FAULT RUPTURE ANALYSIS OF BURIED PIPELINES

Abstract

Pipelines have been acknowledged as the most reliable, economic & efficient means for the transportation of water and other important commercial fluids such as oil and fuel gases. The designation of pipeline systems as "Lifelines" signifies that their operation is essential in maintaining the public safety and well being. A pipeline transmission system is a linear system, which traverses a large geographical area and soil conditions thus is susceptible to a wide variety of seismic hazards. Ruptures or severe distortions of the pipeline are most often associated with relative motion arising from fault movements, landslides, liquefaction, loss of support, or differential motion at abrupt interfaces between rock and soil. Notably the most catastrophic damages are the ones resulting from faulting or ground rupture. Owing to these facts the performance of buried and aboveground pipeline structures subjected to faulting and other seismic hazards have become important subject of study. Soil-Pipeline interaction has always been a major consideration in such studies and analysis. The present dissertation report gives the details of attempt to study the behaviour of pipelines against major seismic hazard i.e. faulting (or ground rupture). The study is based on the Finite Element Analysis carried out using "ANSYS" software package. As a matter of fact, the study of affect of fault crossing is most crucial for the case of buried pipelines. Representative model of a buried pipeline along with the surrounding soil was modeled and fault rupture was simulated using appropriate boundary conditions. Herein soil was modeled using springs with an equivalent stiffness characteristics. Nonlinear behaviour of the soil was incorporated into the equivalent soil spring models. As most of the previous analyses have been carried out on models using elastic beam elements, initial model of the pipe was done using similar elements. The analysis was then extended to study the affect of various parameters such as magnitude of fault displacement, pipe diameter, thickness, material & cover depth. Many important aspects of the pipe behaviour came to surface. It was observed that all of those could be explained by affect of the parameters on stiffness of pipe and/or soil. Further it was noted that the problem, although static in nature, deals with large deformations, thus the modelling was modified using plastic beam elements and material nonlinearity was further incorporated. It was appreciated that results obtained now differed significantly from those obtained earlier considering elastic behaviour. Finally the affected length of the above pipeline was modeled once again using shell elements and the cross-sectional deformations were studied. Results were found to be very well in accordance with those observed during past earthquakes.