Method and Apparatus for Corrosion Allowance Mitigation

Abstract

A design and construction method reduces the amount of structural material (e.g., steel) required when applying the corrosion allowance to the design of floating offshore structures. The (hull) structural elements involved are typically flat or curved panels where at least one side is wet, e.g.; inside a ballast tank or exposed to seawater. The method minimizes the area to which the largest corrosion allowance is applied. One principle of this method is to have a maximum of one wet side for each hull watertight plating element. The stiffening of this hull structural element is applied to the dry side, i.e., the side that requires the lesser amount of corrosion allowance. Practice of the method typically results in a hull design wherein ballast tanks do not share a common structural element with either another ballast tank or the hull external shell.

What is claimed is: 
     
         1 . A ballast tank for a floating vessel comprising:
 a first, interior, wetted surface substantially devoid of structural reinforcements; and,   an opposing, second, exterior non-wetted surface having one or more structural reinforcements.   
     
     
         2 . The ballast tank recited in  claim 1  wherein the one or more structural reinforcements are selected from the group consisting of stiffeners, girders, gussets and bulkheads. 
     
     
         3 . A method of fabricating a ballast tank for use on a floating vessel said ballast tank having a wet surface that is in contact with seawater when the floating vessel is used in normal operations and a dry surface that is not in contact with seawater when the floating vessel is used in normal operations comprising:
 attaching first structural reinforcements to the wet surface, said first structural reinforcements having a first corrosion allowance; and,   attaching second structural reinforcements to the dry surface, said second structural reinforcements having a second corrosion allowance that is less than said first corrosion allowance.   
     
     
         4 . The method recited in  claim 4  wherein the structural reinforcements are selected from the group consisting of stiffeners, girders, gussets and bulkheads. 
     
     
         5 . A floating, offshore vessel comprising:
 a ballast tank;   a first hull shell substantially covering an external face of a first side of the ballast tank;   a second hull shell substantially covering a second side of the ballast tank;   a third hull shell substantially covering a bottom surface of the ballast tank;   wherein the ballast tank comprises
 a first, wetted surface substantially devoid of structural reinforcements; and, 
 an opposing, second, non-wetted surface having one or more structural reinforcements. 
   
     
     
         6 . The floating, offshore vessel recited in  claim 5  wherein the one or more structural reinforcements are selected from the group consisting of stiffeners, girders, gussets and bulkheads. 
     
     
         7 . The floating, offshore vessel recited in  claim 5  further comprising
 a fourth hull shell substantially covering an external face of a third side of the ballast tank; and, 
 a fifth hull shell substantially covering an external face of a fourth side of the ballast tank. 
 
     
     
         8 . The floating, offshore vessel recited in  claim 5  wherein the offshore vessel is a tension leg platform having a plurality of buoyant columns and the ballast tank is located within a column. 
     
     
         9 . The floating, offshore vessel recited in  claim 5  wherein the offshore vessel is a semi-submersible vessel having a plurality of buoyant columns and the ballast tank is located within a column. 
     
     
         10 . The floating, offshore vessel recited in  claim 5  wherein the offshore vessel is a tension leg platform having at least a pair of buoyant columns connected by a buoyant pontoon at least a portion of which comprises the third hull shell substantially covering the bottom surface of the ballast tank. 
     
     
         11 . The floating, offshore vessel recited in  claim 5  wherein the offshore vessel is a semi-submersible vessel having at least a pair of buoyant columns connected by a buoyant pontoon at least a portion of which comprises the third hull shell substantially covering the bottom surface of the ballast tank. 
     
     
         12 . The floating, offshore vessel recited in  claim 5  wherein the first hull shell substantially covering a first side of the ballast tank is comprised of an upper section and a lower section having at least one wall separating the lower section from the upper section. 
     
     
         13 . The floating, offshore vessel recited in  claim 5  wherein the first hull shell substantially covering a first side of the ballast tank is comprised of an upper section and a lower section having at least one wall separating the lower section from the upper section and,
 the second hull shell substantially covering a second side of the ballast tank is comprised of an upper section and a lower section having at least one wall separating the lower section from the upper section. 
 
     
     
         14 . The floating, offshore vessel recited in  claim 5  wherein the external surfaces of the ballast tank are not in contact with seawater when the vessel is used in normal operations.