| Sichuan to East Gas Transmission Pipeline is the second West to East Gas Transmission Pipeline and a natural gas pipeline accrossing Eastern and Western china, therefore, its safe operation is very important. After operating for2years, leak detect and repair of external anticorrosion coating indicate that the pipeline at the point of coating defects has different degrees of corrosion, which means cathodic protection(CP) system does not protect the pipeline perfectly. So it is important to detect the CP status of Sichuan to East Gas Transmission Pipeline and perform an effective evaluation for CP of the pipeline.In this paper, according to CP current situation of Sichuan to East Gas Transmission Pipeline, through setting up a CP potential mathematical model, numerical simulation software Beasy CP based on BEM is chosen to carry out coupon field tests’plan analysis. At the same time, the pipeline carbon steel polarization curve is used to design the coupon. Then coupon field tests of Sichuan to East Gas Transmission Pipeline are made according to coupon field tests’plan analysis and coupon design plan. Finally the CP effectiveness of Sichuan to East Gas Transmission Pipeline is evaluated, which is based on coupon data of field tests and the current CP criteria, and targeted corrective measures are proposed for CP substandard pipe segments. The main research findings are as follows:(1) The CP potential distribution model of buried pipeline is established. Numerical simulation software Beasy CP based on BEM is chosen to conduct research by comparing the commonly used numerical solutions, which lays the foundation of the analysis of coupon detection plan.(2) According to the mathematical model, specific parameters of CP system of Sichuan to East Gas Transmission Pipeline is considered. Software Beasy CP is used to simulate the influence on potential of pipeline and CP coupon while changing the parameters of the shape of coupon, the size of coupon, the location of coupon, pipeline coating damage rate and soil resistivity, the coupon field detection plan is analyzed, and suggestions are put forward to coupon design and field test.(3) Based on the coupon design principle, the range of application for coupon detection is summarized, and the CP coupon test period of Sichuan to East Gas Transmission Pipeline is determined. After testing the X70steel polarization curve in the soil solution at different pH values, and calculating the areas of coupon according to consumptive current value of coupon, the sizes of coupon are determined. At the same time, the IR drops of designed coupons are predicted, which proves that the sizes of CP coupon do not affect the measurement of real potential. Then the design plan of CP coupon group for Sichuan to East Gas Transmission Pipeline is determined.(4) Based on the plan analysis and design plan of coupon, coupon field test is carried out. Coupon groups are produced and40test points along the pipeline are selected, then a set of CP detection practice procedure is formed, including burying coupon, regular excavation, measurement of on-potential and off-potential, cleaning, SEM and XPS. (5) Based on the coupon detection data, analysis, effectiveness evaluation and suggestions to CP system of Sichuan to East Gas Transmission Pipeline are carried out.1) According to the coupon test data, the distribution of soil corrosion along the pipeline39detection points is analyzed. By analyzing corrosion products with macroscopic and microscopic observation, CP of pipeline is proved resistant to corrosion from the perspective of corrosion products.2) The effect of pipeline CP is assessed based on degree of CP, which concludes that its CP pass rate is44%. By associating CP degree with on-potential or off-potential, it is concluded that the average IR drop of Sichuan to East Gas Transmission Pipeline is220mV, which means the on-potential (Uon) criterion should be-1070mV (CSE). When the real potential (UOff) reaches-850mV (CSE), the minimum degree of CP is no less than65%, which provides basis for daily CP management and maintenance.3) According to the criteria of corrosion rate, on-potential and off-potential, Sichuan to East Gas Transmission Pipeline CP status along the39test points is analyzed and evaluated, which concludes that its CP pass rate is74.4%, and judgements are made about the10segments of CP invalid pipes.4) Risk assessment and criticality matrix are used to analyze risk rank by drawing a criticality matrix picture of atmospheric environment corrosivity and "unprotected degree", which concludes that no test segment is located in the highest risk area (V), and number5,22,27, and30segments are in higher risk area (IV).5) Analyzing the factors that influence Sichuan to East Gas Transmission Pipeline CP, targeted corrective measures for CP substandard pipe segments are proposed, and the reliability of coupon detection technology is verified through the pipeline potential monitoring and the field excavation. |
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