Comprehensive detection and evaluation method for external pipeline corrosion of oil and gas fields
A comprehensive evaluation of the external corrosion situation for the aged Sanhua crude oil pipeline in Xinjiang oilfield was carried out by combined techniques of alternating current attenuation method (PCM), DC potential gradient (DCVG) and close interval potential survey (CIPS). The evaluation includes such as soil corrosion survey, coating integrity survey, effectiveness of cathodic protection (CP), DC/AC stray current interference and site excavation. Results revealed that coatings are aged with total 592 of serious damaged spots along the piprline; the potential of 39.2 km pipeline does not meet CP potential standard. Based on the comprehensive evaluation results, measures related with coating restoration and CP adjustment were proposed.
With the rapid development of pipeline construction in China, the mileage of oil pipelines in service will increase rapidly. At the same time, many existing buried pipelines in China have entered the ageing stage. Due to the aging and corrosion of the pipeline itself, the normal operation of the pipeline has been seriously affected. Occurs, not only brings huge economic losses to the production enterprises, but also has serious consequences for the society and the surrounding natural environment. Regular inspection and evaluation of pipeline anti-corrosion protection system, timely and accurate understanding of the corrosion status of oil and gas pipelines, is important to ensure the safe operation of oil and gas pipelines.
The External Corrosion Direct Evaluation (ECDA) technique [1-7] is a method to evaluate the effect of external corrosion on pipeline integrity, and technical standards have been formed [3, 6]. In accordance with the standardized procedures, ECDA obtains the status information of the external corrosion and anti-corrosion system through external detection means, and combines the results of excavation verification and related data analysis to systematically and comprehensively evaluate the external anti-corrosion system. ECDA can be used to judge the weak links, external corrosion and related influencing factors of the external anti-corrosion system.
Exploring the external corrosion detection technology and evaluation method applicable to the old pipelines in China’s oil and gas fields, forming a detection-evaluation-repair system, reducing the pipeline failure rate and improving the safety level, and establishing a set of external corrosion evaluation system suitable for China’s oil and gas fields has become an oil and gas production operation system. An important part of security management. To this end, China National Petroleum Corporation organized the General Planning Institute, Daqing Oilfield, Changqing Oilfield, Xinjiang Oilfield, Southwest Oil and Gas Field, Huabei Oilfield, Dagang Oilfield and other units to carry out a pilot project for comprehensive detection of oil and gas pipeline corrosion. However, the complete ECDA technology requires complete pipeline historical data, and the process is complex and costly. These determine that ECDA may not be fully suitable for a large number of old pipelines in China’s oil and gas fields.
This project is part of the pilot project of Xinjiang Oilfield. It has selected the North China-Wuhua Petrochemical (Sanhua) crude oil pipeline of Xinjiang Oil and Gas Storage and Transportation Company. The Sanhua pipeline passes through high-voltage transmission lines and densely populated areas such as farmland and villages. At the same time, the Sanhua pipeline is the only crude oil pipeline that Zhundong Oilfield transports to the Urumqi Petrochemical Plant. The safe operation is particularly important. Therefore, the pipeline was selected as a pilot. This project draws on the detection technology and ideas of ECDA. The main contents of external corrosion detection and evaluation include: pipeline laying environment investigation, corrosion environment evaluation, anti-corrosion layer integrity, cathodic protection effectiveness, pipe corrosion defect detection and excavation direct inspection, etc. . Among them, the corrosion environment survey includes: soil resistivity, tube ground potential, redox potential, pH value, soil texture, soil water content, soil salinity, chloride ion content and other soil parameters detection, soil corrosion evaluation and direct / exchange Stray current detection, etc.
1 Environmental and external corrosion detection methods along the line
The Sanhua crude oil pipeline was put into operation in 1993 and has been in service for 23 years. It starts at Changsan Station in Changji Prefecture and ends at Urumqi Petrochemical Station. It has a total length of 106.7 km. The whole line uses 16Mn (T 52K) steel, and the petroleum asphalt is strengthened (four oil and three cloth). Layer, applied current cathodic protection. The pipe diameter is 426 mm × 7 mm, the designed annual output is 2.4×106 t/a, the operating temperature is 56 °C, and the operating pressure is 6.2 MPa. Along the line 0~80 km, the aeolian yellow landforms, the main soil layer is loess-like clay, sub-clay, light sub-clay, 80~85 km is low mountainous landform, and 85~100 km is hilly hilly area. The depth of the pipeline is 0.96 m shallow and 3.0 m deep. A total of nine test piles (2, 9, 39, 64, 66, 88, 89, 95 and 98 km) were found along the pipeline and were lost. The section of the pipeline crosses 3 villages, 8 farmland (crossing distance of about 11.5 km), spans 3 large gullies, 12 small water ditches, 10 times through the highway, 1 railway, and 10 pipeline crossing sections. The distance between the five pipelines and other buildings (structures) and the pressure were found along the route. No pipeline leakage occurred during the pipeline test. The water protection facilities such as slope protection, pipe culvert and fortification along the pipeline were good and no damage was observed.
Detection methods include AC current decay (PCM), DC potential gradient test (DCVG), and dense interval potential test (CIPS). The PCM applies an excitation current signal of a specific frequency to the pipeline, and detects the intensity and variation law of the alternating electromagnetic field generated by the excitation current along the pipeline along the pipeline. The change of the current in the tube is calculated from the magnetic field strength of the ground above the pipeline. The insulation resistance of the anti-corrosion layer is obtained through the analysis of the corresponding mathematical model, the damage point position and the damage degree of the anti-corrosion layer are determined, and the level of the anti-corrosion layer is determined. The insulation resistance value of the anti-corrosion layer was obtained by using ESTEC XP software, and the level of the anti-corrosion layer of the inspection pipeline was evaluated according to GB/T19285-2014 . The CIPS test is to periodically interrupt the cathodic protection current through the GPS synchronous current interrupter, and measure the energization potential (EOn), the instantaneous power-off potential EOff and the position information at each point of the pipeline at a small equal interval on the ground surface to evaluate the effectiveness of the cathodic protection. Determine the defects of the anti-corrosion layer. The instrument used for CIPS detection in this work is the CIPS dense-pitch potential detection system produced by British DCVG Company. The cathodic protection current cycle on-off cycle is 12 s, 3 s off. The DCVG test is to measure the potential difference of the anti-corrosion layer, the current direction, the size of the anti-corrosion layer defect, the shape of the defect and the position of the tube in which the defect is located on the surface of the pipeline with two reference electrodes [1, 2].
2 Pipeline corrosion environment evaluation
2.1 Soil corrosion evaluation
The soil corrosion test was carried out every 10 km of the soil along the Sanhua pipeline and every 1 km of the end pipeline. The test items included soil resistivity, tube ground potential, pH value, redox potential, water content, salt content, and Cl-. And SO42- content and so on. The soil corrosion evaluation rating is shown in Table 1. The soil outside the Sanhua line is mostly Gobi desert soil and white alkaline soil, and a small amount of farmland yellow clay. The soil resistivity is low (2.64~15.04 Ωm); the natural corrosion potential of the pipeline is between -462~642 mV (CSE), and the soil redox potential is higher, between 301~367 mV (CSE); soil pH The value is between 7.68 and 8.84, which is basically alkaline; the soil moisture content is between 9.69% and 16.75%; the Cl- content is higher, between 0.01% and 0.45%; and the SO42- content is between 0.05% and 0.31%. The total amount of soluble salts in the soil is between 0.31% and 2.28%. According to the soil detection and test results along the pipeline, the soil corrosion classification standard in GB/T19285-2014  can comprehensively determine that the soil corrosion grade along the line is strong.
Table 1 Analysis results of soil corrosivity at a series of test sites along the Sanhua pipeline
2.2 Stray current detection
Taking the three oil depots in the north as the starting point, the grounding potential and AC/DC stray current interference detection were carried out every 10 km test pile along the Sanhua line. The stray current interference detection uses the maximum forward displacement of the ground potential and the surface potential gradient to detect the position, interference level and distance of the pipeline interfered by the stray current. Determine the degree of stray current interference according to GB/T19285-2014 .
The test of the energization potential of the test pile along the pipe section shows that the energization potential of the pipe section is stable and the disturbance by the stray current is weak. The results are shown in Tables 2 and 3. Except for the positive offset value of the ground potential at the test points of 80 km and 100 km, the forward offset of the ground potential of other test points is between 7 and 89 mV. The ground potential gradient of each test point is between 1.12~4.56 mV/m, the AC interference potential is between 0.012~0.235 V, and the AC current density is between 0.72~5.94 A/m2. The stray current test results in Tables 2 and 3 show that the test points of 100.3 km of the three lines are moderately weak due to DC stray current interference, and weakly affected by AC stray current interference.
Tabel 2 Test results of DC stray current interference at a series of test sites along Sanhua pipeline
Table 3 Test results of AC stray current interference and the interference level
3 Anti-corrosion layer damage detection
The PCM and DCVG combination method is used to detect the defects of the outer coating of the pipeline. The test results show that some of the anti-corrosion layers along the line are severely aging and have many damage points. A total of 592 damage points of the outer anti-corrosion layer were detected on the whole line, with an average of 5.8/km. The attenuation rate of the PCM AC current signal in these damage points was 15 at 55 dB/km, and 123 at 40-55 dB/km, 30~40. 334 between dB/km, 120 locations less than 30 dB/km (Figure 1). As far as the distribution of damage points is concerned, the damage points of the anti-corrosion layer in the 3 km and 65~68 km sections are dense, with 52 and 42 respectively, and the average damage points of the other sections are 5-7/km. The serious damage of the anti-corrosion layer within 3 km of the exit may be due to the excessive oil temperature at the exit of the refined oil.
Fig.1 Distribution of coating defects along the 100.3 km Sanhua pipeline from the Beisantai oil depot
Table 4 lists the insulation resistance values of the PCM measured on the 853.5 m pipeline between 0 and 1 km of the three-line line. There are 29 damages in this section, and the average insulation resistance value is 0.6 kΩm2. The overall evaluation grade of the section is level 4.
Table 4 Statistics of the coating insulation resistance at 0~1 km Sanhua pipelines
According to the national standard [8,9], the grade of anti-corrosion layer is determined. In the 103767.3 m anti-corrosion layer of the tested pipe section, the pipe section of grade 1 is 0 m, the pipe section of evaluation grade 2 is 47719.52 m, accounting for 45.9% of the total pipe section, and the evaluation grade is 3. The pipe section is 38622.41 m, accounting for 37.2% of the total pipe section, and the pipe section of the evaluation grade 4 is 17425.35 m, accounting for 16.7% of the total pipe section (Table 5).
Table 5 Statistical results of the evaluation grade of coating along Sanhua pipeline
4 Cathodic protection
CIPS testing was performed on 65~68 km and 81~86 km sections (8 km total). The GPS synchronous current interrupter is used to measure the instantaneous power-off potential EOFF of the IR drop, which is the effective polarization potential of the pipeline . The NACE recommended standard , for the establishment of effective cathodic protection, the instantaneous power-off potential to eliminate IR drop should be lower than -850 mV (CSE), or the power-off potential should be offset 100 mV from the natural corrosion potential.
The energization potential of 104 test points along the line of the Sanhua pipeline is shown in Fig. 2. The cathodic protection potential along the pipeline fluctuates greatly, with the lowest potential -0.498 V, 45 km from the pipeline. Keep away from the cathodic protection station pipeline, the protection potential is greatly attenuated, indicating that the quality of the coating is not good. The ground potential of the pipe section with a total length of about 39.2 km along the line 36~59 km, 67~83 km, 101.8~103 km can not reach the cathodic protection minimum potential standard and is under-protected.
Fig.2 Cathodic protection potential distribution of test sites along Sanhua pipeline
Figure 3 shows the ground potential distribution of the CIPS tube in the 67~68 km section. It can be seen that the power supply in this section fluctuates between -850 and 950 mV. After most of the pipe sections are eliminated, the EOff is lower than -850 mV (CSE). The cathodic protection rate of this section is about 2%. The CIPS measurement distance of the 82~83 km pipe section is 1061.6 m, and the CIPS pipe ground potential distribution is shown in Fig. 4. The power supply of the pipe segment fluctuates between -840 and 900 mV (CSE), and the power-off is fluctuated near -850 mV (CSE). The pipe section that meets the cathodic protection and the pipe that does not reach the protection are each half. The overall cathodic protection rate of this section of the pipeline is about 50%.
Fig.3 CIPS pipeline-to-soil potential distribution of the 67~68 km segment (a) and 82~83 km segment (b)
Fig.4 Coating and body morphologies for the Φ426 pipeline at excavation sites of 65 km+284.97 m, 67 km+678.25 m and 99 km+45.32 m
5 Excavation inspection
Refer to the external corrosion direct evaluation (ECDA) idea, determine the corrosion condition of the pipe body through the direct evaluation method of excavation, verify the accuracy of the indirect detection method of the anti-corrosion layer, and complete the analysis of the degree of corrosion defects and corrosion causes of the pipe body (Table 6). At the same time, the soil corrosion, the condition of the anti-corrosion layer and the corrosion of the outer wall of the pipeline were tested and analyzed.
|EP/SV||dB||RgΩm2||Appearance corrosion||External wall corrosion||
|-1.034||31||≤0.1||The large area of the anti-corrosion layer 2 is broken and damaged, and the maximum area is 350×200 mm. The damaged layer is aging, the hardening is severe, and a large amount of solution is accumulated in the gap between the anti-corrosion layer and the pipe body.||Large number of rust pits, maximum pit depth 0.84 mm||0.84|
|-0.802||–||≥10||The root of the reed is plunged into the anti-corrosion layer, causing a large number of damage points in the anti-corrosion layer, resulting in a gap between the anti-corrosion layer and the steel pipe.||Large number of rust pits, maximum pit depth 0.76 mm||0.76|
|-0.570||31||2.6||The anti-corrosion layer has three large areas, and the maximum area is 150 mm×100 mm. The damaged layer is aging, hardening is severe, and a large amount of aqueous solution is accumulated in the gap between the anti-corrosion layer and the pipe body.||Large-scale corrosion rust layer||0.2|
|-1.038||32||≤0.1||The anti-corrosion layer has many areas falling off, the crack is longitudinally cracked, the hardening is serious, and a large amount of aqueous solution is accumulated in the gap between the anti-corrosion layer and the outer wall of the pipeline.||Large-scale corrosion rust layer with a rust layer area of 500×250 mm||0.2|
|-0.933||33||≤0.1||The coating layer is partially damaged, and the soil medium penetrates the coating layer.||Corrosion pit depth of 0.85 mm||0.8|
|-1.077||31||≤0.1||Partial damage of the coating||Large area corrosion product rust layer||0.3|
According to the indirect test results, comprehensively consider the size of the anti-corrosion layer, corrosion activity, cathodic protection, AC and DC interference, soil environmental characteristics, relationship with other metal structures and actual needs on site, determine the number of excavation points and position priority criteria Finally, six defect points were selected for excavation inspection. Direct testing after excavation includes: soil corrosivity (such as soil condition, soil resistivity, collection and analysis of soil and water samples, etc.); anti-corrosion layer conditions (such as appearance and aging, thickness and adhesion, defect size) , defect morphology, EDM detection of anti-corrosion layer pinhole defects, etc.; pipe corrosion (such as defect type, size, morphology, corrosion product collection, etc.); tube residual strength evaluation required data (such as pipe corrosion defects The remaining wall thickness of the position, etc.); the ground potential of the pipe, such as the pipe potential at the excavation point, as measured by the near reference method.
On-site excavation test found that 0 km+586.17 m, 55 km+7.29 m and 65 km+284.97 m of the excavation point of the anti-corrosion layer was aging and severely damaged: many large areas fell off, cracked, longitudinal cracking; the adhesion of the anti-corrosion layer was poor A large amount of water is stored between the peeling anti-corrosion layer and the outer wall of the tube, and a large number of rust pits appear on the tube wall, and the maximum pit depth is 0.84 mm. The 67 km+678.25 m and 99 km+45.32 m excavation points have serious damage to the anti-corrosion layer, and the soil penetrates the anti-corrosion layer, and the deepest part of the corrosion pit of the pipe body reaches 0.85 mm. The excavation point of 49 km+130.8 m is reed pond, and the root of reed is plunged into the anti-corrosion layer, causing peeling off of the anti-corrosion layer. The external inspection points were found in the 6 direct inspection points of this test, indicating that the indirect detection process is effective and the defect location is accurate, and the comprehensive evaluation process is effective overall.
6 Conclusions and recommendations
(1) A total of 592 damage points of the anti-corrosion layer were detected in the 100.3 km pipeline of the Sanhua Line. The outer protective layer was aged and damaged, resulting in a total length of 39.2 km along the line of 36-59 km, 67-83 km, and 101.8-103 km. The ground potential of the pipe section cannot reach the cathodic protection – the lowest potential standard of 0.850 V, and it is under-protected.
(2) The anti-corrosion layer of Sanhua pipeline is aging and there are a lot of damage points, which causes the cathodic protection to fail to meet the standard. It is recommended to repair the damage point of the pipeline anti-corrosion layer in time, and continue to detect the cathodic protection power-off potential of the pipeline after the repair is completed, and compare the test data. Subsequent measures provide basic information.
(3) It is recommended to immediately repair the 12189.8 m anti-corrosion layer with evaluation grade 4 to improve the efficiency of cathodic protection; plan to repair the 6805 m anti-corrosion layer of grade 3, and grade 1 and 2 anti-corrosion layers. Carry out key monitoring and temporarily not repair.
(4) For the old pipeline with severe aging damage of the anti-corrosion layer, it is recommended to use the 100 mV polarization criterion when evaluating the effectiveness of cathodic protection. It is recommended to combine the corrosive environment outside the pipeline, the integrity of the anti-corrosion layer, the cathodic protection status and the direct verification results of the selected excavation points, and carry out targeted analysis on the detected damage points of the pipeline anti-corrosion layer, and formulate the maintenance of the pipeline accordingly. , maintenance plans and preventive measures.
The authors have declared that no competing interests exist.
Source: China Oil and Gas Pipeline Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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