Patent ID: 8764346

Claim:
A floating tension based tension leg platform system for use in offshore deepwater applications in a body of water to support dry-tree oil and gas production, comprising: a buoyant ballastable tension base structure submerged a selected distance below the surface of the body of water, said tension base structure connected to a foundation secured on the sea floor and maintained in tension a distance thereabove by a plurality of tensionable lower tendons each attached at an upper end to said tension base and at a lower end to said foundation such that said tension base structure functions as an artificial seabed, said tension base structure closed on the perimeter sides defining an open section for passage therethrough of production risers having a lower end connected to wells on the sea floor; a floating tension leg platform including a hull having a plurality of spaced apart buoyant vertical columns, a deck structure at an upper end of said vertical columns having an opening therethrough, an open framework of horizontal spaced truss pontoon members interconnecting lower ends of said vertical columns together in spaced apart relation, each of said vertical columns having a submerged portion and a non-submerged portion and one or more buoyant tanks or chambers disposed inside said each of said vertical columns or enclosed by bulkheads including ballast control means for selectively adjusting the buoyancy thereof, said floating tension leg platform connected to said submerged tension base structure and maintained in tension a distance thereabove by a plurality of tensionable upper tendons each attached at an upper end to a lower portion of a respective one of said vertical columns and at a lower end to said tension base structure such that said tension base structure functioning as an artificial seabed provides vertical and horizontal resistance to said upper tendons attached to said platform, and resists platform heaves due to wave forces; and an elongate buoyant vertical riser support tower of tubular truss frame construction floating independently of said tension leg platform hull and extending movably through said opening in said floating tension leg platform deck structure to extend above and below said deck structure, said buoyant vertical riser support tower having vertically spaced horizontal riser support members along the length of said tower to receive, secure, surround, and support upper portions of the production risers and support the riser weight along the length of said tower; said independently floating buoyant vertical riser support tower movably coupled with said tension leg platform to allow axial vertical movement of said tower and said upper portions of production risers secured along the length thereof as a unit relative to said tension leg platform while preventing relative horizontal movement therebetween such that said tension leg platform is free of vertical riser loads and vortex induced vibration fatigue of said supported risers.