Seabed Liquefaction Research Articles

A numerical model for assessing the effect of low clay content on wave-induced seabed liquefaction

In the marine environment, the seabed contains a certain amount of clay. Experimental studies show that the liquefaction susceptibility of the sandy seabed increases as clay content (CC) rises to a certain threshold, beyond which further increases in CC reduce liquefaction susceptibility. However, numerical models that describe the effect of CC on seabed liquefaction are very limited. This study proposed a dynamic poro-elasto-plastic finite element method model for analyzing liquefaction in the sandy seabed with CC below the threshold. Based on a series of undrained triaxial compression tests on sand-clay mixtures from existing literature, a unified constitutive framework was demonstrated to be effective for describing the liquefaction behavior of sand with low CC using one set of model parameters. Existing wave flume model tests validated the effectiveness of the proposed seabed model in describing the effect of low CC on excess pore water pressure (EPWP). Numerical results confirmed that adding a small amount of clay to the seabed increased the soil contraction and thus its liquefaction susceptibility. Wave-induced liquefaction was limited to a certain depth of the seabed, and the liquefaction depth was significantly affected by the CC. Adding a low content of clay the sandy seabed significantly increased both horizontal and vertical displacements under wave action, potentially leading to the instability of the seabed. This study provides a new method for accurately assessing the wave-induced stability of marine structures built on the sandy seabed containing certain amounts of clay.

A Methodology for Susceptibility Assessment of Wave-Induced Seabed Liquefaction in Silt-Dominated Nearshore Environments

Wave-induced seabed liquefaction significantly jeopardizes the stability of marine structures and the safety of human life. Susceptibility assessment is key to enabling spatial predictions and establishing a solid foundation for effective risk analysis and management. However, the current research encounters various challenges, involving an incomplete evaluation system, poor applicability of methods, and insufficient databases. These issues collectively hinder the accuracy of susceptibility assessments, undermining their utility in engineering projects. To address these challenges, a susceptibility assessment method with the safety factor was developed as the key assessment parameter, allowing for a comprehensive susceptibility assessment across the silt-dominated nearshore environment using Empirical Bayesian Kriging (EBK). The safety factor is determined by combining the cyclic stress ratio (CSR) and the cyclic resistance ratio (CRR), which characterize wave loadings and sediment properties in the study area, respectively. This method was applied in the Chengdao region of the Yellow River Estuary, China, a typical silt-dominated nearshore environment where wave-induced liquefaction events have been reported as being responsible for multiple oil platform and pipeline accidents. By collecting the regional wave and seabed sediment data from cores spanning from 1998 to 2017, the safety factors were calculated, and a zonal map depicting the susceptibility assessment of wave-induced seabed liquefaction was created. This study can serve as a valuable reference for the construction and maintenance of marine engineering in liquefaction-prone areas.

Effect of Wave Nonlinearity on the Instantaneous Seabed Liquefaction

Effect of Wave Nonlinearity on the Instantaneous Seabed Liquefaction

Fluid characteristics of wave-induced liquefied silty seabed and the resulting wave attenuation

Wave-induced liquefaction of silty seabed is a common submarine geological hazard that poses a significant risk to the safety of marine structures. The liquefied seabed exhibits fluid properties and fluctuates with the overlying wave. To investigate the interaction between waves and the liquefied seabed, a wave flume experiment was conducted to simulate the wave-induced liquefaction process. During the experiment, the variation in the fluctuation trajectory and viscosity coefficient of liquefied silt with the wave intensity and liquefaction process were measured. Additionally, the wave energy dissipation due to seabed liquefaction was analyzed. The results indicate that the viscosity coefficient of the liquefied seabed is closely related to the wave intensity and liquefaction process, with the viscosity coefficient of the liquefied seabed being larger in the liquefaction deposition stage than in the liquefaction development stage. Seabed liquefaction can lead to a considerable increase in wave energy dissipation, up to 15 times greater than when the seabed is stable. Furthermore, wave energy dissipation is greater for high waves than for low waves conditions. The primary factors causing wave energy dissipation are the viscosity friction inside the liquefied seabed and the suspended sediment in the upper water column.

Experimental study on the dynamic response of a silty seabed under waves

Scouring and liquefaction of silty seabeds under waves have been recognized as two major modes of the seabed instability around marine structures. A series of wave experiments were conducted to investigate the wave-induced dynamic response in a silty seabed. The seabed was composed of light sand with and without geotextile covering under various wave conditions. When the seabed was not covered with geotextile, the seabed was liquefied under the action of waves, accompanied by sediment incipience and suspension. The results show that with increasing wave height, the seabed develops a series of processes such as sand ripples, ripple attenuation, sheet flow and seabed liquefaction. After seabed liquefaction occurs, an upward force gradient of the accumulated pore pressure is generated, which increases the suspended sediment concentration near the bottom and further enhances the scour of the seabed. When it is scoured, the sediment particles are continuously removed, and the stress in the soil is released, which intensifies seabed liquefaction. When the seabed is covered with geotextile, the concentration of suspended sediment near the bottom is zero. The maximum liquefaction depth of the seabed decreases along with the development rate of the liquefaction depth. The amplitude of the pore water pressure in the seabed decreases by approximately 5%–20% and seabed liquefaction is reduced. Experimental results clarify that liquefaction and scour promote each other.

Pore-water pressure response of a silty seabed to random wave action: Importance of low-frequency waves

Wave-induced pore pressure in the seabed may cause seabed liquefaction and lead to geohazards such as submarine landslides. Under natural conditions, waves generally consist of different components with various frequencies, and the seabed response to each wave component can vary considerably. However, little is known concerning these differences. In this study, a series of flume experiments were conducted to investigate the pore pressure responses in a silty seabed to various components of random waves. According to the experimental results, the short wave (SW) component had high energies, but the SW-generated high-frequency hydrodynamic pressure (HF p0 ) decayed rapidly in the seabed. The long wave (LW) component had low energies, but the LW-generated low-frequency hydrodynamic pressure (LF p0 ) was effectively propagated in the seabed. The transmittance coefficient of the low-frequency pore pressures (LF p ) was two times that of the high-frequency pore pressures (HF p ). The energy ratio of LF p /HF p was enhanced with increasing soil depth, and the energy of LF p was greater than that of HF p at soil depths ≥15 cm, indicating that the contribution of LWs to the cumulative pore pressure is enhanced with an increase in soil depth. An important implication of our findings is that the use of statistical wave parameters (e.g., significant wave height and average wave period) to determine pore pressure responses of the seabed can underestimate liquefaction risk, and in turn, lead to unsafe engineering designs. • Long wave component generates significantly stronger pore pressure. • High-frequency pore pressures propagated poorly in the seabed. • Low-frequency pore pressures readily propagated within the seabed. • Long wave component had significantly larger contribution to deep soil pore pressure.

A computational framework for fluid–structure interaction with applications on stability evaluation of breakwater under combined tsunami–earthquake activity

AbstractIn this article, an improved impervious solid boundary condition of the coupled method called smooth particle hydrodynamics and discrete element method (SPH‐DEM) is proposed, which prevents the fluid particles from penetrating solid boundary under earthquake action. And an improved transmitting boundary condition of SPH‐DEM is designed in order to conquer the reflection of seismic waves on the boundary. Meanwhile, the effective stress method is proposed to be applied to the SPH‐DEM for simulating seabed liquefaction. Based on these, a new computational framework for the SPH‐DEM is put forward. Dynamic triaxial test of seabed soil samples indicate that our proposed computational framework can well reproduce the seismic liquefaction process of the seabed soil. Moreover, our proposed computational framework is used to numerically reproduce the failure mechanisms of a breakwater built in liquefied seabed under combined tsunami–earthquake activity and meantime the centrifuge test is carried out. And the experimental results demonstrate the effectiveness of our proposed computational framework, in which numerical results of it are consistent with results of the centrifuge test.

Experimental Study on the Influence of Pipeline Vibration on Silty Seabed Liquefaction

Free-spanning submarine pipelines are usually affected by vortex-induced vibration (VIV). Such vibration could influence the liquefaction of the supporting soil at both ends of the free spans and could have catastrophic consequences, including the failure of the local seabed and the displacing, sinking, or floating of pipelines. The influence of pipeline vibration on soil liquefaction has not been studied sufficiently. Therefore, we explored the influence of vortex-induced pipeline vibration on the excess pore pressure of silty soil around a pipeline using flume experiments. Our results showed that pipeline vibration could induce the buildup of excess pore-water pressure, even without wave loading. A fully liquefied zone was found close to the pipeline, where excess pore pressure reached the soil liquefaction criterion, which was surrounded by a partially liquefied zone. The extent of liquefaction depended on the vibration conditions and the weight and burial depth of the pipeline. The pipeline vibration amplitude increased after soil liquefaction. Unlike wave-induced liquefaction, pipeline-induced vibration liquefaction occurred at a critical value smaller than the initial mean normal effective stress. Considering the possibility of pipeline-vibration-induced seabed liquefaction, conventional approaches could underestimate the potential risks to pipeline stability and result in unsafe maintenance practices.

Towards the Numerical Modelling of Residual Seabed Liquefaction Using OpenFOAM

Knowledge of the interaction between free surface waves and the seabed is required for the reliable design of marine structures, preventing severe structural failures. To that end, this paper presents the numerical modelling of wave-induced residual liquefaction of seabed soil. An OpenFOAM® finite volume solver is developed to simulate the behaviour of pore pressure and shear stress in the soil and is validated against analytical reference data. The soil is considered as a poro-elastic solid and an additional equation is solved for the pore pressure buildup. The governing equations are valid only up to the onset of liquefaction. A criterion based on the accumulated pore pressure is applied in order to predict the onset of residual liquefaction. The results show that the pore pressure and shear stresses are in good agreement with the analytical results and the relative errors are less than three percent. Also, the numerical results indicate that the wave induced residual liquefaction originates from the mudline and progresses slowly down the soil which is consistent with the analytical results. The pore pressure buildup for a seabed with stone columns shows that the liquefaction potential is very low near the stone column.

Numerical study of Articulated Concrete Mattresses (ACMs) for offshore pipeline protection

In offshore pipeline projects, inevitably long sections of the transportation pipeline network require protection from natural hazards. Under such environmental conditions, concrete mattresses are recognized as a well-proven technology to surpass challenges in offshore pipeline construction, umbilical deployment and seabed protection/stabilization. To have a better understanding of the physical process of Fluid–Structure–Seabed-Interactions (FSSI) with a ACMs–Pipe–Seabed system, a workable numerical model is proposed to simulate wave–current induced soil responses around the system. Based on the proposed model, this paper discusses the momentary seabed liquefaction near the offshore pipeline and the stability of ACMs. Numerical examples show that ACMs have a significant positive effect on preventing seabed liquefaction around the pipeline by pushing the liquefaction zone to both sides of the shaft. Compared with unprotected conditions, the instability of the protective layer is mainly concentrated in its edge area, and the liquefaction depth can be reduced by around half of the pipe diameter. For the double ACMs–Pipe-System, it is found that the stability of the nearby seabed foundation is related to the mattress spacing and interaction angle.

Seismic Analysis of 10 MW Offshore Wind Turbine with Large-Diameter Monopile in Consideration of Seabed Liquefaction

With the increasing construction of large-scale wind turbines in seismically active coastal areas, the survivability of these high-rated-power offshore wind turbines (OWTs) in marine and geological conditions becomes extremely important. Although research on the dynamic behaviors of OWTs under earthquakes has been conducted with consideration of the soil-structure interaction, the attention paid to the impact of earthquake-induced seabed liquefaction on OWTs supported by large-diameter monopiles remains limited. In view of this research gap, this study carries out dynamic analyses of a 10 MW OWT under combined wind, wave, and earthquake loadings. This study uses a pressure-dependent multisurface elastoplastic constitutive model to simulate the soil liquefaction phenomenon. The results indicate that the motion of the large-diameter monopile leads to more extensive soil liquefaction surrounding the monopile, specifically in the zone near the pile toe. Moreover, compared with earthquake loading alone, liquefaction becomes more severe under the coupled wind and earthquake loadings. Accordingly, the dynamic responses of the OWT are apparently amplified, which demonstrates the importance of considering the coupling loadings. Compared with wind loading, the effect of wave loading on the dynamic response and liquefaction potential is relatively insignificant.

Experimental Study on Silty Seabed Liquefaction and Its Impact on Sediment Resuspension by Random Waves

Seabed liquefaction and sediment resuspension under wave loading are key issues in marine engineering, but are usually regarded as independent processes (instead of coexisting and interacting processes). Here, we analyzed random wave-induced seabed liquefaction and its impact on sediment resuspension using flume experiments. Results show that in a nonliquefaction scenario, excess pore pressure in the seabed oscillates with wave fluctuations, but pressure accumulation is low, while a consistent upward pressure gradient promotes sediment suspension. Wave-induced shear stress was the key driver of sediment resuspension in a nonliquefaction scenario. In the liquefied state, waves with different amplitudes differently responded to excess pore pressure; small-amplitude waves accumulated pressure, while large-amplitude waves dissipated it. Liquefied soil formed mud waves, creating elliptical motion along with random waves. Seabed liquefaction accelerated sediment resuspension in the following ways: reducing soil critical shear stress; forming seepage channels inside the seabed; forming mud waves, resulting in increased turbulent kinetic energy; dissipating excess pore pressure and releasing porewater, expelling fine-grained sediment from the liquefied soil. Our study reveals the variation in excess pore pressure in silty seabed under random waves and its effect on sediment resuspension, which is significant for understanding soil liquefaction and sediment movement of silt.

Three-Dimensional Modelling of Non-Linear Wave-Induced Seabed Response around Offshore Open-Ended Pile

This paper analyses the fluid–seabed–structure interactions (FSSI) around the open-ended pile by applying the in-house solver established on the open-source Computational Fluid Dynamics (CFD) platform. The Reynolds-averaged Navier–Stokes (RANS) equations are solved to simulate the hydrodynamic interactions between waves and open-ended piles. Biot’s poro-elastic theory (quasi-static model) is used to reproduce the wave-induced seabed responses. The parameter analysis indicates that the wave period, degree of saturation of seabed and pile diameter have a great influence on the development of the transient seabed liquefaction depth around the pile. In addition, the distribution of the pore water pressure vs soil depth in the inner zone of the pile presents a “V” shape rotated 90 degrees counterclockwise.

Combined wave–current induced seabed liquefaction around buried pipelines: Design of a trench layer

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.

Experimental Study on Mechanism of Wave-Induced Liquefaction of Sand-Clay Seabed

In this study, a series of laboratory experiments for the response of wave induced clay-sand seabed were carried out to clarify the mechanism of liquefaction of clayey seabed. The experiments were conducted in an 80 m long wave flume. In the tests, the sand-clay beds were mixed with various clay contents (CC) from 0.5% to 15% and were tested for given wave conditions. The pore water pressure and the water elevation were measured in each test. Soil properties tests and scanning electron microscope (SEM) experiments on different seabed samples were carried out to further explore the mechanism of liquefaction. The experimental results indicated that the amplitude and accumulation of the excess pore water pressure (EPP) varied with different CC in the sand-clay bed. With the introduction of CC, micro-structure and properties (such as permeability and compressibility) of bed soils changed. Sand-clay bed presented more susceptibility to liquefy compared with pure sand bed. CC promoted seabed liquefaction, even if the added amount was very small (CC is 0.5%), however when CC exceeded a certain value (10% in this study), the mixed bed will not be liquefied. This phenomenon can be well explained by the micro-structure of sand-clay bed. CC within a sandy seabed, does not only affect the permeability, but also change the compressibility of seabed soils. For example, the microfabric of seabed vulnerable to liquefaction is loose. Clay aggregations generally gathered at the sand particle contact points. This microfabric is easily compressed under wave loads and allowed pore water to flow, resulting in the accumulation of pore water pressure. On the other hand, the microfabric of seabed that was resistant to liquefaction appeared to be more compact. Due to clay-filled gaps between the sand particles, the pore water is more difficult to flow when seabed was compressed. Furthermore, the tendency of seabed liquefaction is closely related to CC.

Laboratory experimental study of ocean waves propagating over a partially buried pipeline in a trench layer

Seabed instability around a pipeline is one of the primary concerns in offshore pipeline projects. To date, most studies focus on investigating the wave/current-induced response within a porous seabed around either a fully buried pipeline or a thoroughly exposed one. In this study, unlike previous investigations, a series of comprehensive laboratory experiments are carried out in a wave flume to investigate the wave-induced pore pressures around a partially embedded pipeline in a trench layer. Measurements show that the presence of the partially buried pipeline can significantly affect the excess pore pressure in a partially backfilled trench layer, which deviates considerably from that predicted by the theoretical approach. The morphology of the trench layer accompanied with the backfill sediments, especially the deeper trench and thicker backfill (i.e., b≥1D,e≥0.5D), provides a certain degree of resistance to seabed instability. The amplitude of excess pore pressure around the trench layer roughly exhibits a left-right asymmetric distribution along the periphery of the pipeline, and decays sharply from the upper layer of the trench to the lower region. Deeper trench depth and thicker buried layer significantly weaken the pore-water pressures in the whole trench area, thus sheltering and protecting the submarine pipeline against the transient seabed liquefaction.

A Three-Dimensional Model for the Seabed Response Induced by Waves in Conjunction with Currents in the Vicinity of an Offshore Pipeline Using OpenFOAM

To better understand the physical processes involved in the wave–seabed–pipeline interactions (WSPI), a three-dimensional numerical model for the wave-induced soil response around an offshore pipeline is proposed in this paper. Seabed instability around an offshore pipeline is one of the key factors that need to be considered by coastal engineers in the design of offshore infrastructures. Most previous investigations into the problem of WSPI have only considered wave conditions and have not included currents, despite the co-existence of waves and currents in natural ocean environments. Unlike previous studies, currents are included in the present study for the numerical modeling of WSPI, using an integrated FVM model, in which the volume-averaged Reynolds-averaged Navier–Stokes (VARANS) equation is used to solve the mean fluid field, while Biot’s consolidation equation is used to describe the solid–pore fluid interaction in the porous medium. Numerical examples demonstrate a significant influence of ocean current direction and angle on the wave-induced pore pressures and the resultant seabed liquefaction around the pipeline, which cannot be observed in two-dimensional (2D) numerical simulation.

Numerical Simulation of a Sandy Seabed Response to Water Surface Waves Propagating on Current

An integrated numerical model is developed to study wave and current-induced seabed response and liquefaction in a flat seabed. The velocity-inlet wave-generating method is adopted in the present study and the finite difference method is employed to solve the Reynolds-averaged Navier-Stokes equations with k-ε turbulence closure. The model validation demonstrates the capacity of the present model. The parametrical study reveals that the increase of current velocity tends to elongate the wave trough and alleviate the corresponding suction force on the seabed, leading to a decrease in liquefaction depth, while the width of the liquefaction area is enlarged simultaneously. This goes against previous studies, which ignored fluid viscosity, turbulence and bed friction.

Experimental study on soil response and wave attenuation in a silt bed

When ocean waves propagate over porous seabed, they cause variations of the pore pressure within seabed, leading to the possible wave attenuation and soil liquefaction. In order to advance and improve our understanding of the process of wave-induced seabed liquefaction and its impact on wave propagation, systematical experiments are carried out in a wave flume with a soil basin filled with silt. Both the pore pressures and water surface elevations are measured simultaneously, while the seabed liquefaction is videotaped using a high-speed camera. Laboratory measurements show that the pore pressure in surface layer mainly oscillates over time, while the wave period averaged pore pressure has little change. In the deep layer, however, the wave period averaged value of the pore pressure builds up dramatically. The results show that the wave height decreases rapidly along the direction of wave propagation when seabed liquefaction occurs. Such a wave attenuation is greatly enhanced when the liquefaction depth further increases. The experiments also demonstrate that the conditions (wave height and wave period) of incident waves have significant impacts on the wave-induced pore pressures, liquefaction depth and wave attenuation in a silt bed.

Numerical testing on wave-induced seabed liquefaction with a poro-elastoplastic model

Dynamic seabed response under wave loading is one of key factors for the design and construction of offshore structures. Most previous studies were based on poroelastic seabed model. In this paper, based on a unified elasto-plastic constitutive model that can describe the liquefaction of sand and two-phase u-p theory for saturated soils, numerical tests are conducted to analyze the dynamic responses of a sandy seabed subjected to cyclic wave loads. The development of liquefaction zone, the change of excess pore water pressure (EPWP), the effective stress path, and the displacement vector are investigated. Numerical tests show that the proposed method is able to capture the mechanical behaviors of wave induced liquefaction of a sandy seabed. The calculated effective stress path and change of EPWP are similar to those of earthquake-induced liquefaction. In other words, the mechanism of wave-induced and earthquake-induced liquefaction are similar, despite of the loading forms. The liquefaction depth increases with the number of wave cycles. Meanwhile, a phase lag is observed between the liquefied seabed and wave motion. A comparison between the dynamic response of elastic and elasto-plastic seabed is presented to underline the importance of considering the plastic deformation of seabed.