Method and apparatus for conversion of semi-submersible platform to tension leg platform for conducting offshore well operations

In a mobile offshore marine platform provided with means for temporarily anchoring said platform in semsubmersible catenary anchor mode and at a selected draft for conducting exploratory drilling, and provided with means for permanently anchoring said platform in tension leg mode for conducting production activities, the provision of:

• means for anchoring said platform in semisubmersible catenary anchor mode and at a selected draft;
• means for converting said platform from semisubmersible catenary anchor mode to a platform in tension leg mode;
• and including mass trap means operable in semisubmersible mode and inoperable in tension leg mode,
• said converting means including anchor pile guide means carried by said platform and adapted to be lowered to the sea floor;
• means on said platform for installing anchor members in said guide means by laterally shifting said platform in catenary anchor mode;
• and including anchor setting means operable thru said guide means;
• tension leg means interconnecting said anchor members and said platform;
• means for uniformly tensioning said tension leg means;
• means for releasing said platform from said catenary anchor mode;
• and means for positioning said platform at a selected draft in tension leg mode.

In a method of conducting offshore well operations including exploration, drilling, and production from a single floatable vessel provided with drilling and production equipment on its deck comprising the steps of:

• towing said vessel to an ocean well site;
• mooring said vessel in a catenary mooring mode over said site with temporary anchor means;
• conducting exploration operations from said vessel at said site to determine production capabilities of said well site;
• setting anchor means at selected anchor locations beneath said vessel while said vessel is in a catenary moored mode;
• shifting said vessel in catenary anchor mode for drilling at each anchor means to set anchor members;
• connecting a tension leg means to said anchor means and to the vessel;
• tensioning said tension leg means to a selected tension;
• releasing said vessel from said catenary mooring mode;
• and tensioning said tension leg means for production operation in taut mode.

In a marine platform construction adapted for operation in semi-submersible mode and tension leg mode, the combination of:

• means including buoyant members having displacement relationships tending to neutralize vertical forces and to stabilize the platform in heave, pitch, and roll,
• said buoyant members including horizontal buoyant members and vertical buoyant members connected to said horizontal buoyant members and adapted to support a platform deck,
• and virtual mass trap means associated with said vertical buoyant members in spaced relation to said horizontal buoyant members adapted to minimize heave response when said platform is operated in semi-submersible mode,
• said mass trap means being rendered inoperable when said platform is operated in tension leg mode.

In a marine platform construction adapted for operation in semi-submersible mode and tension leg mode, the combination of:

• means including buoyant members having displacement relationships tending to neutralize vertical forces and to stabilize the platform in heave, pitch, and rol;
• virtual mass trap means associated with certain buoyant members adapted to minimize heave response when said platform is operated in semi-submersible mode,
• said mass trap means being rendered inoperable when said platform is operated in tension leg mode;
• said virtual mass trap means including plate-like members between said certain buoyant members;
• means for mounting said plate-like members on said certain members;
• and wherein said mounting means is adapted to receive said plate-like members in a vertical direction, said plate-like members being provided neutral buoyance during approach to landing surfaces on said mounting members and being provided slightly negative buoyancy upon landing thereon.

In a method of conducting offshore well operations including exploration, drilling, and production from a single floatable vessel provided with drilling and production equipment on its deck, comprising the steps of:

• towing said vessel to an ocean well site;
• mooring said vessel in a catenary mooring mode over said side with temporary anchor means;
• conducting exploration operations from said vessel at said site to determine production capabilities of said well site;
• setting anchor means at selected anchor locations beneath said vessel while said vessel is in a catenary moored mode;
• shifting said vessel in catenary anchor mode for drilling at each anchor means to set anchor members;
• connecting a tension leg means to said anchor means and to the vessel;
• tensioning said tension leg means to a selected tension;
• releasing said vessel from said catenary mooring mode;
• tensioning said tension leg means for production operation in taut mode;
• providing said vessel with horizontal buoyant members having a cross-sectional area aspect ratio greater than that provided for in an optimum tension leg platform construction;
• and
• providing virtual mass trap members in spaced relation above said horizontal buoyant members.

In a marine platform construction, the combination of:

• a platform deck
• a plurality of submergible buoyant members supporting said platform deck;
• the displacement ratio of said buoyant members tending to neutralize vertical forces acting on the platform construction when in tension leg mode which includes
• tension legs extending between said buoyant members and leg anchor means in the sea floor;
• virtual mass trap means extending horizontally, positioned below said deck, and associated with said buoyant members;
• said virtual mass trap means including mass trap members having relatively large horizontal flat surface areas operative as mass trap means when said platform construction is operated without said tension leg in semi-submersible mode which includes
• catenary anchor lines extending between said platform structure and said sea floor;
• and means for mounting said mass trap members for rendering said mass trap members inoperable when said platform construction is in tension leg mode.