Patent Application
20050238439
This invention relates in general to offshore floating platforms, and in particular to a tension leg platform utilizing tendons with stepped outer diameters and internal sealed compartments.
One technique for offshore drilling and production, particularly in deeper water, utilizes a tension leg platform (“TLP”). A TLP is secured by a number of tendons that attach to pilings in the sea floor. The TLP is de-ballasted to create a desired tension in each of the tendons. The tendons limit lateral movement of the TLP due to waves and currents.
Each of the tendons is preferably close to being neutrally buoyant so that it is substantially self supporting prior to connection to the TLP. Being approximately neutrally buoyant reduces the amount of buoyancy required by the TLP and thus the hull size. To provide buoyancy, the tendons have hollow interiors sealed from sea water. Typically, each tendon is made up of a plurality of joints of pipe, each being approximately 60 to 90 feet in length.
It is important to maintain the buoyancy, because if an interior of one of the tendons filled with sea water, the loss in buoyancy would result in excessive weight being applied to the TLP at the point of connection. It is known to mount sealed bulkheads in the joints of pipe to form separate sealed compartments in the interior of the tendon. Leakage of one compartment would not be as catastrophic as the entire interior of the tendon filling with sea water.
Each tendon must withstand the hydrostatic pressure of the surrounding sea water, which increases with depth. A greater wall thickness will increase the ability of a pipe to withstand hydrostatic pressure. However, a greater wall thickness throughout the length of the tendon would also increase the weight of the tendon, thus requiring a larger and more buoyant hull for the TLP. U.S. Pat. No. 6,851,894 discloses a stepped diameter tendon having upper, intermediate, and lower sections. The upper section has a greater diameter and thinner wall than the intermediate section. Similarly, the intermediate section has a greater diameter and thinner wall than the lower section. This patent does not disclose sealed bulkheads in the interiors of any of the sections.
In this invention, stepped diameter tendons are provided with bulkheads to define a plurality of sealed compartments in the interior. Each tendon has an elongated tubular portion. An upper section of the tubular portion has a larger diameter than a lower section of the tubular member. Both the inner and outer diameters are larger in the upper section. The lower section also has greater wall thickness. Preferably, the cross-sectional areas of the walls of the upper and lower sections are substantially the same.
The tubular portion has a hollow interior that is sealed for preventing entry of sea water. Sealed bulkheads are mounted in the interior of the tubular portion at selected intervals. Preferably, the bulkheads are located both in the upper section and in the lower section of the tubular portion of each tendon. In the preferred embodiment, the tubular portion comprises a plurality of joints of pipe secured together. Each of the joints of pipe has at least one of the bulkheads and preferably two, one located at each end.
Referring to
Upper tendon supports 19 are mounted to platform 11 at each corner. In this embodiment, each upper tendon support 19 is located on an end of one of the horizontal sections 15. Normally, two tendons 21 are supported at each tendon support 19, thus a platform 11 with four corners would have eight separate tendons 21. The lower end of each tendon 21 is secured to a piling 23. A riser 25 is shown extending from wellhead assembly 27 to platform deck 17. Riser 25 may be a drilling riser through which a drill string extends for drilling a well. Riser 25 could also be a production riser. In that instance, a Christmas tree (not shown) may be located at the upper end of riser 25 for controlling well fluid flowing upward from riser 25. If surface Christmas trees are employed, a number of production risers 25 will extend parallel to each other from the sea floor to platform 11, each riser 25 being connected to a separate wellhead. Alternately, subsea trees could be employed.
Referring to
As shown in
By having the smallest outer diameter section and thickest wall, tendon lower section 41 is better able to withstand the higher hydrostatic pressure of the sea water in which it is located. The larger diameter and thinner wall of the upper section 35 increases the buoyancy of tendon 21 by providing more volume for trapped air. The increased buoyancy in upper section 35 helps to support the weight of tendon 21, allowing for a reduced size of platform 11. Preferably, the diameters and wall thicknesses of upper, intermediate, and lower sections are selected to provide a slightly positive or neutral overall buoyancy for tendon 21, such as from 0.95 to 0.97. The slightly positive buoyancy avoids any part of tendon 21 going into compression prior to connection and tensioning with platform 11. Also, when tensioning, platform 11 does not have to initially lift the weight of tendons 21 if they are slightly positive in buoyancy.
A plurality of bulkheads 47 are mounted in each tendon 21 to reduce the consequences of accidental flooding of tendon 21. Bulkheads 47 separate the buoyancy volume into several sealed air compartments so that any leak along the length of tendon 21 will damage only one compartment. The compartment's lengths are selected so that if one or two flood, for example, the remaining compartments would provide sufficient buoyancy to support the weight of tendon 21. Preferably bulkheads 47 are located in each of the sections 35, 37 and 41.
The number of bulkheads 47 may vary. For example, bulkheads 47 could be located at the upper or lower ends of each pipe within upper section 35, intermediate section 37 and lower section 41. Each pipe is typically 60 to 90 feet in length. Alternately, bulkheads 47 could be spaced at greater intervals. Each bulkhead 47 may be secured within the inner diameter of one of the sections of tendon 21 by welding or in a variety of other manners.
Tendons 21 are installed and platform 11 deployed in a conventional manner. Tendons 21 are lowered into the sea and the lower ends latched into bottom connectors 45. Tendons 21 are self supporting, enabling platform 11 to be moved over tendons 21. Columns 13 and horizontal sections 15 are then ballasted until upper terminations 29 are attached to top connectors 33. Then columns 13 and horizontal sections 15 are de-ballasted, causing platform 11 to rise and apply the desired tension to tendons 21.
The invention has significant advantages. The sealed compartments within the stepped diameter tendons avoid catastrophic failure due to leakage. The larger volume of trapped air within the upper section provides additional buoyancy. The smaller diameter lower section better withstands hydrostatic pressure.
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.
This application claims the benefit of provisional application Ser. No. 60/561,831, filed Apr. 13, 2004.
| Number | Date | Country | |
|---|---|---|---|
| 60561831 | Apr 2004 | US |
