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UBC Theses and Dissertations

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UBC Theses and Dissertations

Stability of buried pipelines subjected to wave loading Siddharthan, Rajaratnam

Abstract

The response of buried offshore pipelines to wave loading is shown to depend mainly on the stability of the seafloor. Two analyses to investigate the stability of the seafloor are presented; one is based on Biot's theory of a poro-elastic solid, the other on the theory of residual wave induced porewater pressures. Poro-elastic analysis gives the distribution of the transient porewater pressures and the effective stress field induced by the wave loading. The stability of the seafloor can be estimated by application of the Mohr-Coulomb failure criterion to the computed stress field. For all deep deposits except for hard fine sands, it is shown that a much simpler analysis can be used. The effective stresses can be determined by computing the total stress field elastically and pore pressure field by simple solutions of the Laplace equation. Residual pore pressure analysis is based on computing the residual pore pressures generated by the cyclic action of wave induced shear stresses and then using the Mohr-Coulomb failure criterion to investigate the stability. In many cases, residual pore pressures can lead to instability or even liquefaction of the seafloor. Because of the duration of typical storms, dissipation of residual pore pressures during a storm has been considered in this analysis. The degradation in effective stress dependent properties of the deposit is also taken into account. Two computer programs, STAB-MAX and STAB-W, have been developed to perform the poro-elastic and residual pore pressure analyses respectively. An example problem in pipeline stability has been presented to illustrate the factors that should be considered in the analysis of the stability of buried offshore pipelines.

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