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, wetted surface substantially devoid of structural reinforcements; and,   an opposing, second, non-wetted surface having one or more structural reinforcements.   
     
     
         2 . A ballast tank as 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 designing a tank for use on a floating vessel comprising:
 applying a first, greater corrosion allowance to structural reinforcements attached to walls of the tank that may be in contact with seawater when the floating vessel is used in normal operations; and,   applying a second, lesser corrosion allowance to structural reinforcements attached to walls of the tank that are not in contact with seawater when the floating vessel is used in normal operations.   
     
     
         4 . A method as 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 surrounding a first side of the ballast tank;   a second hull shell substantially surrounding 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 . A floating, offshore vessel as recited in  claim 5  wherein the one or more structural reinforcements are selected from the group consisting of stiffeners, girders, gussets and bulkheads. 
     
     
         7 . A floating, offshore vessel as recited in  claim 5  further comprising
 a fourth hull shell substantially surrounding a third side of the ballast tank; and, 
 a fifth hull shell substantially surrounding a fourth side of the ballast tank. 
 
     
     
         8 . A floating, offshore vessel as recited in  claim 5  further comprising
 a compartment adjacent to the ballast tank having at least one hatch sized and spaced for worker ingress, at least one flight of stairs and at least one inspection port. 
 
     
     
         9 . A floating, offshore vessel as recited in  claim 8  wherein the inspection port comprises an opening into the ballast tank. 
     
     
         10 . A floating, offshore vessel as recited in  claim 5  further comprising
 a compartment adjacent to the ballast tank having at least one hatch sized and spaced for worker ingress, at least one internal structure for climbing up and down and at least one inspection port. 
 
     
     
         11 . A floating, offshore vessel as recited in  claim 10  wherein the at least one internal structure for climbing up and down is a ladder. 
     
     
         12 . A floating, offshore vessel as 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. 
     
     
         13 . A floating, offshore vessel as 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. 
     
     
         14 . A floating, offshore vessel as recited in  claim 5  wherein the offshore vessel is a tension leg platform having a plurality of buoyant columns connected by buoyant pontoons and the bottom of the ballast tank is substantially covered by portions of one or more pontoons. 
     
     
         15 . A floating, offshore vessel as recited in  claim 5  wherein the offshore vessel is a semi-submersible vessel having a plurality of buoyant columns connected by buoyant pontoons and the bottom of the ballast tank is substantially covered by portions of one or more pontoons. 
     
     
         16 . A floating, offshore vessel as recited in  claim 5  wherein the first hull shell substantially surrounding 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. 
     
     
         17 . A floating, offshore vessel as recited in  claim 5  wherein the first hull shell substantially surrounding 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 surrounding 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. 
 
     
     
         18 . A floating, offshore vessel as recited in  claim 5  wherein the external surfaces of the ballast tank are not in contact with seawater when the vessel is being used in normal operations.