1. Field of Invention
This invention relates in general to devices for tensioning risers for offshore well production, and in particular to a device that causes the upper portion of the riser to rotate along with the vessel if the vessel rotates due to waves and currents.
2. Description of Related Art
Offshore production platforms must support production risers from oil or gas wells which extend to the platform from subsea wells. For platforms that are fixed to the ocean floor this is readily accomplished and is well known in the art. However, for subsea completions in deep water that require the use of floating platforms, such as tension leg platforms or semi-submersible platforms, supporting risers present significant problems. These platforms move under the influence of waves, wind, and current and are subjected to various forces, including rotational forces. Thus, the riser tensioning mechanism must permit the platform to move relative to the riser and care must be taken that the tensioning mechanism can withstand rotational forces.
The riser tensioning mechanism must also maintain the riser in tension so that the entire weight of the riser is not transferred to the wellhead and so that the riser does not collapse under its own weight. The tensioning mechanism must therefore exert a continuous tensional force on the riser. Also, this force must be maintained within a narrow tolerance. The use of a hydraulic cylinders attached between a platform and a riser to support the weight of the riser is well known in the art. The tensioner cylinders of the prior art are exposed to torque resulting from vessel rotation in response to environmental forces such as waves, currents, and wind. Improvements to protect the tensioner cylinder from this torque are desired.
The present invention disclosure comprises a tensioner anti-rotation device and method for transferring rotational force from the deck of a floating platform to a riser, thereby protecting tensioner cylinders from torque produced by vessel rotation in response to environmental forces such as waves, currents, and wind.
The apparatus comprises a key adapted to be mounted either to the riser or the deck; with a slot adapted to be located on the other of the riser or the deck, in vertical sliding engagement with the slot. The apparatus preferably includes two brackets mounted on the vessel's deck. Each bracket can have either a key or a slot on its inner side and is preferably located 180° apart from the other. Each bracket is in proximity to the riser and tensioner cylinders. An axially extending key or slot is located on the riser, or on an attached sleeve. The key and slot arrangement between the riser and deck-mounted bracket loosely engage, allowing axial movement of the arrangement with respect to the riser. The system allows for axial movement that is greater than the stroke length of the tensioner cylinders.
Where the key comprises a rib adapted to be mounted to the riser, the rib has a greater longitudinal length than slot. However, if the slot is adapted to be located on the riser, the slot then has a greater longitudinal length than the key.
When the vessel rotates, the key and slot arrangement on the riser and bracket engage, with the key contacting the inner portion of the slot. The contact between the key and slot transfers the torque from the deck, the bracket, through the key and slot arrangement, and to the riser. The tensioner cylinders are thus protected from torque that could disturb their alignment or result in damage.
The apparatus could also include a guide roller adapted to engage riser in lieu of brackets. The guide roller is mounted to the deck and has a slot, with a key adapted to be mounted to the riser.
Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which:
Referring to
In this embodiment, a cylindrical sleeve 25 is mounted around riser 13. Sleeve 25 is rigidly attached to and surrounds an inner cylinder of the riser 13 so that it will not move axially or rotationally relative to riser 13 and thus may be considered as part of the riser 13. Sleeve 25 has a length greater than the maximum stroke of cylinder assemblies 19 from the contracted to the extended positions so that rollers 23 remain in engagement with sleeve 25.
In this example, sleeve 25 has an inner diameter larger than an outer diameter of riser 13, defining an annular clearance between them. Sleeve has a flange 27 at its upper and lower ends that extends radially outward. An axially extending key or rib 29 is mounted on the exterior of sleeve 25 and extends from the lower flange 27 (
Sleeve 25 may be secured to riser 13 in a variety of manners so as to be rigidly attached. In this embodiment, as shown in
Referring to
Other devices may be employed to cause riser 13 to twist if the vessel rotates. For example, the embodiment in
In
Referring to
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. These embodiments are not intended to limit the scope of the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.
This application claims priority from the provisional application Ser. No. 60/988,188 filed Nov. 15, 2007 entitled “Tensioner Anti-Rotation Device,” which is incorporated by reference herein in its entirety.
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