Corrosion and Protection of Seamless Pipeline for Oil and Natural Gas

First, the corrosion characteristics of seamless pipeline for oil and gas are mostly in a complex soil environment, the transmission of the corrosive media, therefore, the seamless pipeline inside and outside walls are likely to be corroded. Once the pipeline is corroded and perforated, oil and gas will be leaked. Not only the transportation is interrupted, but also the environment will be polluted, and even catastrophic accidents such as fires will be extremely dangerous.

Oil, gas pipeline due to the environment and the transmission medium, caused by the corrosion situation is also different. Its corrosion mechanism can be divided into chemical corrosion and electrochemical corrosion, especially electrochemical corrosion-based. Internal corrosion is the medium of water in the pipeline wall and the formation of the original film electrochemical corrosion occurred for the gas pipeline, some of the harmful impurities such as H2S, C02 and other metal directly caused by chemical corrosion. Corrosion mostly concentrated in the elbow, low-lying water, air-liquid interface and so on. External corrosion can be divided into overhead lines and buried pipelines to analyze the two parts, the former vulnerable to atmospheric corrosion, the latter susceptible to soil erosion, bacterial corrosion and stray current corrosion. There are many factors affecting soil erosion, such as soil pH, redox potential, soil resistivity, salt type and amount, water content, porosity, organic matter content and temperature. One of the most common and most harmful factors is due to differences in soil media caused by macro-corrosion cells, such as due to different soil salt content, the difference in permeability of the soil and the different temperature of the soil formed by the concentration of various types of batteries, the pipeline In the high salinity, weak permeability of the soil has a strong tendency to corrosion, and sometimes this macro-cell two-part potential difference between the steel pipe up to about 0.3V, the distance between the two poles up to several kilometers, this system Batteries are the main reason for the severe corrosion of buried pipes.

Second, the corrosion control of oil and gas pipelines corrosion control methods are: selection of corrosion-resistant materials, coating protection, plus corrosion inhibitors; electrical protection and improve the environment. This paper mainly discusses the corrosion control of pipelines. The current common pipeline anti-corrosion program is the outer coating plus the cathodic protection of the joint protection, joint protection so that corrosion control means complement each other to reduce the cost of corrosion control, economic and reasonable, safe and reliable, has become the world's developed countries on the ground, water pipelines Corrosion protection of the common law.

Buried pipe coating to the steel pipe and soil corrosion and other isolated environment, increasing the metal corrosion of the circuit resistance of the battery, reducing the corrosion current, which carried out corrosion control. However, in the anticorrosive construction of pipeline, the coating can not be completely damaged due to many factors such as pipeline transportation, construction conditions and the like, and the wall at the coating damage is easily corroded, and due to the formed corrosion cell small anode / large cathode conditions, corrosion current density, corrosion rate is often doubled. It can be seen only using the coating can not completely prevent pipe corrosion, according to the electrochemical corrosion principle, as long as the metal to be protected into a cathode, you can prevent corrosion. Therefore, through the auxiliary anode to the pipeline through the DC, the pipeline potential reached -0.85V, you can make the pipeline to be adequate protection. That is to say, the method provides enough cathodic polarization current to the pipe body at the damaged coating and the pipe is effectively protected. Cathodic protection investment is generally about 1% of the total pipeline investment, can greatly improve the service life of the pipeline. Many developed countries in the world have explicitly stipulated in the industrial regulations since the 1970s that long-distance pipelines must adopt cathodic protection. China's former Ministry of Oil 1984 in the "steel pipe and tank anti-corrosion engineering design code" SYJ7-84 provides: oil, gas pipelines must be cathodic protection.

The use of outer coating so that steel pipe and soil corrosion and other environmental isolation is the basic means of corrosion of buried pipelines. As pipelines extend to deserts, swamps, permafrost and oceans, and the diversification of conveying media, coatings are required to meet more demanding environmental and process conditions and have longer useful lives. The development of the chemical industry has promoted the progress of pipeline anticorrosive technologies. New materials, new processes and new equipment are constantly emerging. As a result, the single monolithic asphalt coating that has dominated the world has been shattered and the molecular polymer-based multi-species, multi-standard system of materials and composite systems, the formation of a variety of anti-corrosion materials coexist the situation for the pipeline under different environments more effectively preservative provided the possibility.

Technical feasibility is the basic starting point for the selection of anticorrosion coating outside the pipeline. Other factors must be subject to this premise. The technical feasibility of applying various anticorrosive coatings on buried long-distance pipeline usually includes two aspects: Operational requirements to meet the pipeline operation requirements is to ensure that the coating used in the design life of the full and effective. Pipeline in operation, environmental factors will have a variety of destructive coatings, such as: the relative movement between the anti-corrosion layer and the soil shear effect on the anti-corrosion layer: extrusion of soil hard objects and plant roots mechanical damage to the coating. The difference between the thermal behavior of the anticorrosion coating and the steel pipe and the peel effect of the anticorrosive coating due to the cathodic protection and the applicability of the anticorrosion coating to the temperature of the conveying medium and the like so as to deteriorate the performance of the anticorrosive coating and deteriorate or even deteriorate the anticorrosive coating. Therefore, the selection of pipeline coating should first consider its adaptability to the transmission medium and the environment, we must take a comprehensive and specific consideration of the choice of coating options.

Meet the pipeline construction process of the environment and the requirements of the construction process conditions a coating must have sufficient hardness, bending resistance, impact resistance and wear resistance to resist transport, laying process of the pipeline to produce elastic or plastic deformation, cold bending, impact, backfill soil hard objects and soil friction on the role of anti-corrosion coating; anti-corrosion coating layer should have a certain degree of weather resistance to prevent storage, transportation and the entire construction cycle before the backfill, the atmospheric environment on the destruction of the coating; Coating material must have a complementary fillings and wounding materials, and complementary fill material should be able to meet the filling and running of the operation of the basic requirements of the coating, and easy to construct; Coating process to the extent of the pollution of the environment possible small.

Feasibility of the construction process in the selection of anti-corrosion program, in addition to considering the performance of the material itself, but also need to examine the feasibility of the construction process. Excellent performance of the coating needs mature supporting construction technology for protection, the current pipeline commonly used three PE, FBE, coal tar enamel and other types of coating, the construction equipment and construction technology in China with the international level.

Economic rationality in many programs to ensure that technology is feasible, economic rationality analysis and comparison must be carried out. Economic rationality also includes operation and construction of two aspects, in general, the maintenance of long-life design of pipelines should pay attention to running and maintenance costs of consideration; and maintenance conditions are good, short design life pipeline can be properly laid emphasis savings for initial investment. Long-distance pipelines can adopt different anticorrosive layers by different stages when they are subjected to different geological units or operating parameters. However, this section should not be too large. Otherwise, ① it is not convenient for the organization to apply and the daily management in the future. The cost of anticorrosive construction machinery is more amortized, leading to the increase of the cost of the whole anticorrosive project. ② The compatibility of the material of the pipeline filling material with that of the main anticorrosive coating materials should be considered: ③ The cathodic protection design needs to consider the insulation resistance different issues.

The legalization of environmental protection has also put forward new requirements for pipeline construction. Especially in densely populated areas, the choice of outer coating should pay more attention to environmental protection factors.

Pipe commonly used outer coating fused epoxy powder (FBE) coating epoxy powder from the solid epoxy resin, curing agent and a variety of additives by mixing, crushing processing, is a thermosetting material. The epoxy powder by electrostatic spraying in the preheated steel pipe, the use of heat pipe wall to make it melt and solidification molding. Its main advantages are: excellent adhesion to steel: good resistance to cathodic disbonding and cathodic shielding; excellent resistance to soil stress; low levels of environmental pollution. The main drawback is that due to the thin coating, the impact resistance is poor, easy to scratch: poor weather resistance, especially the long-term exposure to UV light: the polarity of the epoxy resin, water resistance is relatively poor. It is suitable for most soil environments, especially clayey soil with strong soil stress and directional drilling. Rock, swamp and high water level with caution. In the construction process should pay attention to the reasonable arrangement of the construction cycle, FBE anticorrosive pipe placed in the open time should not exceed 2 months, or need to take cover measures.

Three-layer PE coating is composed of epoxy powder, adhesive intermediate layer and polyolefin outer protective layer. The coating process is: electrostatic spraying of epoxy powder, in its gel time squeeze the adhesive melt outside the epoxy, the two produce chemical bonds and then the molten polyolefin extrusion in the uncured ethylene copolymer adhesive Outside, the two have good compatibility, thus forming a three-in-one three-layer PE coating.

Its main advantages are: excellent corrosion resistance epoxy powder and polyolefin materials, water resistance, abrasion resistance and impact resistance; corrosion resistance of chemical media is good; surface smooth, resistant to stress soil damage; Hydrocarbon structures have high insulating properties. Make the protection of the current required to protect low: a small degree of environmental pollution. The main drawbacks are: the coating process is complicated, the control of the process parameters more stringent; higher cost; the outer layer of polyolefin, it is difficult with the filler material to achieve the best bond. Suitable for all kinds of soil environment. Particularly suitable for demanding mechanical strength demanding environment. Such as stone section, broken (eggs) stone soil and other regions.

Double-layer epoxy powder coating is sprayed on the outer layer of the traditional single-layer epoxy powder with a polyolefin modified epoxy powder, making the water vapor permeability and impact resistance significantly improved. The coating process is: the inner layer of epoxy powder coating during the outer layer of powder coating, process parameters require more stringent. Its main advantages are: waterproof permeability, mechanical properties better than single-layer FBE: impact resistance, abrasion resistance, scratch resistance is very prominent; resistance to cathodic disbonding, no cathodic shielding. Resistance to soil stress: suitable for large diameter range. The main drawback is: more costly; less practical applications in China, the lack of large-diameter pipeline construction experience. Used in a variety of soil environments, especially for directional drilling and W square segments. Because of the high cost, generally only selected in special locations.

Coal tar enamel by coal tar pitch added coal tar, coal tar fractions and mineral pigments and fillers by heating boil made. The coating process is hot-lacquered enamel wound glass fiber reinforced material. Its main advantages are: good chemical stability, strong corrosion-resistant environment; resistance to plant penetration and anti-microbial erosion; low water absorption, and with the increase of water absorption slow increase; excellent anti-aging properties, long service life. The main disadvantage is: Susceptible to soil stress damage and mechanical damage. With high temperature softening, cold brittle tendency, not suitable for tropical and boreal use. Suitable for moderate temperature conditions, less demanding mechanical strength, poor corrosion conditions, especially in high groundwater levels, plants rhizome lush, biological activity frequent swamps and shrubbery lots. Consider environmental requirements, used with caution in densely populated areas.