Xiaoben Liu

Tensile Strain Capacity of Pipelines for Strain-Based Design

Traditional pipeline design methods presented in various codes are usually based on limit stress criteria. However, these methods may be inapposite to modern steels, especially for displacement controlled loads such as ground displacement load. The design of pipelines for plastic strain should account for both tension strain limit and compression strain limit along the axial direction of the pipe. In tension, the issues relate to the failure modes of plastic collapse or fracture. Tensile axial strain of the pipe often results in rupturing. The capacity of tensile axial strain of the pipe is affected by a large number of factors: D/t ratio, Y/T ratio, internal pressure, girth weld effect, and defect size and location. Consequently, full solutions for tensile strain limits related to above-mentioned factors do not yet exist in codes and standards. In recent years, a number of projects have been funded to develop a quantitative determination of tensile strain limits in China. This paper covers the technical basis of the procedures. The development of the quantitative approach to tensile strain limits involves both experimental tests and finite element analyses, and the process is as follows. Firstly, a series of curved wide plate tests under the axial tensile strain have been done, especially including more than 60 girth weld specimens with not only buried or surface defects but also various defect-sizes and defect-locations. Based on these test data and other available experiment data of full scale tests under the axial tensile strain and internal pressure loading, a valid finite element model has been found. Then a total of 110 finite element analyses produced a lot of data for a wide range of material, D/T ratios, various defect sizes or locations, buried or surface defects, and various internal pressures. So some parametric equations can be developed from finite element analyses. The safety factors and appropriate limits for the parametric equations have been identified against much more experimental data. It is believed that the approach to axial tensile strain limit presented in this paper may lay the initial basis for the quantitative determination of tensile strain limits to pipelines.Copyright

Compressive Strain Capacity of Pipelines for Strain-Based Design

Traditional pipeline design methods presented in various codes are usually based on limit stress criteria. However, these methods may be inapposite to modern steels, especially for displacement controlled loads such as ground displacement load. The design of pipelines for plastic strain should account for both tensile strain limit and compressive strain limit along the axial direction of the pipe. In compression, the failure modes relate to several varieties of buckling. Compressive axial strain of the pipe often results in local buckling and its amplification in wrinkles. The capacity of compressive axial strain of the pipe is affected by a large number of factors: D/t ratio, Y/T ratio, internal pressure and girth weld effect. Consequently, full solutions for compressive strain limits related to above-mentioned factors do not yet exist in codes and standards. In recent years, a number of projects have been funded to develop a quantitative determination of compressive strain limits in China. This paper covers the technical basis of the procedures. The development of the quantitative approach to compressive strain limits involves both experimental tests and finite element analyses, and the process is as follows. Firstly, a series of curved wide plate tests under the axial compressive strain with or without girth weld have been done and the buckling processes of the specimens and the compressive strain limit have been got. Based on these test data and other available experiment data of full scale tests under the axial compressive strain and internal pressure loading, a valid finite element model has been found. Then a total of 144 finite element analyses produced a lot of data for a wide range of material, D/T ratios and various internal pressures. So some parametric equations can be developed from finite element analyses. The safety factors and appropriate limits for the parametric equations have been identified against much more experimental data. It is believed that the approach to compressive axial strain limit presented in this paper may lay the initial basis for the quantitative determination of compressive strain limits of pipelines.Copyright

Strain Prediction for X80 Steel Pipeline Subjected to Strike-Slip Fault Under Compression Combined With Bending

Strike-slip fault is one main kind of PGD faced by long distance gas pipelines. Based on non-linear finite element method, a numerical model for buried pipeline under strike-slip fault was proposed. The model was proven to be reasonable by comparing the numerical results with previous researcher’s experiment results. By using the FE model, peak compressive strain of X80 steel pipeline subjected to strike-slip fault under compression combined with bending was studied. The sensitivities of the diameter, wall thickness, soil rigidity, fault displacement and crossing angle on the peak compressive strain of the pipeline are examined in detail. Furthermore, based on numerous numerical results, a regression equation for predicting peak compressive strain of X80 steel pipeline is proposed. The applicable range of the formula is given. 15 true design cases in the Second West to East pipeline Project in China were investigated to demonstrate the accuracy and applicability of the proposed methodology by comparing the predicting peak compressive strain results with FEM results. The proposed method can be referred in the strain-based and reliability-based design for X80 steel pipelines subjected to strike-slip fault.Copyright