Arctic offshore structure and installation method therefor

An arctic offshore platform, comprising:

• a deck structure mounted onto one or more independent support legs, each of said support legs being anchored to the seafloor at an offshore site, each of said support legs including:
• a substructure unit, said substructure unit having a base mounted around and attached to a lower portion of a central column, said central column having a lowermost end substantially flush with the bottom end of said base, said central column extending upwardly from said base to a position above a water line at said offshore site;
• at least one pile extending through said substructure unit into said seafloor to anchor said substructure unit to said seafloor;
• and
• a sloped-sided member supported on said substructure unit and extending upward to a position above said water line, said central column extending upwardly through said sloped-sided member.

An arctic offshore platform, comprising:

• a deck structure supported a spaced distance above the ocean surface by a plurality of support legs, each of said support legs including:
• a base element having a bottom surface, an upper surface at a position intermediate said ocean bottom and said ocean surface, and a wall portion joining said upper and lower surfaces, said base element defining a recess extending vertically through said base element;
• a central column having a lower end secured within said base element recess and an upper end projecting upward from said base element to a location a spaced distance above the ocean surface, said central column projecting downward through said base element to a position no deeper than said base element bottom surface;
• and
• a sloped-sided member supported on said base element, said sloped-sided member having a lower end, an upper end and sloped walls joining said lower and upper ends, said sloped-sided member defining a recess extending vertically through said sloped-sided member, said recess being sized to permit said central column to extend therethrough, said sloped-sided member being fabricated as a module separate from said base and central column whereby said sloped-sided member can be positioned over said central column and lowered onto said base element following installation of said base element and said central column.

A method of constructing and installing a support leg for an arctic offshore platform at an offshore site, comprising the steps of:

• fabricating a substructure unit at an onshore location, said substructure unit including a base mounted around and attached to a lower portion of a central column, said central column extending upwardly from said base;
• constructing a separate sloped-sided member for said substructure unit at said onshore location;
• transporting said substructure unit and said sloped-sided member to said offshore site;
• submerging said substructure unit so that said base rests on the seafloor of, and said central column extends above a water line at said offshore site;
• extending at least one pile through said substructure unit to anchor said substructure unit to said seafloor;
• and
• placing said sloped-sided member upon said substructure unit, wherein said sloped-sided member extends above said water line, and wherein said central column of said substructure unit extends upwardly through said sloped-sided member.

A method of constructing and intalling an arctic offshore platform at an installation site, said method comprising the steps of:

• (a) installing at least three support leg substructure units at said installation site, each of said substructure units including a base element resting on the ocean bottom and a central column extending upward from said base to a position a spaced distance above the ocean surface, said support leg substructure units being spaced a sufficient distance from one another that a barge can be floated to a preselected position bounded on at least two opposite sides by said supporting substructure units;
• (b) constructing a plurality of slope-sided members each defining a central aperture adapted for receiving a corresponding one of said central columns;
• (c) transporting said sloped-sided members to said installation site;
• (d) placing each of said sloped-sided members upon a corresponding one of said base elements;
• (e) transporting a deck to said installation site on said barge;
• (f) positioning said barge between said support leg substructure units;
• and
• (g) placing said deck atop said central columns.

A method of constructing and installing an arctic offshore structure at an offshore installation site, comprising the steps of:

• fabricating a single substructure unit at a fabrication yard, said substructure unit including a base mounted around and attached to a lower portion of a central column, said central column extending upwardly from said base;
• fabricating a separate sloped-sided member adapted to be placed upon said substructure unit with said central column extending through a central aperture of said sloped-sided member;
• transporting said substructure unit to said offshore installation site and submerging said substructure unit at said offshore installation site so that said base rests on the seafloor and said central column extends above the water line;
• securing said substructure unit to the seafloor with piles extending into the seafloor through said base;
• transporting said sloped-sided member to the offshore installation site and positioning it on said substructure unit after said substructure unit has been secured to the ocean bottom with piles;
• and
• placing a deck structure atop said central column.