Semi-submersible multicolumn floating offshore platform
View Patent ↗A semi-submersible, multicolumn, deep draft, floating offshore oil and gas drilling and production platform comprises a floating hull having an adjustably buoyant base, a plurality of columns vertically upstanding from the base, and an equipment deck that is supported atop the columns when the platform is operationally deployed. Each of the columns comprises a cellular structure that includes a plurality of elongated tubes having a variety of cross-sectional shapes extending from the base to the top of the column. Each of the tubes defines one or more closed compartments. At least one of the compartments has a buoyancy that is fixed, and at least another one of the compartments has a buoyancy that is adjustable. The buoyancy of the compartments and the base can be controllably adjusted with pressurized air to provide a safer and less costly method for deploying the platform for offshore operations.
1. A hull for an offshore semi-submersible platform, comprising:
a base having an adjustable buoyancy; and,
a plurality of upstanding columns connected to the base, each column comprising a plurality of elongated, closely-spaced cylindrical tubes connected together by a plurality of vertically elongated webs to form a plurality of non-cylindrical interstitial tubes interspersed with the cylindrical tubes, each of the cylindrical tubes defining a plurality of vertically-arranged, substantially cylindrical compartments, and each of the interstitial tubes defining a plurality of vertically-arranged, non-cylindrical interstitial compartments;
wherein at least one of the cylindrical compartments has a buoyancy that is fixed and at least one of the interstitial compartments has a buoyancy that is adjustable.
2. The hull of claim 1 , wherein at least one of the webs is substantially planar.
3. The hull of claim 1 , wherein at least one of the webs is arcuate.
4. The hull of claim 1 , further comprising a horizontal bulkhead subdividing the cylindrical and interstitial tubes into cylindrical and interstitial compartments, respectively.
5. The hull of claim 1 , wherein the at least one adjustably buoyant compartment comprises:
an opening to ambient sea water at a lower end of the compartment;
means for introducing pressurized air into an upper end of the compartment; and,
means for venting the upper end of the compartment to atmospheric pressure.
6. The hull of claim 1 , wherein at least one of the cylindrical tubes comprises a shaft providing access to at least some of the compartments.
7. A method for deploying a floating deep-draft semi-submersible offshore platform, comprising:
providing a hull having an adjustably buoyant base and a plurality of upstanding, adjustably buoyant columns at an inshore water site;
de-ballasting the hull to a shallow draft configuration;
towing the hull in the shallow draft configuration to an intermediate site having deeper water that is shielded from wind and waves;
ballasting the hull such that top ends of the columns extend slightly above the surface of the water;
providing a deck supported by a buoyant deck barge;
transferring the weight of the deck from the barge to the top ends of the columns;
securing the deck to the columns to form the semi-submersible platform;
de-ballasting the hull to an intermediate draft configuration;
towing the platform to an operation site in the intermediate draft configuration;
ballasting the hull to an operational draft configuration; and,
anchoring the platform at the operational site with a mooring system.
8. The method of claim 7 , wherein transferring the weight of the deck from the barge to the top ends of the columns comprises:
floating the barge between the columns such that the deck is disposed over the top ends of the columns; and,
de-ballasting the columns such that the deck is lifted off the barge by the columns.
9. The method of claim 7 , wherein transferring the weight of the deck from the barge to the top ends of the columns comprises:
floating the barge between the columns such that the deck is disposed over the top ends of the columns; and,
ballasting the barge down such that the deck is lifted off the barge by the columns.
10. The method of claim 7 , wherein ballasting the hull comprises:
selecting a first set of ballast tanks in the base of the hull to be completely flooded with sea water ballast;
selecting a second set of ballast tanks in the base to be partially filled with sea water ballast;
pressurizing air in the second set of tanks to a pressure that is about the same as the hydrostatic pressure of the sea water at a depth equal to the height of the columns of the hull;
opening the first set of tanks to ambient sea water such that all air in the tanks is completely displaced with sea water;
opening the bottoms of the second set of tanks to ambient sea water; and,
venting the top of the second set of tanks to the atmosphere in a controlled manner.
11. The method of claim 7 , wherein the columns comprise adjustably buoyant compartments, and wherein de-ballasting the hull comprises connecting pressurized air in lower ones of the compartments to higher ones of the compartments.