Patent Application
20100147530
1. Field of Invention
The device described herein relates generally to the production of oil and gas. More specifically, the device described herein relates to an expandable and/or contractable tensioning device for a tie-back assembly.
2. Description of Related Art
Some offshore platforms have a production tree or trees above the sea surface on the platform. In this configuration, a casing string extends from the platform housing to a subsea wellhead housing disposed on the seafloor. Production casing inserted within the wellbore is supported on the subsea floor by a hanger in the subsea housing. The casing string between the subsea and surface wellhead housings is tensioned to prevent flexure that may be caused by thermal expansion from heated wellbore fluids or vibration from applied side loads. Additionally, the string length or height is typically adjusted to seat or land the upper casing hanger within a surface wellhead.
A sub assembly can be attached to the casing string and used to tension the casing string and adjust its length. The sub assemblies typically comprise a pair of mated housings that in response to an applied force are mechanically retractable in length The adjustable sub assemblies connect inline within the string or on its upper end and when retracted impart a tension force on the casing string and by its retraction, shortening the casing string length.
Disclosed herein is a tubular assembly for connection between a platform and a subsea wellhead assembly. In one embodiment the tubular assembly comprises an annular riser for connection between the platform and the subsea wellhead assembly and an axially expandable and contractable member connected to the annular riser. The expandable and contractable member includes a tubular having a wall formed to axially expand and contract a greater amount per linear increment than the riser. The tubular can be formed from a uni-body construction. When expanding and contracting, the expandable and contractable member wall maintains an axial force therein. The wall may include a series of slots along the wall length alternatingly formed about the wall inner circumference and about the wall outer circumference, each slot lying in a plane substantially perpendicular to the member axis. Alternatively, the member wall may be made up of annular foldable segments coaxially stacked along the member axis. The foldable segments may have an “S” shaped cross section and the segment outer and inner diameter can vary along the member axis length. The wall may have a bellows like shape. A helix formed in the member may selectively shape the wall.
Also disclosed herein is a method of connecting a subsea wellhead assembly and a surface platform. The method may involve providing an axially expandable tubular member, connecting the tubular member into a riser extending between the subsea wellhead assembly and the surface platform, and applying tension to the riser and the tubular member, the tubular member being more expandable per linear increment than the riser.
In the present art of adjustable subs, there exists a plurality of seal elements to accommodate the lengthening or shortening of the casing string. The device described herein eliminates the need for sliding seal elements and hence the design can be used for higher elevated temperatures and pressures of the produced fluids or gases. Alternately, the bellows type sub will accommodate higher temperature injection of liquids or gases into a reservoir.
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:
While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims.
The present invention will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.
With reference now to
The compensating member 36 is axially compressive or axially expandable in response to an applied axial force. The member 36 compresses or expands depending on the magnitude of the applied force and its direction. As noted above, a tieback casing string 34 typically remains in tension during operation. Accordingly, the member 36 can be compressed in response to casing string 34 (or other riser) elongation without removing tension from the casing string 34.
With reference now to
Incorporating the slots 40 alters the wall 38 cross sectional structure. As illustrated in an enlarged view in
Unlike a solid tubular, an axial force F initially applied to the wall 38 does not produce an evenly distributed stress across the wall thickness. Instead the resulting stress concentrates at the cantilevered connections C between the member 44 and web element 46 thereby exerting a bending moment B about the connection C. A sufficient bending moment B on a member 44 deflects the member 44 toward an adjacent slot 40 that in turn shortens the wall 38 and member 36 length. Similarly, an axial force applied in a direction opposite to the force F produces oppositely oriented bending moments that increase the slot 40 width to lengthen the member 36. It should be pointed out that the compensating member 36 configuration described herein is designed to deflect, either in compression or tension, before applied forces approach the yield strength of the riser 34 or other components. As such, the compensating member 36 expands or compresses at a linear increment less than the linear expansion/compression of the riser
Due to the dynamic nature of the expanding and contracting riser 34, the wall 38 material should be sufficiently elastically deformable to accommodate such dynamic loading. As is known, the number of members 44 deflecting, and by how much depends on the force F magnitude, the wall 38 and slot 40 dimensions, and wall 38 material. Thus the body 37 material, slot 40 dimensions, number of slots 40, and wall 38 thickness depend on the anticipated tieback attachment operating conditions. However, those skilled in the art are capable of estimating these variables. In the embodiment shown, the body 37 primarily comprises a single member thereby having a uni-body construction. In this embodiment, the body 37 itself expands and contracts to maintain riser tension without relative movement between two or more coupled members.
Referring still to
Shown in a sectional perspective view in
In one example of use of the device described herein, casing string 34 and compensating member 36 are affixed between seafloor wellhead 30 and surface wellhead 32 and axially tensioned. Sufficient tension in the compensating member 36, 36a elastically deforms the wall 38, 38a and increases the slot/space 40, 52 thickness that in turn elastically elongates the compensating member 36. Since the compensating member 36, 36a is elastically deformed, the compensating member 36, 36a can compress to a less elongated state and compensate for casing string 34 elongation due to high temperature fluid exposure. Optionally, the actual tension applied to the casing string 34 and compensating member 36, 36a may exceed the required casing string 34 stabilizing value. Thus the casing string 34 tension can remain above its required value after any tension force reduction experienced by compensating member 36 compression.
One of the advantages presented by the compensating member described herein is that it can be comprised of a single member formed into a uni-body construction. Moreover, each of the compensating member embodiments presented are formable into a single unit. The uni-body construction eliminates additional components that can complicate manufacture as well as increase failure modes and percentages of failure.
It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the invention is therefore to be limited only by the scope of the appended claims.
