Quantifying the impact of soft surface soil layers on fault rupture propagation and kinematic distress of offshore and onshore pipelines
Proceedings Publication Date
Nikolaos Makrakis
Nikolaos Makrakis, Prodromos Psarropoulos, Anastasios Sextos, Yiannis Tsompanakis
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Offshore and onshore natural gas pipelines constitute critical facilities that often cross seismic prone regions, thus, they are exposed to earthquake-induced Permanent Ground Displacements (PGDs). The excessive PGDs that are often developed in the vicinity of a fault scarp may affect pipeline integrity and safe functioning. On the other hand, the complete avoidance of fault rupture zones may be economically and technically unfeasible. Pipelines subjected to PGDs usually exhibit large levels of strains, and consequently they may experience local buckling and even rupture failures. For this reason, the impact of overburden soil layers on the propagation of tectonic faulting to the surface and the developed PGDs is crucial for the reliable assessment of pipeline distress.

This study investigates numerically the complex phenomenon of seismic fault rupture propagation from base rock to surface, focusing on the problem of fault-pipeline intersection. The main aim is to correlate earthquake magnitude with: (i) the critical engineering parameter of ground surface inclination with respect to the developed PGDs, and (ii) the associated kinematic distress of pipelines in terms of strains. In this light, a decoupled finite element (FE) modeling approach is adopted, consisting of two separate numerical models for the simulation of soil and pipeline response. Furthermore, soil non-linearities are taken into account utilizing Mohr-Coulomb constitutive model with isotropic strain softening. A detailed parametric investigation has been performed considering various loading conditions (directly related to earthquake magnitude), different faulting mechanisms and dip angles, as well as overlying soil properties. Accordingly, useful conclusions are derived that can be utilized in the preliminary seismic design of offshore and onshore pipelines against fault rupture.

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