The Optimization Of Hull Cathodic Protection And Control Of Corrosion Related Electromagnetic Field Under Seawater

Coating and cathodic protecton are generally applied to anti-corrosion of hull, the later can induce corrosion related electromagnetic signatures under seawater. The optimization of impressed current cathodic protection (ICCP) system design is important in order to achieve uniform potential profile and minimum electromagnetic characteristics under seawater.The methods, including physical scale model (PSM) and numerical simulation (NS) are used to investigate optimization of cathodic protection and minimum of the electromagnetic characteristics. The anodes position and current output are optimized as account for uniform potential distribution, minimum of static electric (SE) and corrosion related magnetic field (CRM) in the ICCP which composed of2and3group of anodes respectively. Not only the hull corrosion is effectively controlled, but also corrosion related electromagnetic signature is minimized by optimized ICCP.The four anodes are located on frame230and130(S&B) in depth2m under seawater for the ICCP where the Ag/AgCl reference electrode is arranged on frame186and potential controlled at-0.85V after the positions optimized. And then, the hull potential range from-0.80V to-1.00V (Ag/AgCl electrode, SSC in short) in sufficient protection while SE and CRM module are decreased41%and45%respectively after current output optimized. The SE and CRM are effectively controlled by the double optimizations.The six anodes are positioned on frame110,200and230(S&B) in depth2m under seawater after the positions optimized for the ICCP composed of a triple of anode groups and current controlled in order to minimize SE peaks. Secondly, three piece of anodes belonging to the triple of anode groups are adjusted to current output1.82A,1.65A and3.65A respectively after the current output optimized, and where hull is best protected with potential-0.80V～-0.90V (SSC). SE and CRM are effectively decreased72.7%and63.5%respectively after the double optimizations.The results of PSM and onboard demonstrated that extremely low-frequency electromagnetic field (ELEF in short) signature under seawater is generated in condition of current flowing in shaft and the resistance of shaft to hull being modulated as shaft rotation. ELEF can be totally eliminated by the active shaft grounding (ASG) that equivalent resistance less than1.37×10-5O。It is developed that the transformer-rectifier (TR) with ripple less than0.1%,1/50of commercial equipments, can significantly reduce industrial frequency electromagnetic signature. The ASG and the TR are integrated to be a cathodic protection and corrosion electromagnetic controller (CPEM) based on IGBT with module design and volume/weight decreasing. The two modules can ordinate in function and controlling.SE and CRM are rapidly attenuated nearly around hull according to E(B)=a+brc(a, b, c being constant, where c<0) while ship’s potential controlled, and E=d/r2(d being constant) as anodes current controlled.The potential distributions and protection current of hull extremely depend on coating damage. The optimized ICCP which composed of4anodes are achieved by PSM and uniform test arrangement methods in coating damage ratio from2%to10%. The protection current is increasing with coating damage ratio, linear increasing in reasonable anode positions and even potential distributions, and no-linear increasing in fore anodes located at frame30.BP neural network method is used to predict the influence of anode position and given potential on even value of potential profile.