Wave-induced Seabed Instability Research Articles

3D numerical modeling of wave–monopile–seabed interaction in the presence of a scour hole

Coastal engineers are deeply concerned about wave-induced seabed instability around monopile foundations. Most prior studies have primarily focused on the seabed response around monopiles on flat seabeds, often neglecting the presence of scour holes commonly found in engineering practice. This study thoroughly investigates seabed response and liquefaction around monopiles of scour holes. The examination involves analyzing 3D flow velocities, dynamic wave loads, pore pressures, soil effective stresses, and wave forces around a monopile with a scoured profile. The analysis uses a model that integrates the interactions between waves, seabed, and structures, and it is implemented within the OpenFOAM framework. The study meticulously assesses the impact of scour holes on seabed responses and liquefaction zones around monopile foundations. Numerical results reveal that: (1) The magnitude of the pore pressure within the scoured seabed is reduced relative to a flat seabed; (2) Considering the scour hole effect leads to a more uniform distribution of liquefaction depth around the monopile; (3) This effect intensifies with a higher saturation degree, while it exhibits a pattern of increase, followed by a decrease with an increase in permeability.

Meshfree Model for Wave-Seabed Interactions Around Offshore Pipelines

The evaluation of the wave-induced seabed instability around a submarine pipeline is particularly important for coastal engineers involved in the design of pipelines protection. Unlike previous studies, a meshfree model is developed to investigate the wave-induced soil response in the vicinity of a submarine pipeline. In the present model, Reynolds-Averaged Navier-Stokes (RANS) equations are employed to simulate the wave loading, while Biot’s consolidation equations are adopted to investigate the wave-induced soil response. Momentary liquefaction around an offshore pipeline in a trench is examined. Validation of the present seabed model was conducted by comparing with the analytical solution, experimental data, and numerical models available in the literature, which demonstrates the capacity of the present model. Based on the newly proposed model, a parametric study is carried out to investigate the influence of soil properties and wave characteristics for the soil response around the pipeline. The numerical results conclude that the liquefaction depth at the bottom of the pipeline increases with increasing water period (T) and wave height (H), but decreases as backfilled depth ( H b ), degree of saturation ( S r ) and soil permeability (K) increase.

Wave-induced seabed residual response and liquefaction around a mono-pile foundation with various embedded depth

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.

Evaluation of storm wave-induced silty seabed instability and geo-hazards: A case study in the Yellow River delta

Models based on the theoretical framework of soil mechanics are presented to evaluate storm wave-induced silty seabed instability and geo-hazards through a case study in the Yellow River delta. First, the transient and residual mechanisms of wave-induced pore pressure are analyzed. Three typical models (i.e., elastic model, pore pressure development mode and elasto-plastic model) are proposed to calculate wave-induced stresses in the seabed. Next, mechanisms and calculation methods of wave-induced seabed instability modes such as scour, liquefaction, seepage instability and shear slide are proposed. Typical results of storm wave-induced excess pore pressure and seabed instability are given and relevant discussions are made. At last, the formation mechanism of geo-hazards in the Yellow River delta is analyzed based on the proposed mechanism and calculated results. Results and analysis indicate that both transient and residual mechanisms are important to storm wave-induced response of silty seabed and hence the elasto-plastic model is more appropriate. Complete liquefaction does not happen, while other types of instability occur mostly within 2–6m under the seabed surface. Wave-induced scour, seepage instability and shear slide are all possible instability modes under the 1-year storm waves, and scour is predominant for the 50-year storm waves. The formation mechanism of geo-hazards such as shallow slide and storm wave reactivation, pockmarks, silt flow and gully, disturbed stratum and hard crust in the Yellow River are well explained based on the proposed mechanisms and calculated results of storm wave-induced silty seabed instability.

Numerical study of wave-induced soil response in a sloping seabed in the vicinity of a breakwater

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.

3D Modeling of Wave-Seabed-Pipeline in Marine Environments

In this study, a three-dimensional numerical model is developed, based on the Finite Element Method, to ana- lyse the behaviour of soil under the wave loading. The pipeline is assumed to be rigid and anchored within a trench. The influence of wave obliquity on seabed responses, the pore pressure and soil stresses, are studied, which cannot be handled by the existing 2D models. It is revealed that three-dimensional characteristics systematically affect the distribution of soil response around the circumference of the underwater pipeline. Based on new 3D model, the effects of wave and soil char- acteristics and trench configuration on the wave-induced seabed instability are discussed in detail.

Analysis of seabed instability using element free Galerkin method

Wave-induced seabed instability, either momentary liquefaction or shear failure, is an important topic in ocean and coastal engineering. Many factors, such as seabed properties and wave parameters, affect the seabed instability. A non-dimensional parameter is proposed in this paper to evaluate the occurrence of momentary liquefaction. This parameter includes the properties of the soil and the wave. The determination of the wave-induced liquefaction depth is also suggested based on this non-dimensional parameter. As an example, a two-dimensional seabed with finite thickness is numerically treated with the EFGM meshless method developed early for wave-induced seabed responses. Parametric study is carried out to investigate the effect of wavelength, compressibility of pore fluid, permeability and stiffness of porous media, and variable stiffness with depth on the seabed response with three criteria for liquefaction. It is found that this non-dimensional parameter is a good index for identifying the momentary liquefaction qualitatively, and the criterion of liquefaction with seepage force can be used to predict the deepest liquefaction depth.

A 3-D model for ocean waves over a Columb-damping poroelastic seabed

The evaluation of the wave-induced seabed instability in the vicinity of a breakwater is particularly important for coastal and geotechnical engineers involved in the design of coastal structures. In this paper, an analytical solution for three-dimensional short-crested wave-induced seabed instability in a Coulomb-damping porous seabed is derived. The partial wave reflection and self-weight of breakwater are also considered in the new solution. Based on the analytical solution, we examine (1) the wave-induced soil response at different location; (2) the maximum liquefaction and shear failure depth in coarse and fine sand; (3) the effects of reflection coefficients; and (4) the added stresses due to the self-weight of the breakwater.

Effects of surface waves and marine soil parameters on seabed stability

Based on the Yamamoto's soil model by considering the Coulomb friction effects, the wave-induced seabed instability has been investigated. An analytical solution is derived for soil response of a finite depth seabed under surface water wave. The effects of wave parameters and soil characteristics on the seabed instability are addressed for three types of soil. Finally, the roles of Coulomb friction stability are then analyzed as well.

Mechanism of the wave-induced seabed instability in the vicinity of a breakwater: a review

A detailed knowledge of the wave-induced seabed instability is particularly important for engineers involved in the design procedure of many marine structures and offshore installations. In this paper, the basic aspects of such instability will be examined. The current understanding of the mechanism of the wave–seabed interaction phenomenon and available approaches will be reviewed. Based on the framework of simplified analysis, the potential for such instability will be formulated that will help engineers to identify potential unstable sediments in the vicinity of a marine structure.

Effects of a cover layer on wave-induced pore pressure around a buried pipe in an anisotropic seabed

In engineering practice, a cover layer of coarser material has been used to protect a buried marine pipeline from wave-induced seabed instability. However, most previous investigations of the wave–seabed–pipe interaction problem have been concerned only with such a problem either in an isotropic single layer or a rigid pipe. This paper proposes a two-dimensional finite element model by employing the principle of repeatability to investigate the wave-induced soil response around a buried pipeline. The elastic anisotropic soil bahavior and geometry of cover layer are included in the present model, while the pipe is considered to be an elastic medium. This study focuses on the effects of a cover layer (including thickness B and width W of the cover layer) on the wave-induced pore pressure in the vicinity of a buried pipeline.

Wave-induced seabed instability around a buried pipeline in a poro-elastic seabed

The subject of the wave–seabed–structure interaction is important for civil engineers regarding stability analysis of foundations for offshore installations. Most previous investigations have been concerned with such a problem in the vicinity of a simple structure such as a vertical wall. For more complicated structures such as a pipeline, the phenomenon of the wave–seabed–structure has not been fully understood. This paper proposes a finite-difference model in a curvilinear coordinate system to investigate the wave-induced seabed response in a porous seabed around a pipeline. Based on the present numerical model, mechanism of the wave-induced soil response is examined. Employing Mohr–Coulomb failure criterion, the wave-induced seabed instability is also estimated. The numerical results indicate the importance of the effect of pipeline on the seabed response.

Wave-Induced Seabed Instability: Difference Between Liquefaction and Shear Failure

The mechanism of wave-induced instability in a permeable seabed have been studied for more than two decades. The distinction between shear failure and liquefaction, however, has not been clearly defined. This paper presents a fundamental study on the differences in two failure modes for a fully saturated seabed of both finite and infinite thickness. The wave-induced effective stresses and pore pressure, obtained from an analytical solution of Biot’s pore-elastic consolidation theory, were employed to examine the failure modes under a two-dimensional plane strain condition. A case study is presented to examine the failure modes with respect to several parameters, such as excess pore pressure, seepage flow, seepage force, failure areas and stress path in the seabed. The conclusions obtained from this study were as follows; (1) the thickness of a permeable seabed affects the pore pressure and effective stress response to ocean waves and the failure mode of the seabed, (2) either a liquefaction or shear failure, or both, occur in the seabed, even in the saturated seabed, (3) the Mohr-Coulomb’s failure criterion, when combined with elastic stresses, can not be employed to estimate the liquefaction failure in the seabed, (4) the liquefaction can be evaluated by a criterion in terms of the excess pore pressure, (5) The liquefied zone in the seabed is significantly different from the shear failure zone. The shape beneath the seabed surface for the former is almost identical to the contour where the upward seepage flow is concentrated.

Nonlinear Wave-Induced Seabed Instability Around Coastal Structures

Seabed liquefaction at the toe of coastal structures is one of the most serious problems threatening their stability. Various simplified techniques have been commonly used to quantify this phenomenon, but often with shortcomings. An integrated technique, using numerical models developed by the authors, is presented to analyze the conditions and extents of seabed liquefaction and slip failure. This includes: a BEM-FEM model for the dynamic interaction among wave, structure and the seabed; a poro-elastic FEM model for the dynamic interaction between structure and the seabed; and a model for the wave-induced seabed response based on Yamamoto's analytical solution (1977). The proposed technique is applied to the seabed subject to progressive waves, around vertical seawall with/without a rubble toe as well as in the vicinity of submerged and composite breakwaters. In this work, the permeability and porewater compressibility have been found to play a key role in controlling the seabed instability. The nonlinear wave harmonics have also been found to experience different phase delays and may interact constructively in the seabed under coastal structures leading to a higher possibility of seabed liquefaction. Higher harmonics seem to vanish in the surface layer of the seabed while the main harmonic penetrates into the seabed further.

Wave-induced seabed instability in front of a breakwater

The wave-induced soil response in a porous seabed has become an important factor for the stability of offshore facilities, because many marine structures may have failed due to seabed instability and concomitant subsidence. An analytical solution is presented for the wave-induced soil response under the action of a three-dimensional wave system. Based on this general solution, the mechanism of seabed instability is then investigated. The general solutions for pore pressure and effective stresses are readily reducible to two dimensions for progressive waves, and are compared to theoretical and experimental work available. Some dominant factors affecting the wave-induced seabed instability are discussed; including permeability, seabed thickness and degree of saturation.

The effects of variable permeability on the wave-induced seabed response

The topic of wave-seabed interaction is important for civil engineers with regard to stability analysis of foundations for offshore structures. Most previous investigations of such problems have simply assumed a seabed with uniform permeability, even if the evidence of variable permeability has been reported in the literature. This paper presents a finite-element model for investigating the wave-induced seabed response in a porous seabed, with variable permeability as a function of burial depth. The present finite formulation is established by using a combination of semi-analytical techniques and the Galerkin method. Based on the present numerical model, together with the Mohr-Coulomb failure criterion, the wave-induced seabed instability is estimated. The numerical results indicate that variable permeability affects the wave-induced seabed instability significantly, especially for gravelled seabeds.