Predicting failure conditions of SMAW girth welded X70 Pipelines subjected to soil movement.

P. FAZZINI; J,L. OTEGUI; H. KUNERT

Bs. As. - Argentina

Conferencia; World Gas Conference; 2009

International Gas Union

All pipelines presented in this paper failed within the first two years of having been laid out, under conditions of longitudinal traction and internal pressure at moderate to high room temperatures. Geotechnical instability in all cases gave rise to a number of failures by crack propagation along Heat Affected Zone of (in-field) girth welds.  Crack propagation was caused by large external forces generated by  soil movement and loss of pipe support. Progressive soil loading was identified as the likely substantial factor leading to crack propagation and inducing an overload failure of the pipelines. Incremental, stable tearing explains the type of staged crack growth involved in cracks in one case. Cracks grew in a ductile manner, mostly by micro void coalescence. Further increase of crack growth required further increase in stress, the crack eventually stops until a new increase of stress reactivates crack growth. These and other similar failures prompted a risk management methodology that includes risk prediction tools, monitoring soil stability (e.g. inclinometers), and early detection of axial strains (vibrating strain gauges in places with high risk of soil movement. Low tolerance to land forces by pipelines as diameter and thickness are reduced is the main reason for these pipe integrity studies. The ability of the currently employed ILI inspection tools to reliably detect pre-existing circumferential cracks at a size that allows effective future risk mitigation is still too limited to be considered in the near future. A most promising way to assure pipeline integrity in possibly geotechnical unstable regions is performing a comprehensive pipe-soil interaction study utilizing nonlinear FEM methods with proper modeling of pipe and weld resistance. This modeling should include criteria to be used for acceptable corrosion and external damage of pipes that are subjected to external soil loading. A description of the model, and a detailed review of the analysis is presented in the last part of the paper. This modeling includes criteria to be used for acceptable corrosion and external damage of pipes that are subjected to external soil loading.