A 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 the first liquefaction wave period. This 2D pattern became one-dimensional after one wave period, decreasing progressively to a constant value after a number of wave cycles. For the case with standing wave loadings, a 2D liquefaction zone occurred first in the region where the shear strains are highest. Eventually, this 2D pattern becomes continuous, which implies that even the soil in the antinode section can be liquefied. Compared with the seabed response under linear wave loading, the pore pressure more easily accumulates to a higher value under nonlinear wave loading because of the higher peak in the shear strains. Parametric studies indicate that both the wave characteristics and soil properties affect the maximum relative liquefaction depth ($z_L/h$ ) significantly. In general, the maximum liquefaction depth increases as the wave height and wavelength increase in shallow water within the seabed, which has a lower permeability and lower relative density.

In this study, a mathematical integrated model is developed to investigate the wave-induced sloping seabed response in the vicinity of breakwater. In the present model, the wave model is based on the Volume-Averaged/Reynolds Averaged Navier–Stokes (VARANS) equations, while Biot's consolidation equation is used to govern the soil model. The influence of turbulence fluctuations on the mean flow with respect to the complicated interaction between wave, sloping seabed and breakwater are obtained by solving the Volume-Averaged k − ϵ model. Unlike previous investigations, the phase-resolved absolute shear stress is used as the source of accumulation of residual pore pressure, which can link the oscillatory and residual mechanisms simultaneously. Based on the proposed model, parametric studies regarding the effects of wave and soil characteristics as well as bed slopes on the wave-induced soil response in the vicinity of breakwater are investigated. Numerical results indicate that wave-induced seabed instability is more likely to occur in a steep slope in the case of soil with low relative density and low permeability under large wave loadings. It is also found that, the permeability of breakwater significantly affect the potential for liquefaction, especially in the region below the breakwater.Griffith Sciences, Griffith School of EngineeringFull Tex

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 D_r) 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.

The evaluation of seabed response around an offshore pipeline has been studied since the 1980s. Most previous investigations considered full backfilled conditions. However, partial backfill is normally used in engineering practice, rather than full backfill. In this study, a two-dimensional (2D) integrated numerical model (PORO-WSSI-PIPE) is proposed to investigate the wave-induced residual liquefaction in a seabed around a partially backfilled trenched pipeline. The present model is validated through the comparison with previous laboratory experiments to ensure its accuracy and effectiveness. Sensitivity analysis indicates that the thickness of backfills significantly affects the wave-induced pore pressure around a trenched pipeline, and liquefaction may occur in the underlying trench layer with shallow backfill under large waves and in shallow water. Based on the numerical examples, a simplified approximation for the design of the thickness of backfills ($H_b$ ) with given wave and soil parameters is suggested for engineering practice, in which the pipeline can be prevented from seabed liquefaction.