Floating offshore drilling/producing structure

Abstract

A floating offshore drilling/producing structure. The structure is formed from a plurality of closely spaced vertically oriented buoyant columns on which one or more modules or decks may be placed to support process equipment, a drilling rig, utilities, and accommodations for personnel. The columns are held in the spaced relationship by a plurality of horizontal plates spaced along the length of the columns and vertical plates located near the bottom of the columns and near the top of the columns. Drilling and/or producing is accomplished through risers located approximately in the center of the structure. The structure includes fixed ballast, an oil storage area, and voids and variable ballast for offsetting the lighter weight of the stored oil. The columns have a smaller water plane area than the horizontal plates.

What is claimed as invention is:  
     
         1 . A floating offshore drilling/producing structure, comprising: 
 a. a plurality of vertically oriented buoyant columns in closely spaced arrangement adjacent each other;    b. a plurality of horizontal plates attached to said columns and spaced apart along the length of said columns, said horizontal plates being located below the water level at a distance below significant wave energy; and    c. at least two vertical plates attached between said columns and spaced apart along the length of said columns, with one of said vertical plates positioned adjacent the lower end of said columns and the second vertical plate being positioned adjacent the upper portion of said columns.    
     
     
         2 . The structure of    claim 1   , wherein the spacing between said columns is approximately one tenth to three times the diameter of the columns.  
     
     
         3 . The structure of    claim 1   , wherein the center of gravity is below the center of buoyancy.  
     
     
         4 . The structure of    claim 1   , wherein said columns include variable ballast means.  
     
     
         5 . In an offshore structure designed to drill for and produce hydrocarbons and having a riser independently supported by a buoyancy module, the improvement comprising a sleeve rigidly attached to the offshore structure and received around the riser and buoyancy module, said sleeve being open at both ends and extending from above the water line downward to a depth beyond significant wave energy.  
     
     
         6 . The sleeve of    claim 5   , further comprising: 
 a. a first plate rigidly attached to the inside of said sleeve below the water line and closely received around the riser;    b. a second plate rigidly attached to the riser above the buoyancy module and closely received inside said sleeve; and    c. means on said first plate for controlling the rate of change in water volume between said plates in response to movement of the buoyancy module.