Numerical investigation of dip-slip fault propagation effects on offshore seabeds for pipeline applications -

Abstract

The present thesis addresses the behavior of seabed sands subjected to underlying dip-slip fault movement. This problem is of interest in reference to subsea oil-gas pipelines connecting offshore platforms to the shoreline, crossing known existing dip-slip faults in the eastern Mediterranean. The propagation of the faulting offset in seabed sediments is explored using 2D finite element modeling. Abaqus© is used as a numerical platform which allowed the modeling of this complex problem, accounting for nonlinear material behavior with strain softening. Different dip angles and vertical fault displacements up to 10percent of the soil layer thickness were considered. The results presented in this thesis include the effect of the relative density of the seabed sands on the extent and magnitude of ground surface deformations. The required bedrock displacement-offset for the rupture to reach the surface and the length and location of the distorted zone are reported. The effects of the overlying soil layer thickness and the width of the faulting breccia zone are also investigated. The results show that loose sand case and larger fault breccia zone widths, and larger soil layer thicknesses result in larger distorted zones and higher bedrock displacements for the fault base rupture to propagate to the surface. At larger relative densities and shallower dip angles, the formation of back-thrust conditions are observed for normal fault case and pop-up structures are observed for reverse fault case at certain bedrock displacements. Based on the parametric analyses and results presented in this paper, observations related to the potential magnitudes and extents of surface deformations for various conditions of seabed densities and thicknesses are provided. These would be of critical need-use in determining likely effects of distortion-loading on pipelines crossing the fault zone.