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.
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.
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.
Referring to
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
Referring to
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 (
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.
Referring to
Referring to
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 (
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
As the vessel rises from the trough of the large wave, decks 17, 19 move upward relative to conductor 25, as shown by comparing
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.
This application claims priority to provisional application 60/892,166, filed Feb. 28, 2007.
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5758990 | Davies et al. | Jun 1998 | A |
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Number | Date | Country | |
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20080205992 A1 | Aug 2008 | US |
Number | Date | Country | |
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60892166 | Feb 2007 | US |