Subsea structure and method for installing the structure and recovering the structure from the sea floor

A method for maneuvering large subsea structures comprising the steps of:

• arranging said structure to float in water adjacent a drilling vessel;
• connecting lines between said vessel and said structure;
• keelhauling said structure to beneath said vessel;
• ballasting said structure to level it in the water;
• lowering said structure to the sea floor;
• geographically orienting said structure prior to placement on the sea floor;
• sinking piles through said structure to anchor it to the sea floor;
• cementing said piles in place;
• and
• leveling said structure.

A subsea structure comprising:

• a plurality of horizontal and vertical structural tubes arranged to provide support for subsea oil and/or gas drilling and production equipment;
• certain of said tubes being segregated to form compartmented ballast chambers capable of being selectively flooded or dewatered to achieve desired negative or positive buoyancies for said structure;
• certain other of said structural tubes forming pile sleeves;
• said horizontal tubes forming peripheral circumferential ring members as well as interstitial supports;
• and
• the uppermost of said ring members being larger than said other ring members to provide a large water plane area for floating stability and to provide, also, a high center of buoyancy for submerged stability;
• at least a part of said uppermost ring member extending outwardly of said other ring members to provide a fender to protect equipment surrounded by said uppermost ring member from damage by submerged objects.

A method for maneuvering large subsea structures comprising the steps of:

• arranging said structure to float in water adjacent a drilling vessel;
• ballasting said structure to trim said structure level at the water surface;
• ballasting said structure to a negative buoyancy;
• keelhauling said structure to beneath said drilling vessel;
• lowering said structure to the sea floor;
• and
• orienting said structure geographically.

A method for maneuvering a large subsea structure comprising the steps of:

• arranging said structure to float in water adjacent a drilling vessel;
• connecting lines between said vessel and said structure;
• keelhauling said structure to beneath said vessel;
• ballasting said structure to level it in the water;
• lowering said structure to the sea floor;
• geographically orienting said structure prior to placement of it on the sea floor;
• sinking piles through said structure to anchor it to the sea floor;
• cementing said piles in place;
• leveling said structure;
• said structure including cylindrical pile sleeves located at peripheral points on said structure through which said piles are sunk;
• the steps of cementing said piles in place and leveling said structure including:
• lowering a drill string containing a drill bit through the lowest one of said pile sleeves and drilling a pile hole to a desired depth;
• removing said drill string and drill bit to said vessel;
• lowering a first pipe containing slips into said pile hole through said pile sleeve and engaging said slips with the inner wall of said pile sleeve to permit upward movement of said pile sleeve but not downward movement thereof relative to said first pipe;
• pumping cement down said first pipe and up the annulus of said pile hole surrounding said first pipe until said cement is just below the bottom of said pile sleeve;
• removing the upper portion of said first pipe above said slips to said vessel;
• lowering a second pipe containing a lifting tool into said pile sleeve and engaging the lifting tool with said pile sleeve;
• pulling up on said second pipe to raise said pile sleeve and thereby said structure until said structure is as near level as can be achieved;
• removing said lifting tool and said second pipe;
• repeating the above cementing and leveling operations on each of said other pile sleeves until said structure is level;
• and
• pumping cement through the second pipe into the top of each of said pile sleeves until cement fills said pile sleeves.