Method and Apparatus for Offshore Drilling in Ice-Infested Waters

Abstract

An offshore structure for placement on a sea floor in a body of water containing moving ice masses, which impart Environmental Loads to the structure; the offshore structure includes a base which contacts the sea floor, the base having sufficient horizontal cross-sectional area such that a bearing pressure and a horizontal shear force, which result from Environmental Loads and which are transferred to the sea floor foundation by the foundation base, are less than at least one of a predetermined shear capacity and a predetermined bearing capacity for the sea floor foundation; and a hollow, watertight topside structure located at or above the waterline of the body which is supported by a support shaft extending vertically from the base through the topside structure.

1 . A method for utilizing over open water a drilling platform comprising a base, a hollow, watertight topside structure, and a support shaft extending vertically from the base through the topside structure, the method comprising the steps of: 
 (a) maintaining a positive buoyancy of the topside structure;    (b) lowering the base to the sea floor; and thereafter    (b) raising the topside structure along the support shaft until it reaches the top of the support shaft.    
   
   
       2 . The method of  claim 1 , wherein the topside structure is maintained at about the surface of the water during lowering step (b).  
   
   
       3 . The method of  claim 2 , further comprising the step of ballasting the base during lowering step (b).  
   
   
       4 . The method of  claim 3 , wherein the support shaft comprises a guiding tube.  
   
   
       5 . The method of  claim 4 , wherein a pile is installed through one or more guiding tube into the sea floor.  
   
   
       6 . A method for transiting over body of water a drilling platform having a hollow topside structure, a cylindrical support shaft and a base, the method comprising the steps of: 
 (a) lowering the topside structure to the surface of the body of water;    (b) maintaining positive buoyancy of the topside structure over the body of water; and    (c) raising the base along a longitudinal axis of the support shaft wherein the base is maintained below the surface of the open water.    
   
   
       7 . The method of  claim 8 , further comprising the step of deballasting the base.  
   
   
       8 . The method of  claim 7 , wherein the topside structure is watertight.  
   
   
       9 . An offshore structure for placement on a sea floor in a body of water containing moving ice masses, which impart Environmental Loads to the structure, comprising: 
 (a) a base which contacts the sea floor, the base having sufficient horizontal cross-sectional area such that a bearing pressure and a horizontal shear force, which result from Environmental Loads and which are transferred to the sea floor foundation by the foundation base, are less than at least one of a predetermined shear capacity and a predetermined bearing capacity for the sea floor foundation; and    (b) a hollow, watertight topside structure that is supported by a support shaft extending vertically from the base through the topside structure.    
   
   
       10 . The offshore structure of  claim 9 , wherein the topside structure is designed to maintain a positive buoyancy of the offshore structure in the body water.  
   
   
       11 . The offshore structure of  claim 10 , wherein the topside structure is moveable along a longitudinal axis of the support shaft.  
   
   
       12 . The offshore structure of  claim 11 , wherein a guiding tube is mounted along a surface of an inner diameter of the support shaft.