Abstract
As the offshore production of oil and gas shifts to deeper and colder waters throughout the world, the design criteria for cathodic protection systems developed from the broad experience base in shallow water in the Gulf of Mexico become less reliable. Several factors that increase the amount of cathodic protection required to control corrosion change with water depth and temperature. The lower-temperature waters found in the North Sea, Australia, and at great depths have higher resistance, which decreases the current output from the anodes. The solubility of calcium carbonate, one of the principal constituents of the calcareous coating, also increases with decreasing temperature. This increase in calcium carbonate solubility reduces the insulating qualities of the coating and thus increases the amount of current required to achieve protection. The rate at which a structure polarizes and the degree of polarization that is ultimately achieved are also adversely affected by wave action and the dissolved-oxygen content of the water. Each of these factors also serves to increase the extra number of anodes required to provide complete protection to the structure. To compensate for these environmental changes, we have developed a program that allows the anode shape, size, and core diameter to vary in order to optimize the number and size of the nodes needed for protection. The structure weight and the construction cost associated with anode installation are minimized by using these optimized anodes.
