Integrity and Fracture Response of Offshore Pipelines Subject to Large Plastic Strains

Abstract

Steel pipelines are widely used in offshore oil/gas facilities. To achieve economically feasible designs, regulatory codes permit utilization of the pipelines well past their elastic response limit. This requires thorough integrity check of the pipeline subject to large scale yielding (LSY). Engineering criticality assessments (ECA) are used to justify the integrity of a cracked pipeline against fracture failure. The currently used ECA crack driving force equation was developed for load-controlled components subject to very limited crack-tip plasticity. Moreover, fracture toughness data are extracted from deeply-cracked laboratory specimens that produce the lowest margin of toughness values. Therefore, the current framework can be overly conservative (or include non-uniform inaccuracies) for ECA of modern pipelines that undergo LSY and ductile crack growth prior to failure. The two main goals of this thesis are: (i) Development of an alternative crack driving force estimation scheme, (ii) Justification of the use of use of shallow-cracked single edge notch tensile (SENT) specimens for the ECA. Strain concentration in concrete coated pipelines, and effect of Lüders plateau on the fracture response are also investigated.

A new reference strain J-estimation scheme is proposed and calibrated to 300 nonlinear parametric FE models, which takes advantage of the linear evolution of the J with LSY bending strains. The scheme is hence strain-based and needless of limit load solutions, providing additional accuracy and robustness.

The near-tip stress and strain fields of cracked pipelines were also investigated and compared to those obtained from a K-T type formulation. It is shown that the J-Q constraint theory can satisfactorily characterize these fields up to extreme plastic bending levels. Similar J-Q trajectories were also observed in the SENT and pipeline models. Subsequently, FE models utilizing a voided plasticity material were used to parametrically investigate ductile crack growth and subsequent failure of pipelines subject to a biaxial stress state. Plastic strain and stress triaxiality fields ahead of the propagating crack, along with R-curves, were compared among SENT and pipeline models. It is concluded that the SENT specimen could be a viable option for ECA of such pipes based on the observed crack tip constraint similarity.