Polynikis Vazouras

Structural behavior of buried pipe bends and their effect on pipeline response in fault crossing areas

Pipe bends, often referred to as “elbows”, are special pipeline components, widely used in onshore buried steel pipelines. They are sensitive to imposed deformations and their structural behavior is quite flexible and associated with the development of significant stress and strain, which may lead to failure. In the present paper, the mechanical performance of buried steel pipeline bends is investigated first, using rigorous finite element models that account for the pipe-soil interface. Three 36-inch-diameter pipe elbows are considered, subjected to pull-out force and embedded in cohesive soils. The elbows have bend angles equal to 90°, 60° and 30°, and bend radius-over-diameter ratio (R/D) equal to 5. The results show the increased flexibility of the pipeline bend with respect to the straight pipe, and are reported in the form of force–displacement diagrams. Furthermore the deformation limits of each elbow are identified in terms of appropriate performance criteria. The second part of the paper investigates the effect of pipe bends on the response of pipelines crossing active faults using a three-dimensional rigorous finite element model. The numerical results refer to a 36-inch-diameter pipeline crossing a strike-slip fault, and show that the unique mechanical response of pipe bends, in terms of their flexibility, may offer an efficient tool for reducing ground-induced deformations. The three-dimensional model employs the load–displacement curves of the first part of the paper as end conditions through nonlinear springs. Furthermore, the results show that there exist an optimum distance of the elbow from the fault plane, which corresponds to the maximum allowable ground displacement. Therefore, pipeline elbows, if appropriately placed, can be employed as “mitigating devices”, reducing ground-induced action on the pipeline at fault crossings.

STRUCTURAL PERFORMANCE OF BURIED STEEL PIPELINES CROSSING STRIKE-SLIP FAULTS

The performance of pipelines subjected to permanent strike-slip fault movement is investigated by combining detailed numerical simulations and closed-form solutions. A closed-form solution for the force-displacement relationship of a buried pipeline subjected to tension is presented and used in the form of nonlinear springs at the two ends of the pipeline in a refined finite element model, allowing an efficient nonlinear analysis of the pipe-soil system at large strike-slip fault movements. The analysis accounts for large deformations, inelastic material behaviour of the pipeline and the surrounding soil, as well as contact and friction conditions on the soil-pipe interface. Appropriate performance criteria of the steel pipeline are adopted and monitored throughout the analysis. It is shown that the end conditions of the pipeline have a significant influence on pipeline performance. For a strike-slip fault normal to the pipeline axis, local buckling occurs at relatively small fault displacements. As the angle between the fault normal and the pipeline axis increases, local buckling can be avoided due to longitudinal stretching, but the pipeline may fail due to excessive axial tensile strains or cross sectional flattening.

Numerical Simulation of Buried Steel Pipelines under Strike-Slip Fault Displacements

The paper examines the structural response of buried butt-welded steel pipelines, crossing active strike-slip tectonic faults, which are vertical and perpendicular to the pipeline axis. The interacting soil-pipeline system is simulated with finite elements, accounting for large strains and displacements, nonlinear material behaviour, as well as for contact and friction on the soil-pipe interface. Steel pipelines of various diameter-to-thickness ratios, and typical steel material for pipeline applications (API 5L grade X65) are considered. The paper investigates the effects of various soil and pipeline parameters on the mechanical response of the pipeline, with emphasis on pipe wall “kinking” or fracture. The effects of cohesive and non-cohesive soils are also investigated. Furthermore, the influence of internal pressure on the structural response of the steel pipeline is examined. The numerical results are aimed at determining the fault displacement at which the pipeline failure occurs, they are presented in a graphical form showing the critical fault displacement, the corresponding critical strain versus the pipe diameter-to-thickness ratio, and can be used for efficient pipeline design against significant ground-induced deformations due to active strike-slip faults.

Mechanical Behavior of Buried Steel Pipelines Crossing Strike-Slip Seismic Faults

The present paper investigates the mechanical behaviour of buried steel pipelines, crossing active strike-slip tectonic faults. The fault plane is vertical and perpendicular to the pipeline axis. The interacting soil-pipeline system is modelled rigorously through finite elements, which account for large strains and displacements, nonlinear material behaviour and special conditions of contact and friction on the soil-pipe interface. Steel pipelines of various diameter-to-thickness ratios, and typical steel material for pipeline applications (API 5L grades X65 and X80) are considered. The paper investigates the effects of various soil and pipeline parameters on the mechanical response of the pipeline, with particular emphasis on pipe wall failure due to “local buckling” or “kinking” and pipe wall rupture. The effects of shear soil strength and stiffness, are also investigated. Furthermore, the influence of the presence of pipeline internal pressure on the mechanical response of the steel pipeline is examined. Numerical results aim at determining the fault displacement at which the pipeline failure occurs, and they are presented in a graphical form that shows the critical fault displacement, the corresponding critical strain versus the pipe diameter-to-thickness ratio. It is expected that the results of the present study can be used for efficient pipeline design in cases where active faults are expected to impose significant ground-induced deformation to the pipeline.Copyright