US20050147473A1 - Riser tensioner with shrouded rods - Google Patents

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 has a piston, a piston chamber, and a piston rod extending from the piston and away from the piston chamber. A shroud encloses and moves in unison with the piston rod.

Description

RELATED APPLICATIONS
• This nonprovisional patent application claims the benefit of co-pending, provisional patent application U.S. Ser. No. 60/534,831, filed on Jan. 7, 2004, which is hereby incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• This invention relates in general to hydro/pneumatic tensioners for applying tension to a riser supported from a floating platform.
• 2. Background of the Invention
• 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.
• 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.
• 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. Previous ram style assemblies are positioned vertically upward relative to the deck. This arrangement can have stability problems, especially when one of the piston assemblies fails. 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.
SUMMARY OF THE INVENTION
• 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.
• 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.
• The surface assembly can alternatively include a frame extending circumferentially around the riser above the working deck. The frame is connected to the riser so that the frame moves axially with the riser. The surface assembly also includes a hydraulic tensioner assembly having an end connected to the working deck and another end connected to the frame. The hydraulic tensioner assembly inclines radially inward from the deck to the frame. The hydraulic tensioner assembly can define a frame angle between the working deck and the frame. The frame angle varies as the working deck moves relative to the riser.
• The surface assembly can alternatively include a tensioner assembly connected between the working deck and the frame. This tensioner assembly has a contracted positioned when the work deck moves axially downward relative to the riser. The tensioner comprises a piston, a piston rod, and a shroud surrounding a part of the piston rod while in the contracted position. The shroud has at least one end open to atmospheric pressure. The tensioner assembly can also have a retracted position when the work deck moves axially upward relative to the riser. The shroud surrounds a larger portion of the piston rod while in the retracted position than in the contracted position.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a schematic side view of a riser tensioner constructed in accordance with this invention and shown in a retracted or extended position.
• FIG. 2 is a schematic side view of the riser tensioner inFIG. 1, shown in an extended position.
• 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.
• 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.
• 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.
• 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.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
• Referring toFIGS. 1 and 2, afloating platform deck11 is schematically shown.Deck11 may, for example, be a deck of a barge, a tension leg platform, a spar or other types. However, the arrangement ofFIG. 1 is particularly suited for a spar.Deck11 has anopening13 through which ariser15 extends.
• Riser15 is connected on its lower end to a subsea well. In this embodiment,riser15 is a production riser. Typically, a production tree (not shown) is mounted to the upper end ofriser15. Well fluids flow from the subsea wellhead ofproduction riser15 to the tree. Typically, the floating platform will support a number ofrisers15.
• A tensioner assembly comprising a plurality of hydro/pneumatic cylinder assemblies17 supplies tension to eachriser15 asdeck11 moves upward and downward. Twocylinder assemblies17 are shown inFIG. 1, but preferably, at least twomore cylinder assemblies17 will provide tension to eachriser15. Eachcylinder assembly17 includes acylinder19 and apiston21 that strokes withincylinder19.Piston21 has arod23 that protrudes from one end ofcylinder19. In this embodiment,rod23 is located on the upper end ofcylinder19 abovedeck11. 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 oraccumulator24 is shown. If desired, fluid under atmospheric or low pressure may be placed in the annularspace surrounding rod23 abovepiston21 to serve as lubricant forpiston21. The lubricant may lead to a reservoir for maintaining a constant supply aspiston21 strokes up and down.
• In the preferred embodiment, a plurality ofseals22 surround the circumference ofpiston21. In the embodiment shown inFIGS. 1 and 2seals22 engage an interior surface ofcylinder19. A piston chamber is defined bypiston21, seals22 andcylinder19. In the embodiment shown inFIGS. 1 and 2, a plurality ofseals26 also extend fromcylinder19 to sealingly engagerod23.
• Cylinder19 is connected on its lower end to abrace27 by apin25. In the preferred embodiment,pin25 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.Brace27 in this embodiment is secured todeck11, and the lower ends ofcylinders19 are located approximately at the same level asdeck11.
• Eachcylinder assembly17 inclines relative toriser15 anddeck11 in the embodiment shown inFIGS. 1 and 2. The upper ends ofrods23 are closer toriser15 than the lower ends ofcylinders19.Rods23 are secured byspherical pins29 to atop frame31.Top frame31 is mounted to atension ring33 that is clamped or otherwise secured toriser15 for movement therewith. The radial distance from the axis ofriser15 toupper pins29 is less than the radial distance from the riser axis to lower pins25. The angle of eachcylinder assembly17 relative to theriser15 will change asrods23 stroke from a retracted position as shown inFIG. 2 to an extended position shown inFIG. 1. InFIG. 2, a wave or tidal variation has causeddeck11 to rise relative toriser15, causingcylinder assembly17 to retract. InFIG. 1,deck11 has moved downward from that shown inFIG. 2 due to wave movement or tidal action. The pressurized gas over fluid (FIG. 1) maintains pressure on the lower side ofpiston21 to causecylinder assemblies17 to extend.
• Ashroud35 encloses the exposed portion ofrod23 of eachcylinder assembly17.Shroud35 is a cylindrical member having a closedupper end37 and an openlower end39. Eachrod23 extends through a hole inclosed end37 that is preferably sealed to prevent corrosive fluids from contactingrod23.Shroud35 protectsrod23 and seals26 from any debris falling ontocylinder assemblies17 from above. The length ofshroud35 is selected so thatlower end39 will be close to the lower ends ofcylinders19 whilecylinder assembly17 is fully retracted as shown inFIG. 2. When fully extended, as shown inFIG. 3,lower end39 of eachshroud35 is spaced below the upper end ofcylinder19. The interior ofshroud35 is at low or atmospheric pressure.
• Sets ofguide rollers41 are employed to engageriser15 and maintainriser15 generally centralized in opening13 but allow for angular offset of the riser relative to the platform. Although only twoguide rollers41 are shown, preferably more would be employed for eachriser15. Eachguide roller15 is mounted to anarm43 that is fixed in length in the preferred embodiment.Arm43 has an outer end that is secured by apin45 to alug47.Lug47 mounts todeck11 in this embodiment. Pivot pins45 allowrods43 to be pivoted and rotated away fromdeck opening13 for other operations, such as when a larger diameter drilling riser is employed in a preliminary operation. In this embodiment,arms43 are spaced abovedeck11 only a short distance, thus provide centralizing toriser15 atopening13.
• Anupper deck51 is located below tensioningring33 and abovedeck11 in this embodiment. Mounting guide rollers todeck51 reduces any moment arm onguide rollers41 due to the failure of acylinder assembly17. Preventing angular movements are desirable during many workover and intervention operations. Preferably, pivot pins45 allowrods43 to be pivoted and rotated so thatrollers41 connected toupper deck51 may be disengaged and pivoted away fromriser15. This may be desirable during operations where angular movements are allowable, or when a larger diameter drilling riser is employed.
• The embodiment ofFIG. 3 is the same as the embodiment ofFIGS. 1 and 2 except for placement ofguide rollers41 andupper deck51. Consequently, the same numerals will be used except for the different structure. In this embodiment,upper deck51 is mounted abovetension ring33 and a considerable distance abovedeck11.Arms43 forguide rollers41 are mounted toupper deck51. An advantage of the embodiment ofFIG. 3 occurs if one of thecylinder assemblies17 loses pressure. A loss in pressure causes a bending moment arm to be applied toriser15, which is resisted byguide rollers41. Because of the placement abovetension ring33, the force applied by the moment arm is reduced over that which would exist ifrollers41 were placed as inFIGS. 1 and 2.
• The embodiment shown inFIG. 4 includes the use of a sleeve orconductor53.Conductor53 is mounted totop frame31 and extends concentrically aroundriser15.Conductor53 extends downward a distance that is at least equal to the total stroke ofcylinder assemblies17.Guide rollers41 engageconductor53 rather than directly engagingriser15.Conductor53 provides wear protection toriser15 due to contact withrollers41.
• Referring to the embodiment shown inFIG. 5,cylinder assemblies17 are inverted in this alternative embodiment.Piston21 sealingly engages the interior surface ofcylinder19 which contains pressurized gas as in the previously discussed embodiments.Cylinder19 has an open lower end for receivingpiston21, but it does not sealingly engagerod23 in this embodiment. Accordingly, the lower end ofpiston21, below seals22 is open to atmospheric pressure. Any fluid or debris dripping ontocylinder assembly17 from above lands oncylinder19, which protects the sealing region betweenseals22 and the interior surface ofcylinder19. There is no separate surroundingrods23 in this embodiment.
• Referring to another alternative embodiment shown inFIG. 6,cylinder assemblies17 andtension ring33 are located belowdeck11.Cylinder assemblies17 extend downward at an angle so that the lower ends ofcylinder assemblies17 are radially inward and below the upper ends ofcylinder assemblies17.Shroud35 continues to protectrod23 from any debris falling ontocylinder assemblies17 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 betweenrod23 andhousing19 to pullhousing19 upward.
• In operation of the embodiments inFIGS. 1-5,tension ring33 is mounted toriser15, and guiderollers41 are mounted in engagement withriser15 or conductor53 (FIG. 3). Gas pressure incylinder19 exerts a desired upward force onriser15 to maintain a desired tension inriser15. Asdeck11 moves upward relative toriser15,cylinder assemblies17 retract. Asdeck11 moves downward relative toriser15,cylinder assemblies17 extend.
• In each of the embodiments, seals22 are protected from drippings and debris from above while in both the contracted and retracted positions. Moreover, in the embodiments shown inFIGS. 1-4, and6,shroud35 also protectsrod23 and seals26, in addition to the sealing region located betweenpiston21 and the interior surface ofcylinder19.
• 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.

Claims (18)

1. A surface assembly for subsea wells, comprising

a working deck on a floating structure, the working deck having an aperture extending axially therethrough;

a riser extending from a subsea location to the working deck and through the aperture;

a frame extending circumferentially around the riser, the frame being connected to the riser so that the frame moves axially with the riser; and

a tensioner assembly connected between the working deck and the frame comprising 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 and movable in unison with the piston rod.

2. The surface assembly ofclaim 1, wherein the piston chamber is sealed from the shroud.

3. The surface assembly ofclaim 1, wherein the interior surface of the shroud is substantially at atmospheric pressure.

4. The surface assembly ofclaim 1, wherein the frame is located above the working deck and the tensioner assembly inclines radially inward from the working deck to the frame.

5. The surface assembly ofclaim 1, wherein the shroud telescopes over the piston chamber while the tensioner assembly moves between contracted and extended positions.

6. The surface assembly ofclaim 1, further comprising upper and lower sets of guide rollers engaging the riser at axially spaced apart locations below the frame.

7. The surface assembly ofclaim 1, further comprising an upper set of guide rollers engaging the riser at an axial location above the frame, and a lower set of guide rollers engaging the riser at an axial location below the frame.

8. The surface assembly ofclaim 1, wherein the tensioner assembly inclines radially inward from the working deck to connect with the frame.

9. The surface assembly ofclaim 1, wherein an annular clearance exists between the piston rod and the shroud.

10. A surface assembly for subsea wells, comprising:

a working deck on a floating structure, the working deck having an aperture extending axially therethrough;

a riser extending from a subsea location to the working deck and through the aperture;

a frame extending circumferentially around the riser, the frame being connected to the riser so that the frame moves axially with the riser; and

a tensioner assembly connected between the working deck and the frame and inclining radially inward between the working deck and the frame, the tensioner assembly having a contracted position and an extended position, the tensioner assembly comprising a piston slidably carried in a piston chamber, a piston rod extending from the piston chamber, and a shroud surrounding at least part of the piston rod while in the contracted and extended positions position, the shroud having at least one end open to atmospheric pressure and being movable in unison with the piston, the piston chamber containing a pressurized fluid on an end of the piston opposite from the piston rod.

11. The surface assembly ofclaim 10, further comprising upper and lower sets of guide rollers engaging the riser at axially spaced apart locations below the frame.

12. The surface assembly ofclaim 10, further comprising an upper set of guide rollers engaging the riser at an axial location above the frame, and a lower set of guide rollers engaging the riser at an axial location below the frame.

13. The surface assembly ofclaim 11, further comprising:

a secondary deck on the floating structure, the secondary deck being spaced apart from the working deck and having a secondary aperture for the riser to extend therethrough; and

wherein the lower set of guide rollers extend from the working deck, and the upper set of guide rollers extend from the secondary deck and engage the riser at an elevation spaced apart from and above the lower roller assemblies extending from the working deck.

14. A surface assembly for subsea wells, comprising:

a working deck on a floating structure, the working deck having an aperture extending axially therethrough;

a riser extending from a subsea location to the working deck and through the aperture;

a frame extending circumferentially around the riser above the working deck, the frame being connected to the riser so that the frame moves axially with the riser;

a hydraulic tensioner assembly having an end connected to the working deck and another end connected to the frame, the hydraulic tensioner assembly inclining radially inward from the deck to the frame.

15. The surface assembly ofclaim 14, wherein the hydraulic tensioner assembly comprises a piston, a piston chamber, a piston rod extending from the piston out of the piston chamber, and a pressurized fluid carried in the piston chamber on an end of the piston opposite the piston rod.

16. The surface assembly ofclaim 14, wherein the hydraulic tensioner assembly comprises a piston, a piston chamber, a piston rod extending downward from the piston and out of the piston chamber, and a pressurized fluid carried in the piston chamber on an end of the piston opposite the piston rod.

17. The surface assembly ofclaim 14, wherein the hydraulic tensioner assembly comprises a piston, a piston chamber, a piston rod extending upward from the piston and out of the piston chamber, and a pressurized fluid carried in the piston chamber on an end of the piston opposite the piston rod.

18. A surface assembly for subsea wells, comprising:

a working deck on a floating structure, the working deck having an aperture extending axially therethrough;

a riser extending from a subsea location to the working deck and through the aperture;

a frame extending circumferentially around the riser above the working deck, the frame being connected to the riser so that the frame moves axially with the riser;

a tensioner assembly having an end connected to the working deck and another end connected to the frame with the tensioner assembly inclining radially inward from the deck to the frame, the tensioner comprising a piston, a piston chamber, and a piston rod extending upward from the piston and out of the piston chamber, the piston chamber containing a pressurized fluid on a lower side of the piston, and a shroud having at least one end open to atmospheric pressure enclosing and moving in unison with the piston rod and at least the sealing portion of the piston assembly; and

upper and lower sets of guide rollers engaging the riser at axially spaced apart locations below the frame.

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