摘要
水下船体的防腐蚀采用涂层和阴极保护技术,阴极保护技术涉及到防腐蚀效果和水下电磁隐身性能,阴极保护系统的设计优化是关键环节,其目标是充分均匀的船体表面电位/电流分布和最小的水下电磁特征。开展阴极保护优化设计研究,同时获得充分的腐蚀控制和最小的水下电磁场信号,具有重要的理论意义和现实意义。
本文利用均匀试验设计物理缩比模型方法和边界元阴极保护数值模拟优化方法,研究了由2组和3组阳极构成的阴极保护系统中,阳极位置和阳极输出电流对船体表面电位分布、水下稳态电场和腐蚀相关电磁场的影响规律,获得了阳极的最佳位置和最佳输出电流,优化的阴极保护系统不仅具有充分的腐蚀控制能力,而且可使得水下稳态电场、腐蚀相关磁场最小化。
对于2组阳极构成的阴极保护系统中,经阳极位置优化和输出电流优化后,2组阳极分别布置于艉部水下2m/230#肋骨处、舯部阳极布置于水下2m/130#肋骨处,参比电极固定2组阳极之间水下1.5m/186#肋骨处,控制电位-0.85V时,船体阴极保护电位处于-0.80V~-1.00V(SSC,下同)范围,水下船体得到充分保护;稳态电场模值减小幅度达到41%,腐蚀相关磁场模值减小幅度达到45%。二步优化减小水下腐蚀电磁信号的效果非常明显。
对于3组阳极构成的阴极保护系统,3组阳极分别位于110#、200#和230#肋骨时,稳态电场强度峰值最小。优化阳极输出电流后,3组阳极单只输出电流分别为1.82A,1.65A和3.65A。二步优化对降低水下稳态电场具有明显的效果,稳态电场最大值降低幅度高达72.7%;腐蚀相关磁场最大特征峰值降低幅度63.5%。
缩比模型试验和实船试验证明,轴频电场产生的前提条件是:①艉轴中有电流流动,即轴-地间存在因电流流动导致的电压降;②螺旋桨轴转动使得轴-地之间接触电阻发生周期波动。设计研制的有源接地装置可完全消除轴频电场,其等效接地电阻RASG=1.37×10-5O。
设计研制的低纹波恒电位仪纹波系数小于0.1%,为现有商用恒电位仪的1/50,可有效降低工频电磁场。利用IGBT技术,设计集成了兼具阴极保护恒电位仪功能模块和有源接地功能模块的“腐蚀电磁控制器”,二功能模块协同工作性能好,实现了模块化设计,方便使用维护,减小了体积重量。
水下稳态电场和腐蚀相关磁场在船体周围迅速衰减,控制电位时,稳态电场和腐蚀相关磁场衰减符合E(B)=a+brc;控制电流时,稳态电场按照距离的二次方衰减E=d/r2。
涂层破损严重影响着阴极保护电位分布和保护电流。利用均匀设计试验的缩比模型试验方法获得了优化的二组阳极组成的阴极保护系统,在涂层破损2%-10%范围内,保护电流随着涂层破损量的增加而增大,电极布置合理获得较均匀电位分布时,保护电流随着涂层破损率的增加呈线性增大;艏部阳极过于靠近船头时,保护电流随着涂层破损率的增加呈非线性的加速增大。利用BP人工神经网络可以有效预测电极位置和控制参数对电位分布均匀性的影响。
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.
引文
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