Not Applicable
This application relates to offshore platforms for the exploration for, and production of, undersea petroleum deposits, and, in particular, to the various types of platforms generically known as spars, whether of the classic, truss, or cell spar variety. More specifically, the present invention relates to a spar-type platform, of the type having a buoyant upper hull structure and a lower buoyant section or module that supports the mooring and risers when the upper hull structure is detached, in which the lower section is constructed as a sub-sea mooring buoy (SSMB), wherein the upper hull section is detachably connected to the lower (SSMB) section.
The development of sub-sea petroleum and natural gas deposits in Arctic deep water regions presents special challenges for offshore platform designs. Specifically, platforms in these regions must be able to resist local and global loads from ice in addition to loads conveyed by wind, waves, and currents. In some cases, a platform must be moved to avoid contact with or collision with sea ice and icebergs.
One type of platform that has become widely used for the development of deep water deposits is the spar, especially spars that provide for the storage of petroleum or natural gas. The threat of ice would make it advantageous for the hull of the spar, containing storage or not, to be disconnectable or detachable from its mooring and riser system to avoid impact from the ice. Also, the staged development of a particular deposit may be facilitated by changing out topside facilities (by the detachment of the upper hull structure) as development progresses.
Broadly, the present invention is a spar-type platform comprising an upper hull structure that supports the topside facilities and equipment and that provides the buoyancy and ballasting functions and (optionally) a storage function (as in a typical spar), and a lower hull structure or module that forms part of the mooring system and that functions as a sub-sea mooring buoy (SSMB). The upper hull structure and the SSMB module are connected by a detachable connection mechanism, whereby the upper hull structure can be detached from the SSMB module and moved, either by towing or by an on-board propulsion system, to avoid or evade an environmental threat (e.g., floating ice or an iceberg), and then reattached to the SSMB module when the threat has passed. The SSMB module is sized for buoyantly supporting the mooring lines and the riser system that are left behind when the upper hull section is detached and removed.
In the attached condition, the two hull structures are connected by a plurality of connection lines (which can be chains, steel ropes, cables, or combinations thereof) that run from a plurality of chain jacks or fairleaders on the deck of the upper hull section, and through a fairleader/pulley mechanism mounted on the SSMB module. These connection lines can be part of the mooring system, or additional lines can be used to make the connection, or a combination of both can be used. (If additional lines are used in conjunction with the mooring lines, they are referred to as “tie lines.”) During the disconnection process, these lines are slackened at the deck level and removed from the fairleader/pulleys on the SSMB module. These lines are then carried away by the upper hull structure.
In a specific embodiment in which mooring lines are used to connect the two hull structures, the mooring lines are run through chain jacks and chain stoppers mounted on the upper hull structure in the traditional configuration, and they are run down the outside of the spar. Another set of chain stoppers is provided on the SSMB module to maintain tension in the mooring lines between the upper hull structure and the SSMB module. These lines are run through fairleaders mounted on the SSMB module. During the disconnection process, the mooring lines are lowered using guide lines and locked off at the fairleaders. They remain supported by the SSMB module during disconnection. The guide lines are dropped from the upper hull structure and allowed to hang from the SSMB fairleaders.
The weight of the mooring lines and risers, now unsupported by the buoyancy provided by the upper hull structure, causes the SSMB module to separate from the upper hull structure. The SSMB module, carrying the mooring lines and risers, moves downward, controlled by a chain in the centerwell, until the effective weight of the risers and mooring lines is decreased as they lay on the sea floor, until the weight of the mooring lines and risers is equal to the buoyancy of the SSMB module.
The upper side of the SSMB module carries a plurality of upwardly-extending guide posts and a guiding structure that is rigidly fixed to the top of the SSMB module. The guide posts and guiding structure provide the proper alignment between the upper hull structure and the SSMB. The guiding structure also includes a pneumatically- or hydraulically-controlled interface template for the connection of the risers between the upper hull structure and the SSMB module, and it provides structural strength to the connection by fitting inside the underside of the upper hull structure.
For reconnecting the two hull sections, the upper structure is maneuvered above the SSMB module. A haul-in line (chain or steel rope) is lowered and connected to the apex of the guiding structure using a remotely-operated vehicle (ROV). To assist in aligning the upper hull structure and the SSMB, guide post lines are lowered through guide post receptacles on the upper hull structure from the deck of the upper hull structure. The guide post lines are attached to the tops of the guide posts by an ROV. In combination with the haul-in line attached to the apex of the guiding structure, the guide post lines pull the guide posts into their respective receptacles, thereby aligning the upper hull structure and the SSMB. A winch on the upper hull structure draws the SSMB module into a docking bay in the bottom of the wellbay of the upper hull structure, with the guide posts engaging the receptacles to guide the SSMB module into place. The mooring lines are retrieved using an ROV, and they are winched back to the chain jacks. The tie-lines, if used, are reconnected around the fairleaders and tensioned to the deck of the upper hull structure.
Referring now to the drawings, and particularly
The upper hull structure 10 comprises a plurality of interconnected elongate, hollow cells 16, each of which may be divided by a series of vertically-spaced, transverse bulkheads (not shown) into a plurality of compartments (not shown), as is well-known in the art. The uppermost compartments are typically air-filled to provide buoyancy, while the lowermost compartments are typically filled with seawater to provide ballast, so as to keep the platform upright. The intermediate compartments may be used for the storage of petroleum. The tops of the cells 16 support a deck 18, on which are installed the topside facilities and equipment (not shown) that are typical for such platforms.
Spaced around the periphery of the deck 18 are a plurality of upper line holding elements 20, which may be chain jacks or fairleaders. Similarly spaced around the periphery of the SSMB module 14 are a plurality of lower line holding elements 22, which may likewise be chain jacks or fairleaders. A plurality of mooring lines 24 are anchored in the seabed 26 by anchors 28. Each of the mooring lines 24 is run through one of the lower chain jacks or fairleaders 22, then up the side of the upper hull structure 12 and through a guide element 30, and then through one of the upper fairleaders 20. The mooring lines 24 are secured to the upper hull structure 12 by means of upper chain stoppers or cable locks 32, and to the SSMB module 14 by means of lower chain stoppers or cable locks 34. Thus, the extended mooring lines 24 serve as connection lines for connecting the upper hull structure 12 to the SSMB module 14.
Alternatively, the mooring lines 24 may extend only between the SSMB module 14 and the anchors 28, with the connection between the upper fairleaders 20 and the lower fairleaders 22 being provided by tie lines, as discussed below with reference to
As best shown in
The SSMB module 14 comprises a plurality of buoyancy cells or chambers 46 (
As shown in
A plurality of guide posts 58 are spaced around the periphery of the SSMB module 14 and extend upwardly therefrom. A plurality of guide post receptacles 60 are located around the periphery of the upper hull structure 12, near the lower end thereof, so as to receive the guide posts 58 when the SSMB module 14 is connected to the upper hull structure 12. As shown in
The process of disconnecting and removing the upper hull structure 12 from the SSMB module 14 is illustrated in
The weight of the mooring lines 24 and the bottom portions of the risers 52, now unsupported by the buoyancy provided by the upper hull structure 12, causes the SSMB module 14 to sink, controlled by the haul-in line 66 and the guide post lines 74, and thus to separate from the upper hull structure 12. The SSMB module 14 continues to sink as the effective weight of the riser bottom portions and the mooring lines 24 decreases as they settle on the sea floor, until the weight of the mooring lines and risers is equal to the buoyancy of the SSMB module 14.
As shown in
When it is desired to re-connect the upper hull structure 12 to the SSMB module 14, the upper hull structure 12 is positioned over the SSMB module, as shown in
While a preferred embodiment of the invention has been described herein, it has been set forth by way of example only, and is meant to encompass a wide range of equivalent structures. It will be appreciated that a number of variations and modifications will suggest themselves to those skilled in the pertinent arts, and that many of the components and mechanisms specifically described in this specification will find equivalents in the technical arts that are applicable to the present invention. Thus, for example, as mentioned above, the present invention will be readily adaptable to the various types of spar-type platforms known in the art, and the modifications necessary or advantageous to accommodate the invention to various types of spars will be easily understood by those skilled in the pertinent arts. Also, as will be appreciated by those skilled in the pertinent arts, the term “line” as used in this specification, is meant to encompass a cable, a chain, a steel rope, or any functional equivalent thereof. Likewise, the line holding mechanisms described herein may encompass any suitable mechanism available in the art that may accomplish the functions ascribed to these mechanisms. These and other modifications and variations should be considered within the spirit and scope of the present invention, as defined in the claims that follow.
This application claims the benefit, under 35 U.S.C. §119(e), of co-pending provisional application No. 60/617,346, filed Oct. 8, 2004, the disclosure of which is incorporated herein by reference.
Number | Name | Date | Kind |
---|---|---|---|
6012873 | Copple et al. | Jan 2000 | A |
6092483 | Allen et al. | Jul 2000 | A |
6227137 | Allen et al. | May 2001 | B1 |
6263824 | Balint et al. | Jul 2001 | B1 |
6666624 | Wetch | Dec 2003 | B2 |
6786679 | Huang et al. | Sep 2004 | B2 |
6968797 | Persson | Nov 2005 | B2 |
20020154954 | Huang et al. | Oct 2002 | A1 |
20030031517 | Welch | Feb 2003 | A1 |
Number | Date | Country | |
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20060075953 A1 | Apr 2006 | US |
Number | Date | Country | |
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60617346 | Oct 2004 | US |