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.