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The problems of deep-water geohazards encountered by human being have become increasingly prominent as the marine resources and energies development has gradually moved into the deep sea. Among them,submarine landslide is the most typical one because it will lead to the chain disasters and seriously endanger the safety of underwater infrastructure. To such end,this paper focuses on the chain disasters process from landslide formation to movement evolution until impact on infrastructures. First of all,the formation mechanism of submarine landslides in response to various triggering factors is briefly reviewed,and the movement process of submarine landslides and the criteria for identifying different evolutionary stages are stated in depth. Then,the mechanism of environmental water and soil coupling action in the process of landslide movement evolution is examined,and a coupled CFD-DEM method applicable to the mesoscale and small scale movement evolution process is proposed. Furthermore,the current application scope and technical bottleneck of the experimental modelling technology,and in-situ monitoring methods of submarine landslide movement evolution process are enough discussed. Based on this,the quantitative evaluation methods and research tools of submarine landslide impact effects are summarized,with a focus on the problem of landslide impact on underwater infrastructure such as submarine pipelines and cables. Finally,the current research deficiencies and future development directions in the subject of submarine landslides,are pointed out to provide a useful reference for simulation,prediction,and early warning of submarine landslide disaster chains. The problems of deep-water geohazards encountered by human being have become increasingly prominent as the marine resources and energies development has gradually moved into the deep sea. Among them,submarine landslide is the most typical one because it will lead to the chain disasters and seriously endanger the safety of underwater infrastructure. To such end,this paper focuses on the chain disasters process from landslide formation to movement evolution until impact on infrastructures. First of all,the formation mechanism of submarine landslides in response to various triggering factors is briefly reviewed,and the movement process of submarine landslides and the criteria for identifying different evolutionary stages are stated in depth. Then,the mechanism of environmental water and soil coupling action in the process of landslide movement evolution is examined,and a coupled CFD-DEM method applicable to the mesoscale and small scale movement evolution process is proposed. Furthermore,the current application scope and technical bottleneck of the experimental modelling technology,and in-situ monitoring methods of submarine landslide movement evolution process are enough discussed. Based on this,the quantitative evaluation methods and research tools of submarine landslide impact effects are summarized,with a focus on the problem of landslide impact on underwater infrastructure such as submarine pipelines and cables. Finally,the current research deficiencies and future development directions in the subject of submarine landslides,are pointed out to provide a useful reference for simulation,prediction,and early warning of submarine landslide disaster chains.
Deep-sea polymetallic nodules are widely distributed on the global seabed,with abundant resources and great potential for exploitation. Since 1960 s,the mining methods of continuous chain-fighting,shuttle-type,pipe lifting and so on have been proposed around deep-sea polymetallic nodule mining. The exploitation of polymetallic nodules on the surface of seabed sediments would inevitably cause the disturbance of surface sediments,which would affect the chemical properties and marine biological activities of seawater. Many scholars have studied the resources and reserves of the nodules,the engineering geological conditions,mining technology and environmental impact of the deep-sea polymetallic nodules. Based on a large number of domestic and foreign literature,the research progress of the CC zone(the seabed area between the Clarion-Crippaton fault zone in the eastern Pacific Ocean) is collated. The following points are recognized for the potential engineering geological environmental impact of deep-sea polymetallic nodules mining by pipeline lifting: (1)During the process of nodule mining,the surface sediments are disturbed and resuspended,and the concentration of resuspended particles is the main reason affecting the chemical properties and biological activities of seawater; (2)The engineering geological properties of surface sediments in mining area are the key control factors of the ecological environment impact degree in the process of deep-sea polymetallic nodule mining,which determines the quality and spatial distribution characteristics of sediment resuspension during nodule mining; (3)At present,the environmental impact assessment of polymetallic nodule mining is mostly based on the qualitative assessment of the degree of impact on the biological community. There is no environmental impact assessment based on the changes of sediment engineering geological properties and the temporal and spatial distribution of resuspended sediment. The quantitative assessment system of environmental engineering geological impact of deep-sea polymetallic nodule mining in the future needs to be established. Deep-sea polymetallic nodules are widely distributed on the global seabed,with abundant resources and great potential for exploitation. Since 1960 s,the mining methods of continuous chain-fighting,shuttle-type,pipe lifting and so on have been proposed around deep-sea polymetallic nodule mining. The exploitation of polymetallic nodules on the surface of seabed sediments would inevitably cause the disturbance of surface sediments,which would affect the chemical properties and marine biological activities of seawater. Many scholars have studied the resources and reserves of the nodules,the engineering geological conditions,mining technology and environmental impact of the deep-sea polymetallic nodules. Based on a large number of domestic and foreign literature,the research progress of the CC zone(the seabed area between the Clarion-Crippaton fault zone in the eastern Pacific Ocean) is collated. The following points are recognized for the potential engineering geological environmental impact of deep-sea polymetallic nodules mining by pipeline lifting: (1)During the process of nodule mining,the surface sediments are disturbed and resuspended,and the concentration of resuspended particles is the main reason affecting the chemical properties and biological activities of seawater; (2)The engineering geological properties of surface sediments in mining area are the key control factors of the ecological environment impact degree in the process of deep-sea polymetallic nodule mining,which determines the quality and spatial distribution characteristics of sediment resuspension during nodule mining; (3)At present,the environmental impact assessment of polymetallic nodule mining is mostly based on the qualitative assessment of the degree of impact on the biological community. There is no environmental impact assessment based on the changes of sediment engineering geological properties and the temporal and spatial distribution of resuspended sediment. The quantitative assessment system of environmental engineering geological impact of deep-sea polymetallic nodule mining in the future needs to be established.
This paper aims to explore the microscopic pore structure characteristics and qualitative and quantitative analysis of coral skeleton limestone distributed on a certain island reef. We established the three-dimensional digital models for six representative coral skeleton limestone samples through CT scans and studied the pore structure characteristics. The results show that the overall average porosity of the six rock samples is only 3.30%,which indicates a small degree of pore development. The porosity of the rock samples fluctuates greatly from layer to layer. The porosity is small,but its standard deviation is large,which reflects the heterogeneity of the rock sample. We marked and screened the pores extracted from the 3D model of the rock sample,and carried out statistical analysis of the pore volume and quantity. The data shows that although the number of macropores in coral skeleton limestone is small,the volume of macropores accounts for a large proportion. There are some super large pores. Through the ball-and-stick model analysis,it can be found that the main distribution range of coordination numbers is 0~2,indicating that the pore connectivity of coral skeleton limestone is poor. The pores are mainly independent pores and single connected pores. The pores have no connectivity in the x/y/z direction,which means that there is no potential fluid migration channel. This paper aims to explore the microscopic pore structure characteristics and qualitative and quantitative analysis of coral skeleton limestone distributed on a certain island reef. We established the three-dimensional digital models for six representative coral skeleton limestone samples through CT scans and studied the pore structure characteristics. The results show that the overall average porosity of the six rock samples is only 3.30%,which indicates a small degree of pore development. The porosity of the rock samples fluctuates greatly from layer to layer. The porosity is small,but its standard deviation is large,which reflects the heterogeneity of the rock sample. We marked and screened the pores extracted from the 3D model of the rock sample,and carried out statistical analysis of the pore volume and quantity. The data shows that although the number of macropores in coral skeleton limestone is small,the volume of macropores accounts for a large proportion. There are some super large pores. Through the ball-and-stick model analysis,it can be found that the main distribution range of coordination numbers is 0~2,indicating that the pore connectivity of coral skeleton limestone is poor. The pores are mainly independent pores and single connected pores. The pores have no connectivity in the x/y/z direction,which means that there is no potential fluid migration channel.
The scale of reef engineering in the South China Sea is gradually increasing. The construction of some structures with large loads can inevitably lead to the crushing of calcareous sand that can be the main material of foundation. Therefore,it is important to study the compression and the crushing mechanism of calcareous sand under high stress. In this paper,the basic geometric parameters of calcareous particles were obtained based on the microscopic image acquisition and processing technology. The shape of particles was quantitatively characterized by two shape parameters of sphericity(S) and convexity(C). A series of confined compression tests,based on a high-pressure oedometer instrument with a termination pressure of 16 MPa,was carried out to investigate the compression and breakage characteristics of calcareous sand in high stress. The effect of particle size,particle distribution,and particle shape on the compressive and crushing properties of calcareous sand were also discussed respectively. The result showed that with the increase of average particle size,the shape of calcareous sands was more irregular,and the corners were also developed. With the increase of stress,the compression curves of calcareous sand with different particle sizes tend to converge on a straight line. The influence of initial particle size on the compression characteristics of the samples decreased and disappeared eventually. Meanwhile,for calcareous sand of different grades,the compression curve also converged but did not intersect in a straight line. The compression characteristics of samples were still influenced by the initial grades. It's worth noting that when the non-uniformity coefficient(Cu) wass close,the compression and a crushing amount of calcareous sand gradually increased with the increase of average particle size(d50). However,when d50 was close,its compression and crushing amount gradually decreased with the increase of Cu. The scale of reef engineering in the South China Sea is gradually increasing. The construction of some structures with large loads can inevitably lead to the crushing of calcareous sand that can be the main material of foundation. Therefore,it is important to study the compression and the crushing mechanism of calcareous sand under high stress. In this paper,the basic geometric parameters of calcareous particles were obtained based on the microscopic image acquisition and processing technology. The shape of particles was quantitatively characterized by two shape parameters of sphericity(S) and convexity(C). A series of confined compression tests,based on a high-pressure oedometer instrument with a termination pressure of 16 MPa,was carried out to investigate the compression and breakage characteristics of calcareous sand in high stress. The effect of particle size,particle distribution,and particle shape on the compressive and crushing properties of calcareous sand were also discussed respectively. The result showed that with the increase of average particle size,the shape of calcareous sands was more irregular,and the corners were also developed. With the increase of stress,the compression curves of calcareous sand with different particle sizes tend to converge on a straight line. The influence of initial particle size on the compression characteristics of the samples decreased and disappeared eventually. Meanwhile,for calcareous sand of different grades,the compression curve also converged but did not intersect in a straight line. The compression characteristics of samples were still influenced by the initial grades. It's worth noting that when the non-uniformity coefficient(Cu) wass close,the compression and a crushing amount of calcareous sand gradually increased with the increase of average particle size(d50). However,when d50 was close,its compression and crushing amount gradually decreased with the increase of Cu.
This paper aims to reveal the compressive deformation,particle breakage characteristics,and acoustic emission laws of calcareous sand. The one-dimensional compression-resilience tests and synchronous acoustic emission tests were carried out on calcareous sand with different particle size fractions under three different relative densities. By sieving analysis,the relative breakage(Br) was obtained based on the particle size distribution after the test. The experimental results showed that the compressive deformation of calcareous sand is caused by the particle rearrangement and particle breakage. The particle breakage is the major factor behind this phenomenon. The rebound curve is approximately a straight line,indicating that the compressive deformation is an irreversible plastic deformation. Under the same stress,the larger the particle size,the greater the Br. The different shapes of the particles result in different interparticle filling and interlocking effects,which affects the sliding and rearrangement of the particles,and then influences the compressive deformation of the particles. The acoustic emission counts rate of the two kinds of sand increases with the increase of the particle size,and both appear mainly in the compression phase between 800 kPa and 3200 kPa. The compressive deformation and breakage characteristics of calcareous sand are consistent with their acoustic emission laws. The relation curves between counts rate and time are in good agreement with the stress-time curves. The mechanical characteristics of the calcareous sand can be reflected based on the counts rate-time curves. For calcareous sand,a "critical void ratio" exists with the fewest acoustic emission events. In this study,its value of calcareous sand with a particle size of 1~2 mm is 1.33~1.41. When the initial void ratio of the sample deviates from this critical value,the acoustic emission activities would increase in varying degrees. This paper aims to reveal the compressive deformation,particle breakage characteristics,and acoustic emission laws of calcareous sand. The one-dimensional compression-resilience tests and synchronous acoustic emission tests were carried out on calcareous sand with different particle size fractions under three different relative densities. By sieving analysis,the relative breakage(Br) was obtained based on the particle size distribution after the test. The experimental results showed that the compressive deformation of calcareous sand is caused by the particle rearrangement and particle breakage. The particle breakage is the major factor behind this phenomenon. The rebound curve is approximately a straight line,indicating that the compressive deformation is an irreversible plastic deformation. Under the same stress,the larger the particle size,the greater the Br. The different shapes of the particles result in different interparticle filling and interlocking effects,which affects the sliding and rearrangement of the particles,and then influences the compressive deformation of the particles. The acoustic emission counts rate of the two kinds of sand increases with the increase of the particle size,and both appear mainly in the compression phase between 800 kPa and 3200 kPa. The compressive deformation and breakage characteristics of calcareous sand are consistent with their acoustic emission laws. The relation curves between counts rate and time are in good agreement with the stress-time curves. The mechanical characteristics of the calcareous sand can be reflected based on the counts rate-time curves. For calcareous sand,a "critical void ratio" exists with the fewest acoustic emission events. In this study,its value of calcareous sand with a particle size of 1~2 mm is 1.33~1.41. When the initial void ratio of the sample deviates from this critical value,the acoustic emission activities would increase in varying degrees.
The exploitation of methane hydrate from seafloor could degrade the strength properties of the reservoir,and further induce the geological disasters,threatening safety of the drilling platform and the exploitation well. This paper aims to explore the mechanical properties of hydrate-bearing sediments and reveal the mechanism of mechanical strength deterioration of hydrate reservoirs induced by hydrate exploitation. It develops a set of hydraulic-mechanical united experiment apparatus for hydrate-bearing sediment. This apparatus consists of the pressure chamber,pressure-control system,supply/exhaust gas system,temperature control system,data acquisition and human-computer interaction management system. Based on these components,this apparatus can be used to synthesize the methane hydrate-bearing sediment sample,and perform the permeability measurement test,the isotropic compression test and the triaxial compression test under different stress paths. For illustrating the functions of this apparatus,methane hydrate-bearing fine sand specimens are prepared by excess-gas method. The hydraulic-mechanical characteristics are measured. The experimental results are analyzed briefly,and confirm the function and reliability of the apparatus for measuring the hydro-mechanical properties of hydrate bearing sediments. The exploitation of methane hydrate from seafloor could degrade the strength properties of the reservoir,and further induce the geological disasters,threatening safety of the drilling platform and the exploitation well. This paper aims to explore the mechanical properties of hydrate-bearing sediments and reveal the mechanism of mechanical strength deterioration of hydrate reservoirs induced by hydrate exploitation. It develops a set of hydraulic-mechanical united experiment apparatus for hydrate-bearing sediment. This apparatus consists of the pressure chamber,pressure-control system,supply/exhaust gas system,temperature control system,data acquisition and human-computer interaction management system. Based on these components,this apparatus can be used to synthesize the methane hydrate-bearing sediment sample,and perform the permeability measurement test,the isotropic compression test and the triaxial compression test under different stress paths. For illustrating the functions of this apparatus,methane hydrate-bearing fine sand specimens are prepared by excess-gas method. The hydraulic-mechanical characteristics are measured. The experimental results are analyzed briefly,and confirm the function and reliability of the apparatus for measuring the hydro-mechanical properties of hydrate bearing sediments.
The mechanical parameters of hydrate-bearing sediments are the key parameters for the stability evaluation of hydrate formation. The hydrate-bearing sediments in Shenhu area of South China Sea contain large amounts of clay mineral. It is of great significance to understand the effects of clay minerals on the mechanical properties of sediments for hydrate mining. Based on triaxial compression simulation in the PFC code,we first analyzed the mechanical effects of clay mineral to the sediment without hydrate. Secondly,we analyzed the cementation effect of hydrate to mineral grains and the influence of confining pressure on the mechanical properties of the sediment. Our results indicated that the deviator stress-strain curve of the model without hydrate shows obvious strain hardening characteristics. The clay mineral content,grain shape and grain arrangement have significant effects on the triaxial compression characteristics of the sediment. The increase in the content of clay mineral has a significant effect on reducing the mechanical strength of sediments. The peak strength and elastic modulus of the sediments with strip-shaped clay grain are significantly higher than those of the sediment with round-shaped clay grain,which are related to the average co-ordination number in the microscopic view. The directional arrangement of the strip-shaped clay grains makes the mechanical parameters of the model to be anisotropic. The cementation effect of hydrate on the grains can significantly increase the peak strength and elastic modulus of the model. With the increase of the interparticle cementation degree and the decrease of confining pressure,the failure mode of the hydrate-bearing sediments changes from plastic failure to brittle failure. The mechanical parameters of hydrate-bearing sediments are the key parameters for the stability evaluation of hydrate formation. The hydrate-bearing sediments in Shenhu area of South China Sea contain large amounts of clay mineral. It is of great significance to understand the effects of clay minerals on the mechanical properties of sediments for hydrate mining. Based on triaxial compression simulation in the PFC code,we first analyzed the mechanical effects of clay mineral to the sediment without hydrate. Secondly,we analyzed the cementation effect of hydrate to mineral grains and the influence of confining pressure on the mechanical properties of the sediment. Our results indicated that the deviator stress-strain curve of the model without hydrate shows obvious strain hardening characteristics. The clay mineral content,grain shape and grain arrangement have significant effects on the triaxial compression characteristics of the sediment. The increase in the content of clay mineral has a significant effect on reducing the mechanical strength of sediments. The peak strength and elastic modulus of the sediments with strip-shaped clay grain are significantly higher than those of the sediment with round-shaped clay grain,which are related to the average co-ordination number in the microscopic view. The directional arrangement of the strip-shaped clay grains makes the mechanical parameters of the model to be anisotropic. The cementation effect of hydrate on the grains can significantly increase the peak strength and elastic modulus of the model. With the increase of the interparticle cementation degree and the decrease of confining pressure,the failure mode of the hydrate-bearing sediments changes from plastic failure to brittle failure.
The stratigraphic configurations for the design of offshore shallow foundations usually involve multi-layer soil profiles due to their large sizes. The stiff-soft-stiff clay deposit is one of the most prevalent configurations. Design methods are not available in the current API and ISO standards for shallow foundations to estimate the bearing capacity of a footing on stiff-soft-stiff clays. Based on the "bottom-up" approach recommended for spudcan foundations of jack-up rig,this paper assumes an analytical model that sums the resistances from the punching shear of the stiff-soft layering system and the squeezing of the soft-stiff layering system. The rationality of this predictive model is verified based on comprehensive FE analyses. The comparison between the predictions from "bottom-up" approach and FE analyses indicates that the assumed model is reasonable and realistic. However,the resistance due to squeezing response is significantly underestimated by the current design formula. Detailed investigations are carried out. It is revealed that this is mainly because the effect of the overlying stiff clay on the squeezing mechanism is neglected. Based on the findings,an improved design formula is proposed for the squeezing response and hence for the design of circular foundations on stiff-soft-stiff clays. The stratigraphic configurations for the design of offshore shallow foundations usually involve multi-layer soil profiles due to their large sizes. The stiff-soft-stiff clay deposit is one of the most prevalent configurations. Design methods are not available in the current API and ISO standards for shallow foundations to estimate the bearing capacity of a footing on stiff-soft-stiff clays. Based on the "bottom-up" approach recommended for spudcan foundations of jack-up rig,this paper assumes an analytical model that sums the resistances from the punching shear of the stiff-soft layering system and the squeezing of the soft-stiff layering system. The rationality of this predictive model is verified based on comprehensive FE analyses. The comparison between the predictions from "bottom-up" approach and FE analyses indicates that the assumed model is reasonable and realistic. However,the resistance due to squeezing response is significantly underestimated by the current design formula. Detailed investigations are carried out. It is revealed that this is mainly because the effect of the overlying stiff clay on the squeezing mechanism is neglected. Based on the findings,an improved design formula is proposed for the squeezing response and hence for the design of circular foundations on stiff-soft-stiff clays.
Compared with pile foundation,suction caisson has the advantages of convenient construction and recyclability,and is more suitable for deep sea area. Horizontal cyclic response is important for the stability analysis of suction caisson as it suffers the long-term wave loads in service period. This paper presents the results of a series of 1g model tests under horizontal cyclic loading in sand. The cumulative deformation,rotation angle and rotation center of model caisson are discussed in detail. Test results indicates that the horizontal displacement of model caisson in sand accumulates with increase of cyclic numbers and horizontal loading. Under the horizontal cyclic loads of ζb=0.33 and 1.0,the change rule of bucket caisson rotation angle and rotation center is consistent with the cumulative deformation. After 50 cycles,the cumulative rotation angle is 1.43 times and 1.76 times of the first cycle under different cyclic load ratios. With the increase of the number of cycles,the position of the rotation center gradually moves upward,and finally stabilized at about 0.8L below the sand surface. This study can provide theoretical basis for the design of suction caisson as offshore wind power foundation in sand. Compared with pile foundation,suction caisson has the advantages of convenient construction and recyclability,and is more suitable for deep sea area. Horizontal cyclic response is important for the stability analysis of suction caisson as it suffers the long-term wave loads in service period. This paper presents the results of a series of 1g model tests under horizontal cyclic loading in sand. The cumulative deformation,rotation angle and rotation center of model caisson are discussed in detail. Test results indicates that the horizontal displacement of model caisson in sand accumulates with increase of cyclic numbers and horizontal loading. Under the horizontal cyclic loads of ζb=0.33 and 1.0,the change rule of bucket caisson rotation angle and rotation center is consistent with the cumulative deformation. After 50 cycles,the cumulative rotation angle is 1.43 times and 1.76 times of the first cycle under different cyclic load ratios. With the increase of the number of cycles,the position of the rotation center gradually moves upward,and finally stabilized at about 0.8L below the sand surface. This study can provide theoretical basis for the design of suction caisson as offshore wind power foundation in sand.
This paper compares four design methods of horizontal loaded monopile under monotonic loading in clay. They include API method based on p-y spring model,Zhang et al.(2017b) method based on p-y spring model,Wang et al.(2020) method based on double-spring model,and Fu et al.(2020) method based on triple-spring model. Two typical length-diameter ratios of large diameter monopile(L/D=5 and 10) are considered. The four methods are analyzed by comparing with the results of 3D finite element analysis. The influence of different parameters on the prediction results is discussed. The results show that:(1)For the deflection and rotation of a large diameter monopile,the current API method is the most conservative,while Wang et al.(2020) method provides the lowest predictive results. (2)Fu et al.(2020) method not only incorporates the shear force at pile tip and the distributed moment along the pile induced by frictional resistance,but also is capable of capturing the effects of initial shear modulus and soil ductility on horizontal response,which makes it superior to the other three methods; (3)In the design of monopile using Fu et al.(2020) method,the larger the interface roughness coefficient α is,the greater the pile deflection and rotation are. The influence is more significant with larger length-diameter ratios. The effect of coefficient ξp2 related to the distributed moment along the pile is relatively limited,which is negligible for L/D≥10. This paper compares four design methods of horizontal loaded monopile under monotonic loading in clay. They include API method based on p-y spring model,Zhang et al.(2017b) method based on p-y spring model,Wang et al.(2020) method based on double-spring model,and Fu et al.(2020) method based on triple-spring model. Two typical length-diameter ratios of large diameter monopile(L/D=5 and 10) are considered. The four methods are analyzed by comparing with the results of 3D finite element analysis. The influence of different parameters on the prediction results is discussed. The results show that:(1)For the deflection and rotation of a large diameter monopile,the current API method is the most conservative,while Wang et al.(2020) method provides the lowest predictive results. (2)Fu et al.(2020) method not only incorporates the shear force at pile tip and the distributed moment along the pile induced by frictional resistance,but also is capable of capturing the effects of initial shear modulus and soil ductility on horizontal response,which makes it superior to the other three methods; (3)In the design of monopile using Fu et al.(2020) method,the larger the interface roughness coefficient α is,the greater the pile deflection and rotation are. The influence is more significant with larger length-diameter ratios. The effect of coefficient ξp2 related to the distributed moment along the pile is relatively limited,which is negligible for L/D≥10.
The wave-induced seabed liquefaction is an important factor that causes instability of submarine pipelines buried in sediments. The accumulated excess pore water pressure may cause deeper soil liquefaction,which leads to severer influence on stability of pipelines. Therefore,many researchers pay their attention to the residual response of seabed-pipeline system under waves. This study uses the numerical model that considers the coupling effect of pore pressure accumulation and seabed stresses and simulates the seabed residual response with a buried pipeline under nonlinear progressive waves. The results are compared with those of the decoupled numerical model. The results have shown that for the case of considering the coupling effect of residual pore pressure and seabed stresses,the non-uniform distribution of residual pore pressure in the vicinity of the pipeline can cause an increase of the cyclic shear stress,which can significantly accelerate the development of residual pore pressure in the associated area and enlarge the influenced range of the pipeline. Neglecting the coupling effect of residual pore pressure and seabed stresses can underestimate the depth of liquefaction around the buried pipeline to some extent,which is conductive to the safety of pipelines. The wave-induced seabed liquefaction is an important factor that causes instability of submarine pipelines buried in sediments. The accumulated excess pore water pressure may cause deeper soil liquefaction,which leads to severer influence on stability of pipelines. Therefore,many researchers pay their attention to the residual response of seabed-pipeline system under waves. This study uses the numerical model that considers the coupling effect of pore pressure accumulation and seabed stresses and simulates the seabed residual response with a buried pipeline under nonlinear progressive waves. The results are compared with those of the decoupled numerical model. The results have shown that for the case of considering the coupling effect of residual pore pressure and seabed stresses,the non-uniform distribution of residual pore pressure in the vicinity of the pipeline can cause an increase of the cyclic shear stress,which can significantly accelerate the development of residual pore pressure in the associated area and enlarge the influenced range of the pipeline. Neglecting the coupling effect of residual pore pressure and seabed stresses can underestimate the depth of liquefaction around the buried pipeline to some extent,which is conductive to the safety of pipelines.
Dynamic pore water pressure would occur due to the wave-current acts on the seabed. If it cannot be eliminated in time,cumulative pore water pressure would grow in the seabed. The hydraulic gradient caused by the difference of pore water pressure between two adjacent points can produce seepage force,and then cause water flow. The seabed surface is a drainage interface,so an upward seepage force would form in the seabed and act on the sediment particles. It leads to the sediment transport and movement to the seabed surface,thus forming a certain range of coarse-grained layer. In this paper,numerical simulation is used to study the cumulative pore water pressure under different velocity. The influence of the hard shell layer on the cumulative pore water pressure is analyzed. The calculation method of critical scour depth of seabed established by Wang et al.(2014) is used to analyze the final depth of hard shell layer under different velocity. The results show that when the direction of wave and current is the same,it would promote the cumulative pore water pressure. The greater the velocity is,the greater the cumulative pore water pressure is. The opposite direction would inhibit the cumulative pore water pressure. The surface hard shell layer can significantly promote the dissipation of cumulative pore pressure. When the velocity U0=0m ·s-1,the thickness of the hard shell layer increases from 1m to 3m,and the depth of the extreme point decreases by 1.38m. The seepage force caused by the cumulative pore water pressure has a significant effect on the seabed sediment movement. When the velocity at both U0=0m ·s-1 and U0=1m ·s-1,the depth of the sediment incipient motion is 1.5m. If the direction of wave and current is the same,it would produce a larger ΔPL value to drive the coarse sediment particles to the surface of the seabed,but has little effect on the maximum depth of the sediment incipient motion. Dynamic pore water pressure would occur due to the wave-current acts on the seabed. If it cannot be eliminated in time,cumulative pore water pressure would grow in the seabed. The hydraulic gradient caused by the difference of pore water pressure between two adjacent points can produce seepage force,and then cause water flow. The seabed surface is a drainage interface,so an upward seepage force would form in the seabed and act on the sediment particles. It leads to the sediment transport and movement to the seabed surface,thus forming a certain range of coarse-grained layer. In this paper,numerical simulation is used to study the cumulative pore water pressure under different velocity. The influence of the hard shell layer on the cumulative pore water pressure is analyzed. The calculation method of critical scour depth of seabed established by Wang et al.(2014) is used to analyze the final depth of hard shell layer under different velocity. The results show that when the direction of wave and current is the same,it would promote the cumulative pore water pressure. The greater the velocity is,the greater the cumulative pore water pressure is. The opposite direction would inhibit the cumulative pore water pressure. The surface hard shell layer can significantly promote the dissipation of cumulative pore pressure. When the velocity U0=0m ·s-1,the thickness of the hard shell layer increases from 1m to 3m,and the depth of the extreme point decreases by 1.38m. The seepage force caused by the cumulative pore water pressure has a significant effect on the seabed sediment movement. When the velocity at both U0=0m ·s-1 and U0=1m ·s-1,the depth of the sediment incipient motion is 1.5m. If the direction of wave and current is the same,it would produce a larger ΔPL value to drive the coarse sediment particles to the surface of the seabed,but has little effect on the maximum depth of the sediment incipient motion.
Wave-induced transient liquefaction seepage leads to the transport of fine-grained sediments from the seabed to seawater. The contribution of wave-induced transient liquefaction to the sediment resuspension cannot be ignored,and the accurate prediction of the contribution is difficult. The observation data in the Yellow River subaquatic delta include water depth,significant wave height,significant wave period,suspended sediment concentration in the benthic chamber,and suspended sediment concentration out of the benthic chamber. These data are treated as the input data sets. Then,a deep learning model of transient liquefaction contribution to resuspension is developed based on a long short-term memory recurrent neural network. To evaluate the performance of the model,the prediction results of the deep learning model based on LSTM are compared with other models. These models include Support Vector Regression and Artificial Neural Network,and have mean absolute percentage error(MAPE),root mean square error(RMSE) and mean squared error-standard deviation(RSR). The results show that the LSTM model has the smallest error for transient pumping resuspension contribution within 3.5 days,with the mean values of MAPE,RMSE,and RSR of 5.87%,1.6730,and 0.1574,respectively. Therefore,the LSTM model can effectively reduce the error superposition problem arising from machine learning methods in continuous forecasting. Wave-induced transient liquefaction seepage leads to the transport of fine-grained sediments from the seabed to seawater. The contribution of wave-induced transient liquefaction to the sediment resuspension cannot be ignored,and the accurate prediction of the contribution is difficult. The observation data in the Yellow River subaquatic delta include water depth,significant wave height,significant wave period,suspended sediment concentration in the benthic chamber,and suspended sediment concentration out of the benthic chamber. These data are treated as the input data sets. Then,a deep learning model of transient liquefaction contribution to resuspension is developed based on a long short-term memory recurrent neural network. To evaluate the performance of the model,the prediction results of the deep learning model based on LSTM are compared with other models. These models include Support Vector Regression and Artificial Neural Network,and have mean absolute percentage error(MAPE),root mean square error(RMSE) and mean squared error-standard deviation(RSR). The results show that the LSTM model has the smallest error for transient pumping resuspension contribution within 3.5 days,with the mean values of MAPE,RMSE,and RSR of 5.87%,1.6730,and 0.1574,respectively. Therefore,the LSTM model can effectively reduce the error superposition problem arising from machine learning methods in continuous forecasting.
Dynamic loads such as waves and earthquakes can easily cause submarine slope instability,and further lead to the occurrence of submarine landslides,endangering the safety of ports,docks and marine engineering constructions. In this study,the submarine slope in the deep trough of south Caofeidian Port is investigated. To quantitatively calculate the stability of the submarine slope,the finite element method combined with the limit equilibrium method is used considering the effects of real wave loads and seismic loads. The mechanism of dynamic effects on the stability of submarine slopes under special circumstances is also investigated. The results show that extreme wave loads and seismic dynamic loads play an important role in the stability assessment of submarine slopes. Wave loads with a recurrence interval of 50 years and seismic dynamic loads with a peak acceleration of 0.15g can lead to the instability of submarine slopes. Seismic loads can produce larger permanent displacements on submarine slopes. In addition,the erosion and weakening of the rock and soil strength induced by dynamic effects can further reduce the safety factor of the slope under various working conditions,which cannot be ignored in stability analysis of submarine slopes. Dynamic loads such as waves and earthquakes can easily cause submarine slope instability,and further lead to the occurrence of submarine landslides,endangering the safety of ports,docks and marine engineering constructions. In this study,the submarine slope in the deep trough of south Caofeidian Port is investigated. To quantitatively calculate the stability of the submarine slope,the finite element method combined with the limit equilibrium method is used considering the effects of real wave loads and seismic loads. The mechanism of dynamic effects on the stability of submarine slopes under special circumstances is also investigated. The results show that extreme wave loads and seismic dynamic loads play an important role in the stability assessment of submarine slopes. Wave loads with a recurrence interval of 50 years and seismic dynamic loads with a peak acceleration of 0.15g can lead to the instability of submarine slopes. Seismic loads can produce larger permanent displacements on submarine slopes. In addition,the erosion and weakening of the rock and soil strength induced by dynamic effects can further reduce the safety factor of the slope under various working conditions,which cannot be ignored in stability analysis of submarine slopes.
The bibliometric analysis platform and visualization technology are used to quantitatively analyze the research characteristics in the field of submarine landslides from 2000 to 2021, and further reveal these research changes and development trends in different periods of time. The results show that: (1)The number of scientific research outputs in the field of submarine landslides has grown rapidly during the past 20 years. China is the country with the fastest growth on the submarine landslides publications,and has relatively closer cooperation with other countries. (2)By analyzing the research institutions of China and the United States,it is found that the latter has the largest number of research institutions. China's research institutions have most cooperation with other research institutions at home and abroad,and are comparatively influential. (3)The research fields on submarine landslides for the scientists at home and abroad in the past 20 years have been basically consistent. However,on the trigger mechanism,the foreign scholars have paid more attention to the tsunami-induced submarine landslides in recent years. Chinese scholars have more focused on those caused by hydrate decomposition,which can be the new trend in the future researches on submarine landslides. The bibliometric analysis platform and visualization technology are used to quantitatively analyze the research characteristics in the field of submarine landslides from 2000 to 2021, and further reveal these research changes and development trends in different periods of time. The results show that: (1)The number of scientific research outputs in the field of submarine landslides has grown rapidly during the past 20 years. China is the country with the fastest growth on the submarine landslides publications,and has relatively closer cooperation with other countries. (2)By analyzing the research institutions of China and the United States,it is found that the latter has the largest number of research institutions. China's research institutions have most cooperation with other research institutions at home and abroad,and are comparatively influential. (3)The research fields on submarine landslides for the scientists at home and abroad in the past 20 years have been basically consistent. However,on the trigger mechanism,the foreign scholars have paid more attention to the tsunami-induced submarine landslides in recent years. Chinese scholars have more focused on those caused by hydrate decomposition,which can be the new trend in the future researches on submarine landslides.
Submarine landslides are one of the most hazardous agents,through which vast volumes of sediments are transported across the slopes under very high velocities. Hydroplaning is attributed to be the main reason for the high mobility of the submarine landslide,in which complicated interactions between the sliding mass and the ambient water is involved. This paper investigates the consequences of the hydroplaning using a novel numerical tool,the material point method(MPM),simulating the runout process of a laboratory and a real case submarine landslide. Considering the mechanical properties on the sliding mass during the runout process,upper drag force,basal resistance,and resistance from the water cushion are imposed on the sliding mass. The obtained runout distances for the two scenarios agree well with the observations. Based on the further analysis of the mobility of the sliding masses,it is found that the hydroplaning effect significantly enhances the runout distance and maximum velocity. Submarine landslides are one of the most hazardous agents,through which vast volumes of sediments are transported across the slopes under very high velocities. Hydroplaning is attributed to be the main reason for the high mobility of the submarine landslide,in which complicated interactions between the sliding mass and the ambient water is involved. This paper investigates the consequences of the hydroplaning using a novel numerical tool,the material point method(MPM),simulating the runout process of a laboratory and a real case submarine landslide. Considering the mechanical properties on the sliding mass during the runout process,upper drag force,basal resistance,and resistance from the water cushion are imposed on the sliding mass. The obtained runout distances for the two scenarios agree well with the observations. Based on the further analysis of the mobility of the sliding masses,it is found that the hydroplaning effect significantly enhances the runout distance and maximum velocity.
Subaqueous landslides are one of the common natural disasters. PFC is difficult to simulate the subaqueous fluid environment. In order to solve this problem, a calculation method using computational fluid dynamics OpenFOAM and PFC is proposed. Aiming at the problem of scale similarity in fluid-solid coupling, a similarity method suitable for this coupling method is proposed and its feasibility is verified. The dynamic characteristics and accumulation morphology of subaqueous landslides based on the coupling method are analyzed through typical cases, and compared with the results of unidirectional coupled subaqueous landslides and upland landslides. The results show that the coupling method can simulate the law of subaqueous landslide movement well, which mainly shows that the front end of the landslide body is thick and elliptical. Subaqueous landslide movement process and accumulation form are quite different from the landslides on land. The OpenFOAM-PFC bidirectional coupling method is superior to unidirectional coupling method. Subaqueous landslides are one of the common natural disasters. PFC is difficult to simulate the subaqueous fluid environment. In order to solve this problem, a calculation method using computational fluid dynamics OpenFOAM and PFC is proposed. Aiming at the problem of scale similarity in fluid-solid coupling, a similarity method suitable for this coupling method is proposed and its feasibility is verified. The dynamic characteristics and accumulation morphology of subaqueous landslides based on the coupling method are analyzed through typical cases, and compared with the results of unidirectional coupled subaqueous landslides and upland landslides. The results show that the coupling method can simulate the law of subaqueous landslide movement well, which mainly shows that the front end of the landslide body is thick and elliptical. Subaqueous landslide movement process and accumulation form are quite different from the landslides on land. The OpenFOAM-PFC bidirectional coupling method is superior to unidirectional coupling method.
Small-scale flume tests and numerical simulations have been widely applied to investigate submarine debris-flow impact on an undersea pipeline. However, the model-prototype similarity has not yet been guaranteed, which prevents trustable application of model results to prototype scenarios. To this end, the Power-law constitutive relation is used to describe the rheological property of submarine debris flow. The scale ratios between model and prototype for various parameters are derived based on Reynolds criterion. The flume tests published in literatures are used to demonstrate the application of derived scale ratios to model-prototype similarity analyses. The parameters of a model can be converted to those of a prototype with reference to derived scale ratios, and vice versa. In addition, the applicability of derived scale ratios is discussed. It is found that, the derived scale ratios are not applicable to Ng geotechnical centrifuge model tests, but to 1g small-scale flume tests and numerical simulations with relatively high shear strain rate of a submarine debris flow. This study will provide theoretical foundation for reaching model-prototype similarity when studying submarine debris-flow impact on an undersea pipeline in a 1g environment. Small-scale flume tests and numerical simulations have been widely applied to investigate submarine debris-flow impact on an undersea pipeline. However, the model-prototype similarity has not yet been guaranteed, which prevents trustable application of model results to prototype scenarios. To this end, the Power-law constitutive relation is used to describe the rheological property of submarine debris flow. The scale ratios between model and prototype for various parameters are derived based on Reynolds criterion. The flume tests published in literatures are used to demonstrate the application of derived scale ratios to model-prototype similarity analyses. The parameters of a model can be converted to those of a prototype with reference to derived scale ratios, and vice versa. In addition, the applicability of derived scale ratios is discussed. It is found that, the derived scale ratios are not applicable to Ng geotechnical centrifuge model tests, but to 1g small-scale flume tests and numerical simulations with relatively high shear strain rate of a submarine debris flow. This study will provide theoretical foundation for reaching model-prototype similarity when studying submarine debris-flow impact on an undersea pipeline in a 1g environment.
Submarine landslides are a common type of marine geo-disasters and can cause huge threat to the safety of oil and gas pipelines. Due to scour effect of seabed currents, submarine pipelines are usually suspended above the seabed surface, leading to poor pipeline stability. When the suspended pipeline is impacted by the submarine landslide, prediction of pipeline dynamic responses and safety evaluation become especially important. In this study, a finite element model of submarine landslide-pipeline interaction is developed through dividing the pipeline into the suspended and burial sections. The model is capable of capturing dynamic responses of oil and gas pipelines subjected to submarine landslides considering different pipeline spanning lengths and heights. Numerical analyses show that the influence of spanning length and height on the plastic strain of the pipeline is conspicuous. The pipeline strain induced by submarine landslides increases with the increase of spanning length and height. Finally, combing the effect of spanning length and height, a safety evaluation method of suspended pipelines under the impact of submarine landslides is proposed. The results can be directly used for dynamic evaluation of oil and gas pipeline safety under the impact of submarine landslides. Submarine landslides are a common type of marine geo-disasters and can cause huge threat to the safety of oil and gas pipelines. Due to scour effect of seabed currents, submarine pipelines are usually suspended above the seabed surface, leading to poor pipeline stability. When the suspended pipeline is impacted by the submarine landslide, prediction of pipeline dynamic responses and safety evaluation become especially important. In this study, a finite element model of submarine landslide-pipeline interaction is developed through dividing the pipeline into the suspended and burial sections. The model is capable of capturing dynamic responses of oil and gas pipelines subjected to submarine landslides considering different pipeline spanning lengths and heights. Numerical analyses show that the influence of spanning length and height on the plastic strain of the pipeline is conspicuous. The pipeline strain induced by submarine landslides increases with the increase of spanning length and height. Finally, combing the effect of spanning length and height, a safety evaluation method of suspended pipelines under the impact of submarine landslides is proposed. The results can be directly used for dynamic evaluation of oil and gas pipeline safety under the impact of submarine landslides.
The double-row steel sheet pile structure(DSSPS) is a soil-retaining and water-stopping composite structure. It composes of two rows of steel sheet piles, soil between the piles and tie rods. Compared with earth-rock dikes, it has a smaller footprint, faster construction speed, better integrity than steel sheet piles, and good dynamic stability against earthquakes and waves. It is widely used in water conservancy, water transportation, and coastal engineering. However, because DSSPS is greatly affected by geological conditions, its application effects and costs are not the same. This article focuses on collecting and investigating the use of double-row steel sheet piles in coastal projects at home and abroad. The effects of double-row steel sheet piles on karst-developed lithological foundations, high-permeability sand foundations and deep silt soft soil foundations have been investigated. Performance impact and design constraints are analyzed. Combined with the original geological conditions and design data, the finite element software is used to supplement the calculation and analysis. The application effects of the double-row steel sheet piles under various types of engineering geological and hydrological conditions are analyzed. The different projects of the double-row steel sheet piles in the coastal zone are summarized. It is found that the construction of double-row steel sheet piles in karst developed stratum needs to prevent leakage problems caused by poor verticality and karst connectivity caused by pile driving. In the sandy soil layer, it is necessary to prevent the inclination and deformation of the steel sheet piles from deteriorating the water-stop performance of the lock, which in turn leads to water leakage and even quicksand. During the construction of silt formations, the construction of the driven pile and the water-stopping performance of the pile body can be guaranteed, but a large overall deformation of the pile body may occur. At the same time, the inner pit bottom reinforcement should be considered to avoid the stability of the kicking foot. The findings of this article has engineering guidance value for the development of coastal ecological engineering geology and coastal resilient hydraulic structures. The double-row steel sheet pile structure(DSSPS) is a soil-retaining and water-stopping composite structure. It composes of two rows of steel sheet piles, soil between the piles and tie rods. Compared with earth-rock dikes, it has a smaller footprint, faster construction speed, better integrity than steel sheet piles, and good dynamic stability against earthquakes and waves. It is widely used in water conservancy, water transportation, and coastal engineering. However, because DSSPS is greatly affected by geological conditions, its application effects and costs are not the same. This article focuses on collecting and investigating the use of double-row steel sheet piles in coastal projects at home and abroad. The effects of double-row steel sheet piles on karst-developed lithological foundations, high-permeability sand foundations and deep silt soft soil foundations have been investigated. Performance impact and design constraints are analyzed. Combined with the original geological conditions and design data, the finite element software is used to supplement the calculation and analysis. The application effects of the double-row steel sheet piles under various types of engineering geological and hydrological conditions are analyzed. The different projects of the double-row steel sheet piles in the coastal zone are summarized. It is found that the construction of double-row steel sheet piles in karst developed stratum needs to prevent leakage problems caused by poor verticality and karst connectivity caused by pile driving. In the sandy soil layer, it is necessary to prevent the inclination and deformation of the steel sheet piles from deteriorating the water-stop performance of the lock, which in turn leads to water leakage and even quicksand. During the construction of silt formations, the construction of the driven pile and the water-stopping performance of the pile body can be guaranteed, but a large overall deformation of the pile body may occur. At the same time, the inner pit bottom reinforcement should be considered to avoid the stability of the kicking foot. The findings of this article has engineering guidance value for the development of coastal ecological engineering geology and coastal resilient hydraulic structures.
The coastal zone is located in the interaction area between land and sea. Its unique geographical, geological and environmental conditions lead to the frequent occurrence of geological disasters, which are high liability and risk. Considering the important economic and social attributes of the coastal zone, it is very important to carry out a geohazard risk assessment in the coastal zone. In this paper, the geohazard risk assessment model based on fuzzy Bayesian network is established and combined with the Analytic Network Process(ANP), to determine the conditional probability of fuzzy Bayesian network and simplify the Bayesian network structure. On this basis, the coastal zone in the eastern part of the Liaodong peninsula is employed as the study area. Five main types of geohazards including rockfall, landslide, ground subsidence, seawater intrusion and coastal erosion are selected as evaluation objects. Further, the susceptibility evaluation, hazard assessment and risk assessment of geohazards based on the fuzzy Bayesian network model coupling with ANP are carried out. The comprehensive geohazard risk assessment map is also achieved. The results show that the southwestern coastal zone of the study area is the highest or higher risk area. The zone has an area of 249km2 and 9.1% of the total area. The research results can provide an important reference for land and resources development, economic construction planning, disaster prevention and mitigation in the coastal zone, and also have certain reference significance for the risk assessment of geohazards in coastal zones of similar areas. The coastal zone is located in the interaction area between land and sea. Its unique geographical, geological and environmental conditions lead to the frequent occurrence of geological disasters, which are high liability and risk. Considering the important economic and social attributes of the coastal zone, it is very important to carry out a geohazard risk assessment in the coastal zone. In this paper, the geohazard risk assessment model based on fuzzy Bayesian network is established and combined with the Analytic Network Process(ANP), to determine the conditional probability of fuzzy Bayesian network and simplify the Bayesian network structure. On this basis, the coastal zone in the eastern part of the Liaodong peninsula is employed as the study area. Five main types of geohazards including rockfall, landslide, ground subsidence, seawater intrusion and coastal erosion are selected as evaluation objects. Further, the susceptibility evaluation, hazard assessment and risk assessment of geohazards based on the fuzzy Bayesian network model coupling with ANP are carried out. The comprehensive geohazard risk assessment map is also achieved. The results show that the southwestern coastal zone of the study area is the highest or higher risk area. The zone has an area of 249km2 and 9.1% of the total area. The research results can provide an important reference for land and resources development, economic construction planning, disaster prevention and mitigation in the coastal zone, and also have certain reference significance for the risk assessment of geohazards in coastal zones of similar areas.
Buried pipelines are widely used, and the extensive liquefiable soil layers will lead to the problem of pipeline uplift and deformation damage during earthquakes. Based on the buried pipeline project, we analyzed the deformation characteristics and dynamic response of pipes in marine liquefied foundations, and investigated the anti-liquefaction effect of gravel layers and drainage panels. The main conclusions are as follows: the marine saturated sand foundation liquefies under the dynamic load, and the acceleration decay decreases with increasing depth. The excess pore pressure in different depth rises rapidly to the peak and remains stable until the vibration stops. During the vibration, the pipe floats significantly and the floating rate decreases gradually. The gravel layer method is not effective in anti-liquefaction of the pipe, and the acceleration and excess pore pressure are basically consistent with the standard working condition. The effect of wide or narrow drainage panels method are more significant, and the liquefaction phenomenon of the overall soil layer is suppressed. The peak excess pore pressure is smaller compared with the standard working condition, and the average peak value is reduced by 48.30% and 38.91%, respectively, while the vertical displacement of the pipe decreases by more than 100% compared with the standard working condition. In practical engineering applications, it is recommended to use the drainage panels to reduce the risk of foundation liquefaction, while the appropriate drainage panels width needs to be selected. Buried pipelines are widely used, and the extensive liquefiable soil layers will lead to the problem of pipeline uplift and deformation damage during earthquakes. Based on the buried pipeline project, we analyzed the deformation characteristics and dynamic response of pipes in marine liquefied foundations, and investigated the anti-liquefaction effect of gravel layers and drainage panels. The main conclusions are as follows: the marine saturated sand foundation liquefies under the dynamic load, and the acceleration decay decreases with increasing depth. The excess pore pressure in different depth rises rapidly to the peak and remains stable until the vibration stops. During the vibration, the pipe floats significantly and the floating rate decreases gradually. The gravel layer method is not effective in anti-liquefaction of the pipe, and the acceleration and excess pore pressure are basically consistent with the standard working condition. The effect of wide or narrow drainage panels method are more significant, and the liquefaction phenomenon of the overall soil layer is suppressed. The peak excess pore pressure is smaller compared with the standard working condition, and the average peak value is reduced by 48.30% and 38.91%, respectively, while the vertical displacement of the pipe decreases by more than 100% compared with the standard working condition. In practical engineering applications, it is recommended to use the drainage panels to reduce the risk of foundation liquefaction, while the appropriate drainage panels width needs to be selected.
The potential threat of earthquake disaster should be considered in the construction of subsea tunnel in offshore areas. Considering the influence of seawater hydrodynamic pressure can be more in line with the actual situation when analyzing the seismic response of subsea tunnel. A seismic analysis model of water-seabed-tunnel is established, which takes into account the nonlinearity of soil and the fluid-structure interaction between seawater and seabed. The seismic response of undersea tunnel under different seismic excitations and water depths is studied. The results show that the acoustic module can simulate the fluid-structure interaction well. Under the action of horizontal earthquake, the stress of tunnel is mainly concentrated at the arch shoulder and arch foot. The maximum hydrodynamic pressure in the calculated area under earthquake action occurs on the left and right sides of the seabed above the tunnel. Under bidirectional seismic excitation, the hydrodynamic pressure on the seabed surface increases significantly, and the stress peak at each point of the tunnel also increases significantly. The seismic response of subsea tunnel under the earthquake with rich low frequency component is much stronger than that with rich high frequency component. Seismic damage of subsea tunnel decreases with the increasing water depth. The results are of some reference value for understanding the actual seismic response law of the undersea tunnel. The potential threat of earthquake disaster should be considered in the construction of subsea tunnel in offshore areas. Considering the influence of seawater hydrodynamic pressure can be more in line with the actual situation when analyzing the seismic response of subsea tunnel. A seismic analysis model of water-seabed-tunnel is established, which takes into account the nonlinearity of soil and the fluid-structure interaction between seawater and seabed. The seismic response of undersea tunnel under different seismic excitations and water depths is studied. The results show that the acoustic module can simulate the fluid-structure interaction well. Under the action of horizontal earthquake, the stress of tunnel is mainly concentrated at the arch shoulder and arch foot. The maximum hydrodynamic pressure in the calculated area under earthquake action occurs on the left and right sides of the seabed above the tunnel. Under bidirectional seismic excitation, the hydrodynamic pressure on the seabed surface increases significantly, and the stress peak at each point of the tunnel also increases significantly. The seismic response of subsea tunnel under the earthquake with rich low frequency component is much stronger than that with rich high frequency component. Seismic damage of subsea tunnel decreases with the increasing water depth. The results are of some reference value for understanding the actual seismic response law of the undersea tunnel.
To accurately simulate the construction mechanical effects of the subsea shield tunneling process, this study develops a three-dimensional numerical model of shield machine-grouting body-surrounding rock-seawater interaction based on the subsea shield section of Xiamen Metro Line 2. By validating the analysis results with measured data of the project, the effects of four main factors(excavation face support pressure, formation loss rate, grouting pressure, and jacking force) are further investigated. The results show that the water and soil pressure of the segment is strongly disturbed by the construction at the initial stage, and then decreases sharply and rapidly at a decrease of about 100kPa, and then decreases slowly at a decrease of about 20kPa, and finally tends to be stable. It is most reasonable to set the support pressure of the excavation face at about 320kPa. The increase of the support pressure only affects the soil deformation within a certain range in front of the excavation. Due to the large buried depth, the vertical displacement of the surface is basically not affected. The ground layer loss rate has a great influence on ground settlement, segment buoyancy, and segment internal force. As the stratum loss rate increases by 1%, the surface subsidence increases by 241.3%, the segment buoyancy decreases by 38.2% and the bending moment decreases by 23.9%. The grouting pressure has a great influence on segment buoyancy and internal force. The grouting pressure increases by 10%, segment buoyancy increases by 32.1%, and bending moment increases by 24.3%. The study also demonstrates that the jacking force has a certain influence on the axial force of the segment along the tunnel axis but has little influence on the segment floating and bending moment. This research provides a reliable analysis of the construction mechanical effects of the subsea shield tunneling process, which has an in-depth influence on the segment structure design and subsea shield construction parameter control. To accurately simulate the construction mechanical effects of the subsea shield tunneling process, this study develops a three-dimensional numerical model of shield machine-grouting body-surrounding rock-seawater interaction based on the subsea shield section of Xiamen Metro Line 2. By validating the analysis results with measured data of the project, the effects of four main factors(excavation face support pressure, formation loss rate, grouting pressure, and jacking force) are further investigated. The results show that the water and soil pressure of the segment is strongly disturbed by the construction at the initial stage, and then decreases sharply and rapidly at a decrease of about 100kPa, and then decreases slowly at a decrease of about 20kPa, and finally tends to be stable. It is most reasonable to set the support pressure of the excavation face at about 320kPa. The increase of the support pressure only affects the soil deformation within a certain range in front of the excavation. Due to the large buried depth, the vertical displacement of the surface is basically not affected. The ground layer loss rate has a great influence on ground settlement, segment buoyancy, and segment internal force. As the stratum loss rate increases by 1%, the surface subsidence increases by 241.3%, the segment buoyancy decreases by 38.2% and the bending moment decreases by 23.9%. The grouting pressure has a great influence on segment buoyancy and internal force. The grouting pressure increases by 10%, segment buoyancy increases by 32.1%, and bending moment increases by 24.3%. The study also demonstrates that the jacking force has a certain influence on the axial force of the segment along the tunnel axis but has little influence on the segment floating and bending moment. This research provides a reliable analysis of the construction mechanical effects of the subsea shield tunneling process, which has an in-depth influence on the segment structure design and subsea shield construction parameter control.
The preliminary study of the Bohai Strait Cross Sea Channel Project has always remained at the planning level. The systematic engineering geological survey is basically blank. By means of data collection, field investigation, offshore drilling, marine geophysical exploration and indoor test, this paper focuses on the engineering geological characteristics, special geotechnical properties, adverse geological processes and main engineering geological problems along the sea crossing channel. The results show the follows. (1)The thickness of the Quaternary overburden in the channel is different. The buried depth near the islands is shallow. The thickness in the hinterland of the channel and the middle of the channel is larger. The middle layer of Lao Tieshan waterway is the thickest. No bedrock is found in nearby ZK1 hole with depth of 186.3m. (2)The lithology along the channel mainly metamorphic rocks including quartz schist, metamorphic sandstone, arkose and gneiss. The lithology is relatively hard, and the basic classification of the rock mass is mainly grade Ⅲ~Ⅳ. (3)Special rocks and soils along the line include fill soil, recent loess, soft soil and argillaceous expansive rock. Adverse geological processes include dangerous rocks and rockfalls, man-made pits, harmful gases and seismic geological disasters. (4)Engineering geological problems mainly include crossing fault zone and water inrush of subsea tunnel. The risk of water inrush in tunnel is concentrated near fault fracture zone. The research results are of great significance to scientifically demonstrate the construction scheme of Bohai Strait Cross Sea Channel Project. The preliminary study of the Bohai Strait Cross Sea Channel Project has always remained at the planning level. The systematic engineering geological survey is basically blank. By means of data collection, field investigation, offshore drilling, marine geophysical exploration and indoor test, this paper focuses on the engineering geological characteristics, special geotechnical properties, adverse geological processes and main engineering geological problems along the sea crossing channel. The results show the follows. (1)The thickness of the Quaternary overburden in the channel is different. The buried depth near the islands is shallow. The thickness in the hinterland of the channel and the middle of the channel is larger. The middle layer of Lao Tieshan waterway is the thickest. No bedrock is found in nearby ZK1 hole with depth of 186.3m. (2)The lithology along the channel mainly metamorphic rocks including quartz schist, metamorphic sandstone, arkose and gneiss. The lithology is relatively hard, and the basic classification of the rock mass is mainly grade Ⅲ~Ⅳ. (3)Special rocks and soils along the line include fill soil, recent loess, soft soil and argillaceous expansive rock. Adverse geological processes include dangerous rocks and rockfalls, man-made pits, harmful gases and seismic geological disasters. (4)Engineering geological problems mainly include crossing fault zone and water inrush of subsea tunnel. The risk of water inrush in tunnel is concentrated near fault fracture zone. The research results are of great significance to scientifically demonstrate the construction scheme of Bohai Strait Cross Sea Channel Project.
Ancient landslides are widely developed in hydrate-rich areas in the northern continental slope of the South China Sea. Imprudent hydrate production may result in the reactivation of the ancient submarine landslides. In order to explore the mechanism of the ancient landslide reactivation induced by hydrate production, we analyzed the slope stability and instability modes of two typical ancient landslides: the underburden-type and the associated-type. The analysis accounted for the changes of the transient pore pressure and the soil shear strength during hydrate production within the limit equilibrium analysis framework. The results suggest that hydrate dissociation results in the reduction of the cementing strength and meanwhile, the released gas may be trapped below the ancient landslide body with low permeability, giving rise to a laterally extending high-pressure zone. The potential slip surface of the underburden-type reservoir goes through the ancient slip surface, showing a slide pattern. In the early stage of production, the slope stability decreases due to the pore pressure build-up. Then, during the middle and late stages of production, the slope stability recovers because of the secondary hydrate formation. The production would not trigger the ancient reactivation with the calculation configuration in this study. The slope stability of the associated-type reservoir is affected by both the soil strength reduction and the pore pressure build-up. Hydrate production from an associated-type reservoir may trigger the reactivation of the ancient landslide, showing a slump pattern. Ancient landslides are widely developed in hydrate-rich areas in the northern continental slope of the South China Sea. Imprudent hydrate production may result in the reactivation of the ancient submarine landslides. In order to explore the mechanism of the ancient landslide reactivation induced by hydrate production, we analyzed the slope stability and instability modes of two typical ancient landslides: the underburden-type and the associated-type. The analysis accounted for the changes of the transient pore pressure and the soil shear strength during hydrate production within the limit equilibrium analysis framework. The results suggest that hydrate dissociation results in the reduction of the cementing strength and meanwhile, the released gas may be trapped below the ancient landslide body with low permeability, giving rise to a laterally extending high-pressure zone. The potential slip surface of the underburden-type reservoir goes through the ancient slip surface, showing a slide pattern. In the early stage of production, the slope stability decreases due to the pore pressure build-up. Then, during the middle and late stages of production, the slope stability recovers because of the secondary hydrate formation. The production would not trigger the ancient reactivation with the calculation configuration in this study. The slope stability of the associated-type reservoir is affected by both the soil strength reduction and the pore pressure build-up. Hydrate production from an associated-type reservoir may trigger the reactivation of the ancient landslide, showing a slump pattern.
Natural gas hydrate has been treated as a potential energy resource for decades. Depressurization is currently the most promising method for hydrate production. However, its efficiency is far from the commercial need. Hydrate production involves heat transfer, multi-phase seepage, phase transition, and reservoir deformation. A thorough understanding of how multiple physical processes evolve during depressurization is of great significance for efficiency enhancement of hydrate production. An experiment was carried out to simulate depressurization induced evolution of the multiple physical processes. Methane hydrate was formed by using the gas excess method under a heterogeneous temperature condition. Evolutions of pore pressures and temperatures were analyzed. A comparison between gas production process and heat transfer process was discussed. Main conclusions are drawn as follow: temperature distribution is parabola-like after hydrate formation, which has higher temperatures in two sides of the sample. In addition, hydrate distribution is inhomogeneous. Pore pressures decrease completely from the outlet to the inlet, and temperatures increase from the two sides into the middle part. The gas production process related to the heat transfer process well, and the stable stage for gas production is controlled by the heat transfer process. It is a feasible way to replace heat conduction by heat convection or choose a slow depressurization strategy to enhance production efficiency for the commercial need. Natural gas hydrate has been treated as a potential energy resource for decades. Depressurization is currently the most promising method for hydrate production. However, its efficiency is far from the commercial need. Hydrate production involves heat transfer, multi-phase seepage, phase transition, and reservoir deformation. A thorough understanding of how multiple physical processes evolve during depressurization is of great significance for efficiency enhancement of hydrate production. An experiment was carried out to simulate depressurization induced evolution of the multiple physical processes. Methane hydrate was formed by using the gas excess method under a heterogeneous temperature condition. Evolutions of pore pressures and temperatures were analyzed. A comparison between gas production process and heat transfer process was discussed. Main conclusions are drawn as follow: temperature distribution is parabola-like after hydrate formation, which has higher temperatures in two sides of the sample. In addition, hydrate distribution is inhomogeneous. Pore pressures decrease completely from the outlet to the inlet, and temperatures increase from the two sides into the middle part. The gas production process related to the heat transfer process well, and the stable stage for gas production is controlled by the heat transfer process. It is a feasible way to replace heat conduction by heat convection or choose a slow depressurization strategy to enhance production efficiency for the commercial need.
Natural gas hydrate(NGH) is a promising clean alternative energy resource for world in future. Based on the analysis of the challenges in the commercial exploitation, the depressurization and backfilling with in-situ supplemental heat method had been proposed to enhance the gas production of methane hydrate reservoir. In this method, the calcium oxide(CaO) powder is injected into the hydrate reservoir, and the natural gas is exploited by depressurization. The water produced by the decomposition of natural gas hydrate will react with the calcium oxide powder rapidly, which would provide amounts heat for supplement thermal energy of the decomposition of natural gas hydrate. This novel method is evaluated by a numerical simulator based on the finite volume method in this work. A three-dimensional reservoir model was constructed. The simulation results indicate that comparing with the conventional horizontal well method and the horizontal well combined fracturing method, this method has a better production performance. Comparing with the horizontal well combined fracturing method, the cumulative gas production of this method is improved, but the cumulative water production has changed slightly simultaneously. Therefore, the recovery efficiency has been significantly improved. The results of the sensitivity analysis of the equivalent permeability of fractures and the mass of CaO injection show that the increasing effect of fracturing on gas production declines with the improvement of equivalent permeability of fractures. In addition, the greater the amount of injected calcium oxide, the more obvious the effect of increasing production. Increasing the amount of injected calcium oxide only increase the gas production, but not significantly increase the water production. Therefore, theoretically the larger the injection, the higher the gas production efficiency. Simultaneously, the feasibility of this method has been testified in reservoirs with different flow capacity. Herein, the improving effect on low-permeability reservoir is more obviously than other cases. Based on the above conclusions, this work quantitatively verifies the potential value of the depressurization and backfilling with in-situ supplemental heat method from the perspective of the theoretical calculation of the three-dimensional model, which looks forward to providing the reference for following work of hydrate recovery. Natural gas hydrate(NGH) is a promising clean alternative energy resource for world in future. Based on the analysis of the challenges in the commercial exploitation, the depressurization and backfilling with in-situ supplemental heat method had been proposed to enhance the gas production of methane hydrate reservoir. In this method, the calcium oxide(CaO) powder is injected into the hydrate reservoir, and the natural gas is exploited by depressurization. The water produced by the decomposition of natural gas hydrate will react with the calcium oxide powder rapidly, which would provide amounts heat for supplement thermal energy of the decomposition of natural gas hydrate. This novel method is evaluated by a numerical simulator based on the finite volume method in this work. A three-dimensional reservoir model was constructed. The simulation results indicate that comparing with the conventional horizontal well method and the horizontal well combined fracturing method, this method has a better production performance. Comparing with the horizontal well combined fracturing method, the cumulative gas production of this method is improved, but the cumulative water production has changed slightly simultaneously. Therefore, the recovery efficiency has been significantly improved. The results of the sensitivity analysis of the equivalent permeability of fractures and the mass of CaO injection show that the increasing effect of fracturing on gas production declines with the improvement of equivalent permeability of fractures. In addition, the greater the amount of injected calcium oxide, the more obvious the effect of increasing production. Increasing the amount of injected calcium oxide only increase the gas production, but not significantly increase the water production. Therefore, theoretically the larger the injection, the higher the gas production efficiency. Simultaneously, the feasibility of this method has been testified in reservoirs with different flow capacity. Herein, the improving effect on low-permeability reservoir is more obviously than other cases. Based on the above conclusions, this work quantitatively verifies the potential value of the depressurization and backfilling with in-situ supplemental heat method from the perspective of the theoretical calculation of the three-dimensional model, which looks forward to providing the reference for following work of hydrate recovery.
Cone penetration detection has the advantages of in-situ, continuity, high efficiency and resolution. The use of marine cone penetration detection results to estimate the bearing capacity of offshore platform pile foundation has a great application potential. This paper takes the pile foundation of a platform in the Chengdao sea area of Shengli Oilfield as an example. The ROSON100 marine cone penetration device was used to conduct in-situ detection around the platform's legs. Simultaneous drilling sampling and indoor geotechnical parameters testing were implemented. The results of cone penetration detection and geotechnical tests were used to estimate the bearing capacity of platform pile foundation, respectively. The similarities and differences between three different calculation methods of bearing capacity of pile foundation were discussed. The results show that the overall trend of the pile side friction resistance obtained by these three methods is basically the same with the buried depth. Based on certain regional engineering experience, the pile tip resistance and single pile ultimate bearing capacity obtained by the drilling specification method and the CPT indirect method are basically consistent. The pile tip resistance and the ultimate bearing capacity of single pile obtained by the CPT direct method are obviously larger than the former two. The ultimate bearing capacity of pile foundation shows good compatibility for these three calculation methods under the premise of considering silty sand as the bearing layer. The findings can provide a new reference for the calculation of the bearing capacity of offshore engineering pile foundation. Cone penetration detection has the advantages of in-situ, continuity, high efficiency and resolution. The use of marine cone penetration detection results to estimate the bearing capacity of offshore platform pile foundation has a great application potential. This paper takes the pile foundation of a platform in the Chengdao sea area of Shengli Oilfield as an example. The ROSON100 marine cone penetration device was used to conduct in-situ detection around the platform's legs. Simultaneous drilling sampling and indoor geotechnical parameters testing were implemented. The results of cone penetration detection and geotechnical tests were used to estimate the bearing capacity of platform pile foundation, respectively. The similarities and differences between three different calculation methods of bearing capacity of pile foundation were discussed. The results show that the overall trend of the pile side friction resistance obtained by these three methods is basically the same with the buried depth. Based on certain regional engineering experience, the pile tip resistance and single pile ultimate bearing capacity obtained by the drilling specification method and the CPT indirect method are basically consistent. The pile tip resistance and the ultimate bearing capacity of single pile obtained by the CPT direct method are obviously larger than the former two. The ultimate bearing capacity of pile foundation shows good compatibility for these three calculation methods under the premise of considering silty sand as the bearing layer. The findings can provide a new reference for the calculation of the bearing capacity of offshore engineering pile foundation.
Ocean engineering has moved towards the deep sea. But in-situ testing technology for undrained shear strength of deep sea shallow sediments is not yet mature. This paper develops a multi-probe in-situ test system that can evaluate the soil mechanical properties of deep-sea shallow sediments. It includes CPT, Ball Full-flow Penetrometer and VST to achieve a rapid, accurate and intelligent evaluation of the soil mechanical properties of deep-sea shallow sediments. Further the resistance coefficient of CPT is determined with CEL large-deformation numerical analysis method and the full-scale geotechnical model test, and the resistance coefficient of Ball Full-flow Penetrometer is given by the centrifugal model test. Then the undrained shear strength evaluation method of deep sea shallow sediments is improved. On this basis, the undrained shear strength test intervals suitable for each of the instruments are discussed. Results indicate that for the evaluation of the deep-sea shallow saturated soft clay undrained shear strength, the recommended penetration coefficients of CPT and Ball Full-flow Penetrometer are respectively 9.5 and 11.1. Ocean engineering has moved towards the deep sea. But in-situ testing technology for undrained shear strength of deep sea shallow sediments is not yet mature. This paper develops a multi-probe in-situ test system that can evaluate the soil mechanical properties of deep-sea shallow sediments. It includes CPT, Ball Full-flow Penetrometer and VST to achieve a rapid, accurate and intelligent evaluation of the soil mechanical properties of deep-sea shallow sediments. Further the resistance coefficient of CPT is determined with CEL large-deformation numerical analysis method and the full-scale geotechnical model test, and the resistance coefficient of Ball Full-flow Penetrometer is given by the centrifugal model test. Then the undrained shear strength evaluation method of deep sea shallow sediments is improved. On this basis, the undrained shear strength test intervals suitable for each of the instruments are discussed. Results indicate that for the evaluation of the deep-sea shallow saturated soft clay undrained shear strength, the recommended penetration coefficients of CPT and Ball Full-flow Penetrometer are respectively 9.5 and 11.1.
The NSFC proposals and grants of engineering geology field(including the engineering geological environment and disaster under the discipline of environmental geoscience, and the engineering geology subordinate to the discipline of geology) in 2021 were analyzed. The number of applications for funding projects in the field of engineering geology has increased steadily, mainly due to the continuous growing of applications for Young Scientist Funds and General Program Funds. The number of applications for other types of programs were relatively stable. The statistics of the last ten years showed that the peer-review referees well handled the peer-review scale on the NSFC proposals. The age structure of applicants for General Program tends to be younger, and that for the Youth Scientists Fund is reasonable. The field of engineering geology has been funded in several different project types, showing strong competitiveness. The NSFC proposals and grants of engineering geology field(including the engineering geological environment and disaster under the discipline of environmental geoscience, and the engineering geology subordinate to the discipline of geology) in 2021 were analyzed. The number of applications for funding projects in the field of engineering geology has increased steadily, mainly due to the continuous growing of applications for Young Scientist Funds and General Program Funds. The number of applications for other types of programs were relatively stable. The statistics of the last ten years showed that the peer-review referees well handled the peer-review scale on the NSFC proposals. The age structure of applicants for General Program tends to be younger, and that for the Youth Scientists Fund is reasonable. The field of engineering geology has been funded in several different project types, showing strong competitiveness.