Stress Analysis of Buried Steel Pipelines Crossing Normal Faults: From Numerical Analyses to a New Analytical Solution

Abstract eng:
Although less “widespread” than seismic wave propagation effects, earthquake-related permanent ground displacements (fault rupture, slope failure) pose a higher threat to buried steel pipelines, as they may impose large axial and bending strains, leading to rupture, either due to tension or due to buckling. This is especially true for step-like deformations resulting from surface faulting, as indicated by a number of case studies reported in modern literature. A rigorous assessment of fault rupture effects on a buried pipeline, in terms of induced stresses, should involve an advanced numerical analysis which can consistently account for the nonlinear stress-strain response of the pipeline steel, the longitudinal and transverse soil resistance – typically idealized as a series of elastic-perfectly-plastic Winkler springs – as well as second order effects induced by large displacements. Beyond any doubt, modern commercial CAE codes render such analyses feasible. However, they are rather demanding with regard to computational effort and expertise, so that their use in practice is justified only for the final design of large diameter, thinwalled pipes and large ground displacements, or for validating special mitigation measures. For more common applications, as well as for preliminary design purposes, it is desirable to apply simplified analytical methodologies, allowing effective predictions of pipeline stresses and strains, at a fraction of the computational cost required for a rigorous numerical analysis. In the light of the above, the mechanisms governing the pipeline response at normal fault crossings are first examined with the aid of 3D elasto-plastic finite element analyses, for a typical high-pressure natural gas pipeline. More specifically, the pipeline’s mode of deformation is identified, while the effect of the applied soil resistance forces on the developing strains, as well as their distribution along the pipeline’s length and over the pipeline’s cross-section is explored. Subsequently, the basic principles of a new analytical methodology, developed on the basis of the insight gained from the numerical analyses interpretation, are outlined, and its step-by-step application algorithm is given in detail. The analytical predictions are compared against results from numerical analyses of a pipeline, perpendicularly crossing the trace of a normal fault, for various dip angles of the fault plane. Furthermore, the proposed methodology is combined with the “twin” methodology developed for strike-slip faults (Karamitros et al, 2007), in order to predict the pipeline response for the general case of oblique crossing between the pipeline axis and the normal fault’s trace. Comparison of the analytical predictions with the results of benchmark numerical analyses, performed over a wide range of fault displacements, fault plane inclinations and intersection angles, proved their good overall agreement, with minor deviations which did not generally exceed about 1020%. References Karamitros D., Bouckovalas G., Kouretzis G., Stress analysis of buried steel pipelines at strike-slip fault crossings. Soil Dynamics and Earthquake Engineering, 27, 200–211, 2007.

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