Fully Constraint Platform in Deepwater

Abstract

A method and apparatus for securing a buoyant, surface structure in deepwater, such as those used in offshore oil and gas drilling and producing, is disclosed. The surface structure is water surface piercing or fully submerged. Buoyancy tanks inside the surface structure are used as liquid vibration dampers to enhance the motion performance. All tethers are sufficiently taut and some or all of them are inclined. The attachment points of the tethers on the surface structure are at same or different elevations. An alternative attachment method, named as “single tether-multiple attachment leg”, is also disclosed. The tether system provides stiffness in both vertical and horizontal direction so that all six degrees-of-freedom motions of the structure, namely, surge, sway, heave, roll, pitch, and yaw are constrained with stiffness. This fully constraint platform (FCP) has minimum motion in winds, currents, swells, and surface and internal waves, and therefore has many practical uses, as outlined herein.

1 . A fully constraint platform in deepwater comprising:
 (a) a buoyant, surface structure, consisting of a single structure or a plurality of such structures in various orientations each of which is a hollow column, with the to center of the formed buoyant, surface structure either solid or hollow to serve as a moon pool, and with deck(s) placed on top of the buoyant, surface structure.   (b) a tether system, consisting of a plurality of tethers, to attach to the said surface structure, each of which is sufficiently taut and has an angle of inclination from 0 degree to approximately 45 degrees from vertical.   (c) a foundation system with anchors to secure each of the tethers, comprising driven piles or suction piles or gravity based weights.   
     
     
         2 . The system of  claim 1 (a), further being water surface piercing or completely submerged, wherein in the latter case, a truss structure is placed on top of the buoyant, surface structure, to support deck(s) along with other functional structures and equipment. 
     
     
         3 . The system of  claim 1 (a), wherein each single structure is further partitioned into one or more buoyancy tanks filled with fluids to function as liquid vibration dampers and as means to balance the tension in the tethers. 
     
     
         4 . The system of  claim 1 (b), wherein the attachment points of tethers to the buoyant, surface structure are at one or more elevations. 
     
     
         5 . The system of  claim 1 (b), wherein some or all of the attachment points are either outside of or inside of the water. 
     
     
         6 . The system of  claim 1 (b), wherein each attachment point to the surface structure is a steel block (a porch) which is separated from and welded to the main structure. 
     
     
         7 . The system of  claim 1 (b), wherein the tethers are made from carbon fiber or polyester or steel. 
     
     
         8 . The system of  claim 1 (b), wherein the tethers are a chain or a rope or a tube or a combination of segments made from a chain or a rope or a tube. 
     
     
         9 . The system of  claim 1 (b), wherein when the tethers are entirely tubular, is also a conduit to transfer fluids or contains power, fluid and communication cables. 
     
     
         10 . The system of  claim 1 (b), wherein one or more tethers have single or multiple attachment legs attached to the buoyant, surface structure, which can further cascade. 
     
     
         11 . The system of  claim 1 (b), wherein further accompanying a method to uniformly distribute tethers in the horizontal plane, and a method to distribute the tethers in the vertical plane, as described in  FIG. 8  and  FIG. 9 , respectively. 
     
     
         12 . The system of  claim 1 (b), wherein for the system of  1 ( a ) with distinct corners, the tethers are divided evenly into a number of small tether groups, with the number of groups matching the number of corners of the said structure. 
     
     
         13 . The system of  claim 1  is used as:
 (a) a stand alone offshore drilling platform with a tender vessel assisting, or 
 (b) an offshore production platform with or without drilling and/or well servicing (work-over) capability, or 
 (c) an anchored platform in product offloading and other offshore operations, which involves more than one structures adjacent to one another, or 
 (d) an offshore work platform, or 
 (e) an offshore platform to support a wind or current turbine or a transmission line tower or a lighthouse, or 
 (f) an artificial island or a group of artificial islands, or 
 (g) a marine terminal or a floating harbor, or 
 (h) an ocean airport with multiple runways, or 
 (i) a host platform for a nuclear reactor.