Sensitivity analysis of hydrostatic pressure collapse of submarine pipelines based on ABAQUS Cui Zhenping, Zhang Zhonghua 2 (1. School of Civil Engineering and Architecture, Tianjin University, Tianjin 300072; 2. National Marine Technology Center, Tianjin 300112) thin, buckling collapse occurs The problem is becoming more and more prominent. Use ABAQUS finite element analysis software to perform nonlinear buckling analysis of the pipeline to determine the hydrostatic pressure of the pipeline under different diameter-thickness ratios, initial ellipticity, axial tension and bending moment, to analyze the hydrostatic pressure on these factors Sensitivity.
The submarine pipeline is the main artery of national energy, and its safe operation has a great impact on the national economic development. With the application of new technologies and new materials, the rigidity of the pipeline is relatively more and more flexible, and the thickness is relatively thinner and thinner, becoming a typical thin shell structure. This thin shell structure may cause local buckling instability and failure under the action of its own defects and external forces, that is, the collapse of the pipeline. When such crushing occurs, buckling may occur if certain conditions are met, which will not only greatly increase the cost of pipeline repair, but also cause serious environmental impact. Therefore, it is of obvious practical significance to analyze the sensitive effects of various factors on the hydrostatic pressure of the pipeline, so as to reasonably control these sensitive factors during the pipeline design, manufacturing, laying, and operation.
In this paper, the ABAQUS finite element analysis software is used to analyze the sensitivity of the submarine pipeline's diameter-thickness ratio, initial ellipticity, axial tension and bending moment to crush pressure to determine the influence of these factors on hydrostatic pressure.
1 Sensitivity factors for hydrostatic collapse of submarine pipelines The sensitivity factors for hydrostatic collapse of submarine pipelines mainly come from two aspects, geometric features and external forces. In terms of geometric characteristics, the analysis in this paper is conducted on the effects of initial ovality and diameter-thickness ratio; on the external force, the analysis is conducted on the effects of axial tension and bending moment. Among them, the influence of the bending moment is reflected by the change relationship of the bending curvature to the crushing pressure. 3.2 Pipeline hydrostatic crushing analysis method based on ABAQUS. The finite element software ABAQUS software has strong non-linear calculation capabilities, which can be adjusted locally It is convenient to modify various sensitive factors for large-scale analysis. In this paper, the application of ABAQUS software, the main goal is to solve the local buckling critical pressure (ie crushing pressure) of the pipeline under different conditions, so the analysis type is buckling analysis.
Regarding buckling analysis, ABAQUS software provides two methods of eigenvalue buckling analysis and nonlinear buckling analysis to determine the critical load of the structure and the characteristic shape of the structure when the buckling response occurs. Eigenvalue buckling analysis is used to predict the theoretical buckling strength (bifurcation point) of an ideal elastic structure. This method is equivalent to the elastic buckling analysis method. For example, the eigenvalue buckling analysis result of a cylindrical structure will be equivalent to the classical Euler solution. However, the defects and nonlinearity of the actual structure will cause the instability before the load reaches the theoretical elastic buckling load, so the eigenvalue buckling analysis usually gives a non-conservative structure. Non-linear buckling analysis uses non-linear techniques. The analysis model can consider the initial defects of the structure, the plastic behavior of the structure, gaps, and large deformation response. It can also track the post-buckling behavior under buckling, so its results are closer to The actual situation. However, the nonlinear buckling analysis also has high requirements on the computer, and the calculation time, memory requirements, and hard disk storage space consumption are much larger than the eigenvalue analysis.
In order to ensure the accuracy of the crushing pressure in this study, a nonlinear buckling analysis method is used for the analysis. In this paper, four commonly used submarine pipelines with diameter and thickness are calculated. They are 203.2mm in outer diameter and 10mm in wall thickness; 325mm in outer diameter and 6mm in wall thickness; 325mm in outer diameter and 10mm in wall thickness and 406mm in wall thickness and 10mm in thickness. The sensitivity analysis of the initial ellipticity, axial force and curvature of the pipeline was conducted respectively.
3Analysis model of submarine pipeline hydrostatic collapse The pipeline is modeled by solid elements (eg). To overcome the effect of shear self-locking, the analysis unit uses a solid non-coordinated unit C3D8I. A small length is extended at one end of the pipeline, so that The axial force and bending moment are loaded using coupled control points, eg.
The enlarged view of the model coupling area is divided into two layers of cells along the pipe wall thickness in a structured way. The cell size in other directions is about 2 to 4 times the pipe wall thickness.
In terms of constraints and loading, the model uses control points to load axial forces and bending moments in the coupled segment, and rigid constraints are imposed on the uncoupled segment, eg.
Schematic diagram of the model loading constraint analysis The material of the analysis object is APIX65 steel, and the material parameters are determined according to the Ramberg-Osgood model curve. The analysis process is implemented by nonlinear analysis, and the minimum step size of the load is controlled to ensure that the analysis results have sufficient accuracy.
4 Initial ellipticity sensitivity analysis When analyzing the effect of initial ellipticity on the hydrostatic pressure of the pipeline, modeling will build the pipeline into a solid model with initial ellipticity. The existence of the initial ellipticity has formed an initial defect in the pipeline. It is not necessary to apply other defects when performing nonlinear buckling analysis, and the analysis can be directly loaded.
For the initial ellipticity sensitivity analysis of crushing pressure, three types of outer diameter wall thickness combinations: 325mmx10mm, 325mmx6mm, 406mmx10mm, and five initial ellipticities (0.1% to 0.5%) were used for buckling analysis under pure hydrostatic pressure. Selection of initial ellipticity The requirements of DNV related specifications on pipeline ellipticity 'are summarized in Table 1 and Table 15.
Table 1 Relationship between initial ovality, diameter-thickness ratio and hydrostatic pressure collapse pressure of the table Table OD wall thickness-diameter-thickness ratio initial ovality critical pressure / MPa 5 Axial force sensitivity analysis of submarine pipeline collapse in order to avoid initial defects The difference affects the analysis results. In the numerical simulation of the hydrostatic pressure of the pipeline under the axial force, the pipe fittings are directly built into a unified solid model with an initial ellipticity of 0.1%. For the axial force sensitivity analysis of crushing pressure, for three combinations of outer diameter wall thickness: 325mmx 325mmx6mm, 406mmx10mim1 initial ovality (0.1%), apply three different axial tensile forces, and then add pure still water Press to determine the crush pressure.
The magnitude of the axial tensile force is based on the tensile force T0 of the tube when the tube is axially yielded. Take 0.3 times, 0.5 times and 0.8 times of this yield tensile force, respectively, and observe the relationship between the critical pressure of the collapse. The calculation results are summarized in Table 2 and.
The application of curvature is reflected in the form of applied bending moments, and the bending moments are reflected in axial tensile forces distributed linearly along the end section. The magnitude of the curvature applied is based on the point at which the yield stress is reached at a certain point on the cross-section under the action of the bending moment. The bending moment and curvature at this moment are set as the reference bending moment M) and the reference curvature, which are 0.3 times, 0.6 times and 0.9 respectively. Times applied. The 12 sets of analysis results are summarized in Table 3 and.
Table 2 Relationship between axial force, diameter-to-thickness ratio and hydrostatic pressure collapse pressure of the pipe Table outer diameter wall thickness diameter-to-thickness ratio axial tension / N axial force loading ratio Critical pressure gauge 3 Curvature, diameter-thickness ratio and pipe hydrostatic pressure The relationship between the collapse pressure and the outer diameter wall thickness to thickness ratio / mm bending moment bending moment plus the corresponding curvature critical pressure. The curve of the change of the collapse pressure of the pipeline with the axial force loading ratio. 6 The sensitivity analysis of the curvature of the submarine pipeline collapse to avoid The difference of the initial defects affects the analysis results. In the numerical simulation of the hydrostatic pressure of the pipeline under different curvatures, the pipe fittings are directly built into a unified solid model with an ellipticity of 0.1 °%. Sensitivity analysis of the curvature of the crushing pressure, respectively for three combinations of outer diameter wall thickness: 325mmx10mm, 325mmx6mm, 406mmx10mm, an initial ellipticity (0.1 °%, three different curvatures are applied, and then pure hydrostatic To determine the crushing pressure.
Variation curve of crushing pressure of different pipelines with bending moment loading ratio7 Conclusion According to the above multiple sets of numerical simulation analysis, it can be seen that the hydrostatic crushing pressure of the pipeline is not only affected by geometric characteristics such as diameter-thickness ratio and initial ellipticity. It will also be affected by the effects of axial forces and bending moments. The details are summarized as follows: (1) The hydrostatic pressure of the pipeline decreases with the increase of the diameter-thickness ratio; (2 The hydrostatic pressure of the pipeline decreases with the increase of the initial ellipticity; (3) The hydrostatic pressure of the pipeline The crushing pressure decreases with the increase of the axial force; (4 The hydrostatic crushing pressure of the pipeline decreases with the increase of the curvature of the pipeline.
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