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Nov 01, 2024

Applicability evaluation technology for steel small diameter pipes containing external surface corrosion defects | Scientific Reports

Scientific Reports volume 14, Article number: 25803 (2024) Cite this article 280 Accesses Metrics details With the continuous operation of oil and gas stations and petrochemical equipment, it has been

Scientific Reports volume 14, Article number: 25803 (2024) Cite this article

280 Accesses

Metrics details

With the continuous operation of oil and gas stations and petrochemical equipment, it has been found that steel small bore pipelines have different degrees of corrosion defects, which have led to pipeline cracking and medium leakage, and have increasingly attracted great attention of their oil and gas and chemical enterprises. In order to ensure the safe operation of the small diameter pipes with external surface corrosion defects, the influence law of the size of corrosion defects on the bearing capacity of the small diameter pipes were studied by finite element analysis and test simulation method, and the damage mechanism and failure form were analyzed, and the failure pressure prediction formula of the small diameter pipes with defects was fitted. It provides technical guidance for the applicability evaluation of corrosion-containing steel small diameter pipes in oil and gas chemical stations or installations.

The pressure pipeline is a critical component for transporting medium in petrochemical enterprises and represents a focal point for enterprise management. However, the steel small diameter pipes connected to the main pipeline body (referred to as the small diameter pipes) are often overlooked during the design and operational stages. Small diameter pipes are an important device for the connection of pressure pipeline with instruments and valves. Once leakage occurs, it is difficult to repair and the safety risk is high, and it is easy to cause major safety accidents. With the continuous operation of oil and gas stations and petrochemical equipment, defects such as different degrees of corrosion have been found in small diameter pipes one after another, which have led to pipeline cracking and medium leakage problems, which have increasingly attracted the attention of the oil and gas and chemical enterprises. Small diameter pipes refers to the pipe with nominal diameter less than 50 mm. In this paper, A333-6 material small diameter pipes are used as the research object for numerical simulation and test, and the applicability evaluation technology of small diameter pipes are discussed.

As shown in Fig. 1, through the analysis of more than 100 articles and reports on failure cases of small diameter pipes in LNG receiving stations, it is found that corrosion defects on the outer surface are the main cause of failure of small pipe1,2,3,4,5,6. Because LNG receiving stations are usually built in the coastal location, affected by the high humidity and high salt atmospheric environment in the coastal area, the small diameter pipes in the LNG receiving station have suffered large-scale external corrosion.

Failure case statistics of small diameter pipes.

At present, the research on small diameter pipes mainly focuses on detection and accident analysis. The relevant safety evaluation standards, such as GB/T 30,5137 and API 5798, are also established for the blasting test data of long distance oil and gas pipelines or large diameter pipelines. Compared with large diameter pipes, the bearing capacity of small diameter pipes is evaluated by existing standards due to various reasons such as thickness to diameter ratio. The results are conservative, and there is no clear safety evaluation method suitable for steel small diameter pipes with defects.

Therefore, the small diameter pipes used in an LNG receiving station is taken as the research object. Referring to ASME B31G9, the external corrosion defects were regularized into rectangles. Through finite element analysis and simulation test, the failure mechanism of the small diameter pipes with corrosion defects on the outer surface is studied, and the applicability evaluation method of the small diameter pipes with corrosion defects is formed. It provides technical guidance for the applicability evaluation of corrosion-containing steel small path pipelines in oil and gas chemical stations or installations.

Six groups of small diameter pipes with different sizes of external corrosion defects were designed for water pressure burst test. The parameters of the test pipes are shown in Table 1. As shown in Fig. 2, the ends of the test pipes were welded with spherical plug heads, and a pressure inlet was set up on one end. The corrosion defect was processed in the middle of the pipe, and the edge of the defect was rounded.

Test piping.

The measured size parameters of corrosion defects of the test pipeline are shown in Table 2. Strain gauges are arranged around the defects of the test pipeline to monitor the strain changes during the test.

The hydraulic blasting failure pressures of the six groups of test pipes are shown in Table 3.

The No. 2 test pipe was selected for analysis. As shown in Fig. 3a, the corrosion defect was obviously swollen and plastic deformation occurred. When the stress exceeded the tensile strength of the material, strength failure occurred at the defect location with weak wall thickness. After blasting, the wall thickness of the defect is 3.9 mm, the wall thickness is reduced by 17%, the axial length of the failure fracture is 40 mm, and the maximum opening in the ring is 21.92 mm. It can be seen from the electron microscope image of the end face of the failure fracture in Fig. 3b, the form of blasting failure is ductile fracture.

No. 2 failure fracture of test pipeline.

The pressure boost curve of the test pipeline No.2 is shown in Fig. 4, with the test time on the abscissa and the internal pressure load on the ordinate. As can be seen from Fig. 4, the internal pressure increases linearly in the early stage of the test. When the internal pressure rises to 120 MPa, the pipeline will yield; with the rise of the internal pressure, the pipeline will begin to undergo plastic deformation; when the internal pressure reaches 139.5 MPa, the pipeline bursts.

No. 2 pressure boost curve of test pipeline.

Annular and axial strain gauges with a measuring range of 50,000 µε are arranged at 6 locations around the defects of test pipeline No. 2. The strain curves at each location are shown in Fig. 5, where the dashed lines are the end time of water pressure blasting and the upper limit value of strain gauge. It can be seen from Fig. 5b that with the increase of internal pressure, the circumferential strain value at position 1/2/4/5/6 becomes larger, showing a tensile state; and the closer the location is to the defect center, the larger the strain value and the shorter the response time; and the circumferential strain value at the defect center (position 2) is the largest, which is consistent with the failure initiation position. Due to the wall thickness at the defect, the circumferential strain amplitude is larger than that at the normal wall thickness. Due to the circumferential deformation and extrusion at the defect, the circumferential compression state occurs at the upper part of the outer defect ring (position 3). As can be seen from Fig. 5c, for axial strain, it is also due to the wall thickness at the defect and the large axial deformation range. The right side of the defect (position 5) is axially compressed, and the other positions are in a tensile state.

No. 2 strain curve of test pipeline.

Under the action of internal pressure load, the small diameter pipes undergoes elastic deformation, local yield, global yield, material hardening until plastic failure occurs. In this paper, ABAQUS software was used for nonlinear analysis of the numerical model. The outer surface corrosion defect was set in the middle of the model. In order to avoid the influence of stress concentration, a rounded corner was set at the defect boundary and a spherical plug head was set at the end of the model. Since the small diameter pipes with corrosion defects are axially and radially symmetric, a 1/4 model is established, as shown in Fig. 6a. The model adopts an eight-node hexahedral linear reduced integration element (C3D8R) grid, which is gradually refined from the periphery to the defect area, as shown in Fig. 6b. The uniform internal pressure load is applied to the inner surface of the model, which is not considered in the model due to the small influence of other loads such as gravity. Considering the symmetry of the model, symmetrical constraints are applied to the cross section passing through the defect center and the section of the longitudinal wall. The displacement of the outer arc on the free end of the model is 0 in the X and Y directions, and the displacement of the inner arc on the longitudinally cut surface is 0 in the Z direction.

Numerical model.

The tensile test was carried out on A333-6 material, and the engineering stress-strain curve of the material was obtained, as shown in Fig. 7. Due to the elastoplastic deformation of the pipeline under internal pressure, considering the hardening phenomenon of the material, it is necessary to convert the real stress-strain and then assign the material parameters to the numerical model. As can be seen from Fig. 7, the true tensile strength of A333-6 is 595 MPa, the yield strength is 302 MPa, and the elastic modulus is 200GPa. The size parameters of the model are consistent with those of the test pipeline, as shown in Table 4.

Stress-strain curve of A333-6.

Taking the corroded length L (mm), depth d (mm) and width β (rad) as variables, the influence of the corrosion defect size on the bearing capacity of the small diameter pipes was studied. The geometric parameters of the defects were shown in Table 5.

At present, most of the commonly used failure criteria are when the Mises equivalent stress reaches 0.8\(\:{\sigma\:}_{u}\), 0.9\(\:{\sigma\:}_{u}\), \(\:{\sigma\:}_{u}\) and \(\:\frac{{\sigma\:}_{y}+{\sigma\:}_{u}}{2}\) (where, \(\:{\sigma\:}_{u}\) is the tensile strength; \(\:{\sigma\:}_{y}\) is yield strength), the model is considered invalid. Model according to the parameters in Table 2. Mises equivalent stress at intermediate nodes along the wall thickness direction was calculated when the Mises equivalent stress was equal to 0.8\(\:{\sigma\:}_{u}\), 0.9\(\:{\sigma\:}_{u}\), \(\:{\sigma\:}_{u}\) and \(\:\frac{{\sigma\:}_{y}+{\sigma\:}_{u}}{2}\), and was compared with the failure pressure of the water pressure blasting test, as shown in Table 6.

It can be seen from Table 6 that when the Mises equivalent stress is equal to \(\:{\sigma\:}_{u}\), the Mises pressure value is closest to the actual failure pressure value, with an average error of 5.4%. Therefore, Mises equivalent stress equal to \(\:{\sigma\:}_{u}\) was used as the failure criterion of the numerical model10.

Modeling and calculation were carried out according to the parameters of the test pipeline No. 2. The test monitored strain values at the left side of the defect (position 1), the defect center (position 2) and 100 mm away from the right side of the defect (position 6) were selected and compared with the calculated strain values at the corresponding positions of the model, as shown in Fig. 8. As can be seen from Fig. 8, the monitored strain values at the three locations showed the same trend as the calculated strain values, and the values were similar. Therefore, it can be shown that the results of the numerical model are valid.

Comparison diagram of test strain and simulated strain.

A small diameter pipe model with defect sizes d/t = 0.3, β/2π = 0.5 and L = 30 mm was selected for stress analysis, as shown in Fig. 9. As can be seen from Fig. 9, when the internal pressure reaches 50 MPa, due to the influence of stress concentration, the stress is the greatest where the wall thickness changes sharply in the circumferential direction of the defect, and the small diameter pipe is about to yield. With the increase of the internal pressure, the small diameter pipe began to undergo plastic deformation. When the internal pressure reached 100 MPa, the maximum stress was located at the defect center and the stress gradually decreased from the defect center to the periphery.When the internal pressure reaches 118.9 MPa, the maximum stress at the defect reaches the tensile strength of the material, and the strength failure occurs at the defect with the thinnest wall thickness.

Stress nephogram of model under different internal pressures.

The defect parameters shown in the first row of Table 5 were selected for numerical simulation to analyze the changing trend of bearing capacity of the small diameter pipes with corrosion depth under different corrosion lengths. The simulation results are shown in Fig. 10, where the horizontal coordinate is the ratio of corrosion depth to wall thickness, and the vertical coordinate is the ratio of failure internal pressure (P, MPa) of a small diameter pipe with corrosion defects to failure internal pressure (P0, MPa) of a pipe without defects.

It can be seen from Fig. 10 that the bearing capacity of the small-diameter pipe decreases linearly with the increase of the corrosion defect depth. The longer the corrosion length, the faster the decline rate. When L is 10 mm, 30 mm, 50 mm and 100 mm respectively, the average reduction rate of bearing capacity is 5.4%, 9.1%, 10% and 10.1% for each 0.1 increase in d/t.

Variation trend of failure pressure with corrosion depth.

ASME B31G stipulates that \(\:L\ge\:\sqrt{20Dt}\) is a long corrosion defect and \(\:L<\sqrt{20Dt}\) is a short corrosion defect. In order to facilitate the division of long and short corrosion defects, the corrosion length is treated without dimension, and the dimensionless parameter is \(\:L/\sqrt{Dt}\). The defect parameters shown in the second row of Table 5 were selected for numerical simulation to analyze the changing trend of bearing capacity of small diameter pipes with corrosion length under different corrosion depths. The variation trend of failure pressure with corrosion length is shown in Fig. 11. For short corrosion defects, the failure pressure decreases with the increase of corrosion length, the deeper the corrosion depth, the faster the decline rate. For long corrosion defects, the bearing capacity of small diameter pipes is basically not affected by corrosion length. The relationship between corrosion length and pressure bearing capacity is an exponential function.

Variation trend of failure pressure with corrosion length.

The defect parameters shown in the third row of Table 5 were selected for numerical simulation to analyze the changing trend of the bearing capacity of the small diameter pipes with corrosion width and the relationship between corrosion width, corrosion length and depth. The variation trend of failure pressure with corrosion width is shown in Fig. 12. As can be seen from Fig. 12, the bearing capacity of the small diameter pipes decreases linearly with the increase of corrosion width, but the influence of corrosion width on the bearing capacity is related to the corrosion length and depth. For the defects with short corrosion length and large corrosion depth, the failure pressure of small diameter pipes decreases rapidly with the increase of corrosion width.

Variation trend of failure pressure with corrosion width.

Existing standards, such as DNV-RP-F10111 and API-579, indicate that the corrosion width has little impact on the bearing capacity of large-diameter pipelines, so the influence of corrosion width is not considered in the evaluation. Due to the small diameter of the small diameter pipes, the sensitivity to the defect size is high. The simulation results show that the influence of corrosion width cannot be ignored in the safety evaluation of the small diameter pipes.

At present, the commonly used corrosion pipeline evaluation methods include AMSE B31G, DNV-RP-F101, PCORRC12, etc., but they are all developed based on the data of large-diameter pipelines.Based on the simulation results, an evaluation formula suitable for small diameter pipes with corrosion defects will be fitted.

The failure pressure evaluation formula in ASME B31G is as follows:

where Q represents the middle parameters (dimensionless); L represents the defect length (mm); D represents the pipe diameter (mm); t represents the pipe wall thickness (mm).

When \(\:\frac{{L}^{2}}{Dt}\le\:20\), it is a short corrosion defect:

where P represents the failure stress (MPa); \(\:{\sigma\:}_{flow}\) represents the flow stress (MPa); d represents the defect depth (mm).

When \(\:\frac{{L}^{2}}{Dt}\ge\:20\), it is a long corrosion defect:

where, \(\:{\sigma\:}_{flow}=1.1{\sigma\:}_{y}\).

The failure pressure evaluation formula in DNV-RP-F101 is:

Where \(\:{\sigma\:}_{u}\) represents the tensile strength (MPa).

The failure pressure evaluation formula in PCORRC is as follows:

where R represents the pipe radius (mm).

According to the above evaluation formula, the failure factors of pressure bearing capacity of corroded pipelines are mainly affected by the depth and length of defects. However, according to the above analysis, the influence of defect width cannot be ignored when evaluating the pressure bearing capacity of small diameter pipes.

Referring to the above failure pressure evaluation formula, the failure pressure calculation formula of the fault-free pipeline can be expressed as follows:

where \(\:{P}_{0}\) represents the failure pressure of the pipeline without defect (MPa).

Numerical models with different specifications of the same material were established to calculate the failure pressure of the fault-free pipeline, as shown in Table 7.

Through Matlab fitting, the failure pressure formula of fault-free pipeline is obtained as follows:

When there is no defect in the pipeline, d = L = β = 0, then the failure pressure is P0 .When L is infinite and \(\:d\approx\:t\), the bearing capacity is equal to zero, which means that it has failed.When \(\:\beta\:=2{\uppi\:}\), the bearing capacity of the pipeline is not equal to zero.Therefore, combined with the existing failure pressure calculation formula, the function form is proposed as follows:

where a and b are dimensionless coefficients. According to 156 sets of simulation results in Chap. 2, formula (12) and (13) are fitted, and it can be obtained that a=-0.02, b = 0.75. Therefore, the failure pressure prediction formula of small diameter pipes was finally determined as follows:

The data of the 6 groups of test pipelines in Chap. 2 were substituted into formulas (15) and (16) to calculate the predicted failure pressure of the small diameter pipes, as shown in Table 8. As can be seen from Table 8, the minimum error of the predicted failure pressure relative to the actual failure pressure is 2.2%, the maximum error is 13.2%, and the average error is 7.5%. Moreover, the predicted failure pressure is relatively conservative.It can be shown that the failure pressure prediction formula of small diameter pipes has high accuracy, which has theoretical guiding significance for the safety evaluation of small diameter pipes with external surface corrosion defects in the production process. Although the bearing capacity of the small diameter pipes is high, the corrosion conditions in the field are complex and diverse.According to the analysis results in Chap. 3, the corrosion depth has the greatest influence on the bearing capacity of the small diameter pipes. Referring to the application range of existing standards such as SY/T 647713 and TSG D700514 for the residual strength of pipelines with corrosion defects, the boundary conditions of the failure pressure prediction formula for small diameter pipes are limited to d/t ≤ 0.8 and t > 2 mm.

The corrosion defect of the outer surface is the main cause of the failure of the small diameter pipes. The effect of corrosion defects on the bearing capacity of small diameter pipes is studied by finite element analysis and experimental simulation, and the failure law and damage mechanism are analyzed. Through numerical simulation and test data, the failure pressure prediction formula of small diameter pipes is modified. The main conclusions are as follows.

The test results show that with the increase of internal pressure, plastic deformation occurs at the defect, and strength failure occurs at the weak wall thickness. The failure form is ductile fracture.

Corrosion depth has the greatest influence on the bearing capacity of the small diameter pipes, and the bearing capacity of the small diameter pipes decreases linearly with the increase of corrosion depth. The bearing capacity of small diameter pipes decreases exponentially with the increase of corrosion length. When the corrosion length \(\:L\ge\:\sqrt{20Dt}\), it is basically not affected by the corrosion length. For short and deep corrosion defects, the corrosion width has a significant effect on the bearing capacity of the small diameter pipes.

When modifying the failure pressure prediction formula of small diameter pipes, the corrosion width variable is added. The average error is 7.5% when the test data is substituted into the calculation, which proves that the revised formula is more accurate. Considering that the small diameter pipes is more sensitive to the corrosion defect size, the boundary conditions of the prediction formula are limited to d/t ≤ 0.8 and t > 2 mm.

The data that support the findings of this study are available on request from the corresponding author upon reasonable request.

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China University of Petroleum Beijing, Changping, Beijing, 102249, China

Qiang Li & Laibin Zhang

Guangdong Dapeng LNG Co., Ltd, Futian, Shenzhen, 518000, Guangdong, China

Qiang Li, Qiang Liang, Donghong Wei & Xinling Liu

China Special Equipment Inspection and Research Institute, Chaoyang, Beijing, 100029, China

Junqiang Wang & Tao Wang

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Author Contributions Statement Q.L. and T.W. are the main authors of the manuscript. L.Z. controls and supervises the article. J.W. and D.W. completed the simulation work. Q.L. and X.L. completed the experimental work. All authors reviewed the manuscript. If you have any queries or requirement of data, please contact Tao Wang.

Correspondence to Tao Wang.

The authors declare no competing interests.

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Li, Q., Zhang, L., Wang, J. et al. Applicability evaluation technology for steel small diameter pipes containing external surface corrosion defects. Sci Rep 14, 25803 (2024). https://doi.org/10.1038/s41598-024-77098-y

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Received: 25 July 2024

Accepted: 18 October 2024

Published: 28 October 2024

DOI: https://doi.org/10.1038/s41598-024-77098-y

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