Patent Publication Number: US-7708498-B2

Title: Soft stop for maximum riser tensioner stroke

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to provisional application 60/892,166, filed Feb. 28, 2007. 

   FIELD OF THE INVENTION 
   This invention relates in general to riser tensioners for offshore drilling and production vessels and in particular to a stop mechanism that cushions impact during a maximum riser tensioner stroke. 
   BACKGROUND OF THE INVENTION 
   Offshore well operations in deep water may employ a riser extending from subsea well equipment on the sea floor to a vessel or floating platform at the surface. During drilling, a drilling riser is connected to the subsea wellhead and extends to the drilling platform. During well production, production risers might extending from subsea well equipment, such as a subsea tree or manifold, to the surface platform. 
   Guides are employed between the riser and the opening in the vessel through which the riser passes. Typically a tubular conductor is mounted to and surrounds the riser. Bearing members, normally rollers, are mounted to the vessel and engage the conductor. 
   It is important to keep tension in these risers as the vessel rises and falls due to wave movement and/or currents. A tensioner assembly having hydro-pneumatic cylinder units is connected between the riser. As the vessel moves toward and away from the subsea wellhead, the cylinder units extend and retract to keep a generally uniform level of tension in the risers. Normally, the waves are not steep enough to cause the cylinder units to reach a maximum stroke position where the pistons bottom out on the cylinders. A possibility exists, however, that such waves could occur during extreme weather, such as hurricanes. If so, damage could occur to the cylinders. 
   SUMMARY 
   In this invention, an apparatus is incorporated with the riser and vessel to reduce shock if the tensioner reaches an extreme stroke position. A stop and a shock absorber are used, one adapted to be mounted to the vessel and the other to the conductor. The stop and the shock absorber are axially movable relative to each other in response to waves and/or currents, so that during an extreme stroke position of the riser tensioner, the stop and the shock absorber impact each other for absorbing shock. The impact of the stop and the shock absorber occur before the riser tensioner piston tops out in the cylinder. 
   In the preferred embodiment, the stop comprises a flange on the conductor, and the shock absorber is adapted to be mounted to the vessel. The flange is preferably on a lower end of the conductor. The shock absorber comprises upper and lower annular frame members that are movable toward and away from each other. At least one resilient member is located between the frame members. In the preferred embodiment, a plurality of resilient members are located between and spaced around the upper and lower frame members. 
   Preferably the frame members have central openings larger in diameter than an outer diameter of the flange. A plurality of dogs are mounted to the lower frame member and movable between an installation position, which allows the flange to pass downwardly through the central opening in the lower frame member, and an operational position, which prevents the bottom frame member from passing downwardly past the flange. The dogs preferably pivot between the installation position and the operational position. During a maximum downward movement of the vessel relative to the riser, an upper surface of each of the dogs contacts the frame member and a lower surface of each of the dogs contacts the flange to pass the impact force to the lower frame member. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view, partially sectioned, of a riser tensioner having a shock absorber in accordance with the invention and shown during a normal operating position. 
       FIG. 2  is a side view of the riser tensioner of  FIG. 1 , shown in a maximum extended position. 
       FIG. 3  is a side view of the riser tensioner of  FIG. 1  shown in a maximum contracted position. 
       FIG. 4  is an enlarged side view of the lower guide rollers and the shock absorber of  FIG. 1 , shown during installation of the conductor of the riser tensioner. 
       FIG. 5  is a further enlarged, partially sectioned view of a portion of the shock absorber of  FIG. 1 , shown after the conductor has been inserted through the shock absorber. 
       FIG. 6  is a side view of the lower guides rollers and the shock absorber of  FIG. 1 , shown with the tensioner in the maximum extended position. 
       FIG. 7  is a perspective view of the lower guide rollers and the shock absorber of  FIG. 1 , shown with the riser tensioner near its maximum stroke position. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , riser tensioner assembly  11  is utilized on offshore drilling and/or production floating platforms, and may be one of several on the same platform. Riser tensioner assembly  11  is employed to maintain a desired tension in a riser  13  that extends from the vessel or platform to subsea well equipment  15  on the sea floor. Riser  13  may be a drilling riser for drilling new wells or it may be a production riser for production fluid flow. Subsea well equipment  15  may be a subsea wellhead housing, a subsea tree, a subsea manifold or other type of hydrocarbon recovery equipment. The vessel is subject to vertical and translational movement relative to subsea equipment  15  because of currents and waves. 
   Riser tensioner assembly  11  is mounted between an upper deck  17  and a lower deck  19  of the vessel. Decks  17 ,  19  are a fixed distance apart and move in unison with the vessel. In this embodiment, riser tensioner assembly  11  has two bearing members, which comprise an upper set of guide rollers  21  mounted to upper deck  17  and a lower set of guide rollers  23  mounted to lower deck  19 . Riser tensioner assembly  11  has a conductor  25 , which is a large diameter pipe that extends through guide rollers  21 ,  23  and is stationary relative to riser  13 . Vessel decks  19 ,  21  and upper and lower guide rollers  21 ,  23  thus move relative to conductor  25 . Conductor  25  has an upper end that is rigidly secured to a top frame  27 . Conductor  25  has a stop that comprises an external flange  29  located at its lower end. Riser  13  extends through conductor  25  and may be centrally supported by a number of centralizers  31 . Alternately, conductor  25  could be mounted to the vessel for movement therewith, and the guide rollers  21 ,  23  could be mounted to the riser  13 . 
   In this embodiment, it is desired to continually maintain tension throughout the length of riser  13 , regardless of movement of decks  17 ,  19 . Each riser tensioner assembly  11  has a plurality of hydro-pneumatic cylinders  33  that in this embodiment are mounted to upper deck  17  and extend downward from upper guide rollers  21  to a point above lower deck  17 . A piston shaft  35  extends from each cylinder  33  to top frame  27 . Fluid pressure acts against a piston within each cylinder  33  for extending and retracting each piston shaft  35  and for applying an upward force to top frame  27 . A clamp  37  at top frame  27  clamps riser  13  to top frame  27 . 
   A shock absorber  39  is mounted to lower guide rollers  23 , thus shock absorber  39  moves in unison with the vessel in this embodiment. While in the normal operating position of  FIG. 1 , shock absorber  39  is positioned well above flange  29  at the lower end of conductor  25 .  FIG. 2  illustrates an extremely low position for the vessel, such as in the trough of a huge wave in a severe hurricane. In this position, shock absorber  39  lands on conductor flange  29  and piston shafts  35  extend to a maximum length stroke to maintain the desired tension in riser  13 . Preferably, the impact of shock absorber  39  on flange  29  occurs before the pistons top out in cylinders  33 .  FIG. 3  shows the vessel moving away from subsea well equipment  15 , such as at the peak of a big wave. In this position, piston shafts  35  are fully retracted to avoid over tensioning riser  13 , and shock absorber  39  is located a full stroke distance above conductor external flange  29 . Alternately, shock absorber  39  could be mounted stationarily on conductor  25  and a stop, such as flange  29 , mounted on the vessel. 
   Referring to  FIG. 4 , lower guide rollers  23  may be of a variety of types. In this type, guide rollers  23  include an upper plate  41  and a lower plate  43 , each of which extends around conductor  25  in a plane perpendicular to the axis of conductor  25 . Braces  45  extend vertically between plates  41 ,  43 , securing them to each other at a fixed distance. A plurality of rollers  47  are mounted between braces  45  for engaging conductor  25 . 
   Shock absorber  39  comprises a top frame  49  and a bottom frame  51  in this example. Top frame  49  is secured to lower plate  43  of lower guide rollers  23  in any suitable manner, such as by bolts. Frames  49 ,  51  comprise circular flat plates similar to plates  41 ,  43  of guide rollers  23 . Each frame  49 ,  51  has a central hole  52  ( FIG. 5 ) through which conductor  25  extends. Bottom frame  51  is movable vertically a short distance relative to top frame  49 . A number of retaining pins  53  extends between frames  49 ,  51  to retain bottom frame  51  with top frame  49 . Each retaining pin  53  is stationarily secured to top frame  49  for movement therewith. Each retaining pin  53  extends through a hole in bottom frame  51 . A nut  55  at the lower end of each retaining pin  53  retains bottom frame  51  while in its lower position relative to top frame  49 , which is the position shown in  FIGS. 4 and 7 .  FIG. 6  shows bottom frame  51  moved upward relative to retaining pins  53  and top frame  49  to an upper position. Alternatively, retaining pins  53  could be mounted stationarily to bottom frame  51  and extend through holes in top frame  49 . 
   A plurality of dampers or resilient members are located between frames  49  and  51  to dampen upward movement of bottom frame  51  to relative to top frame  49 . In this example, each damper comprises a tubular steel housing  57  containing a flexible spring element  59 . Spring element  59  may comprise an elastomeric member or a coil spring and it initially protrudes from an open end of damper housing  57 . In this example, damper housing  57  is mounted to top frame  49  and damper spring element  59  extends downward and is biased into contact with bottom frame  51 . However, housing  57  and spring element  59  could be inverted, if desired.  FIGS. 4 and 7  show spring elements  59  protruding from housings  57  while  FIG. 6  shows spring elements  59  fully compressed within their housings  57 . 
   Referring to  FIG. 4 , shock absorber  39  also has a plurality of load transfer dogs  61 . Dogs  61  are uniformly spaced around the circumference of bottom frame  51 . In this example, each dog  61  comprises a flat plate that is pivotally mounted to a clevis  63  by a pivot pin  65 . Each clevis  63  is welded or otherwise secured to the lower side of bottom frame  51 . Each dog  61  has a lower edge  67  that when in the operational position of  FIGS. 5-7  faces downward for engagement by external flange  29  when tensioner assembly  11  is in the fully extended position of  FIG. 2 . Each dog  61  has an upper edge  66  that contacts the lower side of bottom frame  51  while in the operational position. An inner edge  71  of each dog  61  is closely spaced to the outer diameter of conductor  25  while in the operational position. Each dog has an upward-facing cam edge  70  located radially outward from pivot pin  65 . 
   Referring to  FIG. 5 , for each dog  61 , an adjustment pin  69  is secured to top frame  49  ( FIG. 4 ) and extends downward into a hole  68  in bottom frame  51 . In the assembly position, illustrated by the dotted lines of  FIG. 5 , the lower end of each adjustment pin  69  is recessed within hole  68 . Adjustment pin  69  has a threaded section that engages a threaded hole in top frame  49  ( FIG. 4 ). When rotated, adjustment pin  69  moves downward against cam edge  70  of dog  61  to cause dog  61  to rotate about pivot pin  65  to the operational position shown by the solid lines of  FIG. 5 . Adjustment pin  69  is locked in a desired position by tightening a nut  73  ( FIG. 4 ) against top frame  49 . 
   During assembly of riser tensioner  11  to the vessel, lower guide rollers  23  and shock absorber  39  will be secured to each other and mounted to lower deck  19  ( FIG. 1 ) of the vessel before installation of conductor  25 . In the installation position shown in  FIG. 4  and by the dotted lines of  FIG. 5 , each dog  61  is free to pivot about its pivot pin  65  and will hang downward by its own weight. In this position, the inner diameter circumscribed by the inner edges  71  of dogs  61  is greater than the outer diameter of conductor flange  29 . Conductor  25  is then lowered through upper guide rollers  21  ( FIG. 1 ), lower guide rollers  23  and shock absorber  39 . The freely pivotal dogs  61  allow flange  29  to pass through shock absorber  39  as conductor  25  is lowered even if flange  29  happens to contact inner edges  71 . After the upper end of conductor  25  lands on tensioner top frame  27  ( FIG. 1 ), the operator rotates adjustment pins  69 , causing each dog  61  to pivot about its pivot point  65 . When upper edge  66  contacts the lower side of bottom frame  51 , the operator will tighten nut  73  ( FIG. 4 ). Dogs  61  will then remain in the operational position of  FIGS. 5-7 . The inner edges  71  will define an inner diameter that is smaller than the outer diameter of conductor flange  29  and slightly larger than the outer diameter of conductor  25  above flange  29 . 
   In operation, shock absorber  39  will move in unison with the vessel and its upper and lower decks  17 ,  19 , as can be seen by comparing  FIGS. 1-3 . Downward and upward movement of vessel decks  17 ,  19  relative to conductor  25  cause piston shafts  35  to extend and retract to maintain a desired tension in riser  13 . If the downward movement is great enough, it is possible for shock absorber  39  to impact external flange  29  of conductor  25 , as shown in  FIG. 2 . Referring to  FIG. 7 , when lower edges  67  of dogs  61  contact external flange  29 , an upward force from flange  29  is transferred through upper edges  66  ( FIG. 5 ), bottom frame  51  and to damper spring elements  59 , which absorb shock and collapse within damper housings  57 . When damper spring elements  59  are fully collapsed ( FIG. 6 ), the upward force passes through damper housings  57 , top frame  49  and lower guide rollers  23  to lower deck  19  ( FIG. 2 ). 
   As the vessel rises from the trough of the large wave, decks  17 ,  19  move upward relative to conductor  25 , as shown by comparing  FIGS. 2 and 3 , which moves shock absorber  39  above the external flange  29 . Bottom frame  51  moves back down to the lower position ( FIG. 7 ) relative to top frame  49  and damper spring elements  59  protrude from damper housings  57 . 
   The shock absorber reduces the possibility of damage occurring to the riser tensioner cylinders because it stops extension of the tensioner cylinder units before the pistons top out. The pivotal load transfer dogs facilitate installation of the riser conductor. 
   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, although the shock absorber and stop only operation during maximum extension of the tensioner cylinder units, similar arrangements could be used to restrict maximum contraction.