Clay Seabed Research Articles

Combined VHM bearing capacity of pile leg-mat foundation on marine clay seabed

The pile leg-mat foundation is a new type of jack-up wind power installation platform foundation. Since the foundation needs to bear the horizontal and bending moment loads caused by wind and waves for long period of time, the combined vertical, horizontal and moment (VHM) bearing capacity of the pile leg-mat foundation in the soft clay seabed is crucial in evaluating the safety of the jack-up platform. In this study, the effects of pile leg insertion depth L and the gradient of soil shear strength κ on the VHM bearing capacity of pile leg-mat foundation were analyzed. The numerical results indicate that the pile leg insertion depth significantly enhances the horizontal bearing capacity of the foundation. As the pile leg insertion depth increases, the horizontal load distribution of the foundation gradually shifts from the mat to the pile leg. However, the vertical and moment bearing capacity of the foundation is mainly provided by the mat and not significantly improved by the pile leg insertion depth. Based on the numerical results, the VHM failure envelope of the pile leg-mat foundation was established, and the method for assessing the bearing capacity of the pile-leg mat foundation was proposed.

Development of a new T-bar for the geotechnical centrifuge at TU Delft

An accurate estimation of undrained shear strength of clay seabed is important for interpreting lateral pile-soil interaction response. The cylindrical T-bar is a widely used site investigation tool for profiling the undrained strength (su) of soft soils. As such, a new miniature T-bar penetrometer is designed and fabricated at TU Delft for characterization of the undrained shear strength profile of clay layer in centrifuge models, and OCR profile can be then derived from undrained shear strength profile with known pre-consolidation stress level. The miniature T-bar penetrometer head is a cylinder of 5 mm in diameter and 20 mm in length and the miniature T-bar penetrometer head is connected with a rigid shaft (Figure 1(a)). The T-bar penetration resistance along the depth of clay sample can be obtained and further interpreted into the undrained shear strength profile.
 Rate of penetration is one of the key parameters that govern the drainage behaviour of soil response around the T-bar and the resulting penetration resistance. A very low rate of penetration leads to partial pore water dissipation and thus partial drainage condition. When a very large rate of penetration is applied, the fully undrained condition is achieved. However, the effect of T-bar penetration rate has not previously been fully examined for normally consolidated and over consolicated clay in the centrifuge. In this paper, the tip resistance profile of T-bar penetration tests under different rates of penetration is analyzed to obtain undrained shear strength profile of clay soils.
 A series of T-bar tests are conducted in both normally consolidated and slightly over consolidated clay samples at 100 g. The sample drainage state is tested by varying the rate of penetration from 0.01 mm/s to 5 mm/s. The results are interpreted by two methods: (i) the conventional method by converting the measured penetration resistance to soil strength using a single bearing factor, indicating a full-flow mechanism at failure [1]; and (ii) an approach considering soil buoyancy and a reduced bearing factor arising from the shallow failure mechanism, indicating the shallow correction procedure has a significant influence on the soil strength profile inferred from a T-bar penetrometer test [2]. The interpreted undrained shear strength profile provides soil property and OCR information for further monopile tests in the centrifuge, allowing a comprehensive study of the soil-structure interaction on soft soils.

Experimental and numerical investigations of eccentric uplift capacity of mat foundation on marine clay seabed

The mat foundation is a new type of jack-up wind power installation platform suitable for clay seabed. Because of the large size of the mat foundation, a large suction force must be overcome during the uplift process. In this study, the eccentric uplift of a mat foundation was experimentally and numerically investigated. The effects of the eccentric distance, eccentric position, and foundation shape on the suction force of the mat foundation were discussed. The results revealed that the breakout force of the mat foundation mainly comprised the foundation weight and suction force. Both the uplift position and eccentric distance affected the suction force; however, the effect of the uplift position was considerably smaller than that of the eccentric distance. The dimensionless suction force for the eccentric uplift tended to decrease linearly as the eccentric distance increased. When the dimensionless eccentric distance was 0.5, the suction force was 0.35–0.5 times that in the case of central uplift.

Centrifuge tests and numerical simulations on vertical bearing capacity of mat foundation on marine clay seabed

Mat foundation is a new type of jack-up wind power installation platform suitable for clay seabed, which avoids the problems of excessive insertion and extraction difficulties of spudcan foundation on soft clay seabed. Evaluating the vertical bearing capacity of the mat foundation on the clay seabed is of great significance for the safe operation of jack-up platform in marine environment. In this research, the vertical bearing characteristics of mat foundation on horizontal and sloping clay seabed were studied through centrifuge tests. Then, numerical simulations were conducted to study the vertical bearing capacity of the full-scale mat foundation on clay seabed, in which the effects of the large mat foundation size, pile leg insertion depth, preloading, consolidation degree of soil, and soil plasticity were analyzed. The results show that the vertical bearing capacity of the mat foundations on the sloping seabed is reduced, and the soil under the mat foundation shows obvious sliding failure. The contribution of the insertion depth of pile legs on the horizontal bearing capacity is more obvious than that of the vertical bearing capacity. The enhancement of the vertical bearing capacity is mainly due to the dissipation of the excess pore pressure and the increase of the mean effective stress of the soil. The effects of preloading, consolidation degree, and soil property on the vertical bearing capacity of the mat foundation are significant.

Research on the Initial Settlement Depth of Artificial Reef on the Clay Seabed

The artificial reef is an artificial structure that simulates some characteristics of a natural reef. Its penetration depth in the seabed soil affects the stability of the reef on the seabed. In this paper, based on the CEL (Coupled Eulerian-Lagrangian analysis) method, a finite element model is established for the process of artificial reef entry and penetration, which simulates the speed change law of the artificial reef in the water and the penetration depth in the clay seabed, and The study analyzed the influence of different reef weights, penetration speeds, and seabed soil properties on the penetration depth of artificial reefs.

Numerical study on the suction force of jack-up mat foundation on marine clay seabed

A deep layer of soft clay is widely present on the seabed in the offshore area of China. The traditional jack-up spudcan faces the problem of difficult insertion and extraction in clay seabed. Jack-up mat foundation is more suitable in soft clay than spudcan for its lower requirements on bearing capacity. Due to the poor permeability of clay, the uplifting process of the mat foundation needs to overcome a large soil suction force. In this study, three-dimensional numerical model is established to investigate the suction force of jack-up mat foundation on marine clay seabed. The elastoplastic constitutive model, which can describe the structural and over-consolidation of soft marine clay seabed, was embedded in commercial software ABAQUS through UMAT subroutine. The cohesive contact model was adopted to stimulate the interaction between mat foundation and the seabed soil. Field test was conducted to verify the accuracy of the numerical model. Then, the effects of degree of over-consolidation, degree of structural, depth of clay, interface stiffness and eccentric uplift loading on suction force are studied in detail. Numerical results show that the effects of degree of over-consolidation and structural of soil have significant influence on the suction force. The suction force first increases rapidly and then decrease gradually as the thickness of the upper dimensionless clay layer increases. With the increase of the uplift eccentricity, the suction force reduces continuously.

Lateral and Uplift Capacity of Pipeline Buried in Seabed of Homogeneous Clay

Buried offshore pipes are prone to upheaval and lateral buckling because of the high temperature and pressure of the materials being transported. An attempt has been made in the present study to evaluate uplift and lateral capacity of buried offshore pipes in a soft clay seabed using finite-element software. The numerical model used in the present study has been validated against results available in the literature. Finite-element analyses are carried out to study the upheaval and lateral buckling behavior of pipes for both the no-tension (NT) condition, i.e., when separation occurs between the pipe and soil, and the full-tension (FT) condition, i.e., when the soil beneath the pipe remains attached to the pipe. The variation of both uplift and lateral normalized capacity factors with soil cover depth ratio is presented for different combinations of soil strength and effective weight. Both uplift and lateral capacity factors have been found to increase and reach a constant peak value with increasing depth of soil cover. A transition from global and local failure mechanisms of a pipe is observed when the capacity factors attain their peak values at a limiting soil cover depth. The critical cases for the design of buried offshore pipe in homogeneous clay are established for both NT and FT conditions.

Centrifuge study of the mechanisms of rubble mounds sinking into soft clay seabeds in the construction stage

The construction of a rubble mound breakwater over a soft clayey seabed is a challenging issue in marine engineering. Excessive settlement and the sinking of such structures into the seabed sludge impose high costs on a project and can cause operational problems. The present study evaluated the mechanisms contributing to the large deformation that can occur during the construction of a breakwater using the results of four centrifuge tests. The result showed that the penetration of individual pieces of rubble and the flow of soft clay into the rubble pores were the leading mechanisms contributing to material loss during the construction of the breakwater. According to the conducted tests, the amount of material loss contributing to the penetration of individual rock particles for Cu = 3.2, 4.1, 11.2 kPa, equal 10.3, 7.6 and 2% of the initial breakwater section respectively. Also, the amount of material loss caused by the flow of sludge into rock pores for the three mentioned undrained shear strengths are equal to 15.1, 10.4 and 5.9% of the initial section respectively. So current study indicates that failing to account for these two mechanisms can lead to considerable underestimation of the required material for construction.

Dimensionless Groups of Parameters Governing the Ice-Seabed Interaction Process

AbstractPrediction of subgouge soil deformation during an ice gouging event is a challenging design factor in Arctic subsea pipelines. An accurate assessment of ice keel–seabed interaction requires expensive model testing and large deformation finite element analysis. Proposing reliable analytical/empirical solutions needs a deep understanding of the key parameters governing the problem. In this study, dimensional analysis of subgouge soil deformations was conducted and eight dimensionless groups of parameters were identified to facilitate proposing potential new solutions. A comprehensive dataset was established for horizontal and vertical subgouge deformations in both sand and clay seabed. Using the identified dimensionless groups, linear regression (LR) models were developed to estimate the horizontal and vertical deformation. Moreover, a sensitivity analysis (SA), as well as an uncertainty analysis (UA), was carried out to identify the superior LR models and the most influential parameter group. A high range of correlation coefficient (R), Nash-Sutcliffe efficiency coefficient (NSC), and variance accounted for (VAF) along with a low range of errors was achieved for the best LR model. The results of the superior LR models were also compared with the existing empirical equations. The study showed that the shear strength parameters of the seabed soil and the ratio of gouge depth to gouge width are the governing dimensionless parameters to model the horizontal and vertical subgouge soil deformations.

Model Tests and Parametric Analysis on Pore Pressure Response in Silty Clay Seabed under Vertical Caisson Breakwater

Abstract Model tests aimed at investigating the wave-induced pore pressure responses of a silty clay seabed around a vertical caisson breakwater were conducted with a large-scale bespoke wave-breakwater-seabed system. The effects of water depth, wave parameters, and seabed consolidation conditions on oscillatory pore pressure were investigated. The residual pore pressure development against the number of cycles was also examined. The test results indicated that the oscillatory pore pressure was generally greater in the upper part of the seabed around the caisson at a large water depth and peaked at the front of the caisson at a small water depth. The oscillatory pore pressure increased with wave height and generally remained stable with an increasing number of cycles. In addition, the oscillatory pore pressure showed varying degrees of declination in the silty clay underneath the rubble after a pause period. The residual pore pressure observed in most areas of the seabed was concentrated in the silty clay underneath the caisson and increased with the number of cycles. These experimental observations will contribute to a better understanding of pore pressure responses in silty clay seabeds under vertical caissons and may have practical implications.

Effect of Tension on the Combined Loading Failure Envelope of a Pipeline on Soft Clay Seabed

Offshore pipelines are commonly laid directly on the seabed, which is dominantly soft clayey deposits in deep-water areas. This article presents a numerical finite-element study of the undrained bearing capacity of a pipeline partially embedded in clay soil and subjected to combined vertical (V) and horizontal (H) loading. The effect of tensile capacity between the pipe and soil is considered. Expressions for bearing- capacity envelopes explicitly written in the V-H space and covering a practical range of embedment ratios and uplift tensile capacities are presented. This allows offshore engineers to estimate pipeline behavior under combined loading with consideration of the tension between pipe and soil.

Tests on wave-induced dynamic response and instability of silty clay seabeds around a semi-circular breakwater

Model tests aimed at investigating the wave-induced dynamic response and instability of silty clay seabeds around a semi-circular breakwater were conducted in a large wave flume. The pattern of wave forces was measured and analysed. The seabed responses were presented in terms of pore pressure and vertical stress, which were further characterized in terms of their oscillatory and residual values. The wave-induced settlement and instability of the semi-circular breakwater were studied. Test results reveal that the total horizontal wave force was generally greater than the vertical wave force, and a phase difference existed in the total horizontal and vertical wave force patterns. The trends in the oscillatory pore pressure and vertical stress were very similar: they generally developed rapidly in the seabed on the wave side, tended to be larger in the upper silty clay and smaller in the bottom area and increased with increasing wave height. Furthermore, the oscillatory responses mostly reached stable states but were observed to vibrate with cyclic time in the destroyed silty clay. The residual pore pressure and vertical stress mainly developed in the seabed around the breakwater and could be quite different depending on the location. With the exception of the decrease-increase tendency in some seabed areas, the residual pore pressure generally increased with cyclic time beneath the breakwater. The residual vertical stress generally increased with cyclic time beneath the breakwater, except in some regions beneath the toes of the internal rubble bed. The semi-circular breakwater settled substantially into the silty clay seabed, and the settlement was greater on the wave side. This seabed shear failure is attributed to the strength degradation of the silty clay induced by the development of excess pore pressure and the destruction of the soil fabric.

Effect of Load Eccentricity on the Bearing Capacity of Strip Footings on Non-homogenous Clay Overlying Bedrock

In underwater infrastructure design, foundations are embedded in soft clay seabeds that exhibited non-homogeneity of cohesion, and subjected to combined actions of vertical, inclined or eccentric loads due to the environmental conditions such as self-weight, wind and wave forces acting on substructure. This paper investigates the undrained bearing capacity for surface and embedded strip footings on non-homogenous clay overlying bedrock. Elasto-plastic finite element analyses are carried out for perfectly rough strip footings on Tresca soil under vertical eccentric loads. Meyerhof’s effective width rule is examined for the cases of surface and embedded footings in non-homogeneous clay. The effect of load eccentricity on the distribution of the normal stresses under strip footings is also investigated in this study. It is found that the effective width rule provides a conservative estimate of the bearing capacity. The elasto-plastic calculations predict that tensile stresses occur for surface footings because of the full bonding at the soil-footing interfaces, while for embedded footings, there is no tensile stresses.

Advanced procedure for estimation of pipeline embedment on soft clay seabed

In the present study, the advanced procedure has been proposed to estimate higher accuracy of embedment of pipes that are installed on soft clay seabed. Numerical simulation by OrcaFlex simulation code was performed to investigate dynamic seabed embedment, and two steps, i.e., static and dynamic analysis, were adopted. In total, four empirical curves were developed to estimate the seabed embedment including dynamic phenomena, i.e., behaviour of vessel, environmental condition, and behaviour of nonlinear soil. The obtained results were compared with existing methods (named general method) such as design code or guideline to examine the difference of seabed embedment for existing and advance methods. Once this process was carried out for each case, a diagram for estimating seabed embedment was established. The applicability of the proposed method was verified through applied examples with field survey data. This method will be very useful in predicting seabed embedment on soft clay, and the structural behaviours of installed subsea pipelines can be changed by the obtained seabed embedment in association with on-bottom stability, free span, and many others.

Fatigue performance of deepwater steel catenary riser considering nonlinear soil

The touch down zone (TDZ) and top connection point of the vessel are most critical part of fatigue damage in the steel catenary riser (SCR). In general, the linear soil model has been used to evaluate fatigue performance of SCRs because it gives conservative results in the TDZ. However, the conservative linear soil model shows the limitation to accommodate real behavior in the TDZ as water depth is increased. Therefore, the riser behavior on soft clay seabed is investigated using a nonlinear soil model through time domain approach in this study. The numerical analysis considering various important parameters of the nonlinear soil model such as shear strength at mudline, shear strength gradient and suction resistance force is conducted to check the adoptability and applicability of nonlinear soil model for SCR design.

Penetration of Steel Catenary Riser in Soft Clay Seabed: Finite-Element and Finite-Volume Methods

AbstractThe penetration of steel catenary risers and other cylindrical objects, such as offshore pipelines or T-bar penetrometers, in a soft clay seabed is of practical importance in deepwater oil and gas development. Finite-element (FE) analyses of these large-deformation problems are computationally very expensive. Water can also play a significant role through development of suction behind the riser. The main objective of the present study is to develop an advanced numerical modeling technique to simulate riser–seabed–water interaction near the touchdown zone. Keeping in mind two critical issues, namely the computational cost and modeling of suction, two different numerical modeling techniques are developed. In the first one, the computational fluid dynamics (CFD) approach is used. The CFD modeling is performed using ANSYS CFX 13.0 software. Among the three different types of CFX models developed in the present study, the subdomain modeling technique is found to be the most efficient. In the second num...

Set-up and Pullout Mechanism of Suction Caisson in a Soft Clay Seabed

Based on a specially designed experimental system, a series of model tests have been performed in order to investigate the set-up and pullout mechanism of a suction caisson in soft clay. Test results indicate that the residual “active” suction after caisson installation is beneficial to its pullout capacity. Different set-up times have little influence on caisson capacity as long as the negative pore pressures around the caisson fully dissipate. When the model caisson is pulled vertically, the external side friction first reverses, and then the inner side friction is gradually mobilized at a small displacement; afterwards, the reverse end bearing resistance near the caisson tip starts to work; finally, the “passive” suction under the top cap develops and helps to keep the interior soil plug moving together with the up-lifting caisson. In general, the pullout resistance would reach its peak within a displacement less than 10% of embedded caisson length.

The influence of sea state on as-laid pipeline embedment: A case study

The as-laid embedment of an offshore pipeline is an important parameter for design as it affects lateral and axial stability, exposure to environmental loads, and thermal insulation. For soft clay seabeds, the as-laid embedment can be significantly higher than predicted using methods based on the static penetration resistance due to dynamic lay effects. There are very few published field surveys showing the as-laid embedment of a pipeline following installation. Back-analysis of field survey data has the potential to improve existing design methods, as well as gain insights into the effects of the installation conditions, including the vessel response, the lay angle and tension, weather and sea state, and downtime events. This study describes the as-laid field survey carried out for a 12-inch flowline installed on a soft clay seabed in the North Sea. The dynamic lay effects are examined by studying the influence of sea state, manifested through the vessel response which leads to pipeline motions at the seabed. The findings show that a clear correlation exists between sea state and pipeline embedment. The ratio between the mean observed embedment and the embedment that would be predicted using a conventional static penetration analysis (termed the dynamic embedment factor, F dyn ) ranged from 2.5 up to 4 for normal pipelay conditions, increasing with wave height. Downtime events, during which pipelay is temporarily suspended, significantly increase the embedment. In this study they led to maximum embedment values greater than one diameter, corresponding to F dyn of up to 10, due to the larger numbers of cyclic pipeline motions at the seabed.

Review of research on interactions between deepwater steel catenary risers and soft clay seabeds

Steel catenary risers (SCR) have become an enabling technology for deepwater environments. A comprehensive review was conducted on recent research that examined interactions between deepwater steel catenary risers and soft clay seabeds. This included the STRIDE (steel risers in deepwater environments) and CARISIMA (catenary riser soil interaction model for global riser analysis) joint jndustry jrogram’s test data as well as information from existing papers.

Sliding resistance of grounded ice on clay in a laboratory: Potential implications for offshore design

Abstract Grounded ice pads as a base for exploration drilling platforms are built by artificially thickening the ice cover, made from either level ice or ice rubble, at the desired drilling location. An experimental set-up was assembled with the purpose of addressing outstanding issues regarding the sliding resistance of these structures. These simulations were done on a clay seabed. The ice was contained in a bottomless rectangular box, called the 'wagon'. The bottom of the ice was in contact with the clay, with a 4 m 2 footprint, and normal stress levels up to 23 kPa. The wagon was pulled by an actuator over distances of up to 100 mm per test while monitoring the horizontal forces required to do so. A total of 29 successful tests were conducted with level ice and another 32 with ice rubble. In the case of level ice, an increase in normal stress only resulted in a small increase in shear resistance, when enough time was allowed for the clay to consolidate. With ice rubble, an increase in normal stress systematically led to a significant increase in shear resistance, presumably because of shorter drainage paths for pore water evacuation, suggesting this material should be more suitable than level ice for this purpose. Vertical motion upon increasing and decreasing the normal stress provided evidence for clay consolidation and swelling, respectively. A peak response in shear resistance is attributed to dilatation. The clay's undrained shear strength, measured with a vane, was consistently higher than the sliding resistance of the ice in all tests. The vane data could therefore not be relied upon to provide a measure of sliding resistance. Also, this parameter progressively increased to levels well exceeding the maximum normal stress applied to the clay. An alternative approach, based on the soil's effective internal friction response, is assessed and proposed for estimating sliding resistance of real ice pads and other structures resting on a clay-rich seabed.