Hongyi Zhao

Two-Dimensional Model for Pore Pressure Accumulations in the Vicinity of a Buried Pipeline

In this paper, we presented an integrated numerical model for the wave-induced residual liquefaction around a buried offshore pipeline. In the present model, unlike previous investigations, two new features were added in the present model: (i) new definition of the source term for the residual pore pressure generations was proposed and extended from 1D to 2D; (ii) preconsolidation due to self-weight of the pipeline was considered. The present model was validated by comparing with the previous experimental data for the cases without a pipeline and with a buried pipeline. Based on the numerical model, first, we examined the effects of seabed, wave and pipeline characteristics on the pore pressure accumulations and residual liquefaction. The numerical results indicated a pipe with a deeper buried depth within the seabed with larger consolidation coefficient and relative density can reduce the risk of liquefaction around a pipeline. Second, we investigated the effects of a trench layer on the wave-induced seabed response. It is found that the geometry of the trench layer (thickness and width), as well as the backfill materials (permeability K and relative density Dr) have significant effect on the development of liquefaction zone around the buried pipeline. Furthermore, under certain conditions, partially backfill the trench layer up to one pipeline diameter is sufficient to protect the pipelines from the wave-induced liquefaction.

Two-Dimensional Model for Accumulation of Pore Pressure in Marine Sediments

AbstractA two-dimensional (2D) porous model was developed to investigate the accumulation of pore pressure in marine sediments in which the volume-averaged Reynolds-averaged Navier-Stokes (VARANS) equations were used as the governing equations for the wave motion and the Biot consolidation theory was used for the porous seabed. Unlike most of the previous investigations on the accumulation of pore pressure in which the amplitude of the shear stress over the wave period was used in the source term, in this study, the source term was redefined as a time-dependent function using the phase-resolved oscillatory shear stresses. Overall good agreement of both the oscillatory and residual pore pressures with previous analytical solutions and experimental data demonstrated the reliability of the model for the prediction of wave-induced pore-pressure accumulation. For the case with progressive wave loadings, the liquefaction zone related to the initial incident of the wave phases was formed as a 2D pattern during t...

Numerical Modelling of Pore Pressure Accumulations in Marine Sediments around Submerged Breakwaters under Combined Wave and Current Loadings

ABSTRACT Zhang, Y.; Jeng, D.-S.; Zhao, H.Y., and Zhang J.-S., 2016. Numerical modelling of pore pressure accumulations in marine sediments around submerged breakwaters under combined wave and current loadings. In this study, an integrated model for the wave (current)-induced seabed response around submerged breakwaters is proposed. The Reynolds-Averaged Navier-Stokes (RANS) equations and k − ϵ turbulence model were used for the flow field, whereas an inelastic two-dimensional seabed model was used for pore pressure accumulation in a porous seabed. Unlike previous studies, the residual soil response was considered in the present model, together with the interaction between waves and currents and the preconsolidation process. The present model was validated with previous studies for wave–current interaction model and seabed models. Based on the numerical examples presented, the following conclusions were made: (1) preconsolidation because of static loading of breakwaters and static water pressures significantly affects the seabed response around the breakwaters, (2) the liquefaction depth under combined wave and current loading is less than that under wave-only loading, (3) the maximum liquefaction depth increases as the width and height of the submerged breakwater increase, and (4) liquefaction depth for the submerged, multiple breakwaters is smaller than that for a single, submerged breakwater.

Numerical study for wave-induced pore pressure accumulations around buried pipeline: Effects of back-fill trench layer

In general, a cover layer with applicable backfill materials can reduce the potential of the waveinduced liquefaction around offshore pipelines. The evaluation of wave-induced pore pressure around a trenched pipeline in the marine sediments have been extensively investigated in the past few decades. Most previous studies regarding the effects of a cover layer considered full backfilled conditions, although general engineering practises normally used partially backfill, rather than fully backfill. In this study, an integrated numerical model is proposed to perform a simple scaling analysis to clarify the applicable ranges of the thickness (Hb) of backfill material related to different wave conditions, in which the bottom of the buried pipeline are fully prevented from being liquefied. Based on the numerical results, it is found that the thickness of backfill material significantly affect the wave-induced accumulated pore pressure around the trenched pipeline, and the seabed at the bottom of the pipeline are easier to be liquefied with shallow backfill depth under high wave loading.