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This paper presents a numerical investigation into the vulnerability to liquefaction of an embedded pipeline exposed to ocean storms. In this study a series of irregular waves similar to a real ocean storm were generated using the JONSWAP spectrum, and their motions with the presence of ocean currents are described using the Reynolds-Averaged Navier–Stokes (RANS) equations. Under storm waves and currents, the dynamics of a seabed with an embedded pipeline were reproduced using a coupled fluid-dynamic framework. An advanced constitutive model (modified Pastor-Zienkiewicz Mark-III) was used to describe the cyclic plasticity of soil. This model was calibrated according to the given soil properties; the predicted results match the wave flume test and geotechnical centrifugal test fairly well when the evolution of residual pore water pressure near the pipe and/or further away are considered. The numerical results indicated that seabed soil is more susceptible to liquefaction due to regular waves than irregular waves. The specific gravity of the pipe can significantly affect the onset and spread of liquefaction in the neighbouring region. As a practical guide, a relationship between the critical wave height and the depth to which the pipe is buried is proposed to control the liquefaction at the bottom of the pipe by selecting feasible backfill materials based on operational requirements and ocean storm conditions.

In this study, an integrated model is developed to investigate the potential risk for failure of caisson-type breakwaters caused by liquefaction in its loosely deposited sand foundations. In the present model, the Volume-Averaged Reynolds Averaged Navier-Stokes (VARANS) equations are used for solving flow inside and outside the porous media, while Biot's consolidation equation is used for linking the soil skeleton-pore fluids interactions. A new developed poro-elastoplastic model is implemented to reproduce the behavior of loose sand foundation under cyclic shearing. Verification of the present numerical framework includes (i) comparison with the laboratory experiments in terms of water surface levels around the structure, and the fluctuating component of the pore water pressure inside the rubble mound and seabed foundation; and (ii) comparison with the geotechnical centrifugal wave tests for the wave-induced residual component of pore water pressures in a porous seabed. Based on the present model, the hydrodynamic process and the associated dynamic foundation behavior involved in the interactions between the ocean waves, a composite breakwater and its loose sand foundations was performed and analyzed. Particular focus is put on the residual pore pressure development and the resulting liquefaction around the structure under combined actions of both wave and structure rocking motions. Based on the numerical examples presented, under combined actions of both wave and caisson motions, liquefaction starts on both edges of the breakwater and spreads sideways until reaching an equilibrium state where the maximum liquefaction depth can be observed. The liquefied zone around the breakwater is asymmetric in nature, and the maximum depth of soil liquefaction appears at the seaward side of the structure. Furthermore, the parametric study indicates that the maximum liquefaction depth around the composite breakwater increases with increasing wave steepness (H/L0) and relative water depth (d/L0) and decreasing soil permeability (ks).

In the past several decades, a great number of offshore structures have been constructed on loosely deposited seabed foundation because sometimes there would be no a dense seabed floor could be chosen in planned sites, for example, the breakwaters and oil platforms in the Yellow River estunary area, China. Wave-induced residual liquefaction is easy to occur in loosely deposited seabed, which brings great risk to the stability of offshore structures. In this study, we focus our attention on the 3D interaction mechanism between ocean wave, a caisson breakwater and its loosely deposited seabed foundation. A three-dimensional integrated numerical model FSSI-CAS 3D is taken as the computational tool; and the soil constitutive model: Pastor-Zienkiewicz Mark III (PZIII) proposed by Pastor et al. [16] is adopted to describe the wave-induced dynamic behavior of loose seabed soil. The numerical results indicate that the developed integrated numerical model FSSI-CAS 3D is capable of capturing a series of nonlinear phenomena, such as tilting, subsiding of breakwater, as well as residual liquefaction in loose seabed foundation etc., in the interaction process between ocean wave, a caisson breakwater and its loose seabed foundation. The purpose of this study is to provide coastal engineers with comprehensive understanding of FSSI problme involving loosely deposited seabed soil; and propose a reliable computational method to engineers involved in the design of offshore structures on loose seabed foundation.

In this note, a simplified analytical approximation for the elevation of the wave-induced pore pressure buildup mechanism in marine sediments is derived using a Laplace transformation. In this study, an infinite seabed is considered. A simple scaling analysis is performed to clarify the applicable ranges of the mechanisms of residual and oscillatory pore pressures. Then, a simplified approximation for the prediction of the wave-induced liquefaction potential is proposed for engineering practice.