Failure analysis of gas pipeline in a gas collecting station

The gas collecting line of a gas collecting station failed and had obvious corrosion pits. The failure reasons of the pipeline in this section are studied by means of macroscopic morphology analysis, mechanical property analysis, hardness analysis and microstructure analysis. The results show that the main reason is the mechanical damage caused in the construction process for the damage of the anticorrosive coating on the outer surface of the pipe section. The main reason for the failure of “corrosion pit” inside the pipe section is that the wet gas transported contains CO2, which dissolves in water to generate H2CO3, and the pipe section is corroded in H2CO3 solution by electrochemical corrosion, which gradually develops into a more serious corrosion pit. In order to prevent the occurrence of such failure, 13Cr (martensite) material can be used to meet the corrosion resistance requirements, but also can appropriately increase the gas temperature in the tube to reduce the solubility of CO2 in water to prevent corrosion.


Introduction
Pipeline is the most economical and safe way to transport oil and gas resources, but it is also extremely vulnerable to the influence of external pressure, corrosion, third-party damage and natural disasters [1][2][3]. In oil and gas storage and transportation engineering, the chemical properties and metallographic structure of the gas collecting pipeline itself determine that this iron-based alloy is a kind of corrosion-prone alloy [4][5]. According to a large number of data analysis at home and abroad and field investigation and analysis of oil and gas fields, the main failure of gas gathering pipeline is corrosion failure [6][7][8]. Generally speaking, the failure of the gas gathering pipeline is mainly due to three reasons: 1. The natural gas contains CO 2 , which reduces the pH value of the water, which is weakly acidic and forms the corrosion product FeCO 3 [9]. 2. Natural gas contains H 2 S and CO 2 , which form an electrochemical reaction similar to a galvanic cell and chemical reaction that destroys the metal lattice, and corrodes the gas gathering pipeline [10]. 3. The natural gas contains CO 2 and CL, forming a "sweet gas" environment and corroding the gas gathering pipeline [11].
The external shale gas transmission pipeline in A area is the first shale gas transmission pipeline in China [12], one of the gas collection station gas collection pipeline failure. According to the information provided, the pipeline is put into operation in May 2019, and the design specification of the pipeline is Φ323.9×8 mm seamless steel pipe, and the steel grade is L360N. The design pressure of the gas gathering pipeline is 8.5 MPa, the design gathering and transportation capacity is 260×104 m 3 /d, and the length is 6.65 km. The transmission medium is raw material gas (moisture), uncontained sulfur, and the anti-corrosion material is 3PE. There are nine obvious corrosion pits on the inner wall of the failed pipe sample, and the outer surface anticorrosion layer is partially damaged.
In order to clarify the reasons of the failure of the gas gathering pipeline, this paper analyzes the composition and mechanical properties of the failed pipeline samples, determines the composition and distribution of the corrosion products, and combines the failure theory to analyze the failure reasons of the pipeline.

Macroscopic morphology analysis
The macro morphology of the failed pipeline is shown in Fig. 1 and Fig. 2. It can be seen from Fig. 2 that there are 9 obvious corrosion pits on the inner wall of the tube sample. After cutting the relatively serious corrosion pits, the minimum remaining wall thickness of all the corrosion pits are measured to be 2.72 mm, 2.66 mm, and 2.36 mm, respectively.  The anti-corrosion layer on the outer surface of the pipe sample is partially damaged, and part of the surface is stuck with a "soil layer with small gravel". "Soil layer with small gravel" is stuck on the surface, as shown in Fig. 3. After clearing the "soil layer with small gravel", it was found that the anti-corrosion layer on the outer surface of the pipe sample was not completely damaged. After the anti-corrosion layer was wore away, the metal outer surface of the pipe sample did not appear damaged or corroded (The surface rust appears after the anti-corrosion layer is worn away), as shown in Fig. 4. The anti-corrosion layer on the outer surface of the pipe section is damaged, and the local damage is serious and there is a "soil layer with small gravel", but not completely penetrated the anti-corrosion layer, and there is no corrosion or rust on the outer surface of the pipe section. It is inferred that the main reason for the damage of the anti-corrosion layer on the outer surface of the pipe section may be the mechanical damage during the construction process.

Test analysis of mechanical properties
Take four tensile test specimens from the tube body, the total length of the processed sample is = 70 mm, the width of the parallel length of the specimen = 15, and the transition arc radius = 6 mm, according to GB/228.1-2010 "Metal Material Stretching Test Part 1: Room temperature test method" to carry out the test. The test results are shown in Table 1; the hardness test specimens are taken from the pipe body, the sampling direction is a transverse circular ring, and the test is carried out in accordance with the requirements of the GB/T230.1-2004 Rockwell hardness test standard for metal materials. The test results are shown in Table 2; Take the impact performance test sample from the sample, the sampling direction is transverse (circumferential), and the size of the test piece is 5×10×55 mm. The test was carried out according to GB/T229-2007 "Charpy Pendulum Impact Test Method for Metallic Materials", and the test results are shown in Table 3.
It can be seen from Table 1 that the tensile strength, flexural strength ratio and elongation of the four tensile samples all meet the requirements, but the yield strength of sample 1 and sample 2 are all lower than the standard value, which should be determined to be caused by corrosion. As can be seen from

Chemical composition analysis
A sample was taken from the failed pipe sample, and the chemical composition of the sample was analyzed by a direct-reading spectrometer. The analysis result showed that the chemical composition of the failed pipe sample complied with the relevant regulations of GB/T 9711-2017 for steel pipes for pipeline transportation systems in the petroleum and natural gas industry. See the results Table 4.

Metallographic analysis
In order to improve the judgment of the microstructure condition, sample 1 and sample 2 are taken from the corrosion pits of the pipe section samples for structural analysis, as shown in Fig. 5. According to GB/T13298-1991 "Metal Microstructure Inspection Method", the microstructures of the two samples are evaluated as ferrite plus pearlite, as shown in Fig. 6 and Fig. 7. According to GB/T 10561-2005 "Determination method of non-metallic inclusions in steel", the non-metallic inclusions in the two samples are evaluated as D-type inclusions (spherical oxides), and the grade is 1.5; according to GB/T 6394-2002 "Method for the determination of the content of non-metallic inclusions in steel" to evaluate the grain size of the two samples, the measured grain size is 7 grades.

Scanning electron microscope and energy spectrum analysis
In order to understand the reason for the corrosion of the tube sample, sample 1 was intercepted in the non-corrosive area of the tube sample, and the results of scanning electron microscope and energy spectrum analysis were shown in Fig. 8 and Table 5. The results show that the surface of sample 2 is flat without any layered or massive corrosion products. Sample 1 is mainly Fe, C, O and a small amount of Si and Mn. Sample 2 was intercepted at the corrosion pit of the pipe sample, and the results of scanning electron microscopy and energy spectrum analysis were shown in Fig. 9 and Table 5. The results show that there are gray layered and massively distributed corrosion products in the pits on the inner surface of sample 1. The gray corrosion products at the bottom of the pits are mainly Fe, O, C and a small amount of Al, Si, P, S, and Mn. It shows that the corrosion type of the gas gathering pipeline is pitting corrosion. A block sample is taken from the pipe corrosion pit, and the corrosion product is analyzed for phase. The results show that the corrosion products are mainly FeCO 3 , Fe 3 O 4 , FeO, FeO(OH).

Corrosion mechanism analysis
Based on the above analysis, the corrosion products of the corrosion pits in the gas collector are mainly Fe, O, and C, and also contain a small amount of Al, Si, P, S, and Mn. The corrosion products are mainly FeCO 3 , Fe 3 O 4 , FeO, FeO(OH). Although the corrosion products contain a small amount of S, according to the data, the natural gas in the pipeline uncontained S, which excludes the possibility of the sulfide corrosion. Excluding the possibility of sulfide corrosion, the detected S is likely to come from SO 4 2in the working environment, which has no direct relationship with the corrosion failure of the pipe sample. Since the natural gas composition does not contain O 2 , it is judged that the iron oxides in the corrosion products are produced by secondary atmospheric corrosion and surface corrosion products conversion.
Excluding the possibility of sulfide and O 2 corrosion, combined with the XRD analysis results, it can be judged that the drill pipe corrosion is mainly caused by CO 2 corrosion. The gas collecting pipeline of this gas collecting station conveys wet gas. The CO 2 in the gas gathering pipeline is dissolved in water, which reduces the pH value of the water and becomes weakly acidic, forming the corrosion product FeCO 3 . CO 2 itself does not It does not corrode the pipeline, but it is extremely corrosive to metal materials after being dissolved in water, and the corrosion of low-carbon alloy steel by CO 2 is mainly pitting corrosion [13][14][15]. CO 2 corrosion is also called Sweet Corrosion in the oil and gas industry [16][17][18]. Its essence is that CO 2 dissolves in water to generate H 2 CO 3 , and metal electrochemical corrosion occurs in H 2 CO 3 solution. Regarding the anodic reaction in the CO 2 corrosion process, there is currently no consensus [19][20][21]. It is generally believed that in a solution containing CO 2 , the anode reaction is: Fe+OH →FeOH+e, FeOH→FeOH + +e, FeOH + →Fe 2+ +OH .
Total anode reaction: Fe→Fe 2+ +2e. At the same time, during the CO 2 corrosion process, the corrosion rate of steel is controlled by the cathodic reaction, which is as follows: 2H 2 CO 3 +2e→2HCO 3  Among them, HCO 3has a great influence on the cathode reaction, and the cathode reduction reaction is controlled by the diffusion of HCO 3 -. At the beginning of corrosion, Fe(HCO 3 ) 2 with strong bonding force has already formed on the metal surface, and the film can change: Fe(HCO 3 ) 2 +Fe→2FeCO 3 +H 2 , The formed FeCO 3 film has poor bonding force with the metal matrix. Since the volume of FeCO 3 is smaller than that of Fe(HCO 3 ) 2 , the volume shrinks during the conversion process, forming a poorly protective FeCO 3 film with micropores, which leads to the corrosion of the gas gathering pipeline [22][23][24][25].

Conclusions
1) The chemical composition, metallographic structure, Rockwell hardness and impact performance of the gas gathering pipeline meet the requirements of relevant standards, and the yield strength in the tensile performance is lower than GB/228.1-2010 "Metallic Material Tensile Test Part 1: Room Temperature" The standard value of the test method should be judged to be caused by CO 2 corrosion.
2) The gas collecting pipeline of this gas collecting station is mainly corroded by CO 2 . The FeCO 3 film formed by the gas gathering pipeline has a poor bonding force with the metal matrix, because the volume of FeCO 3 is larger than that of Fe(HCO 3 ) 2 . The volume is small, and the volume shrinks during the conversion process, forming the poorly protective FeCO 3 with micropores. FeCO 3 can have a "DC battery effect" with iron, leading to electrolytic corrosion.
3) Through the experimental analysis of the gas collecting pipeline of the gas collecting station, corresponding tests can be made to analyze the corrosion causes of other gas gathering pipelines.

Suggested improvement measures
1) In order to prevent the CO 2 corrosive environment from damaging the gas gathering pipeline, corrosion-resistant alloys are used as pipeline materials. According to related literature, nickel-based alloys and 22Cr have the best corrosion resistance, but they are not economical. 13Cr (Martensite) can be used. It can fully meet its corrosion resistance requirements.
2) Corrosion inhibitors can be injected into natural gas to alleviate metal corrosion, but corrosion inhibitors are not widely used. They may be effective in one area but not in another area. It is necessary to simulate the actual situation of the oil field before use. Various corrosion inhibitors are evaluated and screened under working conditions. Haochen Wu postgraduate student, studing in Southwest Petroleum University, mainly engaged in pipe string and pipeline mechanics research work.
Jichuan Li bachelor's degree in Process Equipment and control Engineering, graduated from Sichuan university.

Yanjun Chen
Assist engineer in Southest Oil & Gas Field Company, Petrochina Safty, Environment and Tech Supervision Research Institute Southwest Inspection Center for High-risk Devices.