Corrosion Failure Analysis of Pipe Tee in a Natural Gas Field in West China

Corrosion Failure Analysis of Pipe Tee in a Natural Gas Field in West China

A pipe burst accident occurred in a straight-inserted pipe tee of a gathering and transportation pipeline in a western natural gas field. The main pipe of the pipe tee was made of 16Mn steel and the branch pipe was made of 316L+L416 bimetallic composite pipe. By observing the morphology of the failed tees, measuring the wall thickness of the failed pipe tees, using electrochemical methods to study the galvanic corrosion behavior of 16Mn steel and 316L stainless steel in the simulated gas field environment, using the FLUENT software to analyze the in-line, 45° and 30° respectively. Numerical simulation calculation is carried out on the flow characteristics of the internal fluid of the pipe tee, combined with the investigation of on-site corrosion, the reasons for the failure of the pipe tee are analyzed. The results show that there is a defect of galvanic corrosion in the material selection design of the pipe tee, and its in-line structure causes the fluid flow pattern inside the pipe tee to change drastically. Natural gas forms a vortex at the location of the pipe burst, and the produced water from the gas field gradually adheres to the inner wall of the pipeline due to the low-speed vortex, and forms an acidic corrosive environment with CO2, which leads to galvanic corrosion of the pipeline. The wall surface shearing force on the pipe wall at the burst pipe is relatively large, and the fluid forms a scouring effect on the pipe inner wall. The pipe tee fails under the synergistic effect of galvanic corrosion and mechanical erosion.

Introduction

There are abundant natural gas resources underground in western my country. In the process of natural gas extraction, CO2 and other gas and formation water thousands of meters underground are extracted along with natural gas [1]. These gases and formation water are collected by the gas gathering pipeline of the gas field, and then transported to the processing station for gas-water separation to obtain high-purity natural gas. The pipe tee parts of the gas pipeline become weak parts of the pipeline due to the welding of dissimilar materials and the confluence of high-temperature and high-pressure gases, and are more likely to burst accidents. In the gas pipeline, the high-speed flow of the conveying medium will scour the pipeline, and the change of the conveying medium’s flow rate will change the flow state of the medium, which will have an important impact on the corrosion of the pipeline.
The natural gas produced by a gas field in the western region has a water content of about 7.4%, a CO2 partial pressure of about 0.1 MPa, and a chloride ion concentration of about 150 g/L in the formation water. The produced medium is highly corrosive. In order to prevent corrosion and comprehensively consider economic benefits, the pipe tee branch pipe is made of 316L stainless steel, and the main pipe is made of 16Mn steel. The pipe tee structure is in-line, that is, the branch pipe and the main pipe are connected at an angle of 90° along the entrance direction. The inner diameter of the main pipe is 422 mm, the wall thickness is 26 mm, and the inner diameter of the branch pipe is 144.3 mm. The design flow rate of the branch pipe inlet is 5.3 m/s, the medium temperature is 95 ℃, the design flow rate of the main pipe inlet is 5.6 m/s, and the medium temperature is 61 ℃. After the branch pipe transmission medium merges into the main pipe, the velocity of the transmission medium in the pipe is 6.14 m/s and the temperature is 65 ℃. However, during the production process, all the three links of the gathering and transportation pipelines had obvious local thinning. The thinning rate of the pipelines in some locations reached about 8 mm/a, and even a pipe burst accident occurred 40 months after being put into use.

Fig.1 Three views of the pipe tee
It is unclear whether such rapid local thinning is due to corrosion or fluid erosion. Therefore, in this study, the main methods of observing the morphology of the failed pipe tees, measuring the wall thickness, simulating electrochemical experiments, and computer simulating the flow pattern were used to analyze the causes of the failure of the pipe tee.

Experimental method

The experimental material is a failed tee from a gas field in the west. The main pipe is made of 16Mn steel. Its chemical composition (mass fraction, %) is: C 0.144, Si 0.313, Mn 1.548, P 0.014, S 0.012, Ti 0.002, Nb 0.002 , Al 0.028, V 0.005, the rest is Fe. The branch pipe is made of 316L stainless steel, and its chemical composition (mass fraction, %) is: C 0.020, Si 0.504, Mn 1.059, P 0.036, S<0.001, Ni 11.90, Cr 17.19, Mo 2.093, and the rest is Fe.
Macroscopically inspect the failed tee and observe its macroscopic appearance. A wire cutting machine was used to cut a 30 mm×30 mm square sample from the failed tee main pipe, and the surface was analyzed by electron microscopy on the scanning electron microscope. Use a spiral micrometer to measure the wall thickness of the failed tee main pipe. The three views of the failed pipe tee are shown in Figure 1. The line segment starting from A, B, C, D, E (near the pipe tee fillet weld) is measured. Analyze the pipe wall thickness at the corresponding positions of AA’, BB’, CC’, DD’, EE’, and analyze the thinning law of the pipe tee’s main pipe
Process 16Mn steel and 316L stainless steel into electrodes with a size of 10 mm×10 mm (effective working area 1.0 cm2), encapsulate them with epoxy resin, and then grind them step by step with water sandpaper to 2000#. Use CST500 electrochemical noise and galvanic corrosion detector to conduct galvanic corrosion test. The experiment uses a three-electrode system, two working electrodes are 16Mn steel and 316L stainless steel (WE1 and WE2), and the reference electrode is an Ag/AgCl electrode. The galvanic corrosion current density was measured when the working surfaces of the two working electrodes were 0, 10, 20 and 30 cm apart. The data collection frequency was 0.1 Hz and the experiment duration was 36 h. The corrosive medium simulates the formation water of a gas field in the west, and 247.18 g/L NaCl is prepared as the electrolyte solution. Before the experiment, deoxygenate with nitrogen for 0.5 h. During the experiment, the electrode system and the temperature of the solution were controlled by a constant temperature water bath and kept at 65 ℃, and 0.1 MPa CO2 was always introduced.
Use FLUENT software to simulate the internal flow field of the pipe tee. According to the characteristics of the pipe tee pipe fittings, the pipe tee branch pipe is the center, and the 5 m long pipeline upstream and downstream of the main pipe is the research object. The physical model of the in-line pipe tee connection is established using Gambit. Set the boundary conditions according to Table 1, and perform mesh division [2,3], and establish 356,231 mesh volume elements, as shown in Figure 2a. In the calculation process, the right nozzle and branch nozzle of the main pipe are defined as the inlet, and the left nozzle of the main pipe is the outlet. Take the intersection of the main pipe and the branch pipe as the Cartesian origin; take the main pipe’s axis as the X axis, the downstream of the main pipe as the negative direction of the X axis, and the upstream as the positive direction of the X axis; take the branch pipe’s axis as the Y axis and the fluid flow direction in the branch pipe as the Y axis positive Direction: The plane formed by the axis of the main pipe and the axis of the branch pipe is defined as the XOY plane, that is, the Z plane. On the basis of the in-line pipe tee physical model, around the Cartesian coordinate Z axis, the branch pipe is rotated 45° and 60° counterclockwise respectively to create a pipe tee physical model with the inlet direction of the branch pipe and the main pipe at 45° and 30°. Set the boundary conditions according to Table 1, and divide the mesh, as shown in Figure 2b and c. Through iterative calculation, output the velocity cloud graph, turbulence intensity cloud graph, temperature cloud graph and wall shear force cloud graph of the pipe tee specific cross-section fluid with different structures.

Fig.2 Grids of three tee joints with different structures: (a) upright type tee, (b) 45° lateral tee, (c) 30° lateral tee

Fig.3 Field inspection of tube explosion (a), the pipe tee joint with failure (b), inner wall occurred thickness reduction of the pipe tee joint (c) and inner wall directly faced by the mouth of branch pipe (d)

Experimental results

Field observation results

The scene of the accident

The scene of the accident is shown in Figure 3a. The position of the burst pipe is located downstream of the main pipe and along the pipe tee fillet weld (the junction between the branch pipe and the main pipe) at 8 o’clock (Figure 3b). The downstream main pipe near the pipe tee fillet weld from 7 to 11 o’clock is thinning significantly, and as the distance increases, the thinning area gradually expands in a hyperbolic shape (Figure 3c); the branch pipe does not thin out; and the branch pipe mouth Although the opposite supervisor is thinning, it is not serious (Figure 3d).

Morphology of failed tee

Observed by naked eyes, the pipes located in the hyperbolic thinning area are severely corroded and accompanied by fluid erosion. The morphology of the local area is scaly, and the edge of the area is washed out by the fluid to a clear boundary (Figure 4a). The inner wall of the upstream pipeline symmetrical to the hyperbolic thinning zone has obvious pitting corrosion (Figure 4b), which is a common corrosion form of carbon steel in Cl-containing media.

Fig.4 Macro morphologies of the failure tee: the hyperbolic-type thinned area (a), inner wall of the pipe tee upstream (b)

Fig.5 Micro morphologies of the failure tee:inner wall of the pipe tee upstream (a), the hyperbolic-type thinned area with gully (b), wash zone (c) and pitting (d)

Fig.6 Thickness (a) and thinning rate (b) of the main pipe corresponding to the position of AA′~EE′ in Fig.1
The microscopic morphology of the inner wall of the upstream pipeline symmetrical to the hyperbolic thinning zone is shown in Figure 5a. More pitting pits of larger size can be found, and as the pitting corrosion continues to expand, the pitting pits merge with each other. The pipes in the hyperbolic thinning area show ravine-like morphology (Figure 5b), and the surface shows river-like scour marks (Figure 5c). In addition, there are also pitting pits in the fluid wash area (Figure 5c,d).

The thinning rule of the failed tee main pipe

The thickness of the thinnest part of the pipe wall (the blasting point) is only 1.68 mm, which is 100 mm from the fillet weld of the pipe tee and about 175 mm from the axis of the branch pipe. The length of the hyperbolic thinning zone on the inner wall of the main pipe is about 400 mm, and there is no obvious thinning in other positions. The main pipe wall thickness at the corresponding positions of the line segments AA’, BB’, CC’, DD’, EE’ in Figure 1 is shown in Figure 6a. According to the design wall thickness of the pipe tee main pipe (26 mm), the rate of thinning of the wall thickness of the main pipe at the corresponding position of each line segment in Figure 1 since the pipe tee was put into use 40 months is calculated, as shown in Figure 6b. It can be seen from Figure 6 that as the measuring point is far away from the pipe tee fillet weld, the amount of thinning of the main pipe gradually decreases, and the thinning speed also gradually decreases.

Fig.7 Open circuit potential of 16Mn steel and 316L stainless steel

Galvanic corrosion simulation results

The open circuit potential of 16Mn steel and 316L stainless steel is shown in Figure 7. The potential of 16Mn steel is relatively negative, and the potential of 316L stainless steel is relatively positive. When the two are connected, a galvanic couple will be formed and galvanic corrosion will occur. Among them, 16Mn steel is the anode and 316L stainless steel is the cathode. According to Faraday’s law, there is the following relationship between corrosion current density and corrosion rate [4]:
(1) C R = K 1 I E w / ρ
In the formula, the value of K1 is 3.27×10-3[5], mmg/ (µAcma); I is the current density, µA/cm²; ρ is the density, g/cm³.
According to formula (1), the galvanic corrosion current density of different galvanic couples is converted into corrosion rate. The distance between the faces of 16Mn steel and 316L stainless steel increases, the galvanic corrosion rate gradually decreases. Correspondingly, the thinning amount and thinning rate of 16Mn steel due to galvanic corrosion gradually decrease with the increase of the distance from the opposite surface of the galvanic couple.

Fig.8 Contours of wall shear stress of the upright type tee joint

Fig.9 Contours of velocity magnitude on sectionsof Z=0 (a), X=-80 mm (b), X=-200 mm (c), X=-400 mm (d) of the upright type tee

Flow regime simulation results

Figure 8 is the shear force cloud diagram of the internal fluid of the in-line tee on the pipe wall. It can be seen from the figure that the fluid has a relatively large shear force on the wall of the downstream main pipe located in the direction of the pipe tee fillet weld from 7 to 11 o’clock, and this The area expands roughly in a hyperbolic shape. Among them, along the pipe tee fillet weld at 8 o’clock and 10 o’clock, that is, the pipe wall in the 5 o’clock and 7 o’clock direction of the downstream main pipe receives the greatest shear force.
Figure 9a is the velocity cloud diagram of the in-line pipe tee Z=0 mm cross-section fluid. It can be seen that when the branch pipe and the two high-speed fluids in the main pipe merge, a low-speed vortex area is formed on the wall of the downstream main pipe close to the branch pipe. From the pipe tee X=-80 mm cross-section (Figure 9b), X=-200 mm cross-section (Figure 9c) and X=-400 mm cross-section (Figure 9d), the fluid velocity cloud diagram shows that the vortex zone affects the main pipe at 6 o’clock. The direction (9 o’clock direction of pipe tee fillet weld) is the center, including the wall of the main pipe in the direction of 5 to 7 o’clock (8 to 10 o’clock direction of pipe tee fillet weld), and the low-speed eddy current zone is about 500 mm long. With the increase of the absolute value of X, the vortex influence zone on the wall of the downstream mother pipe increases first and then decreases. The cross section of the pipe tee X=-200 mm is the vortex central zone.
From the turbulence intensity cloud diagram of the in-line pipe tee Z=0 mm cross-section fluid (Figure 10a), it can be seen that the turbulence intensity of the fluid near the downstream main pipe wall of the branch pipe is greater. From the pipe tee X=-80mm cross-section (Figure 10b), X=-200 mm cross-section (Figure 10c) and X=-400 mm cross-section (Figure 10d), the turbulence intensity cloud diagram of the fluid shows that the turbulence intensity distribution and velocity distribution of the fluid exist Correlation, the turbulence intensity of the fluid in the center of the vortex is the largest, and the turbulence has a significant impact on the wall of the main pipe in the direction of 5 to 7 o’clock, which is consistent with the vortex influence area.
Figure 11a is the temperature cloud diagram of the fluid in the Z=0 mm cross-section of the in-line tee. It can be seen that there is a high temperature influence zone in the downstream main pipe close to the branch pipe. Observing the pipe tee X=-80 mm cross-section (Figure 11b), X=-200 mm cross-section (Figure 11c) and X=-400 mm cross-section (Figure 11d) the temperature cloud diagram of the fluid shows that the high temperature affected zone is related to the vortex affected zone The temperature gradient of the fluid in the direction of 5 to 7 o’clock in the main pipe changes drastically.

Fig.10 Contours of turbulent kinetic energy on sectionsctions of Z=0 (a), X=-80 mm (b), X=-200 mm (c), X=-400 mm (d) of the upright type tee
If the merging angle of the pipe tee is changed to 45°, the wall shearing force in the pipe tee will change greatly compared with the in-line tee (Figure 12a). The shearing force of the fluid on the wall of the pipeline is significantly reduced, and the area of the region with strong shearing action is significantly reduced. If you continue to reduce the confluence angle to 30°, the area where the fluid has a strong shearing effect on the pipe will continue to decrease, and the shearing effect will continue to weaken (Figure 12b).

Fig.11 Contours of static temperature on sections of Z=0 (a), X=-80 mm (b), X=-200 mm (c), X=-400 mm (d) of the upright type tee

Fig.12 Contours of wall shear stress of the 45° lateral tee (a) and 30° lateral tee (b)
The velocity cloud diagram of the fluid on different sections of the 45° tee is shown in Figure 13. It can be seen that after the two high-speed fluids in the pipe tee merge, the low-velocity vortex intensity of the fluid on the wall of the downstream main pipe close to the branch pipe is significantly weakened, and the influence range of the vortex zone is obvious. Decrease. The velocity cloud diagram of the fluid on different cross sections of the 30° tee is shown in Figure 14. It can be seen that after the two high-speed fluids in the pipe tee merge, the low-speed vortex disappears.

Fig.13 Contours of velocity magnitude on sections of Z=0 (a), X=-80 mm (b), X=-200 mm (c), X=-400 mm (d) of 45° lateral tee
Figures 15 and 16 are the turbulence intensity cloud diagrams of the fluids with different cross-sections of the 45° tee and 30° tee, respectively. It can be seen that the turbulence intensity of the fluid in the pipe remains basically the same, and there is no greater turbulence intensity on the downstream main pipe wall near the branch pipe. area.

Discussion

The material of the failed tee main pipe is 16Mn steel, and the branch pipe is 316L stainless steel. Since the self-corrosion potentials of 16Mn steel and 316L stainless steel are not equal, there is a risk of galvanic corrosion in the design of the pipe tee material [6,7]. The simulation results of the in-line pipe tee flow pattern show that after the two high-speed fluids in the branch pipe merge with the main pipe, a vortex area with a length of about 500 mm with decreasing velocity and temperature is formed in the 5 to 7 o’clock direction of the downstream main pipe. First of all, due to the appearance of turbulence and the decrease of flow rate, the probability of collision between water vapor particles suddenly increases, and they will gather into larger water droplets and condense; secondly, compared to branch pipes, the temperature of natural gas will decrease after entering the gathering and transportation pipeline, and water vapor in natural gas The saturated vapour pressure also decreases, causing the water vapor to condense more easily; finally, the turbulence causes the acidic solution to stay in the area for a long time and interact with the pipe wall in the area for a long time. Under the combined action of these factors, the water vapor adheres to the inner wall surface of the pipeline after condensation, while a large amount of corrosive gases such as CO2 dissolves into the condensed water, thus forming a typical acidic corrosive environment, which provides for the galvanic corrosion of 16Mn steel and 316L stainless steel Necessary conditions.

Fig.14 Contours of velocity magnitude on sections of Z=0 (a), X=-80 mm (b), X=-200 mm (c), X=-400 mm (d) of 30° lateral tee

Fig.15 Contours of turbulent kinetic energy on sec-tions of Z=0 (a), X=-80 mm (b), X=-200 mm (c), X=-400 mm (d) of 45° lateral tee
In addition, compared with the environment on the other side of the gathering and transportation pipeline, the temperature in the low-speed vortex zone is relatively high, and the higher temperature combined with the acidic medium can accelerate acid corrosion more easily. The length of the thinned area of the main pipe is about 400 mm observed on site, and the thinned area is located in the direction of the pipe tee fillet weld from 7 to 11 o’clock, that is, from 5 to 7 o’clock of the downstream main pipe, which is in good agreement with the flow pattern simulation results. The galvanic corrosion simulation test results in the laboratory also show that the thinning rate of 16Mn steel decreases with the increase of the distance between the opposite sides of the galvanic couple, which is also consistent with the thinning law of the main pipe observed on site. The above results show that galvanic corrosion is one of the reasons that cannot be ignored for the thinning failure of the pipe tee main pipe.

Fig.16 Contours of turbulent kinetic energy on sections of Z=0 (a), X=-80 mm (b), X=-200 mm (c), X=-400 mm (d) of 30° lateral tee
However, comparing the results of field observation and galvanic corrosion test, there is a big discrepancy between the two. The thinning rate of the failed tee main pipe is greater than the corrosion rate of 16Mn steel, as shown in Figure 17, which shows that there are other reasons for the failure of the pipe tee. According to the wall shear force cloud diagram of the fluid in the pipe tee (Figure 8), after the two high-speed fluids in the branch pipe and the main pipe merge in the pipe tee, a strong shearing effect is produced on the wall of the main pipe in the hyperbolic region. Under the continuous cutting action of the high-speed fluid, the main pipe in the hyperbolic area will be thinned due to mechanical erosion. The hyperbolic area is in good agreement with the thinning area of the failed tee main pipe observed in the field. The microscopic morphology of the pipe in the hyperbolic region also illustrates the existence of mechanical erosion. Corrosion causes the surface of the pipe wall to be concave and rough, which leads to greater changes in the flow pattern; after the flow pattern changes, the water vapor is more likely to condense, and the wall shear force of the airflow causes the corrosion products to be blown away, making the corrosion products unable to stop the corrosion , The fresh metal is exposed again, which accelerates corrosion. Comparing the upper and downstream inner wall surfaces of the pipe tee main pipe, it can be found that the ravine-like microscopic morphology of the pipeline in the hyperbolic region is formed by the local corrosion pits after being washed by the fluid for a long time. In addition, the acidic solution will collect in the 7 o’clock direction of the downstream main pipe due to gravity, that is, the 8 o’clock direction of the pipe tee fillet weld, which results in the erosion of the main pipe wall surface of the pipe tee X=-200 mm in the 7 o’clock direction. And corrosion is the most serious, resulting in serious thinning, and finally perforation failure. Field observation results strongly confirmed this analysis. The location of the pipe tee burst pipe was located downstream of the main pipe, along the pipe tee fillet weld at 8 o’clock, that is, the main pipe at 7 o’clock, and was 175 mm away from the branch pipe axis. The synergy of galvanic corrosion and fluid scouring is the root cause of the failure of the pipe tee.

Fig.17 Thinning rate of the main pipe corresponding to theposition of CC′ in Fig.1 and corrosion rate of 16Mn steel
Comparing the flow pattern simulation results of the in-line tee, the 45° tee and the 30° tee, it can be seen that the in-line tee structure causes the flow pattern to change drastically after the two high-speed fluids in the branch pipe and the main pipe merge. , A low-speed vortex zone is formed on the wall of the downstream main pipe close to the branch pipe, and the turbulent intensity of the fluid in this area and the shear force on the pipe wall are relatively large, which will eventually lead to galvanic corrosion and mechanical erosion of the pipe tee. As the angle between the branch pipe and the main entrance direction decreases, the low-speed eddy current disappears, and the shear force of the fluid on the pipe wall gradually decreases. The galvanic corrosion-scouring synergy of the pipe tee can be effectively controlled.

Conclusions and recommendations

• (1) There is a galvanic couple in the material selection design of the failed tee, and its in-line structure results in a drastic change in the flow pattern after the two high-speed fluids in the branch pipe merge with the main pipe, which provides for galvanic corrosion and mechanical erosion of the pipe tee pipe condition.
• (2) The thinning area of the failed pipe tee main pipe is the low-velocity vortex area of the fluid in the pipeline. The natural gas associated gas CO2 and moisture adhere to this area to form an acidic corrosion solution, which causes the 16Mn steel and 316L stainless steel to undergo galvanic corrosion, which eventually leads to 16Mn steel is thinned.
• (3) The shear force and fluid turbulence intensity of the fluid on the inner wall of the pipe at the burst pipe is the largest. The inner wall of the pipeline in this area is subject to strong cyclic erosion. In addition, the acid solution formed by the vortex accumulates at the burst pipe due to gravity, making the acid corrosion of the pipeline in this area the most serious, and ultimately leading to the largest amount of thinning of the pipe at the burst pipe.
• (4) Under the synergistic effect of galvanic corrosion and fluid scouring, a pipe burst accident occurred at 7 o’clock in the downstream main pipe, that is, at 8 o’clock in the pipe tee fillet weld, 175 mm from the axis of the branch pipe.
• (5) In order to eliminate galvanic corrosion, dissimilar metals should be avoided when selecting materials for tee branch pipes and main pipes, or a coating should be applied to the inner wall of the pipe tee to avoid direct contact between the electrolyte solution and the metal. At the same time, the structural design of the pipe tee can be optimized by reducing the angle between the branch pipe and the inlet direction of the main pipe, so as to prevent the fluid from forming a low-speed vortex area inside the pipe and effectively weaken the fluid’s scouring effect on the pipe.

Author: HAN Nannan1, LIU Bin1, ZHANG Tao1,2,, WANG Zhongyi3, MENG Guozhe1,2, SHAO Yawei1

Source: China Pipe Tee Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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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