Study On Cathodic Protection Potential Distribution Of Underground Pipelines

As an important anti-crossion method, cathodic protection is of significance to ensure thesafety of oil&gas pipeline system. Whereas, long distance pipelines cross different regionsas well as complicated climates, for instance, China-Burma pipeline crosses the typical tropicseasonal windy area, where dry season and rainy season alternate to make water contentaround the pipeline change, meanwhile, the China-Russia pipeline crosses tundra that isfrozen for years, from which new problems arose to make new requirements on the cathodicprotection techniques. However, present design method of cathodic protection is lack ofsufficient scientific theory to rely on. Therefore, it is meaningful to apply the numericalsimulation to the design of cathodic protection so as to investigate the effect of variation ofcircumstance along the pipeline on cathodic protection potential distribution.In the model built in this paper, soil resistivity and polarization characteristic were takento characterize the circumstance along a pipeline. Both soil resistance and pipe metalresistance were considered in the model. Simulation software was adopted to solve the model.Correctness of model together with its solving method was validated by comparison with theexperimental results obtained by building a lab underground pipeline cathodic protectionsimulation system.Taking into account the prominent effect of soil resistivity along the long distancepipeline on the cathodic protection potential distribution, lab measurement of soil resistivityas well as the contributing factors were analyzed. AC-four-pole method was recommended totake on the measurement of soil resistivity. Based on outdoor Wenner four pole soil resistivitytest data, multilayer soil resistivity model was built to classify soil layer and its real soilresistivity in each layer. Method of Grey Relational Analysis (GRA) was used to study thecontributing factor to find the prominent ones. Then, lab experiments were carried to analyzeand summarize the regularity of soil resistivity changing with the prominent factors.Moreover, BP artificial neural network was built to calculate the soil electric resistivity.Effect of the placement of anode for underground pipeline and variation of soilresistivity both horizontally and vertically on the potential distribution of cathodic protection was studied. The cathodic protection potential distribution change with pipeline turing to orturing away from the anode were studied which will make the simulation get close to theactual situation.Effect of cathodic polarization curve on the potential distribution of cathodic protectionwas investigated. Further, influence of seasonal water content change and temperaturevariation on potential distribution of cathodic protection was analyzed to provide a theoreticalbackup for determining the proper parameters of cathodic protection system during its designand operation. Model of potential distribution of cathodic protection system for pipelines withcoating deficiency was build, together with the lab experiment designed to measure andcharacterize the polarization curve of the pipeline with coating deficiency, to study the effectof size of holiday, disbondment and their positions on the potential distribution of cathodicprotection.Eventually, an example was set to compare the traditional design of cathodic protectionsystem, model for simulating tanks and short pipelines and the model built in this paper forlong pipelines. The cathodic protection potential distribution in a year were studied andsuggested that to regulate the anode data according to the situation will keep the cathodicprotection system in a safe and effective operating state.