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 or 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.