Influence of ring-stiffeners on buckling behavior of pipelines under hydrostatic pressure

• Increasing the number of ring stiffeners decreased the discrepancy between the buckling, buckling propagation, and final failure pressures
• The pre-buckling and post-buckling regions showed nonlinear behavior for most of the specimens.
• The buckling capacity of the specimen increased in two stages: after the initial local buckling began and after the buckling propagation began.
• Geometric (D/L, D/t) and mechanical properties and imperfections have important and intense effects on behavior of pipelines.
• API Standard estimated the experimental buckling propagation pressure almost exactly and underestimated the buckling propagation pressure.

Submarine pipelines are important and influential structures in marine engineering because they transport important, useful, and common fuels around the world. The pipeline structure is affected by different environmental forces from the surrounding conditions such as the external uniform hydrostatic pressure, hydrodynamic pressure, seismic forces, installation forces, free span forces, and forces scouring the pipeline. These conditions cause the pipeline to express complicated behavior.Local buckling and buckling propagation along the pipeline are common examples of collapses under these conditions and loadings, which can destroy thousands of meters of pipeline. These post-buckling phenomena cause great damage and losses to the oil and gas industries and to the environment; the imposed costs for the repair and protection of pipelines and mitigation of environmental problems are high. The use of ring-stiffeners is one way of increasing the buckling capacity of a pipeline in order to prevent and control the buckling propagation in pipelines.In this study, the buckling and post-buckling behaviors of ring-stiffened pipelines were investigated at a small scale through experiments and the finite element method (FEM). Two different ring-stiffeners were attached to the specimens. Only the uniform hydrostatic pressure was considered as the main loading; the axial stress was neglected in this study. The buckling modes, lateral displacement of the pipeline, ring tilt, formation of yielding lines, and torsion of yielding lines were examined. The results indicated that increasing the number of ring-stiffeners greatly increases the buckling capacity and lateral displacement of the pipeline. The buckling modes were changed, the post-buckling region was shortened, and torsion in the yielding lines and tilting in the ring-stiffeners clearly appeared in the tests. The results of this study were compared with those of recent research and reliable guidelines and standards.