Role of Axial Stress in Pipeline Integrity Management

Abstract

In pipeline integrity management, axial stress solely can be a detrimental condition, and it may play an important role in assessment of other threats. Current practice for the assessment of integrity features such as external metal loss, deformation and stress corrosion cracking (SCC) are based on methods validated by burst testing that primarily consider hoop stress to be the maximum principal stress which governs. Observations during recent integrity management practice, however, indicate that axial stress plays an important role in pipeline failures when interacting with integrity features under certain circumstances, and should be carefully considered in integrity engineering assessment. Multiple real-life case studies are described to illustrate the importance of proper consideration of axial stress in integrity management, including: 1) axial compressive stress induced global buckling, 2) yielding of small radius fitting under axial stress, 3) ductile overloading due to axial tensile stress interacting with circumferentially oriented corrosion feature, 4) axial tensile stress and its relationship with formation of circumferential stress corrosion cracking. The details of the case studies, results and findings are summarized in this paper. Determining axial stress for integrity assessment can be critical, depending on site-specific conditions and nature of the loading. In this paper, a multi-level method for calculating axial stress based on finite element analysis (FEA) using various elements and techniques, combined with bending strain measured by in-line inspection (ILI) is described. In addition, a simplified approach for interacting threat analysis with continuum FEA and a simplified assessment based on empirical equations are proposed and discussed.