First, the development direction of steel pipeline inspection technology. During the operation of long-distance oil and gas steel pipelines, they are usually corroded from both internal and external environments. Internal corrosion is mainly caused by the combined action of the transport medium, liquid accumulation in the pipeline, dirt, and internal stress in the pipeline; External corrosion is usually caused by coating damage and failure. Internal corrosion is generally dealt with using condition management and the addition of corrosion inhibitors. In recent years, with the strengthening of pipeline operation management by pipeline owners and strict requirements for transport media, internal corrosion has been largely controlled. At present, the main development direction of corrosion control of long-distance oil and gas pipelines at home and abroad is in the aspect of external corrosion protection, so pipeline inspection also focuses on coating defects and pipeline defects caused by external corrosion. In recent years, with the widespread popularization and application of computer technology, detection technology at home and abroad has developed rapidly, and pipeline detection technology has gradually formed two branches of pipeline internal and external detection technology (coating detection, and intelligent detection). Usually, the coating is damaged and the pipeline below the failure is also corroded. The purpose of the pipeline external detection technology is to detect the effectiveness of the coating and cathodic protection and to detect the corrosion defects of the pipe body through pit inspection. Most current pipelines that lay out north-inner detection conditions are quite effective. In-pipeline inspection technology is mainly used to find defects such as internal and external corrosion, local deformation, and weld cracks in the pipeline, and can also indirectly judge the integrity of the coating.
Second, detection technology outside the pipeline. Buried pipelines usually use a protection system composed of coating and electrical protection (CP) to control external corrosion. These two methods play a complementary role: the coating is cathodic protection, that is, Economical and effective, cathodic protection allows the coating to be controlled where pinholes or damage occur. This method is recognized as the best protection method and has been widely used in the corrosion control of buried pipelines. The coating is the first line of defense to protect buried pipelines from external corrosion, and its protective effect directly affects the working efficiency of the electrical protection current. Paper No. 7 of the NACE993 Annual Conference pointed out: “The correct coating should be Buried components provide 99% of the protection needs, while the remaining % is provided by cathodic protection”. Therefore, the coating is required to have good comprehensive properties such as electrical insulation, adhesion, continuity, and corrosion resistance, and the maintenance of its integrity is very important. The comprehensive performance of the coating is affected by many factors, such as coating materials, filling technology, construction quality, corrosion environment, management level, etc. After a period of operation of the pipeline, the comprehensive performance of the coating will decline in varying degrees, manifested as aging, cracks, peeling, damage, and other conditions, the surface of the pipe body is corroded due to direct or indirect contact with air and soil. If the coating cannot be effectively detected and maintained, it will eventually lead to pipe perforation, rupture, and damage accidents.
Coating detection technology is to use special equipment to detect the comprehensive performance of the coating non-contact on the ground under the premise of not excavating the pipeline, scientifically, accurately, and economically locate the aging and damage defects of the coating, and determine the size of the Carry out classified statistics, at the same time conduct a comprehensive evaluation on the size and quantity of defects and propose a rectification plan to guide the pipeline owner to grasp the condition of the pipeline coating and conduct practical maintenance to ensure the integrity and integrity of the coating. The domestic implementation of pipeline detection technology began in the mid-1980s. The detection methods mainly include standard pipe/ground potential detection, Pearson (Pearson) coating insulation resistance test, and pipe current test. The test results play an important role in the overall evaluation of the coating, but there is still a big gap in the accurate positioning of defects and reasonable guidance for overhaul. In recent years, through World Bank loans and exchanges with foreign pipeline companies, external pipeline testing equipment is relatively cheap and easy to operate. Foreign pipeline external technology has been widely used in domestic long-distance oil and gas pipeline coating testing. The detection technology has reached the level of advanced developed countries, and the external detection technologies that are widely used in actual work mainly include: standard tube/ground potential detection, Pearson detection, close-pitch potential test, multi-frequency audience current test, and DC gradient test.
1. Standard pipe/site position detection technology (P/S) This technology is mainly used to monitor the effectiveness of the cathodic protection effect, using a multimeter to test the potential between the grounded CU/CuSO4 electrode and a certain point on the metal surface of the pipeline, and through the potential distance curve Understand the potential distribution to distinguish the difference between the current potential and the previous potential and measure the coating condition by whether the measured cathodic protection potential meets the standard. This method is fast and simple and is still widely used in the daily management and monitoring of pipeline coating and cathodic protection by pipeline management departments.
2. Pearson monitoring technology (PS) This technology is used to find coating defects and defect areas. Since cathodic protection current is not required, only the AC signal (000 Hz) of the transmitter needs to be loaded on the pipeline, so The operation is simple and fast, and it has been widely used in coating monitoring. However, the accuracy of the detection results is low, because of the interference of external current, different soil and coating section groups can cause signal changes, and the judgment of defects and the size of defects depends on the experience of the operator.
3. Close interval potential test technology (CIS, CIPS) Close interval potential test (Close Interval Survey) and close interval polarization potential (Close Interval Potential Survey) monitoring is similar to the standard tube/ground potential (P/S) test method, its The essence is the tube-to-ground potential encrypted test and encrypted power-off potential test technology. By testing the intensive potential and intensive potential of cathodic protection on the pipeline, the effectiveness of the cathodic protection effect can be determined, and the position and size of the defect can be found indirectly to reflect the coating condition. This method also has limitations, its accuracy rate is low, it depends on the operator’s experience, it is easy to be affected by external interference, and some reading errors can reach 200-300 mV.
4. PCM multi-frequency tube current test The multi-frequency tube midpoint method is a new technology for monitoring coating leakage, and it is an improved coating detection method based on the tube current gradient test method. It selects the relatively advanced PCM instrument at present, measures the current according to the known detection interval, measures the distribution of the current gradient, and depicts the overview of the entire pipeline. It can quickly and economically find the pipeline section with serious current signal leakage, and pass The computer analyzes and evaluates the condition of the coating, and then uses the “A” frame of the PCM instrument to detect the surface potential gradient to accurately locate the breaking point of the coating. This method is compatible with pipelines of different specifications and materials. It can detect the entire pipeline over a long distance and is less affected by changes in coating materials and ground environments. The surface resistance R g value is used to divide the technical grade of the pipeline coating, evaluate the condition of the pipeline coating, and propose the coating maintenance method. Using a dedicated coupling coil, it can also perform coating inspection on underwater pipelines.
5. DC potential gradient (DCVG) method This method detects the potential gradient generated on the soil medium by the cathodic protection current flowing to the damaged part of the buried pipeline coating (that is, the IR drop of the soil) and calculates the coating based on the percentage of IR drop. The size of the layer defect has the advantage of not being disturbed by alternating current, and by determining whether the current flows into or out of the pipeline, it can also be determined whether the pipeline is suffering from corrosion.
6. Comparison of several test methods In recent years, the author has tested the effectiveness of coating and cathodic protection on several pipelines such as Sichuan Long-Cang Line, Gong-Zi Line, Lu-Wei Line, Shen-Dao Line, etc. On the one hand, the above-mentioned methods were compared, and it was found that all kinds of coating defect detection technologies are realized by loading DC or AC signals on the pipeline, and the difference is only in the structure, performance, and function. Each method has its emphasis and is convincing in evaluating the comprehensive performance of the coating, but each has its advantages and disadvantages. To overcome the limitations of a single detection technology, the author found that the combination of several detection methods to detect coating defects can make up for the deficiencies of various technologies during the on-site inspection. For pipelines protected by cathodic protection, first refer to the test value in the daily management record (P/S), and then use CIPS technology to measure the pipe-ground potential of the pipeline. The measured power-off potential can determine the effect of the cathodic protection system. After the coating may be defective, use DCVG technology to determine the cathode and anode characteristics of each defect, and finally use DCVG to determine the center position of the defect, and use the IR drop caused by the measured defect leakage current to flow through the soil to determine the size and severity of the defect, Use this as a basis for choosing repairs. For pipelines without cathodic protection, PCM test technology can be used to determine the pipe section with serious current signal leakage, and then the “A” frame or Pearson detection technology used in PCM can accurately locate the coating damage point and determine the size of the coating damage. PCM testing technology can also be used for pipelines with cathodic protection, and its detection accuracy is slightly lower than that of DCVG technology. Since all coating detection techniques apply electrical signals on the pipeline, there are some deficiencies in various techniques, and some coating defects cannot be found. It cannot flow to the ground to form a loop, and can only be searched by other means; due to the shielding effect, it is not suitable for crossing pipelines with casings; all technologies cannot determine whether the coating is peeled off.
Third, in-pipeline detection technology In-pipeline detection technology is to add various non-destructive testing (NDT) equipment to the island pig (PIG) and changes the non-intelligence that was originally used for cleaning into information collection, processing, storage, etc. The smart pipeline defect detector (SMART PIG) with multiple functions can detect pipeline defects through the movement of the pig in the pipeline. As early as 965, Tuboscopc Company of the United States successfully applied the magnetic flux leakage (MFL) non-destructive testing (NDT) technology to the internal detection of long-distance oil and gas pipelines, and other non-destructive internal detection technologies were also produced one after another. Discover its broad application prospects. At present, the well-known foreign monitoring companies are Tuboscopc GE PII in the United States, British Gas in the United Kingdom, Pipetronix in Germany, and Corrpro in Canada, and their products have reached serialization and diversification. Internal detectors can be divided into caliper gauges for detecting geometric deformation of pipelines, pipeline leak detectors for pipeline leakage detectors, magnetic flux leakage detectors for volumetric defect detection caused by corrosion, and crack-like planar detectors. Eddy current detectors for defect detection, ultrasonic detectors, and crack detection equipment based on elastic shear waves. Several methods that are widely used are briefly introduced below.
1. Diameter measurement technology improvement technology is mainly used to detect the geometric deformation of the pipeline caused by external force and determine the specific location of the deformation. Some use mechanical devices, and some use the principle of magnetic induction, which can detect the geometry of pits, ovality, and inner diameters. Variations and other geometric anomalies affect the effective inside diameter of the pipe.
2. Leak detection technology At present, the more mature technologies are the differential pressure method and the acoustic radiation method. The former consists of an instrument with a pressure measuring device, and the pipeline to be tested needs to be filled with the appropriate fluid. Leaks from the lowest pressure area in the pipeline, and leak detection instruments are installed here; the latter is based on acoustic leak detection, using the unique sound in the range of 20 to 40 kHz generated when the pipeline leaks, through a sensor with appropriate frequency selection The electronic device collects it, detects and locates the leak through the mileage wheel and marking system.
3. Magnetic flux leakage detection technology (MFL) Among all pipeline detection technologies, magnetic flux leakage detection has the longest history, because it can detect volume-type defects caused by internal and external corrosion of the pipe island, and has low requirements for the detection environment, and can It is also used for oil and gas pipelines, and can indirectly judge the coating condition, and its application range is the most extensive. Since flux leakage is a relatively noisy process, anomalous signals are evident in the data record even without any form of amplification of the data, and its application is relatively simple. It is worth noting that when using a magnetic flux leakage detector to detect pipelines, the operating speed of the pig needs to be controlled, and the magnetic flux leakage is very sensitive to the operating speed of its carrier, although the current sensor used instead of the sensor coil reduces the speed of the pig. Sensitivity, but cannot eliminate the effect of speed. When this technology detects the pipeline, the pipe wall is required to achieve complete magnetic saturation. Therefore, the test accuracy is related to the thickness of the pipe wall. The greater the thickness, the lower the accuracy. The applicable range is usually that the pipe wall thickness does not exceed 2 mm. The precision of this technology is not as high as that of ultrasonic, and the determination of the exact height of the defect still depends on the experience of the operator.
4. Piezoelectric ultrasonic testing technology The principle of piezoelectric ultrasonic testing technology is similar.
Post time: Jun-28-2023