Research On The Corrosion Effects Of UHVDC Grounding Currents On Oil And Gas Pipelines Considering The Nonlinear Polarization

With the implementation of China’s "West to East Power Transmission" and "West to East Gas Transmission" project,the situation of the grounding electrode of the UHVDC transmission project adjacent to the oil and gas pipeline has frequently appeared.The proximity distance is getting closer and closer.The corrosion effect of UHVDC grounding electrode current on oil and gas pipelines is directly related to the safety of national energy transmission and needs to be handled carefully.At present,the research on the corrosion effects of DC grounding currents on pipelines is facing many problems.The UHVDC grounding current can reach thousands of amperes at monopole operation,but the operating time is very short.The grounding current is less than 1%of the rated current at the normal long bipolar operation.In the past,only the rated current is used to assess the corrosion effect for the whole life cycle of the pipeline which is extremely unreasonable.When the grounding electrode conducts a large current and is close to the pipeline,the non-linear polarization effect between pipeline metal and soil is significant.The previous corrosion impact calculation rarely considers the impact of polarization,not to mention the non-linear polarization.This will lead to an inaccurate assessment of pipeline corrosion.The existing pipeline cathodic protection system output current is small with a fixed direction.So it is difficult to cope with the corrosion impact of UHVDC grounding current on the pipelines.There is an urgent need to study the assessment method applicable to the corrosion effects of UHVDC grounding electrode current on oil and gas pipelines,the calculation method of voltage and current on oil and gas pipelines considering non-linear polarization,and the pipeline anti-corrosion protection measures.For this reason,this paper studied the corrosion effect of UHVDC grounding electrode current on oil and gas pipelines considering nonlinear polarization.The main work and results are as follows:(1)A study on the assessment method of the corrosion impact of UHVDC grounding current on oil and gas pipelines is carried out.It is proposed to use the equivalent annual corrosion rate to assess the corrosion impact of grounding currents on oil and gas pipelines considering the influence of grounding current amplitude and duration under different operating conditions.Based on the principle that corrosion caused by the equivalent grounding current should be equal to the corrosion caused by the actual grounding current,the calculation methods of equivalent leakage current density and equivalent annual corrosion rate are proposed.It provides technical support for the application of the equivalent annual corrosion rate assessment method.For the first time,the relationship between the equivalent leakage current density and equivalent annual corrosion rate of long straight oil and gas pipelines adjacent to the grounding electrodes in uniform soil and the grounding electrode current under different operating conditions is established,which provides theoretical support for the use of equivalent annual corrosion rate to assess the corrosion impact of oil and gas pipelines,and also provides a rapid assessment method for simpler practical projects.(2)Considering the nonlinear polarization between pipeline metal and soil,the calculation method of the coupling voltage and current of the grounding current on the oil and gas pipeline is studied.The mathematical model of the coupling voltage and current of the grounding electrode current on the pipeline is established considering the nonlinear polarization.The method of calculating the three-dimensional current field containing nonlinear polarization interface condition by combining the method of moments(MoM)and the finite element method(FEM)is proposed.The effectiveness of the calculation method is verified experimentally.The calculation method is applied to engineering case analysis,revealing the influence characteristics of the rated grounding current,pipe parameters,soil resistivity,and changes in proximity distance between grounding electrode and pipelines on the voltage and current distribution and corrosion effects.The quantitative comparison results of pipeline corrosion assessment by considering nonlinear polarization and not considering polarization,using equivalent annual corrosion rate and previous pipeline corrosion assessment methods are obtained.The results show that the maximum value of the leakage current density and the equivalent annual corrosion rate without considering polarization are higher than the results considering nonlinear polarization,and up to 8.2%～51.4%in the case of this paper.The pipe-to-soil potential is artificially set to zero when not considering polarization.It doesn’t match the actual physical phenomenon.The difference error can be as high as several volts.Using the previous pipeline corrosion assessment methods to derive the annual corrosion rate is much higher than the result based on the equivalent annual corrosion rate assessment method.The case of this paper is more than 80 times higher.Without considering the effect of nonlinear polarization or using the previous pipeline corrosion assessment methods,the corrosion effect of grounding current on oil and gas pipelines will be incorrectly overestimated.(3)Based on the operating characteristics of UHVDC grounding current,an adaptive high-power cathodic protection system(HCPS)with a bidirectional output function is proposed to cope with the corrosion impact of grounding current on oil and gas pipelines.The optimal parameters configuration method of the high-power cathodic protection system for oil and gas pipelines is proposed to cope with the impact of UHVDC grounding current.The objective function of oil and gas pipeline high-power cathodic protection system parameter optimization is established with the evaluation of the equivalent annual corrosion rate and the pipe-to-soil potential.The chaotic particle swarm optimization algorithm is applied to the parameter optimization configuration of the oil and gas pipeline high-power cathodic protection system for the first time.The particle swarm stability criterion is proposed based on the Euclidean distance between particles which improves the global optimization performance of the algorithm.The optimization method is applied to engineering cases,and three solutions are analyzed:no cathodic protection measures,existing cathodic protection,and the optimized high-power cathodic protection system.The results show that the optimized solution of a high-power cathodic protection system has the best voltage and current distribution on the pipeline and the lowest equivalent annual corrosion rate of the pipeline,which can economically and effectively control the corrosion impact of DC grounding electrode current on oil and gas pipelines.The research results lay the foundation for accurate calculation,reasonable evaluation,and effective protection of the corrosion on oil and gas pipelines when considering non-linear polarization,and provide important technical support for the site selection and design of the grounding electrode of UHVDC transmission project and oil and gas pipeline project.It is of great significance for the coordinated development of UHVDC transmission projects and oil and gas pipeline projects.