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Offshore pipelines have been commonly used in the transportation of ma-rine energy resources that is one of the main concerns of offshore oil and gas industry. In general, seabed stability around a pipeline and its resultant damage of the pipeline is one of the critical factors to be considered in the design of offshore pipeline project. In this study, an integrated numerical model for wave-seabed interactions around a subsea pipeline is established using OpenFOAM. The numerical examples demonstrate that a significant influence of different configurations of the protective layer on the distribution of excess pore-water pressure near the pipeline, which further affects the occurrence of seabed liquefaction. Based on the proposed model, four different engineering options for offshore pipeline protection are examined through numerical simulation.
Many offshore structures have been commonly constructed over the last a few decades due to the growing energy resources development in the ocean. Offshore pipelines, as one of common marine installations, have been extensively used for the transportation of natural gas and oil from the offshore platforms, and disposal of industrial and municipal waste. To ensure the safety of such subsea pipelines, coastal engineers have to consider the environmental loads including the wave, current, anchor dropping and dredging, which might cause instability of the pipeline and decrease its lifespan. Thus, it is customary to bury the pipeline by trenching and refilling the soil. However, the cost is relatively high and it is time-consuming (Fredsøe, 2016).
To date, a few FEM (Finite Element Method) numerical models have been proposed for investigating a more complicated Wave-Seabed-Structure Interactions (WSSI) involving a trenched pipeline or a multi-layered and anisotropic seabed (Gao et al., 2003; Gao and Wu, 2006). Note that all aforementioned investigations have only examined a subsea pipeline which fully buried within the seabed; and the effects of the linear or non-linear wave was evaluated. Consequently, these models may not be able to fully predict the seabed response around a pipeline, which is partly buried or mounted on a seabed. To remedy these limitations, Zhao et al. (2014) firstly studied the build-up of pore pressures around a trenched pipeline. Later, Zhao and Jeng (2016) further investigated the protection of backfill depth on residual soil liquefaction in a relationship with both soil and wave parameters. Lin et al. (2016) developed an integrated FEM model to investigate the potential of momentary liquefaction around a trenched pipeline with various configurations. This framework was further extended by considering the effects of ocean currents (Duan et al., 2017).
In this paper, a three-dimensional numerical model for wave–seabed interactions around a group of pile foundations is proposed. Unlike in previous studies, both wave and seabed submodels are developed based on the open source library OpenFOAM (version 4.0, foundation). In this model, the wave motion is governed by RANS equations, while the porous flow in the seabed is governed by dynamic poro-elastic u - p approximation. The present model is first validated with the previous laboratory experiments for a single pile. Then, the present model is further applied to the cases of group of pile foundations. Numerical results indicate that the wave characteristics as well as the configurations of the structures can significantly affect the oscillatory pore water pressures and vertical effective normal stresses around a group of pile foundations.
Pile foundations are commonly used to support various offshore infrastructures such as platforms in shallow water, cross-sea bridge piers, offshore wind turbine foundations, etc. For the design of a pile foundation, seabed stability (including soil liquefaction, scour, and shear failure) in the vicinity of the structure needs to be taken into consideration. Furthermore, wave-induced pore water pressures and effective stresses are two key factors for the estimation of wave-induced seabed instability. With an increase of pore water pressures and a decrease of vertical effective normal stresses due to wave loading, part of the seabed may become unstable or even liquefied. Once the liquefaction occurs, the liquefied soil will behave like a heavy fluid and can only provide very little resistance to the pile foundations.
A precise prediction of seabed stability involving the fluid-pipe-soil interaction can lead to significant cost reductions by optimising design. Unlike previous investigations, a three-dimensional numerical model for the wave-induced soil response around an offshore pipeline is proposed in this paper. The numerical model was first validated with 2-D experimental data available in the literature. Then, a parametric study will be carried out to examine the effects of wave, seabed characteristics and confirmation of pipeline. Numerical examples demonstrate significant influence of wave obliquity on the wave-induced pore pressures and the resultant seabed liquefaction around the pipeline, which cannot be observed in 2-D numerical simulation.
Nowadays, many offshore structures have been commonly constructed over the last few decades due to the growing engineering resource in the ocean. Submarine pipelines, as one of the popular offshore infrastructures, have been extensively used for transportation of natural gas and oil from offshore platform, and disposal of industrial as well as municipal waste. To ensure the safety of usage of such submarine pipelines, the coastal engineers have to consider the unexpected loads including the wave, current, and anchor dropping/dredging, which might cause the its stability and decrease its life span. Thus, it is customary to bury the pipeline by trenching and refilling soil whose cost is relatively high and time-consuming (FredsØ e, 2016).
As reported in the literature, two well-known main mechanisms of dynamic wave-induced seabed liquefactions are the momentary liquefaction and residual liquefaction, based on in the field measurements and laboratory experiments (Zen and Yamazaki, 1991). The fist mechanism, momentary liquefaction, can occur beneath wave troughs when the great seepage flow is upward directly. Since this kind of liquefaction may be happen within a short duration as the passage of wave trough, it is also called instantaneous liquefaction. The other mechanism, residual liquefaction, takes place as a result from a compacted and cyclic shearing process that the build-up of excess pore pressure in the seabed (Seed and Rahman, 1978). As mention previously, the waves also can induce shear stress in the soil when the waves propagate, which has been analytically investigated by Yamamoto et al (1978). Whereas the wave-induced shear stress has less impact on seabed instability compared to that caused by the previous two mechanism above. This study only focuses on the wave-induced seabed liquefaction incorporating both instantaneous mechanism.
Soil permeability is one of key factors in the prediction of the wave- induced seabed response, which will directly affect the design of foundation around marine installations. Most previous studies in the field treated the soil permeability as a constant, although it depends on numerous soil parameters. In this study, the soil permeability is considered as a function of pore water pressure as reported in the literature, with this new feature, the governing equation will become non-linear differential equations. Numerical examples demonstrate the significant influence of dynamic soil permeability on the wave-induced pore pressure and effective stresses.
In the past a few decades, considerable efforts have been devoted to the phenomenon of the wave-soil interactions. One of the major reason is that the evaluation of the wave-induced soil response and its resultant seabed instability is particularly important for coastal geotechnical engineers involved in the design of foundation of the offshore installations (Rahman, 1991; Sumer, 2014). Another reason is that the poro-elastic theories for wave-soil interaction have been applied to field measurements such as determination of the shear modulus of seabed (Yamamoto et al., 1991) and the direction spectra of ocean waves (Nye and Yamamoto, 1994), as well as acoustic waves propagating through porous media (Yamamoto and Turgut, 1988).
Based on Biot's poro-elastic theory (Biot, 1941), several classic investigations for the wave-induced soil response have been carried out since the 1970s. Among these, Yamamoto et al (1978) proposed an exact closed-form analytical solutions for the wave-induced oscillatory soil response in an isotropic, poro-elastic and infinite seabed. This model was further extended to three-dimensional cases by Hsu and Jeng (1994) for various soil conditions. Another different approximation, so-called boundary layer approximation, was proposed by Mei and Foda (1981), which provided a simple formulations of the wave-induced soil response. This approximation can provide precise prediction of soil response in fine sand rather than coarse sand, as reported in Hsu and Jeng (1994). Okusa (1985) investigated the effects of degree of saturation on the wave-induced soil response based on plane stress conditions. A detailed review of previous relevant research can be found in Jeng (2003).
The investigation of the wave-induced soil response is extremely significant for sake of submarine pipelines. Unlike conventional approaches, in this paper, a meshfree model for the wave-seabed interactions around an offshore pipeline is established. The pipeline is considered to be fully buried or partially buried in a trench layer surrounding impermeable walls. The proposed model is validated with the analytical solution, laboratory experiments and numerical models available in the literature. Then, a parametric study is carried out to examine the effects of configuration of a pipeline on the wave-induced soil response in the vicinity of a pipeline.
Submarine pipelines play an extremely important role for the transportation of offshore energy resources that is one of main concerns for offshore engineering. In general, the vulnerability of underwater-laid pipelines may be exposed due to wave-induced liquefaction of underlying seabed soil layers.
Generally, the fluctuating pressures acting upon the seabed due to progressive motion of ocean waves will further induce excess pore pressure and reduce the effective stress within seabed soil. When the excess pore pressure increases, the shear resistance surrounding pipelines may be loss due to the liquefaction of soil. Therefore, the evaluation of the wave-induced soil response is particularly important for offshore engineers involved in the design of protection of offshore pipelines (Fredsøe. 2016).
In the past few decades, numerous investigations for the wave-seabedstructure interactions by using traditional numerical methods, such as finite di_erence method, boundary element method and finite element method, have been reported in the literature. Among these, Cheng and Liu (1986) proposed a boundary integrated model for wave-induced soil response propagating over a pipeline fully buried in a trench layer. Jeng and Cheng (2000) developed a two-dimensional finite di_erence model in a curvilinear coordinate system to examine the wave-induced pore pressures and stresses around a pipeline. The numerical results depicted the significant influence of pipelines on the soil response. Jeng and Lin (2000) proposed a finite element model to examine the wave-induced seabed response in the vicinity of a pipeline in an inhomogeneous seabed. These models were based on the assumption of no slipping at the interface between pipeline and soil. Luan et al. (2008) considered inertial forces and soil-pipeline contact e_ects in their model. It was found from the results that the interface between soil and pipeline significantly a_ected the internal stresses. Dunn et al. (2006) applied the poro-elastoplastic model (Chan, 1988) to investigate the problem of wave-seabed interaction around a fully buried pipeline in marine environments. A three-dimensional finite element model was proposed by Shabani and Jeng (2008) to examine the behaviour of soil under various wave obliquity, soil characteristics and trench configuration. Zhao et al. (2014) adopted new definition of source term in their residual model and applied to the problem of wave-soil-pipe interactions. Duan et al. (2017) proposed a 2D coupled model for wave and current induced soil response around a partially buried pipeline in a trench. They investigated the water-seabed-pipeline interaction under wave and current loading system, and the process was fully coupled.
Response in a porous seabed under dynamic environmental loading is a vital engineering issue in marine geotechnics. Lots of investigations for seabed response under dynamic loading have been developed through mathematical, numerical and experimental approaches. Most previous numerical models for seabed response in marine environments were based on finite element models. In this paper, based on local radial basis function collection method (LRBFCM), a meshfree model is proposed for the seabed response in the marine environments. In the present model, partial dynamic approximation (u-p approximation) will be used, and three different types of natural loading will be considered, i.e., wave, current and earthquake loading.
In the last twenty years, more and more marine structures are constructed with the deeper exploration and study for the offshore area. The most important aspect to be considered in engineering practice is the stability after putting in use of those marine structures under the complicated environment loading. In general, three types of the environmental loading needs to be taken into account for the design of marine structures, which are ocean waves, currents, and probable earthquake respectively. The dynamic response under these loading has attracted great attention among coastal and geotechnical engineers due to the growth of activities in marine environments. As the conventional loading, how ocean wave and current affect the marine structure stability is a vital problem for coastal engineers.
In general, the propagating ocean wave will generate the dynamic pressure in the sea floor, which may trigger soil liquefaction of the seabed as reported in the laboratory test (Sassa and Sekiguchi, 1999). Meanwhile, the effect of earthquake is also important for engineering design. Although the probability of earthquake occurred nearby the marine structures is not so high, once the earthquake happened, the damage would be devastating.
As one of the major natural disasters need to be considered in structure design, earthquake is also able to liquefy the saturated soil through seismic shaking effect. The liquefaction phenomena induced by seismic wave was fully aware by the public from the Niigata earthquake in 1964 in Japan, which caused unprecedented damage. The problem of earthquake-induced liquefaction attracted a great deal of attention of geotechnical researcher and great achievements have been made in the past (Seed et al., 2003). However, as pointed by Ye and Wang (2015), most of the studies for earthquake loading are concerned with onshore structures, while only a few studies considered offshore structures whatever by experiment or numerical simulation. For the earthquake loading, Chen et al. (2018) has developed the analytical solution for layered porous seabed under vertical seismic motion.
In this study, a one-dimensional experimental study for the influence of clayey soils on wave-induced liquefaction was reported. The one- dimensional facility was set-up with a vertical cylinder and a 1.8 m thick deposit and 0.2 m thick water above the deposit. Unlike the previous experimental study only use a single soil or clay, this study use sand-clay mixtures as the experimental sample. A series of experiments with 3000 cycles in each test were conducted under numerous wave and soil conditions, which allow us to examine the influence of wave and soil parameters, especially the influence of clay content (CC) on wave-induced liquefaction. It has been confirmed that the deposit will become prone to liquefaction with the increase of CC, while when CC up to 30%, the mixture sand-clay deposit will almost never liquefied.
In this paper, an experimental study for the wave-induced pore pressure in marine sediments was reported. In the experiment, a one-dimensional facility was set-up with a vertical cylinder and a 1.8 m thick sandy deposit and 0.2 m thick water above the deposit. Unlike the previous experiments in Zen and Yamazaki (1990a, b), we added additional static water pressures on the harmonic dynamic wave pressure, which allowed us to simulate the case with larger water depth. A series of experiments were conducted under numerous different wave and soil conditions, which allowed us to examine the influence of wave and soil parameters on the wave-induced pore pressure as well as liquefaction. The experimental results showed the significant influence of liquefaction on sandy seabed in shallow water.
Evaluation of wave-induced residual pore pressure in marine sediments is a key factor in prediction of wave-induced liquefaction around coastal structures. Conventional models for wave-induced soil response have focused on regular wave loading. In this paper, we present a semi-analytical approximation for random wave-induced residual pore pressure. Two spectra, JONSWAP and B-M, will be used for simulations of random wave generation. Numerical results demonstrate significant influence of wave randomness on wave-induced residual pore pressure. INTRODUCTION Recently, considerable efforts have been devoted to WSSI (Wave- Seabed-Structure Interaction) problem due to growing offshore and coastal activities. Foundation failure around coastal structures has been reported in literature (Christian et al., 1974; Miyamoto et al., 1989). Wave-induced pore pressure and resultant liquefaction in a porous seabed has been recognized as a key factor for foundation failure. In general, mechanisms of the wave-induced seabed response may be classified into two categories, depending upon how the excess pore pressure is generated (Nago et al., 1993). One is caused by the residual or progressive nature of the excess pore pressure, which appears in the initial stage of cyclic loading. This type of soil response is similar to that induced by earthquakes, caused by the build-up of the excess pore pressure. The other, generated by the transient or oscillatory excess pore pressures, accompanied by the damping of amplitude and phase lag in the pore pressure, appears periodic response to each wave. In this study, both mechanisms will be considered. Numerous investigations for the wave-induced soil response have been carried out since the 1970s. These include analytical approximations (Yamamoto et al., 1978; Jeng and Hsu, 1996), numerical modeling (Mostafa et al., 1999; Dunn et al., 2006) and physical modeling (Zen and Yamazaki, 1990; Sassa and Sekiguchi, 1999; Sumer, 2006; 2007).