Abstract:
A surface assembly that communicates with subsea structures and includes a working deck on a floating structure. The working deck has an aperture extending axially therethrough. A riser extends from a subsea location to the working deck and through the aperture. The surface assembly includes a frame extending circumferentially around the riser so that the frame moves axially with the riser. The assembly also includes a tensioner assembly connected between the working deck and the frame. The tensioner assembly includes a piston slidably carried in a piston chamber, a piston rod extending from the piston and away from the piston chamber, and a shroud enclosing the piston rod. The shroud has a plurality of segments with at least one of the shroud segments being movable in unison with the piston rod.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates in general to hydro/pneumatic tensioners for applying tension to a riser supported from a floating platform.  
         [0003]     2. Background of the Invention  
         [0004]     An offshore facility includes a riser extending to a subsea facility such as a subsea well or subsea manifold located at the sea floor. Offshore facilities that float, such as a tension leg platform, move up and down and horizontally relative to the subsea well with the oscillations of the waves and currents. It is often desirous to maintain a desired tension on the riser during these oscillating movements. Tensioners are often utilized in order to react to the movements of offshore facilities moving with the wave oscillations and currents.  
         [0005]     Previous tensioner assemblies, like those on tension leg platforms, include a plurality of piston assemblies suspended from a deck that connect to a tension ring surrounding the riser. One type relied upon gas positioned in a chamber surrounding the piston rod to create tension. These piston assemblies are pull-type piston assemblies because they react when the piston is being pulled through the piston chamber and the fluid surrounding the piston rod is compressed. These assemblies require large piston assemblies to accommodate the necessary fluid for creating tension in reaction to the movements of the platform.  
         [0006]     Other previous tensioner assemblies include ram style or push-type piston assemblies that have the reactive fluid on the side of the piston opposite from the piston rod. Ram style piston assemblies react when the piston is being pushed through the piston chamber. This arrangement allows for smaller piston assemblies because there is no piston rod in the chamber containing the fluid. Moreover, in previous assemblies, the piston rod extends downward to the piston housed with the piston chamber. Therefore, drippings and debris from above often fall onto the piston rods which can damage the seals of the piston assembly. Failure and less reactive tensioning can occur when the seals are damaged.  
         [0007]     In other ram style or push-type piston assemblies, the piston rod extends upward to the piston housed with the piston chamber. In these assemblies, drippings and debris fall from above onto the rods. Such an arrangement typically required expensive coatings to be applied to the outer surface of the piston rods that were exposed to the elements.  
       SUMMARY OF THE INVENTION  
       [0008]     A surface assembly that communicates with subsea structures includes a working deck on a floating structure. The working deck has an aperture extending axially therethrough. A riser extends from a subsea location to the working deck. The riser extends through the aperture. The surface assembly includes a frame extending circumferentially around the riser. The frame is connected to the riser so that the frame moves axially with the riser. The assembly also includes a tensioner assembly connected between the working deck and the frame. The tensioner assembly comprises a piston, a piston chamber, a sealing portion between the piston and the piston chamber, a piston rod extending from the piston and away from the piston chamber, and a shroud enclosing the piston rod and at least the sealing portion of the piston assembly.  
         [0009]     In another configuration, the sealing portion is between the piston and an interior surface of the shroud. A piston chamber is defined by the sealing portion, the piston, and the shroud. The tensioner assembly can also include a cylinder. The sealing portion can then be located between the piston and the cylinder. The piston chamber is then defined by the sealing portion, the piston, and the cylinder. The shroud typically has a closed upper end, and an open lower end that exposes a portion of its interior surface to atmospheric pressure.  
         [0010]     In yet another configuration, a surface assembly for subsea wells includes a working deck on a floating structure. The working deck has an aperture extending axially therethrough. A riser extends from a subsea location to the working deck and through the aperture. A frame extends circumferentially around the riser. The frame is connected to the riser so that the frame moves axially with the riser. A tensioner assembly is connected between the working deck and the frame. The tensioner assembly includes a piston slidably carried in a piston chamber, a piston rod extending from the piston and away from the piston chamber, and a shroud enclosing the piston rod. The shroud has a plurality of segments with at least one of the shroud segments being movable in unison with the piston rod.  
         [0011]     The plurality of segments can include an inner shroud segment being stationary relative to the piston rod. The plurality of segments can have an inner shroud segment and an outer shroud segment, with the outer shroud segment telescoping over the inner shroud segment when the tensioner assembly is in a contracted position. A substantial portion of the inner shroud segment can be uncovered when the tensioner assembly is in an extended position. Either the outer shroud segment or the inner shroud segment that is fixedly connected to an end portion of the piston chamber that receives the piston rod.  
         [0012]     The plurality of segments can also include an intermediate shroud segment. The intermediate shroud segment telescoping over the inner shroud segment when the tensioner assembly is in a contracted position, and the outer shroud segment telescoping over the intermediate and inner shroud segments when the tensioner assembly is in a contracted position.  
         [0013]     In another configuration a riser tensioner assembly for maintaining tension in a riser extending from a subsea well through an aperture in a working deck of a floating structure includes a piston slidably carried in a piston chamber. A piston rod extends from the piston chamber. The piston rod and piston are movable between a contracted position and an extended position of the tensioner assembly. A shroud surrounds at least part of the piston rod while in the contracted and extended positions. The shroud has a plurality of shroud segments with at least one of the shroud segments being movable in unison with the piston rod and at least one of the shroud segments being fixedly connected to an end portion of the piston chamber that receives the piston rod.  
         [0014]     In the tensioner assembly, the plurality of shroud segments can include an inner shroud segment and an outer shroud segment. The inner shroud segment can have a flange end connected to either the piston chamber or the piston rod and a telescoping end having an outer lip. The outer shroud can also have a flange end connected to the other of the piston chamber or the piston rod and a telescoping end having an inner lip. When the tensioner assembly is in the extended position, the outer lip of the inner shroud engaging another shroud segment telescoping over the inner shroud and the inner lip of the outer shroud engaging another shroud segment telescoping within the outer shroud.  
         [0015]     The plurality of shroud segments can also include an intermediate shroud segment. The intermediate shroud segment telescopes over the inner shroud segment when the tensioner assembly is in a contracted position. The outer shroud segment telescopes over the intermediate and inner shroud segments when the tensioner assembly is in a contracted position.  
         [0016]     Each intermediate shroud segment can have an extension end and a contraction end. The extension end has an outer lip and the contraction end has an inner lip. When the tensioner assembly is in the extended position the outer lip of the intermediate shroud segment engages an interior lip of either another intermediate shroud segment or the outer shroud segment, and the inner lip of the intermediate shroud segment engages an outer lip of either another intermediate shroud segment or the inner shroud segment.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a schematic side view of a riser tensioner constructed in accordance with this invention and shown in an extended position.  
         [0018]      FIG. 2  is a schematic side view of the riser tensioner in  FIG. 1 , shown in a contracted position.  
         [0019]      FIG. 3  is a schematic side view of an alternate embodiment of a riser tensioner in accordance with this invention and shown in an extended position.  
         [0020]      FIG. 4  is a schematic side view of an alternate embodiment of a riser tensioner in accordance with this invention and shown in an extended position.  
         [0021]      FIG. 5  is a schematic side view of an alternate embodiment of a riser tensioner in accordance with this invention and shown in an extended position.  
         [0022]      FIG. 6  is a schematic side view of an alternate embodiment of a riser tensioner in accordance with this invention and shown in a partially an extended position.  
         [0023]      FIG. 7  is a schematic side view of another alternate embodiment of a riser tensioner in accordance with this invention and shown in an extended position.  
         [0024]      FIG. 8  is a schematic side view of the riser tensioner in  FIG. 7 , shown in a contracted position.  
         [0025]      FIG. 9  is a schematic side view of another alternate embodiment of a riser tensioner in accordance with this invention and shown in an extended position.  
         [0026]      FIG. 10  is a schematic side view of another alternate embodiment of a riser tensioner in accordance with this invention and shown in an extended position.  
         [0027]      FIG. 11  is a schematic side view of the riser tensioner in  FIG. 10 , shown in a contracted position.  
         [0028]      FIG. 12  is an exploded view of a cylinder assembly in the riser tensioner shown in  FIGS. 7-9 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]     Referring to  FIGS. 1 and 2 , a floating platform deck  11  is schematically shown. Deck  11  may, for example, be a deck of a barge, a tension leg platform, a spar or other types. However, the arrangement of  FIG. 1  is particularly suited for a spar. Deck  11  has an opening  13  through which a riser  15  extends.  
         [0030]     Riser  15  is connected on its lower end to a subsea well. In this embodiment, riser  15  is a production riser. Typically, a production tree (not shown) is mounted to the upper end of riser  15 . Well fluids flow from the subsea wellhead of production riser  15  to the tree. Typically, the floating platform will support a number of risers  15 .  
         [0031]     A tensioner assembly comprising a plurality of hydro/pneumatic cylinder assemblies  17  supplies tension to each riser  15  as deck  11  moves upward and downward. Two cylinder assemblies  17  are shown in  FIG. 1 , but preferably, at least two more cylinder assemblies  17  will provide tension to each riser  15 . Each cylinder assembly  17  includes a cylinder  19  and a piston  21  that strokes within cylinder  19 . Piston  21  has a rod  23  that protrudes from one end of cylinder  19 . In this embodiment, rod  23  is located on the upper end of cylinder  19  above deck  11 . A closed system of pressurized gas over fluid is utilized to provide force. The pressurized fluid and gas may be internal or external to the cylinder. Both internal and external sources may be used together. An external pressurized fluid and gas source or accumulator  24  is shown. If desired, fluid under atmospheric or low pressure may be placed in the annular space surrounding rod  23  above piston  21  to serve as lubricant for piston  21 . The lubricant may lead to a reservoir for maintaining a constant supply as piston  21  strokes up and down.  
         [0032]     In the preferred embodiment, a plurality of seals  22  surround the circumference of piston  21 . In the embodiment shown in  FIGS. 1 and 2  seals  22  engage an interior surface of cylinder  19 . A piston chamber is defined by piston  21 , seals  22  and cylinder  19 . In the embodiment shown in  FIGS. 1 and 2 , a plurality of seals  26  also extend from cylinder  19  to sealingly engage rod  23 .  
         [0033]     Cylinder  19  is connected on its lower end to a brace  27  by a pin  25 . In the preferred embodiment, pin  25  is spherical so as to allow pivotal rotation not only in the plane containing the drawing, but also in a Z-plane perpendicular to the plane containing the drawing. Brace  27  in this embodiment is secured to deck  11 , and the lower ends of cylinders  19  are located approximately at the same level as deck  11 .  
         [0034]     Each cylinder assembly  17  inclines relative to riser  15  and deck  11  in the embodiment shown in  FIG. 1  and  2 . The upper ends of rods  23  are closer to riser  15  than the lower ends of cylinders  19 . Rods  23  are secured by spherical pins  29  to a top frame  31 . Top frame  31  is mounted to a tension ring  33  that is clamped or otherwise secured to riser  15  for movement therewith. The radial distance from the axis of riser  15  to upper pins  29  is less than the radial distance from the riser axis to lower pins  25 . The angle of each cylinder assembly  17  relative to the riser  15  will change as rods  23  stroke from a retracted position as shown in  FIG. 2  to an extended position shown in  FIG. 1 . In  FIG. 2 , a wave or tidal variation has caused deck  11  to rise relative to riser  15 , causing cylinder assembly  17  to retract. In  FIG. 1 , deck  11  has moved downward from that shown in  FIG. 2  due to wave movement or tidal action. The pressurized gas over fluid ( FIG. 1 ) maintains pressure on the lower side of piston  21  to cause cylinder assemblies  17  to extend.  
         [0035]     A shroud  35  encloses the exposed portion of rod  23  of each cylinder assembly  17 . Shroud  35  is a cylindrical member having a closed upper end  37  and an open lower end  39 . Each rod  23  extends through a hole in closed end  37  that is preferably sealed to prevent corrosive fluids from contacting rod  23 . Shroud  35  protects rod  23  and seals  26  from any debris falling onto cylinder assemblies  17  from above. The length of shroud  35  is selected so that lower end  39  will be close to the lower ends of cylinders  19  while cylinder assembly  17  is fully retracted as shown in  FIG. 2 . When fully extended, as shown in  FIG. 3 , lower end  39  of each shroud  35  is spaced below the upper end of cylinder  19 . The interior of shroud  35  is at low or atmospheric pressure.  
         [0036]     Sets of guide rollers  41  are employed to engage riser  15  and maintain riser  15  generally centralized in opening  13  but allow for angular offset of the riser relative to the platform. Although only two guide rollers  41  are shown, preferably more would be employed for each riser  15 . Each guide roller  15  is mounted to an arm  43  that is fixed in length in the preferred embodiment. Arm  43  has an outer end that is secured by a pin  45  to a lug  47 . Lug  47  mounts to deck  11  in this embodiment. Pivot pins  45  allow rods  43  to be pivoted and rotated away from deck opening  13  for other operations, such as when a larger diameter drilling riser is employed in a preliminary operation. In this embodiment, arms  43  are spaced above deck  11  only a short distance, thus provide centralizing to riser  15  at opening  13 .  
         [0037]     An upper deck  51  is located below tensioning ring  33  and above deck  11  in this embodiment. Mounting guide rollers to deck  51  reduces any moment arm on guide rollers  41  due to the failure of a cylinder assembly  17 . Preventing angular movements are desirable during many workover and intervention operations. Preferably, pivot pins  45  allow rods  43  to be pivoted and rotated so that rollers  41  connected to upper deck  51  may be disengaged and pivoted away from riser  15 . This may be desirable during operations where angular movements are allowable, or when a larger diameter drilling riser is employed.  
         [0038]     The embodiment of  FIG. 3  is the same as the embodiment of  FIGS. 1 and 2  except for placement of guide rollers  41  and upper deck  51 . Consequently, the same numerals will be used except for the different structure. In this embodiment, upper deck  51  is mounted above tension ring  33  and a considerable distance above deck  11 . Arms  43  for guide rollers  41  are mounted to upper deck  51 . An advantage of the embodiment of  FIG. 3  occurs if one of the cylinder assemblies  17  loses pressure. A loss in pressure causes a bending moment arm to be applied to riser  15 , which is resisted by guide rollers  41 . Because of the placement above tension ring  33 , the force applied by the moment arm is reduced over that which would exist if rollers  41  were placed as in  FIGS. 1 and 2 .  
         [0039]     The embodiment shown in  FIG. 4  includes the use of a sleeve or conductor  53 . Conductor  53  is mounted to top frame  31  and extends concentrically around riser  15 . Conductor  53  extends downward a distance that is at least equal to the total stroke of cylinder assemblies  17 . Guide rollers  41  engage conductor  53  rather than directly engaging riser  15 . Conductor  53  provides wear protection to riser  15  due to contact with rollers  41 .  
         [0040]     Referring to the embodiment shown in  FIG. 5 , cylinder assemblies  17  are inverted in this alternative embodiment. Piston  21  sealingly engages the interior surface of cylinder  19  which contains pressurized gas as in the previously discussed embodiments. Cylinder  19  has an open lower end for receiving piston  21 , but it does not sealingly engage rod  23  in this embodiment. Accordingly, the lower end of piston  21 , below seals  22  is open to atmospheric pressure. Any fluid or debris dripping onto cylinder assembly  17  from above lands on cylinder  19 , which protects the sealing region between seals  22  and the interior surface of cylinder  19 . There is no separate surrounding rods  23  in this embodiment.  
         [0041]     Referring to another alternative embodiment shown in  FIG. 6 , cylinder assemblies  17  and tension ring  33  are located below deck  11 . Cylinder assemblies  17  extend downward at an angle so that the lower ends of cylinder assemblies  17  are radially inward and below the upper ends of cylinder assemblies  17 . Shroud  35  continues to protect rod  23  from any debris falling onto cylinder assemblies  17  from above. This embodiment is particularly useful for replacing tensioner assemblies on existing structures, like existing tension leg platforms, wherein the tension ring is located below the deck. In this embodiment, gas over fluid pressure acts on the annular space between rod  23  and housing  19  to pull housing  19  upward.  
         [0042]     In operation of the embodiments in  FIGS. 1-5 , tension ring  33  is mounted to riser  15 , and guide rollers  41  are mounted in engagement with riser  15  or conductor  53  ( FIG. 3 ). Gas pressure in cylinder  19  exerts a desired upward force on riser  15  to maintain a desired tension in riser  15 . As deck  11  moves upward relative to riser  15 , cylinder assemblies  17  retract. As deck  11  moves downward relative to riser  15 , cylinder assemblies  17  extend.  
         [0043]     In each of the embodiments, seals  22  are protected from drippings and debris from above while in both the contracted and retracted positions. Moreover, in the embodiments shown in  FIGS. 1-4 , and  6 , shroud  35  also protects rod  23  and seals  26 , in addition to the sealing region located between piston  21  and the interior surface of cylinder  19 .  
         [0044]     In the alternative embodiment of cylinder assembly  17 ′ shown in  FIGS. 7-11 , shroud  35  is replaced with a shroud  35 ′ having telescoping shroud portions  35   a ′,  35   b ′,  35   c ′. Similarly, piston and piston rod  22 , 23 , and cylinder  19  from the embodiments shown in  FIGS. 1-6  are replaced with piston  22 ′, piston rod  23 ′, and cylinder  19 ′.  FIGS. 7 and 8  are similar to the embodiment shown in  FIGS. 1 and 2 , with upper deck  51  being positioned below engagement ring  31 .  FIG. 7  illustrates the alternative embodiment in an extended position, while  FIG. 8  illustrates the alternative embodiment in a contracted position. Likewise,  FIG. 9  is similar to the embodiment shown in  FIG. 3  such that upper deck  51  is positioned above engagement ring. Finally,  FIGS. 10-11  are similar to the embodiment shown  FIG. 5 , with cylinder  19 ′ being disposed above piston rod  23 ′ and shroud  35 ′.  FIGS. 10 and 11  illustrate this alternative embodiment in both an extended and a contracted position. The alternative embodiments illustrated in  FIGS. 7-11  also show cylinder assemblies  17 ′ extending substantially vertical rather than extending at an angle radially inward from lower deck  11  to upper deck  51 .  
         [0045]     Referring to  FIGS. 7-12 , shroud  35 ′ includes a plurality of tubular, telescoping shroud portions or segments  35   a ′,  35   b ′, and  35   c ′. In the preferred embodiment, outer shroud segment  35   a ′ has an inner diameter larger enough to receive intermediate shroud segment  35   b ′ and shroud segment  35   c ′. Intermediate shroud segment  35   b ′ preferably has an inner diameter large enough to receive inner or small shroud segment  35   c ′. Outer or large shroud segment  35   a ′ is preferably positioned above intermediate and small shroud segments  35   b ′,  35   c ′ so that shroud  35 ′ shields piston rod  23 ′ from drippings from above when shroud  35 ′ is both extended and contracted, whether cylinder is positioned below shroud  35 ′ ( FIGS. 7-9 ) or below shroud  35 ′ ( FIGS. 10-11 ).  
         [0046]     As is perhaps shown best in  FIG. 12 , shroud segments  35   a ′, 35   b ′, 35   c ′ include upper and lower lips  61 ,  63  for engaging each other when moving from the contracted position to the extended position. Lower lips  63  are preferably formed on an interior surface of the respective shroud segments for engaging an outer surface of another shroud segment disposed therein. Lower lips  63  are typically formed on a contraction end—or the end in the direction of movement of the shrouds during contraction—of each shroud segment. Upper lips  61  are preferably formed on an outer surface of the respective shroud segments for engaging an inner surface of another shroud. Typically, upper lips  61  are formed on an extension end—or the end in the direction of movement of the shrouds during extension—of each shroud segment. As will be appreciated by those skilled in the art, upper and lower lips  61 ,  63  engage each other when shroud  35 ′ is in its extended position and help to define the overall length of shroud  35 ′ when extended.  
         [0047]     In the preferred embodiment, each intermediate segment  35 b′ includes both upper and lower lips  61   b ,  63   b  because each intermediate shroud segment receives a shroud segment, and is received by a larger shroud segment. In the preferred embodiment, large shroud segment  31   a ′ includes only lower lip  63   a,  but has a flange  62  at its upper end for connecting to a piston rod connector flange  67  located on a piston rod connector  65  ( FIGS. 7-9 ), or a flange located at the upper end portion of cylinder  19 ′ ( FIGS. 10-11 ). In the preferred embodiment, small shroud segment  35   c ′ only includes upper lip  63   c.  However, small shroud segment  35   c ′ also includes a flange  64  at its lower end for connecting to a flange located at the upper end portion of cylinder  19 ′ ( FIGS. 7-9 ) or to piston rod connector flange  67  ( FIGS. 10-11 ).  
         [0048]     In the embodiments shown in  FIGS. 7-12 , piston rod  23 ′ is at least one shroud length longer than piston rod  23  in the previous embodiments because no shroud segment telescope over cylinder  19 ′. As will be readily appreciated by those skilled in the art, in the embodiment shown in  FIGS. 7-9 , small shroud segment  35   c ′ could be adapted to telescope over an outer surface of cylinder  19 ′, for example with a lower lip  63  rather than a flange  64 , so that piston rod  23 ′ could have substantially the same length as piston rod  23 .  
         [0049]     In each of the alternative embodiments illustrated in  FIGS. 7-12 , seals  22  are protected from drippings and debris from above while in both the contracted and retracted positions. Moreover, in each of the embodiments shown in  FIGS. 7-12 , shroud  35 ′ also protects rod  23 ′ and seals  26 , in addition to the sealing region located between piston  21 ′ and the interior surface of cylinder  19 . Protecting the outer surface of piston rod  23 ′ allows for a less expensive manufacture of piston rod  23 ′ because a protective layer will not be necessary.  
         [0050]     While the invention has been shown in only three 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, the number of intermediate shrouds  35   b ′ can be increased or decreased, even such that large shroud  35   a ′ registers with small shroud  35   b ′. Furthermore, the telescoping shroud segments could also be utilized with the tensioner assemblies having the piston rod extending radially inward from the working deck to the tension ring.