ABSTRACT

Buckling resistance is required in strain-based design (SBD) for line pipes buried in discontinuous permafrost. This paper describes the potential strain limit for girth-welded UOE line pipes. The strain limit for the buckling resistance or the compressive strain limit εc is affected by not only the work hardening coefficient in the material but also the strength difference between the girth-welded pipes. The εc for commercially manufactured pipes was predicted by the FE-model of the pipe bending, introducing the decisive mechanical properties, the yield strength (YS) and the flow stress ratio. The FEA simulation for the pipe consecutively modeled from UOE pipe forming through bending attempted to solve the effect of the strength variation in the pipe on εc. Consequently, this study reveals the lower bound for the strain limit and it contributes to the reliability analysis for SBD.

INTRODUCTION

Line pipes buried in discontinuous permafrost and seismic areas suffer plastic deformation from ground movement. In order to maintain the structural integrity under the deformation, the plastic design, so-called strain-based design, is applied to such pipelines while the deformability is required for the line pipes. The typical loading condition in the discontinuous permafrost is a bending moment under internal pressure in which the buckling resistance is emphasized. The bending strain is defined as the compressive strain limit εc when the bending moment reaches the peak value. In contrast, the tensile stress generates in the extrados under bending so that necking occurs occasionally (Igi, 2011, Tsuru, 2011). The tensile strain limit is determined from the size of the defect inevitably existing in the girth weld. Since the plastic hinge in the buckling portion increases the tensile stress after local buckling, it is important to evaluate the lower bound of εc for increasing the reliability of SBD.

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