Suction Caissons In Sand Research Articles

Evaluating installation of suction caisson in sand via accounting for soil seepage-deformation coupled process

Suction-assisted installation is one of the special features that distinguishes the suction caisson from other offshore foundations. Suction control during installation can be challenging, as it involves highly coupled processes between seepage flow and seabed soil deformations, including excessive soil deformation and plug formation under strong seepage actions that lead to early termination of the installation, as well as a seepage flow-induced reduction in caisson penetration resistance. However, existing methods for evaluating suction caisson installation are primarily based on pure seepage analysis while disregarding the above-coupled processes. This paper proposes an approach to evaluate suction caisson installation in sand by explicitly accounting for seepage-deformation coupling. For this purpose, pore pressure-deformation coupled finite element analyses are performed to investigate soil failure mechanism and evolving caisson penetration resistance subjected to suction. These analyses result in novel rules for assessing critical suction upon the rapid growth of soil deformation and describing the degradation of caisson penetration resistance. The latter two propositions lead to a new calculation method for determining the required suction for caisson installation. The performance of the calculation method is evaluated against field tests and centrifuge tests to illustrate the usefulness of considering coupled soil deformation-seepage process in designing caisson installation.

Model tests on penetration and seepage behaviors of a scaled suction caisson in sand

This study presents a new type of the offshore mooring foundation named as a scaled suction caisson (SSC), of which concept is inspired from the kinetic function of the scaled skin of snakes. When the SSC penetrates into the soil, the penetration resistance effectively decreases due to decreasing the friction force between the soil and sidewall. However, the bearing capacity increases due to increasing the friction force between the soil and sidewall when the SSC is subjected to pull load. A series of the suction-assisted installation model tests are carried out to investigate the penetration and seepage behaviors of the scaled suction caisson in sand. The result indicates that penetration resistance of the SSC increases with decreasing the aspect ratio H1/D1 of the scale structure. For example, when the aspect ratio H1/D1 equals 10, the penetration resistance decreases by 3.1% compared with the traditional suction caisson. And as the aspect ratio H1/D1 decreases from 10 to 5, the penetration resistance increases by 9.9%. In spite of these cases, it shows that reasonable selection of H1/D1 is helpful or not an obstacle to penetration. Furthermore, it is found that the height of the soil plug effectively decreases by the intermittent suction-assisted installation. The formula of calculating the penetration resistance of the SSC during suction-assisted installation is presented in terms of the excess pore water pressure distribution inside and outside the sidewall. The calculation results agree well with the model test results, indicating that the proposed formula of calculating the penetration resistance is reliable.

Deep learning-based design model for suction caissons on clay

Predicting the non-linear loading response is the key to the design of suction caissons. This paper presents a systematic study to explore the applicability of deep learning techniques in foundation design. Firstly, a series of three-dimensional finite element simulations was performed, covering a wide range of embedment ratios and different loading directions, to provide training data for the deep neural network (DNN) model. Then, hyper-parameter tuning was performed and it is found that the basic Fully-Connected (FC) neural network model is sufficient to capture the non-linear response of suction caissons with excellent accuracy and robustness. Furthermore, the optimized FC neural network model was also successfully applied to a database of suction caissons in sand, demonstrating its broad applicability. By comparing three typical DNNs, i.e., FC, Convolutional Neural Network (CNN) and Long Short-Term Memory (LSTM), it was observed that the FC neural network model excels over others in terms of simplicity, efficiency and accuracy. More importantly, by looking into the model’s generalization performance, the FC neural network model can also identify the change in foundation failure mechanisms. This study demonstrates the DNN’s powerful mapping ability and its potential for future use in offshore foundation design.

Theoretical studies on penetration resistance of a modified suction caisson in sand

The modified suction caisson (MSC) is an innovative offshore foundation, consisting of an internal caisson (IC) and an external skirt (ES). Determining the penetration resistances is a key point to predict the required suction for installing MSCs smoothly in sand. During suction-assisted installation of MSCs, the seepage will greatly change the effective streess in sand, and thus affect the penetration resistance along the caisson wall. Therefore, this study clarifies the distribution characteristics of the seepage field by exploring the excess porewater pressure and hydraulic gradient inside and outside the MSC with different aspect ratios in sand with SEEP/W. Meanwhile, the water fluxes are also investigated. It is found that the minimum absolute values of the excess porewater pressures and the hydraulic gradients outside the IC appear at the “0 seepage line”. The absolute values of water flux reaches the maximum at the tip of the IC and the ranges of the negative water fluxes outside will enlarge with increasing the aspect ratio. Based on the above results, the method of calculating the MSC penetration resistance and the formula of calculating the required suction for the MSC installation in sand is proposed, which is verified by the model test results.

Failure envelope considering the ultimate tensile capacity of suction caissons in sand

Little analytical work has been done to elucidate the ultimate capacity of suction caissons under vertical tensile (V), lateral (H), and moment (M) loads in soils. In this paper, in order to reveal the effect of vertical tensile, lateral, and moment loads on the ultimate capacity of suction caissons in sand, an analytical investigation was made using a traditional bearing capacity theory. Taking account of the vertical equilibrium of an annular element of a skirt, through the vertical tractions inside and outside the skirt of a suction caisson when a vertical tensile load is applied, the vertical displacement of the soils adjacent to the skirt of the suction caisson was presented. The most appropriate bearing capacity equation for predicting the experimental results was shown for suction caissons having an embedment larger than a diameter in sand. For the deformation-load responses of suction caissons with various embedment ratios in sand, subjected to inclined tensile loads, there was a good agreement between the results obtained from laboratory tests and those predicted by the present method. The failure surfaces, considering the ultimate tensile capacity in the H-M, H-V, and M−V planes, and in the H-M−V space, for suction caissons in sand, were presented.

Calculation Method for Uplift Capacity of Suction Caisson in Sand Considering Different Drainage Conditions

Uplift capacity of suction caissons is one of the main concerns in the design of jackets with multi-caissons supported offshore wind turbine. The uplift movement of suction caissons leads to soil stress variation and increases the difficulty to predict the uplift capacity. In this paper, a calculation method considering soil stress release and differential pressure contribution is proposed to predict the uplift capacity of caisson. Firstly, a series of numerical simulations based on the SANISAND model are conducted to study the uplift responses of suction caisson in sand, and it is verified with centrifuge test results. Considering the soil drainage condition during caisson being pulled out, the fully drained, partially drained and undrained are divided, and an equation is provided to assess differential pressure beneath the caisson lid incorporating the effects of main factors. Based on the above simulation results, a calculation method is proposed to calculate the uplift capacity of caissons. The prediction results are compared with the centrifuge model tests and previous studies, which indicate that the prediction accuracy is much improved. This proposed method contributes to the more accurate assessment of uplift capacity of suction caisson in sand.

Accumulated displacement and unloading stiffness variations of modified suction caisson in sand under multidirectional lateral cyclic loading

The modified suction caisson (MSC) is an innovative foundation for offshore wind turbines. Model tests were conducted to study the bearing behavior of the regular suction caisson (RSC) and the MSC in sand under multidirectional lateral cyclic loads. Each test includes two stages: (a) the load was applied in the initial loading direction, and (b) the load was applied in the direction which rotate through a deflection angle from the initial loading direction. Results show that in the first stage, the translational motion direction of suction caisson in plane view coincides with the loading direction. In the second stage, the motion direction of the suction caisson doesn't coincide with the loading direction. This phenomenon becomes more obvious under low loading amplitude. When the deflection angle is less than 90°, total accumulated displacement increases with increasing the load cycles. The maximum normalized total displacements for the RSC (ζb = 0.833) and the MSC (ζb = 0.649, 0.833) were obtained when the deflection angles equal 45°, 45° and 30°. When the deflection angle value is higher than 90°, the total displacement decreases with increasing the deflection angle. Changing the loading direction has little effect on the unloading stiffness values for both the RSC and the MSC.

A full numerical model for the installation analysis of suction caissons in sand

Safe installation of suction caissons requires a reliable preliminary study where soil resistance and other soil conditions are predicted with good accuracy. Past studies did not address the full problem when modelling the installation procedure of suction caissons in sand. Suction was prescribed at the mudline and soil resistance was predicted based on soil properties and the resulting seepage profile around the caisson wall. Suction magnitude was assumed to overcome soil resistance, limiting its magnitude to avoid critical conditions such as piping. Most of these studies did not address other important parameters such as pumping rate and water depth.This paper addresses the following points: Feasibility of the installation depends on the water depth and hence, this must be specified as part of the installation design. Suction magnitude in excess to the initial hydrostatic pressure corresponds to a certain pumping rate, which is the parameter to control for a safe installation.A full numerical model is proposed in this paper to consider a prescribed pumping rate at the center of the caisson lid and the resulting flow inside the caisson cavity. The model was implemented in COMSOL Multiphysics and used to perform simulations of suction assisted installation in sand. Prescribed pumping rates have been investigated in conjunction with mobilized soil resistance, critical piping, water depth and available driving pressure.

Influence factors of extraction behavior of modified suction caisson in sand by reverse pumping water

The suction caisson for offshore engineering features fast installation and reuse. When offshore engineering structure supported by a suction caisson is decommissioned, the suction caisson can be extracted by reverse pumping water for reuse and being replaced by a larger one. Model tests were conducted to investigate effects of the installation methods, extraction methods and pumping rate on the extraction behavior of both the regular suction caisson (RSC) and modified suction caisson (MSC) in sand. It was found that when the seepage failure occurs in and around the caisson, the extraction by reverse pumping water is terminated. Under the same pumping rate, the final extraction height and the maximum overpressure beneath the caisson lid of the MSC are greater than those of the RSC. The final extraction heights of caissons installed by jacking are greater than those installed by suction. Model tests on the extraction of suction caissons by pumping air was also conducted and it was found that final extraction heights of the RSC and the MSC which were extracted by reverse pumping water are greater than those extracted by reverse pumping air. Further, extraction tests of Perspex-made suction caisson were conducted to investigate the difference between the extraction behavior of suction caissons by reverse pumping water and air into the caisson.

Bearing behavior and earth pressure distribution for modified suction caissons in sand under inclined loading

Model tests accompanying numerical analyses on the modified suction caissons (MSCs) in saturated marine sand subjected to inclined loading were conducted to investigate the bearing behavior and earth pressure distribution along the embedded depth. Influences of external structure sizes, inclined loading angles and loading point position on the inclined bearing behavior were discussed. Results show that the inclined bearing capacity significantly decreases with increasing the loading angle when the loading point locates at the outer wall of the internal compartment. It can be also found that the passive earth pressure can resist a considerable portion of the inclined load when the loading point is located at the internal compartment wall. The maximum inclined bearing capacity of MSC is obtained when the loading position locates at the internal compartment outer wall of 2 L 1/3-3L 1/4 below the caisson lid under relatively small loading angle. An equation was proposed to determine the optimum loading point position under different loading angles to obtain the maximum inclined bearing capacity.

Effect of the ordering of cyclic loading on the response of suction caissons in sand

Wind and wave action on offshore wind turbines causes irregular cyclic loading on the foundation that can lead to rotation accumulation of the structure. Such loading is generally considered in design using ‘counting methods’ that decompose the time history of irregular cyclic loads into a series of cyclic load parcels of uniform amplitude. These parcels are ordered in magnitude, with the rotation due to previous cyclic load parcels accounted for by including an equivalent number of cycles of the current cyclic load parcel. Hence, this superposition approach adopts Miner's rule, as the accumulated rotation is considered to be independent of the ordering of the cyclic load parcels. This paper examines the validity of this assumption for moment cyclic loading of suction caissons in sand, through a series of model tests involving both constant and varying amplitude cyclic loads, with each test involving at least one million cycles. In the varying amplitude cyclic tests, the accumulated rotation approximately doubled when the load ordering changed from ascending to descending. This is considered to be due to changes in grain contacts and beneficial densification effects from lower amplitude cyclic loads that are absent when the cyclic loads are arranged in descending order.

Centrifuge observations on multidirectional loading of a suction caisson in dense sand

This paper presents centrifuge tests undertaken to investigate the combined VH capacities of suction caisson in sand under multidirectional loadings following series of low magnitude horizontal cyclic loading. These loading paths are relevant to foundations of array of floating renewables such as wave energy converters or wind turbines. The centrifuge tests mainly provide information about the V–H yield envelopes in different loading directions and the Hx–Hy yield envelopes in the horizontal plane so that the effect of multidirectional loading can be analysed. The evolution of the yield envelope after cyclic loadings is first presented. Yield envelopes for load change angle β of 60°, 90°, 120° from the initial horizontal cyclic loading direction are then presented, and the mechanisms for increased capacity for β > 90° and reduced capacity for β < 90° are discussed.

Capacity of Caissons in Dense Sand under Combined Loading

AbstractThis paper presents the development of an improved plasticity model to define the combined vertical and horizontal capacity of suction caisson in sand under low vertical loads, conditions r...

The Seepage and Soil Plug Formation in Suction Caissons in Sand Using Visual Tests

The rapid development of offshore wind energy in China is becoming increasingly relevant for movement toward green development. This paper presents the results of visual tests of a suction caisson used as foundation for offshore wind turbines. The distribution of hydraulic gradients of sand at the mudline in the caisson was obtained to find out the relationship with the heights of soil plugs. The relationship equation was proposed and obtained by using quadratic regression, guiding project designs, and construction. It was found that there was no soil plug in the caisson when small suction was applied during the suction penetration. The relationship between the heights of the soil plugs and the hydraulic gradient of the soil was proposed and obtained by using quadratic regression to predict (roughly) the height of soil plugs in suction caissons in sand during suction penetration. The influence of settlement outside caissons on the soil plug was found to decrease as the buried depth rose.

Suction caisson foundations for offshore wind energy: cyclic response in sand and sand over clay

This technical note considers experimental data on the long-term response of a suction caisson in sand and sand over clay to lateral cyclic loading. Installation of the caisson under suction in a geotechnical centrifuge provides insight into the contribution of this installation process, as well as the effects that soil drainage and consolidation in the clay layer have on the accumulated caisson rotation and change in stiffness. The tests focused on sand over clay, and considered variations in the cyclic load magnitude and symmetry. One-way cyclic loading in sand over clay is seen to result in higher rotation than two-way loading, which contrasts with findings from previous studies in sand. Excess pore pressure dissipation in the clay layer leads to strength increases that stabilise caisson rotation and increase stiffness. The rate of accumulation in caisson rotation is observed to be the same from centrifuge and single gravity tests, while the initial rotation differs with stress level, drainage regime, loading magnitude, soil profile and installation method. The centrifuge tests are considered collectively with equivalent single gravity tests to form a basis for predicting the long-term response of a monopod suction caisson.

A standard formulation for the installation of suction caissons in sand

Suction assisted installation of caisson foundations in sand relies on the developed seepage around the caisson wall. Seepage is known to produce soil loosening inside the caisson cavity and an overall reduction in soil resistance to caisson penetration. On the other hand, suction must be controlled so that no excessive piping is induced within the sand volume trapped inside the caisson cavity. When it extends over the full embedded length of the caisson wall, piping may lead to the formation of piping channels, which may compromise the established seal between caisson and soil and ultimately cause the installation process to stop. A safe installation process requires a proper design procedure to ensure that a safe suction can be predicted prior to installation. The present paper provides a framework where analytical expressions are obtained for the required suction magnitude, and for the critical suction that causes piping to initiate at the caisson tip. These analytical expressions are derived for a normalized caisson geometry, based on compiled results obtained from finite element analysis of seepage around a caisson wall, at various installation depths. The developed analytical formulation applies independently of caisson dimensions such as diameter, height and wall thickness. Critical suction for piping condition is also obtained under analytical form as a function of normalized penetration depth. The developed formulation can also be easily incorporated into design procedures or used in design codes without a need for a preliminary seepage analysis to be undertaken. The proposed suction predictions for the whole process of caisson installation in sand are validated against field trials reported in the literature.

Drained Capacity of a Suction Caisson in Sand under Inclined Loading

AbstractSuction caissons have recently been considered as potential anchoring solutions for floating renewable devices (wind turbines and wave energy converters) in shallow waters, where—unlike for...

Determination of Maximum Penetration Depth of Suction Caissons in Sand

The suction caisson is a large top-closed cylindrical steel structure in diameter, short in length and much thinner in skirt wall thickness. The total resistance of the suction caisson during installation consists of the tip resistance and the skirt wall friction. However, since the thickness of the skirt wall is very small, the skirt wall friction may produce additional vertical stress and shear stress in soil at the skirt tip level, and this additional vertical stress and shear stress will contribute to the increase in the skirt tip resistance. At the same time, seepage induced by suction also causes the tip resistance to reduce significantly. A modified slip-line field is proposed in this study estimating the tip resistance in terms of the slip-line theory. The expression obtaining the minimum suction to install the suction caisson is also proposed in terms of the force equilibrium. In addition, the critical suction is determined based on the mechanism of sand piping. Thus, the maximum penetration depth of the suction caisson can be reached when the critical suction equals the minimum suction. Results from calculations of the minimum suction and the maximum penetration depth have been proved to be in a good agreement with the measured data.

Evaluation of pullout load capacity of suction caissons in sand using a three-dimensional displacement approach

ABSTRACTAn investigation was conducted to obtain analytical solutions for the pullout behavior of a suction caisson undergoing inclined loads in sand. The inclined load is transformed into an equivalent load system in which the vertical, horizontal, and moment loads are applied on the center of the lid of the suction caisson. The vertical and lateral stiffness coefficients along the skirt of the suction caisson in sands are presented using the new three-dimensional elastic solutions taking into account the nonhomogeneous and nonlinear properties of the sand. The vertical, lateral, and rocking stiffness coefficients on the base of the suction caisson are presented considering the solutions of a hollow rigid cylindrical punch acting on the surface of a soil. The yield, pullout, and failure for sands with the nonhomogeneous and nonlinear characteristics are taken into consideration. The effects of the load inclination, the loading depth, and the aspect ratio on the pullout load capacity of the suction caisson are presented. Behaviour of the suction caisson in sand prior to failure is clarified from the relationship between tensile load, displacement, and rotation and that between depth, vertical pressure, and lateral pressure.

Model tests on installation and extraction of suction caissons in dense sand

ABSTRACTA series of model tests were performed on steel- and Perspex-made suction caissons in saturated dense marine sand to explore installation and extraction behaviors. The extractions of the caisson were conducted by applying monotonic loading or by pumping water into the caisson. Responses of suction caissons to pullout rates, aspect ratios, and extraction manners were examined. Test results show that a cone-shaped subsidence region occurs around the suction caisson during the suction-assisted installation. The pullout bearing capacity of the suction caisson in sand is dominated by the loading rate and the loading manner. For the suction caisson subjected to monotonic loading, the maximum bearing capacity is reached at the pullout rate of about 20.0 mm/s. The mobilized vertical displacement corresponding to the pullout capacity increases with increasing the pullout rate. The passive suction beneath the suction caisson lid reaches the maximum value when the pullout bearing capacity is mobilized. In addition, during the suction caisson extracted by pumping water into the caisson, the maximum pore water pressure in the caisson is obtained under the displacement of approximately 0.04 times the caisson diameter. The absolute values of the maximum pore water pressures for the suction caissons approximately equal those of the maximum vertical resistances at the monotonic pullout rate of 5 mm/s. When the vertical displacements of the suction caissons with the aspect ratio of 1.0 and 2.0 reach 0.92 and 1.77 times the caisson diameter, respectively, the seepage failure occurs around the caissons. Using a scaling method, the test results can be used to predict the time length required for the prototype suction caisson to be extracted from the seabed.