Abstract:
A connection between a riser and a receptacle of a floating platform may be preloaded to reduce fatigue. The platform has a receptacle with upper and lower grooved profiles. A hanger is connected to the upper end of the riser. The hanger has a latch that engages the lower grooved profile in the receptacle to resist downward pull of the riser. A tieback connection inserts from above into the receptacle. The tieback connection has an outer member that lands on the hanger. The outer member has a latch that engages the upper grooved profile. The tieback connection also has an inner member with a set of internal threads. Threads on the inner member engage threads on the latch, rotating the inner member relative to the outer member exerts an outward force on the tieback connection latch. As the latch is pushed into the tieback grooved profile, a downward preload force is created, which passes to the hanger latch. In another version, radial preload is provided by a deflectable lip that engages the receptacle below the hanger latch.

Description:
This invention claims priority from provisional application Ser. No. 60/204,586, filed May 16, 2000 for Connection System for Catenary Riser. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to offshore drilling and production equipment, and in particular to an apparatus for connecting a riser to a platform. 
     BACKGROUND OF THE INVENTION 
     In subsea oil and gas wells, particularly in deep water, the wellheads will be located at the sea floor. Risers connect the wellheads or manifolds to a platform for drilling and production. A variety of systems are employed. In one, the platform floats and is anchored in place. Each wellhead has a riser that extends from the sea floor to the platform. In some systems, the risers will be supported at a lower deck level or keel on the platform, which may be beneath the surface of the sea. Each riser will be supported within a receptacle at the lower deck level or keel. A load shoulder in the receptacle supports the weight of the riser. A tieback connector is lowered from an upper deck level into the receptacle to provide a continuous conduit to the upper deck level. 
     In such systems, the platforms may be anchored such that the risers are curved in a catenary form. Currents and wave movements cause cyclic loading of the connection between the riser and the receptacle. This can result in fatigue damage to the connection. 
     SUMMARY OF THE INVENTION 
     In this invention, the connection apparatus is preloaded to resist fatigue damage. The receptacle at the platform has an upper shoulder and a lower shoulder. A hanger is attached to the riser, the hanger having a supporting shoulder that engages the lower shoulder to resist downward pull of the riser. In one embodiment, an upper member, which may be a tieback connector, lands in the receptacle above and in contact with the riser hanger. The upper member has a latch that engages the upper shoulder to prevent upward movement of the upper member. The upper member also has a tensioner that cooperates with the latch to exert a downward preload force on the supporting shoulder of the hanger. 
     In the first embodiment, the upper member has an outer member that carries a radially expandable latch and lands on the hanger. It also has an inner member that is carried within the outer member. The inner member has a locking surface that engages a locking surface on the latch. These locking surfaces are tapered and threaded in the preferred embodiment. Rotation of the inner member moves the inner member downward against the outer member, pushing the latch member outward. As the latch member moves into engagement with the upper shoulder, a downward force is exerted by the upper shoulder, which creates a downward acting preload that force against the hanger latch. 
     Alternately, the hanger may include a device for mitigating fatigue damage without an independent upper member. The preload member may comprise a radially deflectable lip mounted to the hanger below the supporting shoulder. The lip is forced into radial interference with the receptacle. In one embodiment, a hydraulically actuated wedge member is moved axially upward between the lip and the riser hanger to force the lip outward into engagement with the receptacle. In another embodiment, the lip is sized for radial interference as the hanger is pulled into the receptacle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a general catenary riser system extending from a platform. 
     FIG. 2 is a cross-sectional view of a first embodiment of a connection system constructed in accordance with this invention in an engaged position. 
     FIG. 3 is a cross-sectional view of the connection system of FIG. 2 wherein the hanger is in position to engage the receptacle. 
     FIG. 4 is a cross-sectional view of the connection system of FIG. 2 wherein the hanger is in engagement with the receptacle. 
     FIG. 5 is a cross-sectional view of the connection system of FIG. 2 wherein the tieback connector is being landed in the hanger and receptacle. 
     FIG. 6 is a cross-sectional view of the connection system of FIG. 2 wherein the tieback connector is in engagement with the receptacle and sealed with the hanger. 
     FIG. 7 is a cross-sectional view of the connection system of FIG. 2 wherein the hanger is being lifted above the receptacle. 
     FIG. 8 is a cross-sectional view of the connection system of FIG. 2 wherein the retainer is preventing the hanger from engaging the receptacle. 
     FIG. 9 is a cross-sectional view of a second embodiment of a connection system in accordance with this invention, showing the preload member in a disengaged position. 
     FIG. 10 is an enlarged cross-sectional view of the connection system of FIG. 9, showing the preload member in an engaged position. 
     FIG. 11 is a cross-sectional view of a third embodiment of a connection system in accordance with this invention. 
     FIG. 12 is a partial enlarged cross-sectional view of the connection system of FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to FIG. 1, a production vessel, tanker, or platform  10  for an offshore well is generally positioned on the ocean surface with one or more risers  12  extending downward to transport product to and from platform  10 . Platform  10  will be anchored by lines (not shown), not by riser  12 . Generally, risers  12  are catenary risers extending downward from the platform and curving in a catenary curve to extend horizontally at the sea floor or some intermediate point beneath the surface. However, it is not necessary to this invention that riser  12  be of a catenary type, rather it could be essentially vertical. Riser  12  is a tubular, pressure containing member connected at one end to the destination or source of the product to flow therethrough, such as a pipeline or Christmas tree (not shown) 
     Referring to FIG. 2, platform  10  has a catenary riser receptacle  14  positioned on its hull. Receptacle  14  is cylindrical having an upper conical end  16  and a lower conical end  18  which slope inwardly and toward receptacle  14 . Conical ends  16 ,  18  help guide riser  12  and other lines into receptacle  14 . The inner diameter of receptacle  14  is stepped into a plurality of diameters. In a preferred embodiment, there are four diameters, with a first and largest diameter  20  near the lower end of receptacle  14 . A second, smaller diameter  22  is above the first diameter  20  and there is a first beveled transition  24  between the first and second diameters  20 ,  22 . A third diameter  26  is smaller than and positioned above second diameter  22 , and has a second beveled transition  28 . The fourth diameter  30  is smaller than and positioned above third diameter  26 , and has a third beveled transition  32 . Fourth diameter extends for the remainder of receptacle  14 . 
     Receptacle  14  has an upper shoulder comprising tieback engagement grooves  34  near its upper end and on the fourth diameter  30 . A lower shoulder comprising riser engagement grooves  36  reside beneath tieback engagement grooves  34  on third diameter  26 . Grooves  34  are generally triangular in the preferred embodiment, each having generally upward and downward facing flanks or shoulders that converge and join each other in a valley. Receptacle  14  may have stiffening members  38  on its outer diameter to stiffen receptacle  14  and facilitate mounting to platform  10 . 
     The upper end of riser  12  is supported in receptacle  14  by a riser hanger  40 . Riser hanger  40  has a tubular housing  42  with an axial bore  43  extending through it. Bore  43  has the same diameter as the riser  12 . A recess  44  on the outer diameter of housing  42  slopes downward and inward. A split ring locking member or latch  46  resides in recess  44  and is biased outward and retained by a stop ring  48  secured to housing  42 . Stop ring  48  engages an upper edge of latch  46 . Latch  46  has an upper surface  50  that slopes downward and outward and is adapted to mate with transitions  28  and  32  and force latch  46  inward as it passes from a larger receptacle inner diameter to a smaller inner diameter, such as third diameter  26  to fourth diameter  30 . Latch  46  also has a grooved profile  52  on its outer diameter adapted to engage the riser engagement grooves  36  of receptacle  14 . 
     Latch  46  may be retained within recess  44  in a retracted position by a split ring retainer  54 . Retainer  54  is axially and radially movable relative to housing and latch  46 . Retainer  54  has an outer profile which generally mates with transitions  24 ,  28 , and  32  and their respective diameters  20 ,  22 ,  26 , and  30  to compress retainer to each diameter as it is drawn through receptacle  14 . Retainer  54  also has a lip  56  on its upper end which extends inward and upward toward a corresponding profile  58  on latch  46 . An internal rib  60  extends inward and upward from the inner diameter of retainer  54  and engages a slot  62  in housing  42 . Retainer  54  also has a downwardly extending leg  64  which resides in a cavity  66  formed between the lower end of housing  42  and a cap  68 . Cap  68  is joined to the lower end of housing  42  and extends upwardly concentric around housing  42 . Retainer  54  is biased outward and radially retained against cap  58  by leg  54  when outside of receiver  14 . Other similar retainer configurations will be readily apparent to one skilled in the art, and use of such other configurations are within the scope of this patent. 
     Referring to FIGS. 3 and 4, riser hanger  40  can be drawn up through the bottom of receptacle  14  with a handling tool (not shown), so that latch  46  meets and passes grooves  36 . Sloped upper surface  50  contacts second transition  28  and forces latch  46  inward, allowing latch  46  to slide into fourth diameter  30 . Hanger  40  is then lowered, allowing latch  46  to expand in third diameter  26  and engaging groove profile  52  with riser grooves  36 . Groove profile  52  and riser grooves  36  are biased to only support hanger  40  against downward movement, but slide out of engagement if hanger  40  is moved upward. In this locked position, retainer  54  resides in second diameter  22 . 
     Referring to FIGS. 7-8, hanger  40  can be released from the locked position described above and lowered out of receptacle  14  by first drawing hanger  40  upward out the top of receptacle  14 . Sloped upper surface  50  contacts second transition  28  and forces latch  46  inward, allowing latch  46  to slide through fourth diameter  30 . When hanger  40  exits the top of receptacle  14 , latch  46  expands outward until it contacts stop ring  48 . Retainer  54  expands outward until leg  64  contacts the inner diameter of cap  68 . Hanger  40  is then lowered back into receptacle  14 . The outer diameter of retainer  54  contacts fourth diameter  30  of receptacle  14 , and retainer  54  is forced upward over latch  46 . Lip  56  of retainer  54  overlaps the lower edge of latch  46 , engaging profile  58  and internal rib  60  aligns over slot  62 . As hanger  40  is lowered further into receptacle  14 , retainer  54  is forced inward, which in turn forces latch  46  inward. Internal rib  60  in slot  62  ensures that retainer  54  does not override latch  46 . Also, retainer  54  will abut the lower edge of profile  52  and prevent retainer  54  from overriding latch  46 . When retainer  54  is fully within fourth diameter  30  and as it passes through the other diameters  26 ,  22 , and  20 , retainer  54  holds latch  46  out of contact with receptacle  14 . Thus, grooved profile  52  on latch  46  does not engage riser grooves  36  on receptacle  14 , and hanger  40  can be lowered out through the bottom of receptacle  14 . 
     Latch  46  can be reset to re-engage riser grooves  36  on receptacle  14  by lowering hanger  40  out through the bottom of receptacle  14  and then lifting it back into receptacle  14 . Retainer  54  will expand and be pulled downward away from and off of latch  46  as it contacts first diameter  20 . Latch  46  will expand to contact receptacle  14  and is reset to re-engage riser grooves  36  as described above. 
     Referring to FIGS. 5 and 6, with hanger  40  locked into receptacle  14 , a tubular member  70  can be joined with riser  12 . Receptacle  14  is typically at a lower deck level on platform  10  (FIG.  1 ), normally below water. Tubular member  70  connects riser  12  with an upper deck level (not shown). Tubular member  70  has a tieback connector  72  on its lower end which inserts into hanger  40  and connects to receptacle  14 . Tieback connector  72  has a tubular inner body or member  74 , which serves along with other components as a tensioner to preload the engagement of hanger latch  46 . Inner member  74  may be fitted with an annular metal seal  76  bolted to its lower end designed to make a metal to metal seal or an elastomer type seal. Seal  76  has the same inner diameter as the bore of tubular member  70  and tieback connector  72 , as well as the nominal inner diameter of bore  43  of hanger  40 . 
     A tieback connector outer member  78  is concentrically carried to slide axially on inner member  74 , and is retained with inner member  74  by a ring  88 . Outer member  78  is a sleeve that has an external downward and outward facing conical surface  79  that engages a mating conical rim  81  on the upper end of hanger  40 . Outer member is prevented from rotating relative to hanger  40 , once it lands, by anti-rotation pins  80 . Pins  80  engage vertical slots formed in an upward facing receptacle  90  of hanger  40 . A lower end of tieback outer member  78  is preferably spaced slightly above an upward facing shoulder in receptacle  90  of hanger  40  when conical surface  79  lands on hanger rim  81 . 
     Inner member  74  has a conical threaded portion  82  on its exterior. A split dog ring  84  is carried within an annular internal recess in outer member  78 . Dog ring  84  has threads on an inner conical surface that mate with the threaded portion  82 . Conical threaded portion  82  slopes upward and outward. Split dog ring  84  carries a plurality of segments or dogs  86  joined to its outer diameter by retainers (FIG.  6 ), the dog ring  84  and dogs  86  serving as a latch to engage tieback grooves  34 . Dogs  86  have grooved exteriors to engage tieback grooves  34  of receptacle  14 . Dogs  86  protrude out of windows formed in outer member  78 . 
     Hanger  40  and tieback connector  72  are dimensioned to provide a downward preload force on hanger latch  46 . This dimensioning results in the upper ends of dogs  86  being initially slightly above the upper edge of tieback grooves  34  when tieback connector  72  first lands and prior to preloading. As the dogs  86  are pushed into tieback grooves  34 , the inward and downward facing shoulders of grooves  34  will push downward on dogs  86 , which in turn push downward on outer member  78 . Outer member  78  transfers this downward preload force through surface  79  to rim  81  of hanger  40 , which in turn transfers the force through latch  46  into receptacle  14 . 
     As tubular member  70  is lowered onto hanger  40 , tieback outer member  78  inserts concentrically into receptacle  90  of hanger  40  and lands on rim  81 . When tieback outer member  78  lands on hanger  40 , continued downward movement causes inner member  74  to move downward relative to outer member  78 . Threaded section  82  will ratchet downward relative to split ring  84 . This forces split ring  84  to expand radially outward, pushing dogs  86  into tieback grooves  34 . At the same time, metal seal  76  inserts into a counterbore  92  of hanger bore  43 , forming a metal-to-metal seal. The upward facing shoulder in receptacle  90  will be spaced a short distance below the lower end of inner member  74  and outer member  78 , and seal  76  bridges this gap. 
     Tubular member  70  and connector inner member  74  are then rotated clockwise to preload the engagement of dogs  86  with tieback grooves  34  and apply a downward preload force on hanger latch  46 . As inner member  74  rotates, outer member  78  is held against rotation by antirotation pins  80 . The threaded section  82  will advance farther downward relative to dog ring  84 , forcing dogs  86  more tightly into engagement with tieback grooves  34 . Threaded section  82  will not contact the inner surface of outer member  78  at any point, rather a slight clearance will always exist. Because dogs  86  were initially slightly spaced above and out of alignment with grooves  34 , the inward and downward facing shoulders of tieback grooves  34  exert a reactive downward and inward force on dogs  86  as dogs  86  move outward. Dogs  86  move downward slightly to align with tieback grooves  34 , creating a compressive force that transmits through conical surface  79  of outer member  78  into rim  81  of hanger  40 , which is an upper end of a neck surrounding receptacle  90  of hanger  40 . This downward force is applied to hanger latch  46 , which transmits it to receptacle  14 . The downward force component thus preloads latch  46  in a downward direction. 
     The amount of downward deflection of outer member  78  and hanger rim  81  during preload is within the elastic range of the metal. To disengage connector  72 , inner member  74  is rotated counterclockwise to unscrew it from dog ring  84 , allowing dogs  88  to retract from engagement with grooves  34  when tieback connector  72  is pulled upward. 
     In operation, referring to FIGS. 3-8 in sequence, hanger  40  of catenary riser  12  is drawn up into receptacle  14  with a handling tool (not shown) so that latch  46  passes riser grooves  36  (FIG.  3 ). Hanger  40  is then lowered until latch  46  engages grooves  36  and is supported (FIG.  4 ). Since platform  10  is separately anchored, there is no buoyant force of the platform applying tension to riser  12 . However, hanger grooves  36  must support the weight of riser  12 . 
     Tieback connector  72  of tubular member  70  is then lowered into receptacle  90  of hanger  40  (FIG.  5 ). When tieback outer member  78  lands on rim  91 , tieback inner member  74  will move downward relative to outer member  78 , expanding dog ring  84  and causing dogs  86  to engage tieback grooves  34 . Seal  76  will slide into counterbore  92  of hanger  40 , forming a metal-to-metal seal. Tubular member  70  is then rotated in a first direction to cause inner member  74  to move further downward relative to outer member, pushing dogs  86  further outward into grooves  34 , which force dogs  86  to move downward slightly. This causes deflection of the neck surround hanger rim  91 , exerting a preload force through hanger housing  42 , latches  46  and into receptacle  14 . 
     When it is desired to disassemble the connection, tubular member  70  is rotated in a second direction to disengage dogs  86  from grooves  34 . Tubular member  70  and tieback connector  72  are removed. Hanger  40  is then lifted upward, and latch  46  disengages from grooves  36 . Hanger  40  is lifted above the top of receptacle  14  (FIG. 7) and lowered back in. Split ring retainer  54  impacts receptacle  14  and is forced up over latch  46 , retaining latch  46  out of engagement with receptacle  14  and grooves  36  (FIG.  8 ). Hanger  40  is then lowered out the bottom of receptacle  14 . 
     In the embodiments of FIGS. 9-12, radial preloading is applied rather than axial preloading. Also, there is no separate tieback member lowered from above, rather the riser assembly extends to the upper deck. Referring to FIGS. 9 and 10, hanger  94  is not shown attached to the upper end of the riser, rather the riser will extend upward to an upper deck level. Hanger  94  has a latch similar in construction and operation to latch  46  of the first embodiment. Latch  96  is a split ring biased radially outward for snapping into grooves  98  in the inner diameter  97  of a receptacle  99 . A latch retainer  100  located below latch  96  operates in the same manner as latch retainer  54  of the first embodiment. 
     A lip  102  is formed on the exterior surface of hanger  94  below latch  96 . Lip  102  is spaced radially outward from the exterior surface of hanger  94  and depends downward. Lip  102  may be annular or a segment. Wedging block  104  is mounted to the exterior of hanger  94  for axial movement. Wedging block  104 , which is preferably a segment, but could be annular, moves between the disengaged position of FIG. 9 upward to the engaged position of FIG.  10 . Wedging block  104  has a cam surface on its outer surface that is tapered to provide a greater radial width at the lower end of block  104  than at the upper end. The lower end has a greater radial width than the width of the cavity between lip  102  and the exterior surface of hanger  94  when lip  102  is in its natural undeflected state. Moving block  104  upward into the cavity pushed lip  103  radially outward to radially preload it against the inner diameter  97  of receptacle  99 . In the disengaged position, lip  102  is free to deflect radially back inward. In the disengaged position, the outer surface of lip  102  may be spaced slightly inward from the inner diameter  97  of receptacle  99 . The deflection of lip  102  is preferably elastic, not permanent. 
     Wedging block  102  may be moved upward and downward by various devices. In the version shown in FIGS. 9 and 10, the actuator includes a plurality of rods  106  (only one shown) that are rigidly secured to block  102  and extend downward. Preferably they extend to a point below the lower end of receptacle  99  to allow access by an ROV (remote operated vehicle). Rods  106  are rigidly connected to an actuator ring  108 , which is mounted to hanger  94  for axial sliding movement. A reacting ring  109  is rigidly attached to hanger  94  below actuator ring  108 . 
     An actuator  110  may be removably mounted to reacting ring  109  by an ROV after langer latch  96  has engaged receptacle grooves  98 . Actuator  110  has a pin  112  that slides into a hole in reacting ring  109  and a hydraulic cylinder  114 . An engaging member  115  is mounted to the upper end of hydraulic cylinder  114 . Engaging member  115  has an inward facing profile that engages actuator ring  108 . When hydraulic fluid pressure is supplied, it strokes engaging member  115 , actuator ring  108 , rods  106  and wedging member  104  upward. The taper of cam surface  105  on wedging member  104  is a locking taper, allowing hydraulic pressure to be removed without wedging member  104  sliding downward. In this embodiment, the entire actuator  110  may be removed, leaving only actuator ring  108 , reacting ring  109 , rods  106  and wedging member  104 . There will preferably be three or more assemblies or rods  106  and wedging members  104  spaced circumferentially around hanger  94 . These assemblies would have been installed permanently at the surface. Hydraulic fluid pressure may be delivered by the ROV or from the upper deck of the platform. A permanently installed actuating assembly to move wedging block  104  could also be employed. 
     In the embodiment of FIGS. 11 and 12, the radial preload mechanism is passive, not utilizing any actuators. Hanger  117  is supported in the same manner as the other embodiments, having a latch  119  that engages receptacle grooves  121 . Grooves  121  are formed in bore  123  of receptacle  125 . A lip  127  is formed on a shoulder  129  of hanger  117 . Lip  127  depends downward and is spaced from outer surface  131  of hanger  117  by an annular cavity. The outer surface  133  of lip  127  engages in radial interference an internal shoulder  135  formed in receptacle bore  123 . Shoulder  137  has a lesser radial dimension that the diameter of bore  123  above shoulder  137 , as indicated by dimension  137 . Shoulder  137  has a tapered upper surface that increases in diameter in an upward direction back to the nominal dimension of receptacle bore  123 . Shoulder  137  also has a tapered lower surface that increases in diameter in a downward direction to a counterbore  139  of larger diameter than the upper portion of bore  123 . 
     In a natural undeflected condition, the outer radial dimension of lip outer surface  131  is greater than the inner diameter of shoulder  135 . As hanger  117  is pulled into receptacle  125 , lip  127  will deflect radially inward, creating a radial preload force. The amount of deflection is elastic, not permanent. 
     The invention has significant advantages. Preloading the hanger against the receptacle helps resist fatigue due to wave and current movement of the platform relative to the riser, whether the preloading is axial or radial. 
     While the 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 scope of the invention. For example, different load supporting mechanisms may be used to support the hanger in the receptacle. The riser hanger grooves could comprise a single upward facing shoulder. The riser hanger grooves could comprise a retractable shoulder, allowing the latch on the riser hanger to be a simple downward facing shoulder, rather than a retractable member. Similarly, a single downward facing shoulder could be substituted for the tieback grooves in the receptacle. This shoulder could also be configured to be retractable. Also, rather than segments or dogs, a single split ring could be employed. The axial preload force could pass through a lower end of the outer member into the hanger body rather than into the rim of the hanger body. Threaded engagement and rotary movement to axially preload could be changed to a straight downward action of a cam member employing hydraulic cylinders or other drive mechanisms. The radial preloading of the second embodiment can be accomplished by devices other than a hydraulic cylinder, such as a screw jack. The system may be employed with other types of subsea riser connections rather than a catenary riser.