Seismic Analysis of Buried Continuous Pipelines Crossing Fault Stepovers

Ramachandruni, Avinash and Somala, S N (2019) Seismic Analysis of Buried Continuous Pipelines Crossing Fault Stepovers. Masters thesis, Indian institute of technology Hyderabad.

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Abstract

The response of ductile single degree of freedom (SDOF) structures subjected to spontaneous rupture propagation in two faults with a stepover (both compressional and extensional) is studied. A dynamic rupture simulation is carried by defining the initial stresses on the fault and considering slip-weakening friction laws. Differences in peak ground parameters plots in three orthogonal directions are highlighted. Using the synthetic seismograms that are based on the numerical solution to wave propagation partial differential equation coupled with contact-friction laws, a comparison is made between the right step and left step faulting conditions for variations in contours of elastic (µ=1) and multiple ductility response spectra (µ = 2,4,6,8). Also, an attempt is made to understand the difference in responses of elastic-perfectly-plastic structures by placing stations radially at 5 km (approx.) from the point of origin that lies between the two faults and at different azimuths and performing the time-history analysis. Structures buried inside the ground such as pipelines, tunnels while crossing a fault plane develops both axial and bending strains because of the fault movement. Complex fault geometry like fault stepping at a site can either aggravate or alleviate the response of such structures. In this study, we addressed the influence fault stepping on the behaviour of oil and gas pipeline. Since the existing literature developed pipelines models crossing single fault, considering the fault stepping system as a whole is essential to predict the behaviour of pipeline failures accurately. The seismic analysis was carried out using an analytical approach proposed by Uckan et al. [2015]. Results from the analytical approach clearly show a significant variation in both axial and bending strains along the pipeline axis between the two type of fault stepping – Extensional (also called dilational) and Compressional. It was observed that pipeline develops about 35% and 60% lower bending and axial strains when it crosses a compressional step. Also in the direction parallel to the fault plane, the compressional stepping reduced the pipeline strains. A parametric study of the pipe diameter to thickness ratio (D/t) for critical strains also suggested that pipeline is more likely to buckle within 10 km on either side of the main fault for an extensional case. However, the critical zone length was limited to only 5 km in the case of the compressional stepping, making it a more reliable path of the two alternatives for pipeline design.

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