Seabed Foundation Research Articles

Numerical Analysis of Dynamic Response and Liquefaction Phenomena in Sandy Seabed Foundation around a Semi-Circular Breakwater under Wave Loading

Understanding the stability of the seabed foundation holds paramount significance in guaranteeing the safety and structural soundness of the breakwater alongside additional offshore structures. This study aimed to investigate the stability of a sandy seabed foundation around a semi-circular breakwater under wave loading in nearshore areas. A coupled numerical model of waves, a semi-circular breakwater, and the seabed was developed based on the OpenFOAM platform. The VARANS equations were used to govern the wave behavior. Meanwhile, the Biot’s partially dynamic model was employed to numerically simulate the seabed response considering both consolidation under self-weight and dynamic response under wave loading. The effects of various wave parameters, seabed properties, and the radius of the structure on the dynamic response of the seabed and the depth of liquefaction were investigated. The numerical results indicate that an increase in wave height, period, and permeability coefficient intensifies the dynamic response of the seabed soil. Furthermore, an increase in water depth weakened the soil’s dynamic response. There was a negative correlation between the radius of the semi-circular breakwater and the dynamic response. The influence of Poisson’s ratio on the dynamic response of the seabed was relatively small. Furthermore, a stronger dynamic pore pressure response was observed at the connection between the semi-circular breakwater and the rubble foundation.

3-Dimensional numerical study of wave-induced seabed response around three different types of wind turbine pile foundations

In this paper, preliminary study is conducted on oscillatory wave-induced seabed response around three configuration cases of pile support foundations applying a 3-dimensional integrated numerical model to study and comparatively analyzed them. In the past, numerous studies have been conducted into exploring the wave-seabed-structure interaction (WSSI) mostly around monopile. However, attention on other pile support structure foundations is minimal. In this present study, Reynolds-Average Navier–Stokes equations with k-turbulence closure as well as Biot’s poroelastic theory are employed to govern the wave motion and porous seabed foundation respectively. The present numerical model is compared with available physical experimental data to determine its capability of simulating the WSSI around pile structures. Results analysis indicate that the impact of wave forces and wave pressure on the gravity-based support foundation is relatively higher than that of the monopile and tripod support pile due to the large peripheral area it occupied. Result of the momentary wave-induced liquefaction depth for the three configuration cases of pile structures at the upstream side in the seabed foundation shows that the tripod support pile has higher tendency resistance against wave-induced liquefaction, which may perhaps be due to the additional legs.

Seismic Dynamics of Pipeline Buried in Dense Seabed Foundation

Submarine pipeline is a type of important infrastructure in petroleum industry used for transporting crude oil or natural gas. However, submarine pipelines constructed in high seismic intensity zones are vulnerable of attacks from seismic waves. It is important and meaningful in engineering design to comprehensively understand the seismic wave-induced dynamics characteristics of submarine pipelines. In this study, taking the coupled numerical model FSSI-CAS 2D as the tool, the seismic dynamics of a submarine steel pipeline buried in dense soil is investigated. Computational results indicate that submarine pipeline buried in dense seabed soil strongly responds to seismic wave. The peak acceleration could be double of that of input seismic wave. There is no residual pore pressure in the dense seabed. Significant resonance of the pipeline is observed in horizontal direction. Comparative study shows that the lateral boundary condition which can avoid wave reflection on it, such as laminar boundary and absorbing boundary should be used for seabed foundation domain in computation. Finally, it is proven that the coupled numerical model FSSI-CAS 2D is applicable to evaluate the seismic dynamics of submarine pipeline.

Effects of cross-anisotropic soil behaviour on the wave-induced residual liquefaction in the vicinity of pipeline buried in elasto-plastic seabed foundations

In this paper, a two-dimensional integrated numerical model is developed to examine the influences of cross-anisotropic soil behaviour on the wave-induced residual liquefaction in the vicinity of a pipeline buried in a porous seabed. In the wave model, the RANS (Reynolds Averaged Navier–Stokes) equation is used to govern the wave motion. In the seabed model, the residual soil response in the vicinity of an embedded pipeline is considered with the 2-D elasto-plastic solution, where the phase-resolved shear stress is used as a source for the build-up of residual pore pressure. Classical Biot׳s consolidation equation is used for linking the solid-pore fluid interaction. The validation of the proposed integrated numerical model is conducted by the comparisons with the previous experimental data. Numerical examples show that the pore pressures can accumulate to a large value, thus resulting in a larger area of liquefaction potential in the given anisotropic soil compared to that with isotropic solution. The influences of anisotropic parameters on the wave-induced residual soil response in the vicinity of pipeline are significant. A high rate of pore pressure accumulation and dissipation is observed and the liquefaction potential develops faster as the anisotropic parameters increase. Finally, a simplified approximation based on a detailed parametric investigations is proposed for the evaluation of maximum liquefaction depth (zL) in engineering application.

Numerical simulation of the seismic liquefaction mechanism in an offshore loosely deposited seabed

As a natural foundation of offshore structures, the instability of offshore seabed foundations is the dominant factor for the failure of offshore structures in strong earthquake events. It has been reported that a great number of offshore structures have failed due to soil liquefaction of Quaternary loosely deposited seabed during several recent strong seismic events. At present, countless investigations on seismic wave-induced liquefaction in on-land soil have been conducted. However, investigations on seismic wave-induced seabed soil liquefaction in an offshore environment is limited. In this study, a coupled numerical model [fluid–structure–seabed interaction (FSSI)-CAS 2D] was utilized to investigate the seismic wave-induced liquefaction mechanism in newly deposited seabed soil. The advanced soil constitutive model PZIII was adopted to describe the nonlinear dynamic behavior of loose soil. In computation, the variation of void ratio e, and related variation of soil permeability is taken into consideration; and the hydrostatic pressure acting on the seabed surface, as a boundary condition value, is automatically updated based on seabed deformation . The numerical results indicate that a loose seabed could liquefy completely, and that seabed soil liquefaction initiates at the surface of the seabed, then progresses downward. It is also indicated that the advanced soil constitutive model, Pastor–Zienkiewicz Mark III (PZIII), is capable of describing the post-liquefaction behavior of loose seabed soil to some extent.

Seismic dynamics of offshore breakwater on liquefiable seabed foundation

Offshore structures, such as composite breakwaters, are generally vulnerable to strong seismic wave propagating through loose or medium-dense seabed foundation. However, the seismically induced failure process of offshore structures is not well understood. In this study, seismic dynamics of a composite breakwater on liquefiable seabed foundation is investigated using a fully coupled numerical model FSSI-CAS 2D. The computation results show that the numerical model is capable of capturing a variety of nonlinear interaction phenomena between the composite breakwater and its seabed foundation. The numerical investigation demonstrates a three-stage failure process of the breakwater under seismic loading. In this process, the far-field seabed can become fully liquefied first, inducing excessive settlement of the structure, followed by significant lateral movement and tilting of the structure when the near-field soil progressively liquefies. The study demonstrates great promise of using advanced numerical analysis in geotechnical earthquake design of offshore structures.

Nonlinear interaction between wave, breakwater and its loose seabed foundation: A small-scale case

In offshore environment, newly deposited quaternary loose sediments widely exist in the world. Sometimes, coastal engineers cannot avoid choosing these natural loose sediments as the foundation of marine structures. Under this situation, the interaction between ocean wave, marine structures and their loose sand foundation, and the wave-induced pore pressure built-up in loose seabed foundation are the key factors in which coastal engineers would most concern. In this study, the nonlinear interaction mechanism between ocean wave, a composite breakwater and its loose elasto-plastic sand bed foundation is investigated by utilizing a semi-coupled numerical model FSSI-CAS 2D. In the coupled numerical model FSSI-CAS 2D, VARANS equation for wave model, and dynamic Biot׳s equation for soil model are used for the governing equations. The advanced and excellent elasto-plastic constitutive model PZIII proposed by Pastor et al. (1990) is adopted to describe the nonlinear dynamic behavior of loose sand soil under cyclic wave loading. Taking the parameters of Neveda sand determined in the VELACS project funded by NSF as the property parameters for the loose sand bed, a small-scale computational case like the experimental set-up in Mostafa et al. (1999) is taken as a representative case to investigate the wave-structure-loose seabed foundation interaction mechanism. The numerical results indicate that the pore pressure in loose seabed foundation builts up significantly, and the marine structure subsides and tilts under wave loading. The loose sand bed becomes denser and denser due to soil compaction. Parametric studies show that both the wave characteristics and the soil properties can affect the built-up of residual pore pressure in loose seabed foundation.

Numerical simulation of the wave-induced dynamic response of poro-elastoplastic seabed foundations and a composite breakwater

In this study, an integrated numerical model FSSI-CAS 2D (previously known as POROWSSI 2D) is developed for the problem of wave-elasto-plastic seabed-structure interactions, where the Volume Average Reynolds Average Navier–Stokes (VARANS) equation is taken as the governing equation for wave motion and porous flow in porous medium; the dynamic Biot’s equation known as “u-p” is taken as the governing equation for the dynamics of porous seabed soil under wave loading. The Pastor–Zienkiewicz Mark III proposed by Pastor et al. (1990) [45] is used to describe the dynamic behaviour of poro-elasto-plastic seabed under wave loading. This developed integrated numerical model is validated by a centrifuge test conducted by Sassa and Sekiguchi (1999) [30]. The developed integrated numerical model is applied to investigate the wave-induced dynamic response of a composite breakwater and its elasto-plastic seabed foundation. The numerical results indicate that the pore pressure in an elasto-plastic seabed builds up under wave loading, leading to the reduction of the contact effective stresses between soil particles. The residual liquefaction occurs when the effective stresses decrease to a value approaching zero. The wave-induced residual liquefaction in seabed is progressive downward. A parameter considering the cohesion and friction angle of soil is defined to evaluate the residual liquefaction potential. Analysis results illustrate that the friction angle of soil has significant effect on the soil liquefaction; and Nevada dense sand becomes liquefied if the defined parameter exceeded 0.86. Parametric study shows that wave characteristics and soil properties have significant effects on the wave-induced progressive residual liquefaction in loose elasto-plastic seabed foundation.

Validation of a 2-D semi-coupled numerical model for fluid–structure–seabed interaction

A 2-D semi-coupled model PORO-WSSI 2D (also be referred as FSSI-CAS 2D) for the Fluid-Structure-Seabed Interaction (FSSI) has been developed by employing RANS equations for wave motion in fluid domain, VARANS equations for porous flow in porous structures; and taking the dynamic Biot's equations (known as "u − p" approximation) for soil as the governing equations. The finite difference two-step projection method and the forward time difference method are adopted to solve the RANS, VARANS equations; and the finite element method is adopted to solve the "u − p" approximation. A data exchange port is developed to couple the RANS, VARANS equations and the dynamic Biot's equations together. The analytical solution proposed by Hsu and Jeng (1994) and some experiments conducted in wave flume or geotechnical centrifuge in which various waves involved are used to validate the developed semi-coupled numerical model. The sandy bed involved in these experiments is poro-elastic or poro-elastoplastic. The inclusion of the interaction between fluid, marine structures and poro-elastoplastic seabed foundation is a special point and highlight in this paper, which is essentially different with other previous coupled models The excellent agreement between the numerical results and the experiment data indicates that the developed coupled model is highly reliablefor the FSSI problem.

Seismic response of poro-elastic seabed and composite breakwater under strong earthquake loading

The marine structures in offshore area, such as composite breakwater, are generally vulnerable to the strong seismic wave propagating through their seabed foundation. There are a lot of failure examples during the strong earthquake events in the world in the past 20 years. However, attention given to the seismic response of marine structures under strong seismic wave is limited. In this study, taking the dynamic Biot’s equation “u − p” as the governing equation for porous seabed foundation, the seismic response of a composite breakwater and its porous seabed foundation under the seismic wave recorded in the Japan 311 off the pacific coast of Tohoku earthquake (M L = 9.0) is investigated using a FEM numerical model. The numerical results indicate that the seismic response of composite breakwater is very strong in the earthquake process. The amplification of the input seismic wave occurs both in seabed foundation and composite breakwater; and this amplification is positively related to the buried depth of points. The horizontal seismic response is much strong than the vertical seismic response. The seismic wave induced excess pore pressure and effective stresses in seabed foundation vibrates; the vibration amplitude is also positively related to the buried depth of points. Under strong seismic loading, the surface region of seabed foundation could liquefy. The parametric study shows that the young’s modulus of seabed foundation has significant effect on the seismic response of composite breakwater.

Hydrodynamic response of tension-leg platforms—A model

The tension-leg platform (TLP) is a hybrid type of offshore platform, fixed for the vertical degrees of freedom and compliant for the horizontal degrees of freedom. The platform structure is connected to the sea-bed foundation by vertical moorings (tension legs) which are kept in tension under all conditions by the excess buoyancy over weight of the platform. Hydrodynamic analysis is an important stage of the design of these structures; model tests are usually done for this purpose. This paper gives the details of an experimental study to analyze the behavior of a 1:50 TLP model subjected to the action of regular and irregular waves. Platform motions in surge, sway and heave directions were measured using accelerometers. Tether tensions were measured using specially developed load cells. Details of instrumentation, testing and analysis to obtain response amplitude operators (RAOs) are explained. The connection of tether to the column, arrangement for adjusting pretension from top, connection of load cell to the tether and provision for changing the angle of incidence of waves are some special features of this experimental study. The work reported will serve as a useful practical guide for similar types of experimental study. In addition, the conclusions on the comparison of RAOs of the model for regular and irregular waves will be useful in planning hydrodynamic model tests.

Softening of the Clayey Seabed Foundation due to Earthquakes

Offshore structures are subjected to repeated loads from earthquakes and waves which may cause softening of the clayey seabed foundation. Carrying out a series of model tests on a shaking table, the following results are obtained. Settlement and inclination of a model of the base-part of the structure occur when the excess pore water pressure beneath the model rises to about 5 percent of the initial confining pressure. The earthquake response analysis even taking the nonlinearity of the soil into account cannot predict the results of the model test when the pore water pressure does generate and accumulate. Model tests show that the values of the pore water pressure are about twice as large as those predicted by calculation. From these results, rough evaluation of earthquake stability of the clayey seabed under offshore structures are obtained.

Earthquake Response of Tension Leg Platform with Gravity Anchor

The Tension Leg Platform (TLP) is a new type of oceanic structure developed for exploiting deep water petroleum. Generally the TLP is moored to the sea-bed foundation by vertical taut tethers. When the TLP is compared with a conventional jacket-type or jack-up-type platform, it has an advantage of low cost when constructing in a deep waters.Since the TLP has been said to be an aseismatic structure, an earthquake load has not been considered as an external force to tethers in design. It is true that the horizontal component of earthquake can be ignored, because the bending rigidity of the tether is nearly zero. However, the vertical seismic force is an important factor to be considered in design, because it is added to pretention of the tether directly. The authors have made theoretical and experimental investigations on the earthquake response of the TLP for years.The purpose of this report is to set up an analysis model for determining the response of TLP at it's connection with the gravity anchor when the TLP is subjected to vertical seismic excitation.As a result of a calculation, the weight of the gravity anchor in water must be more than about two and half times as much as the pretension of tethers.

Wave-induced pore pressures and effective stresses in inhomogeneous seabed foundations

Stability analyses of homogeneous and inhomogeneous seabed foundations under attack by storm waves are made by calculating the wave-induced effective stresses. Wave-induced effective stress analysis of homogeneous seabed is made using the theory previously developed by the author which is based on the poro-elastic theory of Biot. Effective stresses in inhomogeneous seabeds induced by waves are calculated by approximating aa inhomogeneous bed by many layers of homogeneous soils each of which has different geotechnical soil properties. A good agreement is obtained between the theory and the pore pressure data from in situ field measurements. For a given wavelength, it is found that there exists a most unstable thickness of homogeneous seabed when the thickness is one-fifth of the wave length. As a realistic example of an inhomogeneous bed, the effective stresses in a typical seabed formation at the Mississippi Delta area of the Gulf of Mexico under the attack of design storm waves are calculated. The numerical results indicate that the storm waves induce a continuous submarine landslide which extends as deep as 9 m from the mud line. Numerical calculations also indicate that such landslides and liquefaction of seabeds can be prevented by placing a layer of concrete blocks or rubble on top of the seabeds.

STABILITY ANALYSIS OF SEAFLOOR FOUNDATIONS

Stability analysis of homogeneous and inhomogeneous seabed foundations under attack by storm waves are made by calculating the wave induced effective stresses. Wave induced effective stress analysis of homogeneous seabed is made using the theory previously developed by the senior author which is based on the poro-elastic theory by Biot. Effective stresses in inhomogeneous seabeds induced by waves are calculated by approximating an inhomogeneous bed by many layers of homogeneous soils each of which has different geotechnical properties of soils. A good agreement is obtained between the theory and the pore pressure data in situ field measurements. For a given wave length, it is found that there exists a most unstable thickness of homogeneous seabed when the seabed thickness is one-fifth of the wave length. As a realistic example of an inhomogeneous bed, the effective stresses in a typical seabed formation at the Mississippi Delta area of the Gulf of Mexico under the attack of design storm waves are calculated. The numerical results indicate that the storm waves induce a continuous submarine landslide which extends as deep as 9 m from the mud line. Numerical calculations also indicate that such landslides and liquefaction of seabeds can be prevented by placing a layer of concrete blocks or rubbles on the top of the seabeds.