Investigation Of Ships' Corrosion Related Static Electric Field Using FEM-BEM Hybrid Method

Corrosion-related static electric field is one of ships' important non-acoustic characteristics.The establishment of simulation model that can handle both internal and external electromagnetic field is the key to this study.To assess the calculation error of Boundery Element Method's(BEM)"equipotential hypothesis",but also to study the underwater spatial distribution of corrosion related static electrtic field within the life-cycle of ships' combined anti-corrosion system,analysis model based on the coupling of Finite Element Method(FEM)and BEM is proposed on the basis of internal and external field's features,and the simulation program is completed and validated,that provides simulation methods for the relevant research.The major contents are summarized as follows:Firstly,analysis model based on FEM-BEM hybrid method is established through the introduction of coupling electrochemical boundary condition at the underwater structure surface and the simulation program is completed.The reasonability of parameter settings and the correctness of the method on the simulation of two typical corrosion/anti-corrosion working conditions: bimetallic corrosion and impressed current cathodic protection(ICCP)are verified through the comparisons with BEM results.Secondly,the control parameters of ships' corrosion related static electric field are combed out and the modeling method is given.The validity of the method is verified by comparing with the results of the model experiment.Finally,a ship's ICCP system design plan is taken as research object,the corrosion related static electric field that under typical working conditions within the life-cycle of the ship's combined anti-corrosion system is studied.Results show that,the static electric field strength that at the end of combined anti-corrosion system's life-cycle is stronger than that at initial state during normal working hours of the ICCP system,the breakdown of coatings at local area will affect the field strength.The electric field distribution is basically consistent within the life-cycle when the ICCP system is shut down,which is a viable wartime measure.The decay rate of electric field's vertical part is bigger than that of horizontal ones at the distance of more than three times of ship length away.The change of resistance has a significant effect on the strength and distribution of underwater electric field when the shaft grounding system failed.The calculation errors of BEM's "equipotential hypothesis" are closely related to the coating state on the electrode surface and the internal voltage differencebetween electrodes.It is feasible by changing the single zone ICCP system's control potential to a certain value to control the electric field characteristics at wartime and can achieve better performance than shut down the system.