Plastic Interaction Relations for Corroded Steel Pipes under Combined Loadings

Interaction relationships can be derived analytically for corroded pipelines under combined loadings, assuming a fully plastic failure mode. A set of closed-form analytical solutions are presented in this paper, describing the interaction relationships between bending resistance, axial force and internal pressure for corroded pipelines. Three different idealized corrosion shapes, namely a constant-depth, an elliptical and a parabolic corrosion shapes, are considered and compared. The presence and shape of corrosion features reduces the ultimate bending capacity significantly, having analyzed the outcome of analytical calculations performed, using the interaction relationships as derived for corroded pipelines under bending, axial force and internal pressure. Good agreement is furthermore observed comparing the calculated bending moment capacity using the interaction curves with the actually measured bending moment from full-size tests performed on corroded pipes. It is shown that pipelines having corrosion features having an (idealized) elliptical or parabolic shapes yield higher plastic bending resistance when compared with a constant-depth corrosion feature. It is further pointed out that the simplification of the actual corrosion geometry by a constant-depth approximation, as commonly assumed in current assessment codes for corroded pipes, will inevitably underestimate the residual plastic strength of the pipe, especially for deep and wide corrosion features.