Analysis on leakage of girth weld of a product oil pipeline

In order to explore the causes of leakage in the process of operation of a finished oil pipeline, the failure samples were analyzed by means of macroscopic observation, nondestructive testing, mechanical property analysis, metallographic analysis, scanning electron microscope and energy spectrum analysis. The results show that the leakage failure crack is located at the arc starting and closing of the circumferential weld. The results show that the reason for the failure of steel pipe leakage is that there are welding hot cracks in the circumferential weld and the inner surface of the circumferential weld is not formed well. During the long-term service, due to the internal and external loads, the stress concentration of the welding hot cracks located near the weld tumors is concentrated, and the expansion causes the leakage failure of the circumferential weld joint.

Failure overview

Oil and gas pipeline transportation is one of the important ways of oil and gas transportation. For pipelines, circumferential welds are weak links. Due to the influence of different climate conditions, deviation of pipe mouth group, welder technical level and other factors, the quality deviation of the pipe girth will be caused. If there is welding defect or poor welding forming, it will easily lead to the failure of the pipeline at the circumferential weld, Causing loss of personnel and property.
During the internal inspection and excavation verification of k675 + 850 of Lanzhou Chengdu Chongqing pipeline, it is found that there is slight oil leakage at the joint weld between the elbow and the straight pipe section, which is Φ 508mm × 7.9MM spiral submerged arc welding pipe, material L415, design pressure 10MPa, medium for oil, normal temperature operation. 3PE anticorrosion is adopted for the pipeline, and the specification of pipe bend is adopted Φ 508mm × 9.5mm, bend angle 24 °, pipeline operation 17 years. The leakage ring weld is cut and replaced and sent to the laboratory for analysis of the leakage reason. The inspection and analysis items include macroscopic inspection, nondestructive testing, physical and chemical property test, metallographic analysis and fracture analysis.

Macroscopic inspection of pipe

Sample delivery section overview

The pipe section sample is shown in Figure 1. The leakage ring weld is about 100cm away from the port of the straight pipe section. The anticorrosive coating of the joint joint of the circumferential weld has been removed, and the anticorrosive coating in other positions is damaged.
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Figure.1 Physical photos of sample delivery pipe section

Macro inspection of steel pipe

After the steel pipe outer anti-corrosion coating is removed and polished and rusted, the residual height, staggered edge and width of the weld are measured. The outer surface of the weld is basically neat and even. Observe the internal surface of weld, the surface of pipe body and the position of the circumferential weld, which basically shows uniform corrosion morphology. There are large weld tumors at 12o’clock on the inner surface. As shown in Figure 2, the circumferential length of the weld tumor is 90mm and the maximum height is 8.2mm, which is larger than the thickness of the pipe wall itself, indicating that the shape and quality of the inner surface is poor.
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Fig.2 appearance of internal solder joint in 12 points of circumferential weld

NDT of circumferential weld

According to SY / t4109-2017 nondestructive testing for oil and gas steel pipeline, magnetic particle testing and X-ray inspection are carried out for leakage ring welds. Through magnetic particle inspection, the crack length of the outer surface of the leakage ring weld is 52mm, and the crack is exactly located at the weld tumor on the inner surface of the ring weld, as shown in Figure 3. X-ray examination found that there was an over standard defect at 12 points of leakage ring weld, which showed that it was a crack defect with a length of 82mm and the evaluation grade of grade IV. The results are shown in Figure 4.
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Figure.3 MT results of leakage ring weld
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Figure.4 X-ray test results of leakage ring weld

Performance test of circumferential weld

In order to analyze the performance of the circumferential weld, according to GB/T 31032-2014 welding and acceptance of steel pipes, the tensile and Charpy impact performance tests were conducted near the parts far away from the failure and failure parts, and the metallographic structure of the circumferential weld was analyzed.

Tensile property test

Tensile properties of 0-3, 3-6, 6-9, 9-0 and circumferential welds near cracks are tested by using uh-f500kn material testing machine, and according to GB/T 228.1-2010. See Table 1 for test results. According to the standard, the tensile properties of each position of the circumferential weld meet the standard requirements.
Table.1 Tensile test results of circumferential weld
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Note: ① if the standard requires that the sample is broken in the base metal and the tensile strength is greater than or equal to the minimum tensile strength specified by the pipe, the sample is qualified; ② The minimum tensile strength of base metal is 520mpa.

Charpy impact test

The Charpy impact test was conducted on the center and heat affected zone of 0-3, 3-6 welds of circumferential weld according to GB/T 229-2007 by ZBC25252-b impact tester. The test temperature is the same as that of the tube body, taking 0 ℃, and the test results are shown in Table 2. The comparison standard shows that Charpy impact work value of circumferential weld meets the requirements.

Table 2 Charpy impact performance test results of circumferential weld J

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Note: the standard requires that the minimum and average Charpy impact work values of each notch position be greater than 30J and 40J respectively.

Metallographic analysis

The metallographic structure of the circumferential weld at 3 points, 6 points and the girth welds near the cracks (near 12 points) was analyzed by using ols4100 laser confocal microscope. The results show that the microstructure of the three, six and the cover near the cracks are granular bainite, and the bottom welding structure is pearlite + ferrite + intracrystal nucleated ferrite, The microstructure of the filler welding is pearlite + ferrite + granular bainite, and the grain size of the circumferential weld near the crack is basically the same as that of 3 and 6.
In the circumferential weld samples near the failure crack, there are many micro cracks, and the decarbonization phenomenon exists around the micro cracks. The metallographic photos of microcracks are shown in Figure 5.
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Fig.5 micro crack morphology near failure crack
The metallographic photos of crack tip are shown in Fig. 6. The metallographic structure of the failure crack tip is visible. The metallographic structure at and near the crack tip is normal, and the crack fracture form is intergranular fracture.
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Fig.6 metallographic structure of failure crack tip

Fracture analysis

Analysis of fracture macromorphology

The fracture analysis is conducted on the leakage of the annular weld of the finished oil pipeline. It is found that the leakage part is located at the weld joint of the ring weld. The fracture at the leakage position is sampled and observed. The macroscopic appearance of the fracture is shown in Fig. 7. The welding arc pattern can be seen clearly from the leakage fracture and nearby shown in Figure 7. The intersection position of arc pattern is located in the center of crack. Here is the starting and closing position of welding construction. There are large welding tumors in this position, which may be caused by too large welding heat input or long welding dwell time.
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Fig.7 macroscopic appearance of fracture at leakage location

SEM analysis of fracture

The cross section and fracture of the crack near the fracture surface of the failure sample pipe are analyzed by SEM. The analysis results are shown in Figure 8. It can be seen from figure 8 that the crack originates from the root of the weld toe on the inner surface of the circumferential weld, and extends along the root of the weld toe to the center of the weld. Due to the long leakage time, the surface of the fracture surface is corroded seriously, and the fracture surface has been covered by dense black material, It is impossible to see the brittle fracture or ductile fracture characteristics.
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Fig.8 crack and fracture morphology near fracture

Fracture energy spectrum analysis

The energy spectrum analysis of inclusions, non inclusion fracture surfaces and cracks inside the crack sections without open fracture is carried out respectively. The results show that the main components of fracture surface are C, Fe and O, which are caused by the long-term immersion of the finished oil, and the fracture is free of welding drug inclusions. See Figure 9 for the specific energy spectrum analysis results.
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Figure.9 fracture energy spectrum analysis results

Result analysis

The cracks are located at the starting and closing positions of 12 o’clock welding of the circumferential weld. The forming quality of the inner surface of the circumferential weld is poor, and there are large weld tumors on the inner surface, and the crack and the welding spot are in the same height. In metallographic observation, it is found that there are many micro cracks near the failure cracks and decarbonization around the microcracks. From the micro crack morphology and the metallographic structure around the crack, these microcracks are welding hot cracks.
The welding hot crack is a cracking phenomenon in the high temperature stage of welding process, which occurs mostly near the solid phase line, and its characteristic is cracking along the austenite grain boundary. During welding, the intergranular ductility of the material is weak, which can not bear the strain of the material at that time. In this state, the cracks will be generated and propagated along the junction of crystallization when the temperature is cooled. The factors that produce welding thermal cracks mainly include the influence of harmful impurities (mainly s and P) and the influence of welding process (such as fusion ratio, welding speed, welding line energy, cooling speed, etc.). At the same time, decarburization near the hot crack also indicates that the welding temperature of the circumferential weld is too high or the stay time is too long in the position.

Conclusion

  • (1) During the welding process of the finished oil pipeline, the long stay time of the arc starting and closing position causes the welding temperature of the circumferential weld in this position to be too high, and the weld melts and falls to form the weld tumor.
  • (2) The overheating of the pool in this position reduces the crack resistance of the weld metal, resulting in the thermal crack, and the decarbonization, which makes the local mechanical properties of the circumferential weld decrease.
  • (3) The original crack leakage shall be the thermal crack at the root of weld toe on the inner surface of the circumferential weld during welding, and there are large weld tumors at the weld toe, which leads to stress concentration in this position. Under the long-term internal and external load, the crack will expand in the stress concentration position, and finally lead to leakage failure.

Author: liukuirong, renuoqi, wanggaofeng, Zhanghao

Source: China Oil Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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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