Ballasting offshore platform with buoy assistance

A method for deploying a floating platform includes storing buoys on a hull of the platform. Tension devices are mounted to the hull, each being connected by a line to one of the buoys. The operator tows the hull to a ballast down site while the buoys are stored on the hull. While adding ballast to the hull, the operator feeds out the lines from the tension devices at a selected tension. The hull moves downward in the water while the buoys float at the surface to maintain stability during the ballasting. The buoys are detached from the lines after the hull is ballasted to a desired depth.

FIELD OF THE INVENTION

This invention relates in general to floating offshore oil and gas platforms, and in particular to a method for ballasting platform while using buoys for assistance.

BACKGROUND OF THE INVENTION

Offshore floating platforms are utilized for hydrocarbon extraction and processing. The platforms have tanks that provide the necessary floatation. Water is pumped into at least some of the tanks to provide ballast for positioning the platform at a desired draft. A certain amount of draft may be necessary to prevent capsizing under the effects of wind and waves during storms. The desired draft might be needed both for towing to a well site as well as while stationed at the well site.

Typically, when a platform is being ballasted to the desired draft, it will undergo a region of instability between the initial draft and the desired draft. While in the region of instability, the righting moment of the platform is insufficient to keep the platform upright if it heels excessively. The ballasting must be carefully controlled while in the region of instability to avoid a catastrophe.

Some platforms have a very deep draft, which may be hundreds of feet. Typically, these platforms have a single cylindrical column and may be called “spars” or “deep draft caisson vessels”. Normally, a single column hull is towed to the well site while in a horizontal position, then ballasted to an upright position. These vessels also undergo a region of instability, thus upending the structure at the well site has associated risks. After being upended and ballasted to the desired depth, a catenary mooring system is used to hold the vessel at the well site. A large barge and crane at the well site lifts a deck structure onto the spar after it is at the desired draft and moored.

U.S. Pat. No. 6,371,697 discloses a single column floater that has a larger diameter lower section to provide stability and buoyancy. This patent discloses towing the single column floater to the well site in an upright position. The vessel is towed to the well site at a towing draft, then ballasted at the well site to a desired draft. A catenary mooring system holds the single column floater on station. The deck and structure may be placed on the single column floater while at the dockside, avoiding a need for a barge and crane at the well site. Even though ballasting occurs while the vessel is upright, instability can still exist during the process.

One proposed method to provide stability during ballasting deals specifically with tension leg platforms (“TLP”). A TLP is not moored with a catenary mooring system, rather it is held on station by tendons under tension. The tendons comprise hollow, buoyant strings of pipe extending vertically upward from the sea floor to the platform. Normally the TLP is towed to the well site at a first draft, then ballasted to a second draft. The operator connects the tendons to the TLP and removes ballast to place the tendons in tension. U.S. patent application Publication 2004/0190999 discloses connecting pull-down lines between upper ends of the tendons and pull down devices on the platform. The operator applies tension to the pull-down lines while ballasting to avoid instability. When the tops of the tendons pass through the top terminations on the platform, the operator connects the tendons to the platform, removes the pull-down lines, and deballasts until the desired tension in the tendons is reached.

SUMMARY OF THE INVENTION

In this invention, at least one tension device is mounted to a hull of the platform. A line extends from the tension device to a buoy. While adding ballast to the hull, the operator feeds out the line from the tension device and maintains a desired tension in the line. The buoy provides stability to the hull as the hull passes through a zone of instability while being ballasted. After passing through the zone of instability, the operator may detach the buoy from the tension device.

In one embodiment of the invention, while at the dockside, a number of the buoys are stored on supports attached to the perimeter of the hull. The operator tows the hull to a ballast down site while the buoys are located on the supports. The operator ballasts the hull to a safe towing draft at the ballast down site, using the buoys to provide stability as it passes through the region of instability. The operator removes the buoys and tows the hull at the towing draft to a well site. At the well site, the operator adds more ballast to reach a desired operating draft. The buoys are not required while at the towing draft or while adding more draft at the well site. The operator moors the hull with a conventional system.

In one embodiment, the hull is a single column type, and catenary mooring is used. Alternately, the hull may be a tension leg platform using pontoons and columns. Tendons are used to anchor the hull.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, platform11is a floating vessel that is used particularly for oil and gas well drilling and production. In this embodiment, platform11has a single elongated hull or column13as shown in U.S. Pat. No. 6,503,023, but it could have a plurality of columns and be of different designs, such as a tension leg platform. Column13has a cylindrical base15of a larger diameter than column13. Column13and base15have a plurality of compartments17that may be sealed from each other for ballasting platform11to a desired depth. A central passage19extends axially within column13. Production and/or drilling risers (not shown) are typically supported by platform11at the well site and pass through central passage19. If platform11is serving as a tender vessel to a production and drilling platform, typically the lower end of central passage19would be closed. One or more decks21are mounted to column13of platform11for supporting drilling and/or production equipment.

Platform11has a plurality of supports or outriggers23spaced around its perimeter. A buoy25is shown inFIG. 1temporarily resting on each outrigger23. Buoy25is a buoyant, airtight member that may be cylindrical, spherical or other shapes. The number of buoys25depends upon their size and the size of platform11. Buoy25could comprise a single tank that surrounds at least a portion of column13or it could be made up of segments that releasably attach to each other to form an annular shape, as described in U.S. Pat. No. 6,786,679.

Each buoy25is attached to a line27that leads through a lower line guide28on outrigger23(FIGS. 2,3) to a lifting or tension device29. Line27may comprise chain, cable or rope. Tension device29may be a winch, chain jack, strain jack, rotating block or other means of applying tension to lines27. Tension devices29are preferably located at the top of column13.

During manufacturing, deck21may be installed while platform11is located beside a dock, or it could be installed at an offshore site. Platform11is designed to be towed to a well site while in a vertical orientation. In the event of storms, platform11has a towing draft deeper than the dockside draft to avoid heeling excessively in high winds. Normally, the water alongside the dock is not deep enough to ballast platform11to its safe towing draft. Depending upon the size of platform11, the water may need to be 200 to 500 feet in depth to accommodate the towing draft.

In this invention, while platform11is at a first or dockside draft, a tug31will tow platform11out to a water depth that is sufficient for it to be ballasted to its safe towing draft. Buoys25will preferably be stored on supports23while being towed from the dock side. Supports23are located near the lower end of column13. Preferably, buoys25are partially submerged while column13is being towed to the staging site. Also, buoys25will be temporarily fastened to supports23by fasteners (not shown) that are readily releasable. The fasteners could be a variety of devices, such as straps or latches.

Once at the staging site, the operator releases the fasteners that hold buoys25on supports23and begins admitting ballast water to compartments17. As column11lowers in the water, the operator feeds out lines27with tension devices29. Buoys25lift upward from outriggers23as vessel11moves downward. The operator determines a tension that is desired for each of the lines27and controls the rate of addition of water ballast and the rate at which tension devices29feed out line27in order to maintain that desired tension. As platform11moves downward, buoys25provide additional stability necessary for platform11by maintaining a positive righting arm through its region of instability. Once platform11is at a sufficient draft to be stable, buoys25may be removed. Tug31tows platform11to a desired well site at its safe towing draft without buoys25.

When at the well site, the operator normally ballasts platform11further to a desired installation draft. In this embodiment, catenary mooring lines33are attached to anchors or pilings35to maintain platform11at the desired location. With a catenary mooring system, the lines extend in long gradual curves to anchors or pilings imbedded in the sea floor outside the perimeter of vessel11. Other types of platforms may require tendons to be placed under tension rather than catenary lines33.

The invention has significant advantages. The buoys and tension devices provide stability when ballasting the vessel to towing and installation drafts. The buoys are readily removable after installation and may be re-used.

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.