moving relative to production riser  13  and wellhead housing  15 . The engagement between the upper dogs  119  and housing  25  produces a preload force through load transfer ring  135  between wellhead housing  15 , riser  13 , and tieback connector  11 . 
     Tieback connector  11  can be unlocked in three different ways. The preferred method of unlocking the connector  11  involves injecting hydraulic fluid through primary release port  157  into lower chamber  109 . The hydraulic fluid exerts an upward force on the lower side  97  of piston  87  that is sufficient to overcome the interference fit between locking surfaces  115  and  125 . As the piston  87  moves upward, the lower portion  91  becomes disengaged from the upper dogs  119 . The upward motion of the piston  87  is accompanied by upward movement of dog retraction ring  145 . The retraction surface  149  of disengagement portion  147  comes in contact with the lower retraction surfaces  123  of the upper dogs  119 . The inclined nature of these surfaces  123 ,  149  causes the dog retraction ring  145  to bias the upper dogs radially inward, thereby disengaging the teeth  127  of the dogs  119  from the grooves  129  of the housing  25 . The piston  87  continues to move up until piston cap  191  is stopped by retainer ring  197 . 
     After the housing  25  is “unlocked” from the upper dogs  119 , the force exerted by the hydraulic fluid on shoulder  165  causes the housing  25  to move downward. The housing  25  continues to move down until the groove  199  in upper end cap  29  engages the retainer ring  197 . The bands  73  associated with the housing  25  realign with the outer grooves  71  of the lower dogs  51  when housing  25  reaches its final downward position. 
     An upward force is applied to production riser  13  and tieback connector  11  to remove them from the wellhead housing. The inclined nature of teeth  55 ,  59  push the lower dogs  51  radially outward as the upward force is applied. The lower dogs  51  become disengaged from grooves  59 , allowing the production riser  13  and the tieback connector  11  to be easily lifted from the wellhead housing  15 . 
     A second way to release connector  11  is to inject hydraulic fluid through secondary release port  195  into upper chamber  193 . The same steps of moving the piston  87  upward and moving the housing  25  downward are involved in this release operation, but the hydraulic fluid supplies force to different parts. Hydraulic fluid entering upper chamber  193  exerts an upward force on piston cap  191  which causes piston  87  to move upwards. After releasing the upper dogs  119 , housing  25  moves downward because of the hydraulic pressure exerted on the inner seal sleeve  171 . 
     Finally, a manual method of moving the piston  87  upward is provided. Release shaft  201  is adapted to be pulled upward by a remote operated vehicle. The vehicle would be used in the event of a hydraulic failure to disconnect the production riser  13  and the tieback connector  11  from the wellhead housing  15 . By supplying a sufficient upward force to the release shaft  201 , the piston  87  could be “pulled” upward in order to unlock the housing  25  from the upper dogs  119 . The vehicle would then be used to supply a downward force to the upper end cap  29  and housing  25  in order to unlock the lower dogs  51 . 
     Referring to FIG. 6 in the drawings, a tieback connector  211  according to an alternate embodiment of the present invention is illustrated. Tieback connector  211  is similar in structure and operation to tieback connector  11 . Tieback connector  211  includes a housing  212 . A lower locking system  214  having lower dogs  215  and a lower dog retainer ring  217  is identical to that of connector  11 . The lower dogs  215  engage a wellhead housing  221  to form a connection between a production riser  223  and the wellhead housing  221 . 
     Tieback connector  211  also includes a primary piston  225  that is analogous to piston  87  in connector  11 . Primary piston  225  is cooperatively used with a dog retraction ring  231  to seat and dislodge a plurality of upper dogs  233  from engagement with housing  212 . Similar to upper dogs  119  used with connector  11 , upper dogs  233  are used to lock housing  212 , thereby preventing the housing  212  from moving axially and preventing disengagement of the lower dogs  215  from the wellhead housing  221 . 
     The primary difference between the tieback connectors  11  and  211  is that connector  211  includes a secondary release port  213  located differently from secondary release port  195  associated with connector  11 . A secondary piston  237  is located in an annulus between housing  212  and production riser  223  just beneath dog retraction ring  231 . When tieback connector  211  is in a locked position, with the upper dogs  233  engaging the housing  212 , hydraulic fluid can be injected through secondary release port  213  to an area just beneath secondary piston  237 . The hydraulic fluid exerts an upward force on the secondary piston  237  which begins to move upward, pushing both the dog retraction ring  231  and the primary piston  225  upward. As the primary piston  225  moves upward, the dog retraction ring  231  forces the upper dogs  233  radially inward and away from housing  212 , thereby allowing the hydraulic fluid to exert a downward force on a shoulder  239  to move housing  212  in a downward direction relative to production riser  223  and wellhead housing  221 . As housing  212  moves downward, the lower dogs  215  disengage the wellhead housing  221  such that the production riser  223  and tieback connector  211  can be removed from the wellhead housing  221 . 
     Referring to FIG. 7 in the drawings, a tieback connector  311  according to another alternate embodiment of the present invention is illustrated. Tieback connector  311  is similar in structure and operation to tieback connector  11  (FIGS.  1 - 5 ). Tieback connector  311  includes a housing  325  similar to housing  25 . An upper locking system  327 , having upper dogs  329 , load transfer ring  331 , dog retraction ring  333  and a piston  335 , that is identical to upper locking system  45  of connector  11 . 
     Tieback connector  311  also includes a lower locking system  337  analogous to lower locking system  47 . Lower locking system  337  has lower dogs  339  analogous to lower dogs  51  that engage wellhead housing  15 . 
     The primary difference between the tieback connectors  11  and  311  is that connector  311  includes a c-ring  341  and a plurality of retaining pins  343 , instead of retaining ring  53 , to hold lower dogs  339  in position. Retaining pins  343  slidingly engages an upper end of dogs  339  such that dogs  339  may move vertically relative to pins  343 . C-ring  341  is secured vertically by pins  343  and is positioned inside an upper portion of dogs  339 . C-ring  341  exerts an outward force on the upper portion of dogs  339  keeping them adjacent outer housing  325  until engaged. As outer housing  325  lowers it engages lower dogs  339  in the same manner as connector  11 , except that c-ring  341  is compressed by the engagement. This configuration prevents lower dogs  339  from interfering when connector  311  is lowered into position or removed from wellhead housing  15 . 
     A primary advantage of the present invention is the use of the housing to effect engagement between the lower dogs and the wellhead housing. Typically, dogs used in other connectors use a piston to directly engage the dogs. The current invention places the piston in an area surrounding the production riser. The piston is used to lock the housing, the housing being the activator of the lower dogs. The result of the above features is that the overall diameter of the connector can be substantially reduced when compared to connectors using a piston in the area near the lower dogs. 
     Another advantage of the current invention includes the use of two separate locking systems, each locking system being activated independently. As explained above, the lower dogs, a passive locking mechanism, serve to connect the production riser to the wellhead housing and are activated by the housing of the tieback connector without having to generate high locking forces. The upper dogs, an active locking mechanism, are used to lock the housing relative to the production riser and the wellhead housing. The upper dogs are activated by the piston. 
     Still another advantage of the present invention involves the multiple methods by which the tieback connector can be unlocked from the wellhead housing. Two of the methods involve using hydraulic fluid to move the piston and housing, hydraulic fluid being injected through the primary release port in one method and being injected through the secondary release port in the other. A third, manual method allows a remote operated vehicle to supply the necessary force to unlock the tieback connector. 
     It should be apparent from the foregoing that an invention having significant advantages has been provided. While the invention is shown in only a few of its forms, it is not just limited but is susceptible to various changes and modifications without departing from the spirit thereof. Furthermore, while the invention is shown attaching a production riser to a wellhead housing, it may be used to connect a drilling riser to a wellhead housing, or almost any tubular member to any wellhead member where a secure connection and a small diameter connector are advantageous.

Summary:
A tieback connector includes a passive lower locking system and an active upper locking system to exert a positive locking force on the connection between a production riser and a high pressure wellhead. The tieback connector is composed of an outer housing which carries lower locking dogs, upper locking dogs and a piston. The piston is located above the lower end of the production riser and controls the movement of the outer housing. As the piston is stroked the outer housing cams the lower dogs into grooved profile in the wellhead housing. As the piston is stroked further the upper dogs exert a force onto the production riser that locks the riser to the wellhead housing.