考虑非线性极化时不同土壤电阻率下直流接地极电流对油气管道的腐蚀影响分析

Corrosion Effects of HVDC Grounding Currents on Oil and Gas Pipelines Under Different Soil Resistivity Conditions Considering Nonlinear Polarization

在特高压直流输电工程接地极对邻近油气管道的腐蚀影响分析和防护设计中,准确计算不同土壤电阻率下管道上的电压电流分布至关重要,然而目前计算中缺乏对油气管道金属-土壤交界面非线性极化影响的考虑。通过试验测量和分析,获得了不同土壤电阻率下X80管道钢–土壤交界面的极化特性,为准确计算管道管地电位差和泄漏电流密度奠定了基础。考虑非线性极化的影响,计算获得了不同土壤电阻率下油气管道上的泄漏电流密度和管地电位差分布、直流接地极与油气管道的防护距离,结果表明,随着土壤电阻率的增大,管道泄漏电流密度最大值呈对数函数增大,管道与接地极间的防护距离呈幂函数增大。对不考虑极化和考虑非线性极化时管道上的电压电流分布、管道与接地极间防护距离进行了对比分析,结果表明,不考虑极化时管地电位差被人为确定为零,与实际物理现象不符,导致直流接地极电流对油气管道的腐蚀程度被错误高估,应用于实际工程时将导致油气管道防护成本显著增大。基于实际工程参数的应用分析表明,与考虑非线性极化影响相比,不考虑极化时将错误地提高油气管道的腐蚀防护要求,不必要地增加防护成本,防护距离或防护措施成本增加量高达30%~40%。研究结果可为特高压直流输电工程接地极和油气管道工程的选址以及干扰防护设计提供技术支撑。

It is essential to calculate the leakage current density and the pipe-to-soil potential(PSP) accurately under different soil resistivities when considering the corrosion effects of UHVDC grounding current on the buried pipelines and the protective design for such pipelines.However,the nonlinear polarization of the interface between the metal pipeline and the surrounding soil has not been considered in the present calculations.Through tests and analyses the polarization characteristics of the steel-soil interface of the X80 pipeline are obtained under different soil resistivities,laying a foundation for the accurate calculation of the interference voltage and current.The leakage current density,the pipe-to-soil potential and the protective distance between the grounding electrode and the pipeline are calculated under different soil resistivities considering the nonlinear polarization.The results show that the maximum leakage current density increases logarithmically as the soil resistivity increases and the protective distance between the grounding electrode and the pipeline grows as a power function.The PSP is determined artificially as zero which is not consistent with the actual physical phenomenon.So the annual corrosion rate is overestimated and the protective cost increases when the polarization effect is not considered.Based on the analysis of a typical engineering case,compared to considering the nonlinear polarization,the protection requirements are mistakenly overestimated in such situation when the polarization is not considered,increasing the unneeded protection costs by 30%~40%.The research results will provide a technical support for the site selection of the UHVDC projects and pipeline projects,and also for the interference protection design.