Vacuum preloading combined with prefabricated vertical drains (PVDs) has the potential to improve... more Vacuum preloading combined with prefabricated vertical drains (PVDs) has the potential to improve the soft sediments under water, however, its development is partly limited by the unclear understanding of the mechanism. This paper aims to extend the comprehension of the influential mechanism of overlapping water in the scenario of underwater vacuum preloading with PVDs. The systematic investigations were conducted by small strain finite element drained analyses, with the separated analysis schemes considering suction-induced consolidation, seepage and their combination. The development of settlement in the improved soil region and the evolution of seepage flow from the overlapping water through the non-improved soil region into improved zone are examined in terms of the build-up of excess pore pressure. Based on the results of numerical analyses, a theoretical approach was set out. It was capable to estimate the time-dependent non-uniform settlement along the improved soil surface i...
Proceedings of GeoShanghai 2018 International Conference: Geoenvironment and Geohazard, 2018
A kind of fine black sands is always hydraulic filled onto the backfilled silt to support the equ... more A kind of fine black sands is always hydraulic filled onto the backfilled silt to support the equipment in reclamation projects in Tianjin, China. Bio-grouting, as a new approach of soil improvement, was adopted to cement black sands on the surface of silt to rapidly increase the carrying capacity. The results showed that the black sands were well cemented by calcium carbonate crystals produced in the process of bio-mineralization. A static load test was carried out on the cemented sand layer, which resulted of an increase of the loading-bearing capacity by about sixty times. The calcium carbonate contents of different positions in the cemented sand layer were detected, which showed an uneven distribution. The micro-structures of the produced calcium crystals were observed by using an electron microscopy. Different forms of crystals were observed, illustrating that the concentrations of bacteria in the sand layer were not uniform. From discussion, the bio-grouting method can cement the black sand layer and the solidification effect can be improved by some reasonable measures, which indicates the feasibility of this method in reclamation projects. Moreover, the bio-grouting method can bring a lot of time saving and material saving compared with traditional methods.
Pipeline walking is a global axial movement caused by the asymmetric distribution of axial force ... more Pipeline walking is a global axial movement caused by the asymmetric distribution of axial force along the pipeline during the repetitive operational loading cycle. The pipeline walking rate, increasing with the loading cycles, may lead to high stress thereby the failure of connectors at PLETs or PLEMs. Current research studies and design guidelines focused on free-end pipelines and neglected changes in the pipeline walking rate and axial resistance with effects of pipeline end structures. However, in practice, several pipelines are adjacent to PLETs or PLEMs, which potentially affect the pipeline walking and expansion behaviour thereby walking rate and axial displacement. This study analyses the walking phenomenon considering the effects of mobilisation of the resistance at the pipeline end structures and develops new analytical solutions for the present problem, in terms of different walking mechanism, maximum axial resistance Pmax, and positions of pipeline end structures. The an...
The wide-shallow bucket foundation proposed by Tianjin University of China is a new type of offsh... more The wide-shallow bucket foundation proposed by Tianjin University of China is a new type of offshore wind turbine foundation. In this paper, the vertical bearing capacity of wide-shallow bucket foundation embedded in two layered sand that contains an underlying medium strength sand layer and a weaker or stronger overlaying sand layer is studied. A parametric study for bearing capacity is carried out with the ratio of unit weight γ1/γ2 (where γ1 and γ2 are the unit weight of the upper and lower sand layers respectively), the ratio of internal friction angle φ1/φ2 (where φ1 and φ2 are the internal friction angle of the upper and lower sand layers respectively) and relative thickness of the top sand layer H1/B (where H1 and B are the thickness of the top sand layer and the bucket foundation diameter). All of the presents were performed by the Finite Element Method and the results show that the thickness of the top layer has a great influence on the vertical bearing capacity of the foun...
Abstract Monopiles are popular solutions as the foundation to support offshore wind turbines (OWT... more Abstract Monopiles are popular solutions as the foundation to support offshore wind turbines (OWTs), which suffer from various amplitudes of continuous lateral cyclic wind and wave loading. The accumulation of tilt and change of the natural frequency of OWTs under cyclic loading have become crucial issues for OWT design. Hence the monopile-tower-soil system was established at the small-scale level based on similarity laws with a suit of constant-amplitude and multi-amplitude tests in dense sand. Two sets of gear-driving assembly were successfully set up to achieve the application of cyclic loading to the model OWT. The amplitude and symmetry of cyclic loading were specified as three indicative regions according to those of real wind farms. On the basis of test results, the two-way loading was found to be the most hazardous situation and the preloading condition led to a high accumulation of tilt. The change of natural frequency was controlled by the load magnitude and tended to increase during cyclic loads, while a sharp decrease was observed when sand subsidence was generated. The post-cyclic lateral capacity seemed to be a slight increase compared with the static capacity. The relevant analyses based on test results can provide practical recommendations for OWT design.
Abstract As a new breakwater foundation form, a bucket foundation with two skirted side semicircl... more Abstract As a new breakwater foundation form, a bucket foundation with two skirted side semicircles and a skirted middle rectangle was applied to the breakwater project at Xuyu Port, China. A series of laboratory tests were performed on the bucket foundation to investigate the vertical bearing capacity and the bucket-sand interaction mechanisms during the load bearing process. The main failure mode of the bucket foundation under vertical loading was general shear failure. The vertical loads increased with penetration displacement, ultimately reaching 330~404 kN with a displacement criterion of 0.06De (equivalent diameter of the foundation) at different incremental load steps. The inner soil pressures were clearly greater than the outer soil pressures induced by the skirt constraint effect. Regarding the bearing capacity, the outer skirt friction decreasing from approximately 0.05 to 0 with increasing penetration displacement, contributed only slightly, while the foundation base provided the majority. The cover plate resistance increased rapidly after the penetration displacement exceeded 25 mm, and its sharing ratio ultimately reached approximately 0.82. The tip resistance was estimated via the assistance of the cone resistance, and its sharing ratio decreased with penetration displacement. The inner friction and corresponding friction coefficient first increased and then decreased with penetration displacement, reaching a final sharing ratio of approximately 0.14.
Abstract Bio-grouting is emerging as a sustainable technology for improving the mechanical proper... more Abstract Bio-grouting is emerging as a sustainable technology for improving the mechanical properties of sand in which CaCO3 is the main product. Previous studies proved that the amount of CaCO3 affects the mechanical properties of bio-cemented sand, but few studies address CaCO3 quantification. In this paper, we describe a novel method to predict the amount of CaCO3 in the process of bio-grouting. A total of 25 bio-grouting tests were carried out with 6 treatment factors considered: bacterial suspension’s concentration, flow rate and retention time, cementing solution’s concentration, flow rate, and reaction time. Based on the mechanism of microbial-induced calcite precipitate (MICP), a theoretical solution to quantify the amount of CaCO3 was established by researching the effects of environmental substances on urea hydrolyzed rate in a liquid environment. This solution was developed by analyzing the difference between sandy and liquid environments. A series of time-dependent equations that evaluate the relationship between the amount of CaCO3 produced with bacterial concentration, the cementing solution’s concentration, and the reaction time, were derived. Moreover, it was found that the bacterial retention time and the flow rates of grouting solutions affect the distributions of reactants in sand. This can be captured by a residual coefficient in the CaCO3 quantification model. Finally, once the linkage between the improved strength of bio-cemented sand and the CaCO3 content is established, the proposed model can be applied to estimate the strength of bio-cemented sand in real time.
Abstract A novel side-rounded suction caisson foundation is proposed in this study. Compared to a... more Abstract A novel side-rounded suction caisson foundation is proposed in this study. Compared to a conventional circular shape in plan, it has a rectangular middle section inserted in between the two circular halves for increased moment capacity. This paper presents an investigation into the bearing performance of this novel foundation in clay under uniaxial and combined loading by means of an extensive finite element parametric analysis. It is found that by adopting a dimensionless equivalent embedment ratio and a dimensionless equivalent strength heterogeneity ratio, which account for the side-rounded shape of the proposed foundation, its design approach can be unified with an existing framework established for conventional skirted circular footings. The advantage of the proposed foundation and the application of the proposed design method is demonstrated through an example application.
Abstract The installation process of caisson foundations causes the seabed soil heaving as well a... more Abstract The installation process of caisson foundations causes the seabed soil heaving as well as the soil strength remoulding. This paper investigates these two effects by examining the installation process of caissons through large deformation finite element analysis. The short-term undrained bearing capacity of caisson foundations in clay after installation, which is a critical engineering scenario because the long term foundation capacity increases with time due to seabed soil consolidation, is subsequently evaluated. Expressed in terms of bearing capacity envelopes under combined loading, the short-term bearing capacity exhibits significant reduction affected by the two effects (i.e. soil heaving and soil remoulding) during installation. The analysis results show that the caisson foundation capacity under uniaxial vertical load reduces by up to 40.95% when the ratio of caisson skirt length (d) to diameter of the caisson (D) is d/D = 4 while the uniaxial moment capacity reduces by up to 42.21% when d/D = 1. Meanwhile, all failure envelopes shrink due to the influence of installation. The maximum capacity degradation factors of all H-M failure envelopes are always greater than those of the uniaxial vertical, horizontal or moment capacity. The influence of the installation effects on the capacity of caisson post installation is more obviously when V ≤ 0.5Vult.
Abstract Suction caisson foundations have been increasingly employed as a primary solution to sup... more Abstract Suction caisson foundations have been increasingly employed as a primary solution to support the offshore fixed- or mobile-structures. Due to harsh environment and complex force transferring of offshore structures, they are still being developed as to satisfy increasing requirements in strict working scenarios. One of emerging challenges is a torsion-governing failure, which has been observed in the oil and gas industry (i.e. significant multiple-inline-force-induced torsion) and in the renewable energy field (i.e. non-coplanar tensile force induced torsion). This paper introduces a novel suction caisson foundation, with anti-rotational fins assembled on the outer skin of caisson. By a comprehensive numerical study, the evolution from local to global failure as the fin numbers from single to multiple, is examined in the clayey soil deposit with effects of soil strength heterogeneity, fins dimension, installation process and foundation-soil interface considered. Based on these, a set of methods to estimate the ultimate torsional capacity of such novel caisson is proposed, which starts from the gain in capacity for a single fin, and evaluate the changes of gains in capacity as fin numbers, then identify the optimised anti-rotational capacity. Finally, three key parameters (i.e. the required fin numbers, the available anti-rotational capacity and the optimised anti-rotational capacity) with some critical considerations, recommendations and implications have been concluded for design practice.
Subsea pipelines are commonly employed in the offshore oil and gas industry to transport high-pre... more Subsea pipelines are commonly employed in the offshore oil and gas industry to transport high-pressure and high-temperature (HPHT) hydrocarbons. The phenomenon of pipeline walking is a topic that has drawn a great deal of attention, and is related to the on-bottom stability of the pipeline, such as directional accumulation with respect to axial movement, which can threaten the security of the entire pipeline system. An accurate assessment of pipeline walking is therefore necessary for offshore pipeline design. This paper reports a comprehensive suite of numerical analyses investigating the performance of pipeline walking, with a focus on the effect of increasing axial soil resistance on walking rates. Three walking-driven modes (steel catenary riser (SCR) tension, downslope, and thermal transient) are considered, covering a wide range of influential parameters. The variation in walking rate with respect to the effect of increased soil friction is well reflected in the development of...
Abstract The concrete-made bucket foundations were employed as emerging solution to support the w... more Abstract The concrete-made bucket foundations were employed as emerging solution to support the wind turbine or breakwater, where the relatively large thicknesses of skirts and the strain softening effect related to the soil sensitivity and ductility were necessary to be explicitly accounted for. However, few of current study concerned this topic. This paper therefore reported an extensive parametric investigation of penetrating behaviour for bucket foundation in nonhomogeneous clay to allow a better estimation of penetration resistance of bucket foundation. The three-dimensional (3D) large deformation finite element (LDFE) analysis model incorporating the effect of strain softening on soil strength was set up and validated by comparing to the published centrifuge data. Then the evolution of soil flow mechanism and the distribution of soil strength softening were examined with the soil plug generated inside skirt assessed. Based on the numerical results, a new analytical model was proposed to predict the penetration resistance of bucket foundation in clay accounting for a partially-remoulded condition.
Abstract Subsea pipelines are important facilities in offshore oil and gas industry to transport ... more Abstract Subsea pipelines are important facilities in offshore oil and gas industry to transport High-Pressure and High-Temperature (HPHT) hydrocarbon. They are often exerted by cyclic thermal loading through the whole operational life, which may trigger asymmetry in the effective axial force (EAF) profile, leading to a global axial movement, defined as ‘pipeline walking’. It may cause the downtime and structural risks, since this directional accumulation in axial movement results in the overstressing of end connection, loss of tension in a steel catenary riser (SCR) and other issues in the field, therefore subsea pipeline design requires a reliable estimate of the global pipeline walking rate. Current design methods adopt a constant soil friction coefficient and ignore variations in soil-pipeline interaction with time during the heating up and cooling down processes. In reality, the overlooked changing soil friction is important, and may alter the pipeline walking behaviours. To achieve an accurate assessment of pipeline walking, this paper advances the conventional design practice, by introducing a time-dependent axial soil friction function, to examine the pipeline walking behaviour over the operational thermal cycles. A suite of time-dependent matrixes is provided to reveal the development of the expansion/contraction of the pipeline, the corresponding mobilised soil frictions and the EAF profile. The significant nonlinear EAF profile in response to the time-dependent soil friction is examined, and is used to well assess the accumulation in pipeline walking rate. The proposed analytical framework is applied into case studies to demonstrate its validity and applicability in practice.
Vacuum preloading combined with prefabricated vertical drains (PVDs) has the potential to improve... more Vacuum preloading combined with prefabricated vertical drains (PVDs) has the potential to improve the soft sediments under water, however, its development is partly limited by the unclear understanding of the mechanism. This paper aims to extend the comprehension of the influential mechanism of overlapping water in the scenario of underwater vacuum preloading with PVDs. The systematic investigations were conducted by small strain finite element drained analyses, with the separated analysis schemes considering suction-induced consolidation, seepage and their combination. The development of settlement in the improved soil region and the evolution of seepage flow from the overlapping water through the non-improved soil region into improved zone are examined in terms of the build-up of excess pore pressure. Based on the results of numerical analyses, a theoretical approach was set out. It was capable to estimate the time-dependent non-uniform settlement along the improved soil surface i...
Proceedings of GeoShanghai 2018 International Conference: Geoenvironment and Geohazard, 2018
A kind of fine black sands is always hydraulic filled onto the backfilled silt to support the equ... more A kind of fine black sands is always hydraulic filled onto the backfilled silt to support the equipment in reclamation projects in Tianjin, China. Bio-grouting, as a new approach of soil improvement, was adopted to cement black sands on the surface of silt to rapidly increase the carrying capacity. The results showed that the black sands were well cemented by calcium carbonate crystals produced in the process of bio-mineralization. A static load test was carried out on the cemented sand layer, which resulted of an increase of the loading-bearing capacity by about sixty times. The calcium carbonate contents of different positions in the cemented sand layer were detected, which showed an uneven distribution. The micro-structures of the produced calcium crystals were observed by using an electron microscopy. Different forms of crystals were observed, illustrating that the concentrations of bacteria in the sand layer were not uniform. From discussion, the bio-grouting method can cement the black sand layer and the solidification effect can be improved by some reasonable measures, which indicates the feasibility of this method in reclamation projects. Moreover, the bio-grouting method can bring a lot of time saving and material saving compared with traditional methods.
Pipeline walking is a global axial movement caused by the asymmetric distribution of axial force ... more Pipeline walking is a global axial movement caused by the asymmetric distribution of axial force along the pipeline during the repetitive operational loading cycle. The pipeline walking rate, increasing with the loading cycles, may lead to high stress thereby the failure of connectors at PLETs or PLEMs. Current research studies and design guidelines focused on free-end pipelines and neglected changes in the pipeline walking rate and axial resistance with effects of pipeline end structures. However, in practice, several pipelines are adjacent to PLETs or PLEMs, which potentially affect the pipeline walking and expansion behaviour thereby walking rate and axial displacement. This study analyses the walking phenomenon considering the effects of mobilisation of the resistance at the pipeline end structures and develops new analytical solutions for the present problem, in terms of different walking mechanism, maximum axial resistance Pmax, and positions of pipeline end structures. The an...
The wide-shallow bucket foundation proposed by Tianjin University of China is a new type of offsh... more The wide-shallow bucket foundation proposed by Tianjin University of China is a new type of offshore wind turbine foundation. In this paper, the vertical bearing capacity of wide-shallow bucket foundation embedded in two layered sand that contains an underlying medium strength sand layer and a weaker or stronger overlaying sand layer is studied. A parametric study for bearing capacity is carried out with the ratio of unit weight γ1/γ2 (where γ1 and γ2 are the unit weight of the upper and lower sand layers respectively), the ratio of internal friction angle φ1/φ2 (where φ1 and φ2 are the internal friction angle of the upper and lower sand layers respectively) and relative thickness of the top sand layer H1/B (where H1 and B are the thickness of the top sand layer and the bucket foundation diameter). All of the presents were performed by the Finite Element Method and the results show that the thickness of the top layer has a great influence on the vertical bearing capacity of the foun...
Abstract Monopiles are popular solutions as the foundation to support offshore wind turbines (OWT... more Abstract Monopiles are popular solutions as the foundation to support offshore wind turbines (OWTs), which suffer from various amplitudes of continuous lateral cyclic wind and wave loading. The accumulation of tilt and change of the natural frequency of OWTs under cyclic loading have become crucial issues for OWT design. Hence the monopile-tower-soil system was established at the small-scale level based on similarity laws with a suit of constant-amplitude and multi-amplitude tests in dense sand. Two sets of gear-driving assembly were successfully set up to achieve the application of cyclic loading to the model OWT. The amplitude and symmetry of cyclic loading were specified as three indicative regions according to those of real wind farms. On the basis of test results, the two-way loading was found to be the most hazardous situation and the preloading condition led to a high accumulation of tilt. The change of natural frequency was controlled by the load magnitude and tended to increase during cyclic loads, while a sharp decrease was observed when sand subsidence was generated. The post-cyclic lateral capacity seemed to be a slight increase compared with the static capacity. The relevant analyses based on test results can provide practical recommendations for OWT design.
Abstract As a new breakwater foundation form, a bucket foundation with two skirted side semicircl... more Abstract As a new breakwater foundation form, a bucket foundation with two skirted side semicircles and a skirted middle rectangle was applied to the breakwater project at Xuyu Port, China. A series of laboratory tests were performed on the bucket foundation to investigate the vertical bearing capacity and the bucket-sand interaction mechanisms during the load bearing process. The main failure mode of the bucket foundation under vertical loading was general shear failure. The vertical loads increased with penetration displacement, ultimately reaching 330~404 kN with a displacement criterion of 0.06De (equivalent diameter of the foundation) at different incremental load steps. The inner soil pressures were clearly greater than the outer soil pressures induced by the skirt constraint effect. Regarding the bearing capacity, the outer skirt friction decreasing from approximately 0.05 to 0 with increasing penetration displacement, contributed only slightly, while the foundation base provided the majority. The cover plate resistance increased rapidly after the penetration displacement exceeded 25 mm, and its sharing ratio ultimately reached approximately 0.82. The tip resistance was estimated via the assistance of the cone resistance, and its sharing ratio decreased with penetration displacement. The inner friction and corresponding friction coefficient first increased and then decreased with penetration displacement, reaching a final sharing ratio of approximately 0.14.
Abstract Bio-grouting is emerging as a sustainable technology for improving the mechanical proper... more Abstract Bio-grouting is emerging as a sustainable technology for improving the mechanical properties of sand in which CaCO3 is the main product. Previous studies proved that the amount of CaCO3 affects the mechanical properties of bio-cemented sand, but few studies address CaCO3 quantification. In this paper, we describe a novel method to predict the amount of CaCO3 in the process of bio-grouting. A total of 25 bio-grouting tests were carried out with 6 treatment factors considered: bacterial suspension’s concentration, flow rate and retention time, cementing solution’s concentration, flow rate, and reaction time. Based on the mechanism of microbial-induced calcite precipitate (MICP), a theoretical solution to quantify the amount of CaCO3 was established by researching the effects of environmental substances on urea hydrolyzed rate in a liquid environment. This solution was developed by analyzing the difference between sandy and liquid environments. A series of time-dependent equations that evaluate the relationship between the amount of CaCO3 produced with bacterial concentration, the cementing solution’s concentration, and the reaction time, were derived. Moreover, it was found that the bacterial retention time and the flow rates of grouting solutions affect the distributions of reactants in sand. This can be captured by a residual coefficient in the CaCO3 quantification model. Finally, once the linkage between the improved strength of bio-cemented sand and the CaCO3 content is established, the proposed model can be applied to estimate the strength of bio-cemented sand in real time.
Abstract A novel side-rounded suction caisson foundation is proposed in this study. Compared to a... more Abstract A novel side-rounded suction caisson foundation is proposed in this study. Compared to a conventional circular shape in plan, it has a rectangular middle section inserted in between the two circular halves for increased moment capacity. This paper presents an investigation into the bearing performance of this novel foundation in clay under uniaxial and combined loading by means of an extensive finite element parametric analysis. It is found that by adopting a dimensionless equivalent embedment ratio and a dimensionless equivalent strength heterogeneity ratio, which account for the side-rounded shape of the proposed foundation, its design approach can be unified with an existing framework established for conventional skirted circular footings. The advantage of the proposed foundation and the application of the proposed design method is demonstrated through an example application.
Abstract The installation process of caisson foundations causes the seabed soil heaving as well a... more Abstract The installation process of caisson foundations causes the seabed soil heaving as well as the soil strength remoulding. This paper investigates these two effects by examining the installation process of caissons through large deformation finite element analysis. The short-term undrained bearing capacity of caisson foundations in clay after installation, which is a critical engineering scenario because the long term foundation capacity increases with time due to seabed soil consolidation, is subsequently evaluated. Expressed in terms of bearing capacity envelopes under combined loading, the short-term bearing capacity exhibits significant reduction affected by the two effects (i.e. soil heaving and soil remoulding) during installation. The analysis results show that the caisson foundation capacity under uniaxial vertical load reduces by up to 40.95% when the ratio of caisson skirt length (d) to diameter of the caisson (D) is d/D = 4 while the uniaxial moment capacity reduces by up to 42.21% when d/D = 1. Meanwhile, all failure envelopes shrink due to the influence of installation. The maximum capacity degradation factors of all H-M failure envelopes are always greater than those of the uniaxial vertical, horizontal or moment capacity. The influence of the installation effects on the capacity of caisson post installation is more obviously when V ≤ 0.5Vult.
Abstract Suction caisson foundations have been increasingly employed as a primary solution to sup... more Abstract Suction caisson foundations have been increasingly employed as a primary solution to support the offshore fixed- or mobile-structures. Due to harsh environment and complex force transferring of offshore structures, they are still being developed as to satisfy increasing requirements in strict working scenarios. One of emerging challenges is a torsion-governing failure, which has been observed in the oil and gas industry (i.e. significant multiple-inline-force-induced torsion) and in the renewable energy field (i.e. non-coplanar tensile force induced torsion). This paper introduces a novel suction caisson foundation, with anti-rotational fins assembled on the outer skin of caisson. By a comprehensive numerical study, the evolution from local to global failure as the fin numbers from single to multiple, is examined in the clayey soil deposit with effects of soil strength heterogeneity, fins dimension, installation process and foundation-soil interface considered. Based on these, a set of methods to estimate the ultimate torsional capacity of such novel caisson is proposed, which starts from the gain in capacity for a single fin, and evaluate the changes of gains in capacity as fin numbers, then identify the optimised anti-rotational capacity. Finally, three key parameters (i.e. the required fin numbers, the available anti-rotational capacity and the optimised anti-rotational capacity) with some critical considerations, recommendations and implications have been concluded for design practice.
Subsea pipelines are commonly employed in the offshore oil and gas industry to transport high-pre... more Subsea pipelines are commonly employed in the offshore oil and gas industry to transport high-pressure and high-temperature (HPHT) hydrocarbons. The phenomenon of pipeline walking is a topic that has drawn a great deal of attention, and is related to the on-bottom stability of the pipeline, such as directional accumulation with respect to axial movement, which can threaten the security of the entire pipeline system. An accurate assessment of pipeline walking is therefore necessary for offshore pipeline design. This paper reports a comprehensive suite of numerical analyses investigating the performance of pipeline walking, with a focus on the effect of increasing axial soil resistance on walking rates. Three walking-driven modes (steel catenary riser (SCR) tension, downslope, and thermal transient) are considered, covering a wide range of influential parameters. The variation in walking rate with respect to the effect of increased soil friction is well reflected in the development of...
Abstract The concrete-made bucket foundations were employed as emerging solution to support the w... more Abstract The concrete-made bucket foundations were employed as emerging solution to support the wind turbine or breakwater, where the relatively large thicknesses of skirts and the strain softening effect related to the soil sensitivity and ductility were necessary to be explicitly accounted for. However, few of current study concerned this topic. This paper therefore reported an extensive parametric investigation of penetrating behaviour for bucket foundation in nonhomogeneous clay to allow a better estimation of penetration resistance of bucket foundation. The three-dimensional (3D) large deformation finite element (LDFE) analysis model incorporating the effect of strain softening on soil strength was set up and validated by comparing to the published centrifuge data. Then the evolution of soil flow mechanism and the distribution of soil strength softening were examined with the soil plug generated inside skirt assessed. Based on the numerical results, a new analytical model was proposed to predict the penetration resistance of bucket foundation in clay accounting for a partially-remoulded condition.
Abstract Subsea pipelines are important facilities in offshore oil and gas industry to transport ... more Abstract Subsea pipelines are important facilities in offshore oil and gas industry to transport High-Pressure and High-Temperature (HPHT) hydrocarbon. They are often exerted by cyclic thermal loading through the whole operational life, which may trigger asymmetry in the effective axial force (EAF) profile, leading to a global axial movement, defined as ‘pipeline walking’. It may cause the downtime and structural risks, since this directional accumulation in axial movement results in the overstressing of end connection, loss of tension in a steel catenary riser (SCR) and other issues in the field, therefore subsea pipeline design requires a reliable estimate of the global pipeline walking rate. Current design methods adopt a constant soil friction coefficient and ignore variations in soil-pipeline interaction with time during the heating up and cooling down processes. In reality, the overlooked changing soil friction is important, and may alter the pipeline walking behaviours. To achieve an accurate assessment of pipeline walking, this paper advances the conventional design practice, by introducing a time-dependent axial soil friction function, to examine the pipeline walking behaviour over the operational thermal cycles. A suite of time-dependent matrixes is provided to reveal the development of the expansion/contraction of the pipeline, the corresponding mobilised soil frictions and the EAF profile. The significant nonlinear EAF profile in response to the time-dependent soil friction is examined, and is used to well assess the accumulation in pipeline walking rate. The proposed analytical framework is applied into case studies to demonstrate its validity and applicability in practice.
Uploads