At present, the domestic oil and gas transmission pipeline business is at a stage of rapid development. The West-East Gas Pipeline Project spans multiple seismically active areas and passes over 100 faults. Its seismic safety is a key issue to be considered in pipeline engineering construction. Therefore, the study of buried pipelines crossing active faults has important engineering application value.

1 Current status of buried pipelines crossing active faults 1.1 Overview of research methods at home and abroad Research on the response of buried pipelines to faults has been in existence for more than 30 years. Scholars have continuously improved and come up with a variety of calculation methods. These Calculation methods can be divided into theoretical analytical methods, numerical finite element methods and.

The theoretical analysis model of buried pipelines crossing faults was first proposed by the famous American seismic engineering scientist Newmark and his student Hall. They proposed a highly simplified calculation method in 1975. This method reduces the pipeline to a continuous rigid pipe, which is assumed to be ignored Inertial force factors, assuming that the pipeline is completely deformed by the axial deformation to absorb the fault displacement, ignoring the bending deformation of the pipeline and the lateral force of the soil, only the axial deformation and strain are considered, and the stress-strain curve of the pipe adopts the tri-fold model Although simple and practical, due to the neglect of the lateral force of the soil and the bending deformation in the pipeline, the results obtained are unsafe.

Table 1. Comparison of the three research methods. The main features of the research methods. The advantages and disadvantages represent the theoretical analysis method. The thin shell of the pipeline is simple in concept, and the calculation of the structure with cables or beams is convenient. Reliability needs to be further verified. The numerical finite element method is based on finite element modeling. The use of numerical analysis can simulate relatively complex engineering problems. The results are also better than the ideal calculation. The beam element model method, the shell element model method, the experimental method, and the reasonable selection of pipeline models and faults Volume model, which simulates the intuitive state of the pipeline under the action of fault dislocation in the actual project, and the strain and displacement response status of the pipeline model is quite different from the actual state, and the accuracy and reliability of the experimental results cannot be guaranteed It is difficult to conduct a more in-depth analysis to study the mechanical properties of the pipeline when Gaotian Zhilang was subsidenced; Feng Qimin ’s soil box test method was proposed by KennedyR.P. In 1977, which considered the lateral effect of the soil on the pipeline and the bending Segment curvature and bending stress, passive earth pressure and other factors. The stress-strain curve of the pipe is not a three-fold line model, but the Ramberg-Osgood stress-strain model. Although the Kennedy method takes into account the lateral pressure of the soil on the pipeline and the bending curvature of the pipeline and the corresponding bending strain, However, because it ignores the bending stiffness of the pipe, the results obtained in most cases are conservative.

He and his student Yaw-hueiYeh made improvements on the basis of the above method, and proposed a more elaborate analysis method than Kennedy method. This method considers the bending strain and stiffness of the pipeline. It is assumed that the deformation of the pipeline near the fault is a circular arc, but the deformation of the pipeline farther away from the fault is simulated by the response of the elastic foundation beam. 2.1998, LeonR.L.Wang And Luo-iaWang further improved the Wang-Yeh analysis model, and proposed a more complicated method. The calculation model of this method is also to divide the pipeline on the side of the fault into two parts (the elastic foundation beam part and the transition zone part near the fault ), The difference is that the deformation of the pipeline in the transition area is assumed to be a deformation of a cantilever beam with spring hinge support. The above several theoretical analytical analysis methods have the following common limitations: only the pipeline can be analyzed under tensile load For reverse faults, the theoretical analysis method is not applicable; it is difficult to analyze the large deformation in the tube section. The pipeline is actually a thin-walled shell structure. The axial strain and bending strain in the tube affect each other, and the axial strain and The bending strain gives the total strain of the pipe. In addition, in the actual situation of the pipeline across the fault, the pipeline will also appear stress residual and stress concentration, etc. These phenomena are best analyzed by the finite element method.

Therefore, since the 1990s, due to the widespread use of computers and the rapid increase in computing speed, researchers have proposed a variety of numerical analysis methods based on finite element models. The finite element method is roughly divided into two categories: dividing the pipeline into beam elements; dividing the pipeline into shell elements. In the beam or shell finite element method, the interaction between the pipe and soil is simulated by the soil spring. Compared with the beam element model, the shell model is closer to the actual structure of the pipe, so it can better analyze the large deformation conditions such as pipe buckling, but the shell finite element model requires much more calculations than the beam finite element model. .

The various methods introduced above are all scholars through theoretical abstraction to establish a suitable model for analysis and calculation, and can not truly reflect the interaction of pipe and soil, and the elastoplastic changes of the soil body cannot be accurately expressed by theory. Therefore, some scholars have tried to simulate real interactions through experiments. Japanese scholars Takada Zhilang H and domestic scholars Feng Qimin and Guo Endong have conducted relevant experiments.

1.2 Seismic measures for buried pipelines At present, the methods commonly used in seismic design at home and abroad include stress-based seismic design, strain-based seismic design and performance-based seismic design. 3 Seismic design in the seismic engineering specifications for oil and gas pipeline engineering The calculation is based on the seismic design of strain. At present, the common seismic measures for oil and gas pipelines buried through active faults include: selecting sections with smaller fault displacement and fault width; the crossing angle of the pipeline crossing faults should be selected at a suitable angle. The specification recommends 30 ~ 70. And cannot exceed 90 ° To avoid buckling failure when the pipeline moves in the fault zone; the thickness of the pipeline cover in the fault area should not exceed 1m; the fixed pier should be set outside the sliding length of the pipeline; the loose pipe trench and loose Backfill soil 1.2 Strain analysis of buried pipeline crossing active fault 2.1 Calculation example 1 A buried pipeline passes through an active fault at an angle of 30. The fault is a strike-slip fault. The fault has a horizontal displacement of 2m and a vertical displacement of 0.5m. The pipeline uses X70 It is made of pipeline steel with a diameter of 529mm, a wall thickness of 6mm, a buried depth of 1m, a backfill density of 1 800kg / m3, and an internal friction angle of 28. For X70 steel, the allowable tensile strain in the elastic phase zone 0.0024, the allowable tensile stress is 503MPa, the elastic modulus in the elastic zone is 2.1x105MPa, the elastic modulus in the elastoplastic zone is 2 The two theoretical analytical methods commonly used in this article are Newmark-Hall method 0 and Kennedy method 65-7. example Make calculations and compare the results of the two. Using the Newmark-Hall method, the strain of the pipeline is 1.31%, and using the Kennedy method, the strain is 1.74%. From this, it can be compared that the Newmark-Hall method is more conservative than the Kennedy method, and the calculated pipeline strain is smaller.

2.2 Analysis of pipeline strain influencing factors Based on the parameters given in Example 1, the pipeline crossing fault angle, pipeline buried depth, backfill soil density, backfill soil internal friction angle and pipeline wall thickness were adjusted. The effect of parameters on pipeline strain.

It is the change of pipeline strain when the angle of the pipeline crossing faults is different. It can be seen that as the crossing angle increases, the strain of the pipeline gradually decreases. It can be seen that the ideal angle of the pipeline crossing the fault is 50. ~ 80 °.

It is the change of pipeline strain when the pipeline depth is different. With the increase of the pipeline depth, the tensile strain of the pipeline gradually becomes larger. It can be seen that the buried depth of the pipeline is less than 1m.

It is the change of pipeline strain when the density of backfill soil is different.

It can be seen that as the density of the backfill soil increases, the strain of the pipeline crosses the fault angle / (°> the impact of the pipeline crossing fault angle on the pipeline strain. The greater the influence of the pipeline depth on the pipeline strain, For the degree of damage, light (low density) backfill soil should be used.

The influence of backfill soil density on pipeline strain is the influence of backfill soil internal friction angle on pipeline strain.

It can be seen that the backfill soil with a small internal friction angle should be used, so that the strain of the pipeline under the action of the fault is small.

The influence of the wall thickness on the strain under the two methods is compared.

It can be seen that when the pipeline is under the action of active faults, the greater the wall thickness of the pipeline (more than 8mm) has a significant effect on the seismic performance of the pipeline. Through analysis of the influencing factors of Example 1, some relevant seismic design of buried pipelines can be drawn. Measures: The crossing angle is 50. ~ 80. It is more appropriate; the buried depth of the pipeline should not be too large, preferably no more than 1m, shallow buried; the use of low density and loose backfill soil can reduce the pipeline strain; increase the pipeline appropriately Wall thickness to enhance the seismic resistance of buried pipelines.

2.3 Calculation Example 2 Through the above calculation and analysis, the various impact parameters are optimized. The crossing angle is 60. The buried depth is 0.7m. The backfill density is 1600kg / m3. The internal friction angle is 22. The pipe wall thickness is changed to 9mm. Recalculated, using Newmark-Hall method, the pipeline strain is 0.14%, which is 89.31% lower than before optimization. Using Kennedy method, the maximum strain is 0.44%, which is 3% lower than before optimization. From the initial linear elastic beam model to a non-linear finite element thin shell model, the research method has transitioned from the initial semi-theoretical and semi-empirical analysis methods to a combination of theoretical, experimental, and numerical simulations, which makes The research method system of underground pipelines is becoming more and more perfect.

To sum up the current situation, the research on buried pipelines crossing active faults still needs to be developed, and the correctness of various methods has yet to be verified. Therefore, there are a few views on the research of buried pipelines crossing active faults: A variety of analytical models have been established for the stress response of buried pipelines in faults, and many research results have been achieved. However, whether these results are consistent with the destruction of real seismic faults, still need to pass through examples of responses to actual crustal activities, Statistical data is analyzed to verify; comprehensive theoretical methods, numerical simulation methods and experimental methods are used to simulate the most suitable situation for damage to oil and gas pipelines crossing faults. Connections and feedback between various methods need to be strengthened; (continued on page 17) The positioning accuracy and leak judgment accuracy of the leak point mainly depend on the measurement error of the propagation speed of the negative pressure wave in the pipeline. In summary, the soft measurement based on the correlation analysis of the negative pressure wave has been obtained in the pipeline leak detection and positioning Very good application, it is an excellent external detection method for pipeline leakage, and is the main direction of pipeline leakage detection and positioning. Of it.

2.3 The application of soft measurement based on wavelet analysis technology in the detection of gas-liquid two-phase flow In the transportation of oil and gas, the research of gas-liquid two-phase flow detection is an urgently needed field to be solved. Real-time and effective detection of various information of the multi-phase flow system is of great significance to the measurement, control and reliable operation of the system. As a new type of process parameter detection technology, soft sensor technology provides a new way of thinking and method for solving the parameter detection of multi-phase flow, which is a complex system with variable nonlinearity, and has received extensive attention in recent years.

Soft measurement based on wavelet analysis technology is to use easy-to-measure auxiliary variables, use wavelet analysis information processing technology, and analyze and process the obtained information to extract the signal feature quantity, thereby realizing the online detection of a parameter or the state of the process Identify. In a multi-phase flow system, the differential pressure fluctuation signal contains rich information, and detection is convenient and reliable. Therefore, the differential pressure fluctuation signal of the two-phase flow is used as an easy-to-measure auxiliary variable. The dominant variable is the first-class important parameter in the gas-liquid two-phase flow.

The design experiment process 4 is as follows: the gas phase (air) and the liquid phase (water) are respectively mixed by an air compressor and a centrifugal pump through a phase mixer, and then flow into the plexiglass test tube sections with inner diameters of 20 mm, 25 mm, and 40 mm, respectively. The differential pressure signal is extracted at a distance of 15 mm, and the sampling frequency is 200 Hz. After filtering and 12-bit A / D conversion, the differential pressure signal is sent to a PC for storage and data analysis. Discrete wavelet transform is used to analyze the data under each flow type, and the time-frequency domain characteristics of the measured data under different flow types can be obtained. Considering the wavelet order and the length of the filter, Sym5 wavelet filter is adopted under the premise of ensuring accuracy. After experimental analysis and verification, the selected wavelet function basically meets the experimental requirements.

Thus, using the soft-sensing model based on wavelet analysis technology, the flow pattern discrimination of gas-liquid two-phase flow is realized.

3 Conclusion In recent years, in order to solve the problem of difficult-to-measure variables in process control, soft measurement technology has been greatly developed. Soft measurement is based on mathematical theory, signal processing theory, and modern control theory, with software as the core means of implementation. The core issue is modeling. At present, soft measurement technology has been applied in the field of oil and gas storage and transportation. With the construction of domestic crude oil reserves and the continuous commissioning of oil and gas pipeline networks6, the application of soft measurement technology in oil and gas storage and transportation projects will increasingly The wider.

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