How to measure the condition of the anti-corrosion coating on pipes that are already buried (leak detection)

1# Research on the Application of Anti-corrosion Inspection Techniques for Buried Pipelines in Changle – Author: Liu Jinming 1. Overview: After nearly forty years of development, the Jilin oil field’s pipeline networks are subject mainly to external corrosion of the buried pipelines; this occurs when the anti-corrosion and insulation layers are damaged or lose their insulating properties (degrade), allowing moisture and corrosive ions from the soil to reach the outer surface of the steel pipes, thereby causing electrochemical corrosion. At the current stage, the main approach for pipeline overhauls is to replace the entire pipeline or specific sections of it, or to carry out overhauls, based on the information provided by the oil production plant regarding pipeline perforations and the age of the pipelines. Judging from the pipes that have been replaced, some sections have damaged or aged anti-corrosion and insulation layers, as well as severe pipe corrosion, and indeed need to be replaced ； In some pipe sections, the anti-corrosion and insulation layers are damaged, and the pipe body is only slightly corroded; repair is sufficient ； The anti-corrosion and insulation layers of some pipe sections are in good condition, as is the pipe body itself; no major repairs are required. Furthermore, the lack of preliminary inspection and diagnosis during major pipeline network repairs prevents the effective detection of potential problems such as sections that have not yet developed perforations but whose anti-corrosion and insulation layers are damaged, which is one of the reasons why buried pipelines need to be repaired repeatedly each year and continue to develop perforations. Performing non-digging leak detection (for damage) on the external anti-corrosion coating of buried pipelines, evaluating their insulation condition, and carrying out targeted replacement and repairs can effectively mitigate losses caused by corrosion, extend the service life of the pipelines, and save substantial amounts in maintenance and repair costs. Using the PCM100 detector from British company Reddy, Type A frames, and data analysis such as pipeline depth measurement, GPS positioning can be employed for long-distance pipelines (using Beijing 54 latitude/longitude coordinates) at the turning points along the pipeline route; this allows for a segmented assessment of the insulation condition of the pipeline’s external anti-corrosion layer. The damaged sections of this outer anti-corrosion layer are excavated to determine the nature of the corrosion occurring in those areas, and repair recommendations are provided for the pipeline—ranging from replacing the entire pipeline, to replacing only certain sections, to carrying out major repairs on specific sections, or to making local repairs. 2. Principles of engineering surveying 2.1 Techniques for identifying damage points in the external anti-corrosion and insulation layers of pipelines Principle: During on-site measurements, the transmitter of the underground pipeline detector sends out detection electromagnetic wave signals of a certain frequency towards the pipeline; the receiver of the detector is used to determine the exact position of the pipeline, as well as to measure the current intensity corresponding to the transmitted frequency and the depth at which the pipeline is buried. The distance Xi from each measurement point and the current value Ii are recorded accordingly. If the anti-corrosion coating between point n and point n+1 within the pipeline is damaged, some of the signal current will flow into the soil through the damaged area. Therefore, the Ii curve will experience abnormal attenuation between these two points, and the extent of damage to the anti-corrosion coating can be determined based on the amplitude of the attenuated current. This is the principle behind using the abnormal attenuation of current to identify the locations of damage to the anti-corrosion coating. 2.1.2 Method of determining the location of damage points using the potential difference method. Principle: The potential difference method also involves using a transmitter to emit detection electromagnetic wave signals of a certain frequency onto the pipeline under inspection. If there is damage to the pipeline’s insulation layer, a spherical electric field will form around the point of damage or the joint area; on the ground, this results in an electric field distribution centered on the leak site, with a potential gradient extending outward from that center. Assuming the electrical conductivity of the soil is uniform, the equipotential lines will be multiple concentric circles. The location of the damage point is determined accurately by measuring the potential difference on the ground surface using two grounded steel probes (A-frame), and the magnitude of the damage or the quality of the overlap can be assessed qualitatively based on the peak value of this potential difference. 2.2 Method for comprehensively determining the location of damage using current signal attenuation and potential difference methods. The current signal attenuation method and the potential difference method each have their advantages and disadvantages, and they complement each other. If these two methods are applied simultaneously to a pipeline under inspection, using the potential difference method to detect leaks in those sections where there is current signal attenuation not only improves work efficiency and solves the problem of conducting inspections simultaneously, but also ensures the accuracy of leak detection. 3. Formulation of the overhaul plan for buried pipelines 3.1 Principles for formulating the overhaul plan for buried pipelines 3.1.1 After the pipeline inspection is completed, based on the insulation condition of the anti-corrosion and insulation layers in the sections indicated in the inspection report, as well as the distribution of pipeline damage, along with actual records of pipeline perforations and the structure of the anti-corrosion and insulation layers, it is determined what length and scope of pipe sections need to be replaced, as well as the length of sections that require local repair. Based on the degree of corrosion of some of the excavated pipe sections, recommendations for repair methods are proposed (replacement, repair of the anti-corrosion and insulation layer, reinforcement of the steel structure along with repair of the anti-corrosion and insulation layer). 3.1.2 To facilitate the suspension of flow, major repairs to pipes with poor economic efficiency can primarily involve replacing sections of the pipe, with localized repairs serving as a supplementary measure. It is not convenient to shut down the pipeline supply; therefore, repairs mainly focus on the anti-corrosion and insulation layers, as well as the reinforcement of the steel structure, with pipe segment replacement being a secondary approach. During the pipe section replacement work, special attention must be paid to ensuring that the junction between the replaced pipe section and the original one, as well as the seal area at the replaced section, are handled in strict accordance with relevant standards, in order to prevent groundwater from seeping in and eliminate the risk of pipe corrosion. 3.1.3 The repair length for a local section of the pipeline at a damaged area should be the length of that section for which the original anti-corrosion and insulation coating is removed from the damaged area until the pipe surface is free of rust. 3.1.4 When evaluating the insulation condition of the anti-corrosion and insulation coating on the pipeline under test in segments, if there are only a few discrete damage points along the pipe section (as determined using the potential difference method) and the insulation condition of the pipeline before and after these damage points is good, the short sections of pipe surrounding these damage points should be considered as separate segments with abnormal conditions. Local repairs should be carried out on the damage points in these abnormal segments (including replacement, repair of the anti-corrosion and insulation coating, reinforcement of the steel structure, and further repair of the anti-corrosion and insulation coating). When evaluating the insulation condition of the anti-corrosion and insulation layer of the pipeline under test in segments, if a certain section of the pipeline has numerous consecutive damage points (as determined using the potential difference method), this section must be evaluated separately regarding its insulation condition. This section requires sectional repairs (replacement of the anti-corrosion and insulation layer, reinforcement of the steel structure, and repair of the anti-corrosion and insulation layer). When evaluating the insulation condition of the anti-corrosion and insulation layers of the pipeline in segments, if the current attenuation in a certain section of the pipeline is high, the insulation resistance Rg is rated as poor or very poor; however, the potential difference method fails to detect any damaged areas. This is due to aging of the insulation layer in that section or the presence of numerous small damage points, which results in a uniform distribution of the leakage field in that section. Such sections require comprehensive repair – including replacement of the anti-corrosion and insulation layers, reinforcement of the steel structure, and further repair of those layers. 3.1.5 Repair of local pipe sections – Reinforcement measures for repairing corroded steel parts. Table 1: Degree of corrosion in the pipe body and corrective measures taken. Corrosion depth | Correction method: 1.5 mm – Welding of the steel plate; Thinning of the entire pipe section, with multiple leakage points – Replacement of the affected pipe section. 3.2 – Major repair recommended. In May 2004, relevant technical inspectors were sent to conduct on-site inspections; they used the PCM-100 detector from the British company Reddy’s, as well as Type A frames, to assess the insulation condition of the outer coating and to determine the depth of the pipe (with data recorded every 50 meters). The non-digging inspection method for pipelines involves applying a low-frequency current to the pipeline, and by recording the attenuation of the current signal, the performance of the pipeline’s anti-corrosion insulation layer can be assessed. Through the statistics and analysis of the data, the performance status of the anti-corrosion insulation layers in the segmented pipelines was determined. By comparing with **standards, a grading system was established to assess the quality of these insulation layers. Subsequently, the 6 locations where damage to the anti-corrosion insulation layers was detected were verified on-site together with the relevant parties involved, confirming the scientific accuracy of the detection results. Through a full-line inspection of the water injection pipeline and oil transport pipeline at valve room -1# of transfer station Ying 148 in Yingtai Oil Production Plant, as well as a partial inspection of the oil transport pipeline at Zhonger Station, and a non-digging partial inspection of the anti-corrosion coating on the water injection pipeline at Yingyi Union outlet station, it was found that overall, the anti-corrosion and insulating layers at the joints where fiberglass pipes are combined with steel pipes are in poor condition, while the quality of the anti-corrosion coating on pipes made of rigid polyurethane foam is better. Through non-invasive pipeline inspection, a total of 3 damage points in the pipeline anti-corrosion coating were found in the water mixing, oil transportation, and water injection pipelines at Yingtai Oil Production Plant; the peak values of these damage points were 53, 62, and 64 respectively. At the middle station – the inlet pipeline between stations 2 and 3 is made of fiberglass-reinforced plastic in the low-lying areas; it becomes steel pipe 0.5 km after leaving that section. Damage to the pipeline’s anti-corrosion coating was found at the junction where the fiberglass-reinforced plastic pipe meets the steel pipe, with a potential peak value of 65 mV. During testing of the steel pipe, the current decay was minimal, indicating that the quality of the polyester foam plastic pipe as well as the quality of its installation are good. Then the oil pipeline was excavated, and defects in the construction techniques used during its construction were identified, along with shortcomings in the construction inspection process. The patching process involves breaking the heat-shrink sleeve at the site during the foaming step, and then repairing the area once foaming is complete; this approach can easily compromise the integrity of the anti-corrosion and insulating layer of the patching sleeve. The construction records do not specify the detailed procedures for this work, and it is necessary to establish such procedures and follow them strictly during construction. On-site inspections revealed no relevant records regarding non-destructive testing of the water-mixing pipeline and oil transmission pipeline crossing the embankment at Valve Chamber Ying 148-1#. According to Article 9.1.8.2 of the GB50253-94 “Code for Design of Oil Pipeline Engineering”, “100% inspection shall be carried out on welds located in residential areas, industrial and mining enterprises, as well as welds crossing rivers, lakes, reservoirs, highways, railways, and other pipelines, including butt welds.” ”Inspections are carried out during the project acceptance process in accordance with standards. It can be seen that the pipeline non-digging inspection technology can be used to establish a construction supervision mechanism, thereby avoiding waste of human and material resources resulting from secondary construction work. As a result of inspections of the external water injection pipes at the combined station, it was found that the anti-corrosion coating on these pipes had good insulation properties. However, since no internal anti-corrosion treatment had been applied to them, internal corrosion was quite severe. On-site, the reinforcement plates on the main external water injection pipe outside the station’s walls cracked due to thinning; the strong impact force caused the cover plate of the water injection valve chamber to be flung over a 2-meter-high wall and into the combined station. Fortunately, no injuries were caused to anyone during the night. An ultrasonic thickness gauge was used to measure the thickness of the f 219 ′ 16 water injection pipe inside the valve chamber; the wall thickness of the pipe was 15.2 mm, 15.8 mm, and 15.2 mm in three measurements, with the thinnest portion being 14.8 mm. The outer surface of the water injection pipe is pitted, with the depth of the corrosion pits being 3 mm. The pipe thinning amount is 1.2 mm. In accordance with SY0453-98 \"Quality Inspection and Evaluation Standards for Petroleum Engineering Projects\", 100% flaw detection shall be carried out on the welds of water injection pipelines, as well as on the welds of reinforcement plates. 3.3 Summary By applying non-digging inspection techniques to buried pipelines, it can serve as an effective method for construction acceptance and quality assessment of such pipelines. If there are defects in the pipeline’s anti-corrosion coating, non-digging inspection methods can be used to identify the causes, thereby enabling appropriate control measures to be taken to mitigate the effects of pipeline corrosion and damage.

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Take a look at this: the Buried Pipeline Corrosion Protection Layer Inspection System (DCVG). DCVG works by detecting the potential gradient (i.e., the IR drop) generated in the soil medium due to the cathodic protection current flowing to the damaged areas of the pipeline coating, and it calculates the size of the coating defects based on the percentage of this IR drop； Its other product, CIPS (Close Interval Polarization Potential) monitoring, is similar to the standard pipe/ground potential (P/S) testing method. It essentially involves enhanced pipe-ground potential testing and enhanced shutdown potential testing techniques; by measuring the dense potentials on the pipeline resulting from cathodic protection, it is possible to determine the effectiveness of this protection system. It also allows for the indirect identification of the location and size of defects, as well as an assessment of the condition of the coating. 　　 　　When a direct current is applied to the pipeline, current can flow through the soil to reach the metal pipe where the anti-corrosion coating is damaged; as a result, the voltage gradient changes. The greater the current, and the closer it is to the area where the anti-corrosion coating is damaged, the more concentrated the voltage gradient becomes. Generally speaking, the greater the damage, the higher the current and the greater the voltage gradient as well. 　　 　　The DC voltage gradient method uses a sensitive millivoltmeter to display the difference between two Cu/CuSo4 electrodes that are placed on the same plane. When these two electrodes are placed 2 meters apart, one of them will be more active than the other, allowing it to be determined the magnitude of the gradient that causes the voltage gradient as well as the size of the current. 　　 　　Advantages of DCVG 　Precise location of damaged anti-corrosion layers (areas with a diameter of less than 10 cm) ； 　　Precise location of pipeline anti-corrosion layer damage (within 10 cm range) ； 　　The degree of damage to the anti-corrosion layer is determined by testing the soil from where DCVG originates; the severity is related to size, but it is also influenced by other factors such as the pH effect ； 　　During the DCVG inspection, the pipeline areas that can be accessed are carefully examined ； 　　In DC traction interference, it is possible to determine whether the current loss is due to a damaged anti-corrosion layer (corrosion process) ； 　　It can be determined that the damage to the anti-corrosion layer in that area accelerated the direct current traction current ； 　　It is possible to determine the location of the damage to the anti-corrosion coating and thus identify the location where cathodic protection is applied ； 　　The settings of the rectifier can be adjusted so that the areas where the anti-corrosion coating is damaged receive sufficient cathodic protection current ； 　　It is possible to determine the effective range of a constant-voltage rectifier (or other constant-voltage power supply) ； 　　 　　The sensitivity between the failing rectifier and the defects in the anti-corrosion layer can be determined ； 　　It is possible to determine whether the metal at the damaged area of the anti-corrosion coating receives sufficient cathodic protection current (degree of protection) ； 　　This technology can be applied to any electrolyte, such as soil, river water, seawater, ice/snow ； 　　This technology can be applied to the inspection of anti-corrosion coatings on various materials, including insulation layers ； 　　Determine the circumferential position of the corrosion damage on the large pipeline’s coating ； 　　It is possible to determine the approximate shape of the damage to the anti-corrosion layer before excavation ； 　　The burial depth of pipelines can be determined through DCVG testing ； 　　Corrosion factors can be used to determine the protective effectiveness of the inorganic coating at the defective areas of the anti-corrosion layer on the metal surface, through DCVG testing ； 　　Devices that are friendly, flexible, and easy to operate ； 　　It is capable of detecting defects with a depth of 2 meters and a damaged area the size of a fingernail ； 　　It can be used to detect buried pipes beneath concrete and asphalt pavements, requiring only some modifications to the normal detection mode ； 　　Precise DCVG technology can detect complex pipeline networks, such as urban pipelines, factories, and oil storage facilities ； 　　It can detect buried pipelines beneath high-voltage lines; alternating current has no effect on the DCVG technology ； 　　It can be operated on land with DC pipelines ； 　　No cables are needed, so there’s no risk of animals or people breaking them ； 　　Without additional equipment, it can detect complex terrains such as cliffs or hills, as well as areas with heavy vegetation.

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