RISER TENSIONER WITH SHROUDED RODS
Related Applications
1] This nonprovisional patent application claims the benefit of copending, provisional patent application United States Serial No. 60/534, 831, filed on January 7, 2004, which is hereby incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
2] This invention relates in general to hydro/pneumatic tensioners for applying tension to a riser supported from a floating platform.
2. Backeround of the Invention [0003] 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.
4] 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.
5] 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
10006] 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.
10007] 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.
10008] The shroud typically has a closed upper end, and an open lower end that exposes apportion of its interior surface to atmospheric pressure.
10009] 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.
0] 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
[00111 Figure 1 is a schematic side view of a riser tensioner constructed in accordance with this invention and shown in a retracted or extended position.
2] Figure 2 is a schematic side view of the riser tensioner in Figure 1, shown in an extended position.
3] Figure 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.
4] Figure 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.
[00151 Figure 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.
6] Figure 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
7] Referring to Figures 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 Figure 1 is particularly suited for a spar. Deck 11 has an opening 13 through which a riser 15 extends.
8] 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.
[00191 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 Figure 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.
0] In the preferred embodiment, a plurality of seals 22 surround the circumference of piston 21. In the embodiment shown in Figures 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 Figures 1 and 2, a plurality of seals 26 also extend from cylinder 19 to sealingly engage rod 23.
10021] 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.
10022] Each cylinder assembly 17 inclines relative to riser 15 and deck 11 in the embodiment shown in Figure 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 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 Figure 2 to an extended position shown in Figure 1. In Figure 2, a wave or tidal variation has caused deck 11 to rise relative to riser 15, causing cylinder assembly 17 to retract. In Figure 1, deck 11 has moved downward from that shown in Figure 2 due to wave movement or tidal action. The pressurized gas over fluid (Figure 1) maintains pressure on the lower side of piston 21 to cause cylinder assemblies 17 to extend.
3] 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 Figure 2.
When fully extended, as shown in Figure 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.
lO024] 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 l operation. In this embodiment, arms 43 are spaced above deck 11 only a short distance, thus provide centralizing to riser 1 S at opening 13.
5] 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 1 S. This may be desirable during operations where angular movements are allowable, or when a larger diameter drilling riser is employed.
6] The embodiment of Figure 3 is the same as the embodiment of Figures 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 S l 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 Figure 3 occurs if one of the cylinder assemblies l 7 loses pressure. A loss in pressure causes a bending moment arm to be applied to riser IS, 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 Figures 1 and 2.
7] The embodiment shown in Figure 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.
10028] Referring to the embodiment shown in Figure 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 l 9. There is no separate surrounding rods 23 in this embodiment.
9] Referring to another alternative embodiment shown in Figure 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.
0] In operation of the embodiments in Figures 1-5, tension ring 33 is mounted to riser 15, and guide rollers 41 are mounted in engagement with riser 15 or conductor 53 (Figure 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 l 7 extend.
1] 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 Figures 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.
2] 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.