Search

Abstract Wave-induced seabed instability caused by the residual liquefaction of seabed may threaten the safety of an offshore foundation. Most previous studies have focused on the structure that sits on the seabed surface (e.g., breakwater and pipeline), a few studies investigate the structure embedded into the seabed (e.g. a mono-pile). In this study, by considering the inertial terms of pore fluid and soil skeleton, a three-dimensional (3D) integrated model for the wave-induced seabed residual response around a mono-pile is developed. The model is validated with five experimental tests available in the literature. The proposed model is then applied to investigate the spatial and temporal pattern of pore pressure accumulation as well as the 3D liquefaction zone around a mono-pile. The numerical simulation shows that the residual pore pressure in front of a pile is larger than that at the rear, and the seabed residual response would be underestimated if the inertial terms of pore fluid and soil skeleton are neglected. The result also shows that the maximum residual liquefaction depth will increase with the increase of the embedded depth of the pile. Highlights • 3D model for wave-induced seabed residual response by considering the inertial terms of pore fluid & soil skeleton. • Effects of pile embedded depth on the seabed residual response and liquefaction around the vicinity of mono-pile. • Effects of wave reflection and diffraction on seabed residual response with various wave and seabed parameters. [ABSTRACT FROM AUTHOR]

This paper proposes a two-dimensional (2D) coupled model for wave and current-seabed-pipeline interactions to examine oscillatory non-cohesive soil liquefaction around a partially buried pipeline in a trench. Unlike previous studies, two new features are included in this model: (1) wave-current interactions around the pipeline; and (2) fully coupled processes for the wave and current-seabed-pipeline system. In this study, the Reynolds Averaged Navier-Stokes (RANS) equations are applied to simulate the flow field around the pipeline, and Biot's poro-elastic theory for porous media is imposed to govern the soil response due to the wave-current loading. After being validated using data available in the literature, the 2D model is used to investigate the effects of the current velocity, the soil properties, and the wave characteristics on oscillatory non-cohesive soil liquefaction. Using the model, a function for the critical backfill thickness and the wave steepness under various flow and soil conditions is proposed to facilitate engineering practice. [ABSTRACT FROM AUTHOR]

Abstract: Analysis of the wave–seabed–structure interaction is of great importance for the design and construction of marine structures. In this paper, a three-dimensional porous model, based on Reynolds-Averaged Navier–Stokes equations and Biot׳s poro-elastic theory, is developed by integrating 3D wave and seabed models to simulate the wave–seabed–structure interaction around the high-rising structure foundation used in the Donghai offshore wind farm. Then parametric studies for wave and seabed characteristics on soil response around the wind turbine foundation are conducted. Liquefaction potential and seabed protection methodology are investigated in the last section. The main results concluded from the numerical analysis are as follows: (i) both larger wave height and longer wave period will result in higher wave pressure on the structure and seabed; (ii) the existence of the structure has a significant effect on the wave transformation and the distribution of wave-induced pore pressure; (iii) wave height, wave period, soil permeability and degree of saturation play critical roles on dynamic soil behavior; and (iv) original seabed can be protected from liquefaction by replacing the existing layers. [Copyright &y& Elsevier]

Abstract: Experiments on three types of soil (d 50=0.287, 0.057 and 0.034mm) with pipeline(D=4cm) either half buried or resting on the seabed under regular wave or combined with current actions were conducted in a large wave flume to investigate characteristics of soil responses. The pore pressures were measured through the soil depth and across the pipeline. When pipeline is present the measured pore pressures in sandy soil nearby the pipeline deviate considerably from that predicted by the poro-elasticity theory. The buried pipeline seems to provide a degree of resistance to soil liquefaction in the two finer soil seabeds. In the silt bed, a negative power relationship was found between maximum values of excess pore pressure p max and test intervals under the same wave conditions due to soil densification and dissipation of the pore pressure. In the case of wave combined with current, pore pressures in sandy soil show slightly decrease with time, whereas in silt soil, the current causes an increase in the excess pore pressure build-up, especially at the deeper depth. Comparing liquefaction depth with scour depth underneath the pipeline indicates that the occurrence of liquefaction is accompanied with larger scour depth under the same pipeline-bed configuration. [ABSTRACT FROM AUTHOR]

Abstract: Wave-induced soil liquefaction can cause serious damages to coastlines and coastal infrastructures. Although liquefaction caused by regular waves over flat sea beds has been extensively investigated, studies of tsunami-induced liquefaction of coastal sandy slopes have been relatively rare. In this work, parametric studies are conducted to investigate the liquefaction potential of coastal sandy slopes caused by breaking solitary waves. The dependence of the maximum liquefaction depth on soil permeability is studied at different cross-shore locations and for different offshore wave heights. The results suggest that tsunami can induce liquefaction failure of coastal sand slopes, and that the soil responses can be characterized by the ratio of the pore pressure diffusion time scale to the wave loading time scale. Under the same wave loading condition, an offshore soil column may experience the largest liquefaction depth, while the longest duration of liquefaction (which may physically correlate to the largest liquefaction volume) occurs in the seepage zone, because of the wave loading and unloading pattern above the seepagezone. [ABSTRACT FROM AUTHOR]

In this paper, a 3D model is developed to investigate the foundation stability around breakwaters at a river mouth, where both ocean waves and river currents are considered. The present model consists of flow and seabed sub-models. In the flow sub-model, the VARANS equations is used to describe hydrodynamic process, while the dynamic u − p approximation is employed in the seabed sub-model. Based on the integrated model, both oscillatory and residual liquefaction phenomenon is investigated by adopting the poro-elastic and poro-elastoplastic seabed models, respectively. The numerical results reveal that the breakwaters at the river mouth with the river current significantly alter the hydrodynamic properties around breakwater heads. Furthermore, the dynamic soil responses and liquefaction zones predicted by the poro-elastic and poro-elastoplastic seabed models show the completely different development trend, which the latter seabed has more intense and severe reactions. The parametric study indicates that the breakwater foundation is more likely to be liquefied in a loosely deposited soil under larger waves. The river currents can exacerbate the liquefaction condition in the region near breakwater heads. In addition, the pore pressure is smaller in front of a milder slope of breakwater flank and a round breakwater head with a smooth slope. • 3D poro-elastoplastic model for stability analysis around breakwater heads at a river mouth. • The soil liquefaction is more serious in a poro-elastoplastic seabed than a poro-elastic seabed. • The breakwater foundation is likely to be liquefied in loosely seabed with poor drainage condition. • The river currents can exacerbate the liquefaction around breakwater heads. • A milder slope of breakwater flank can prevent liquefaction around it to some extent. [ABSTRACT FROM AUTHOR]

Submarine pipeline is a type of important infrastructure in petroleum industry, used for transporting crude oil or natural gas. Understanding of the dynamics characteristics under hydrodynamic loading is crucial for engineers when assessing the stability of offshore pipelines in their designed service period. In this study, taking the integrated numerical model FSSI-CAS 2D as the tool, the nonlinear ocean wave & current-induced dynamics of a shallowly buried submarine steel pipeline and its surrounding loosely deposited seabed soil is numerically investigated. The excellent soil model Pastor-Zienkiewicz-Mark III (PZIII) is adopted to describe the complicated mechanical behaviour of loose seabed soil under cyclic loading. Computational results indicate that the shallowly buried submarine pipeline eventually significantly float up, and extrude the seabed soil over the pipeline driven by the gradually increased buoyancy on the pipeline caused by the accumulation of pore pressure around the pipeline, making the seabed over the pipeline hunch significantly. As a result, considerable deformation occurs in the seabed soil surrounding the pipeline. Two effective stress-based criterion are proposed to judge the occurrence of soil liquefaction. Adopting the two criterion. it is found that the surrounding seabed soil around the pipeline does not become liquefied; only stiffness softening has occurred in it. However, the soil in the upper seabed with shallow depth away from the pipeline becomes liquefied with a liquefaction depth 1.2–1.5 m. Comparative analysis indicates that the pipeline transporting natural gas floats upward with a much greater displacement than that if crude oil is transported. The computational results show that the integrated mode FSSI-CAS 2D has successfully and subtly captured a series of nonlinear physical phenomena of the intensive interaction between pipeline and its surrounding seabed soil. Finally, it is indicated that the integrated model FSSI-CAS 2D has an advantage to investigate the complicated interaction between fluid-structure-seabed foundation. • Wave & current-induced dynamics of a pipeline buried in loose seabed is studied. • The integrated model FSSI-CAS 2D is validated for pipeline-dense seabed interaction. • The third-order formulation is used to apply wave & current induced cyclic loading. • The sinking and floatation of pipeline have been successfully captured. • The soil surrounding pipeline is difficult to become liquefied under hydrodynamic loading. [ABSTRACT FROM AUTHOR]

With the increasing development and utilization of offshore oil and gas resources, seabed instability around pipelines subjected to combined ocean wave and current loadings and protection of the pipelines are becoming increasingly important. As a general practice, it is recommended to use trenching in shallow water region for the protection of submarine pipelines from the danger caused by storm waves and ocean currents changing level of the seabed. For a better understanding of the physical process involved in wave–current–seabed–pipeline interactions (WCSPI), a workable Finite Volume Model (FVM) is proposed to simulate wave–current induced soil responses around offshore pipelines. Based on the established FVM model, this study investigates the momentary soil liquefaction induced by various environmental loadings in ocean environments. In the present model, data exchange is taken place on the seabed surface to couple the flow and seabed sub-models. Unlike most previous studies, ocean currents are included in the present model, in which the Volume-Averaged Reynolds-Averaged Navier–Stokes (VARANS) equation is employed to govern non-linear fluid motions, while Biot's consolidation equation is used to link solid-pore fluid interactions in porous mediums. Numerical examples demonstrate the significant influence of ocean currents, trench geometry, and self-weight of the pipe on the wave-induced pore pressures and on the resultant seabed liquefaction around the pipeline. • OpenFOAM model for wave(current)-seabed interactions in a trench layer. • Significant changes of flow field and seabed liquefaction with a trench layer. • Simplified engineering design of pipeline protection with a trench layer. [ABSTRACT FROM AUTHOR]