Suction Caissons Research Articles

Assessment of cyclic response to suction caisson in clay using a three-dimensional displacement approach

ABSTRACTAn investigation was made to present analytical solutions of cyclic response to suction caisson subjected to inclined cyclic loadings in clay using a three-dimensional displacement approach. A model representing the relationship between vertical load and vertical displacement and that between lateral load and lateral displacement along the skirt of suction caisson subjected to cyclic loadings is proposed for overconsolidated clay. For the effect of vertical load on cyclic load capacity of suction caisson, using the Mindlin solution in the case of a vertical point load, the vertical stress of soil under the base of suction caisson is presented. For the stress state of soil beneath the base of suction caisson subjected to cyclic loading, the Mohr–Coulomb failure line and critical state line are presented and the relationship between total stress, effective mean principal stress, stress difference, and pore-pressure is elucidated. The comparison of results predicted by the present method for a suction caisson subjected to cyclic loadings in clay has shown good agreement with those obtained from field tests. Cyclic behavior of clay up to failure is made clear from the relationship between cyclic tensile load, vertical and lateral displacements, and rotation and that between depth, vertical, and lateral pressures.

Estimating uplift capacity of suction caissons in soft clay: A hybrid computational approach based on model tree and GP

Stability of suction caissons used as foundations or anchors of offshore structures is a critical challenge in marine structures engineering. To this end, many studies have been conducted including those concentrate on implementing computational intelligence methods to model the response of suction caissons under loading. In this regard, this paper aims at formulating uplift capacity of suction caissons using a hybrid artificial intelligence computational tool based on model tree (M5) and genetic programming (GP), called M5-GP. The formulae are developed in terms of several governing parameters using a reliable experimental database from the literature. The results show that the M5-GP based relationships are able to predict the uplift capacity of suction caissons precisely. Furthermore, to consider the safety in the design process, probabilistic equations are also given for various risk levels. The new formulas compare favorably with the existing relationships in the literature regarding prediction performance. In addition, the simplified formulation is compact, easy to use and physically sound. Therefore, it is especially appropriate to be used in design practice.

Experimental Studies on Modified Suction Caissons in Fine Sand Subject to Uplift Loading

A modified suction caisson (MSC), which was reported by the authors of this paper previously, comprises an external short-skirted structure that is added to a regular suction caisson (RSC). It has been proved that MSCs can improve the lateral bearing capacity and limit the deflection of the caisson compared with RSCs. A series of model tests were conducted to investigate responses of MSCs subject to uplift loading in saturated sand. The effects of external skirt dimensions on the uplift bearing capacity of MSCs were considered. In addition, the influences of the sealed top lid of the skirted structure on the uplift bearing capacity and the resulting passive suction of MSCs were also studied. It was found that the uplift bearing capacities of MSCs are 1.4–1.7 times that of RSCs. Moreover, test results in serviceable conditions show that the sealed external skirted structure of perspex-made suction caissons significantly contributed to the uplift bearing capacity as a result of passive suction.

Insight into the lateral response of offshore shallow foundations

Suction caissons are often subjected to cyclic lateral loads caused by the action of wind or waves. As a result of these cyclic loads, excessive lateral deformations may be induced during a caisson’s service life. Although soil-foundation response analysis requires information about stiffness and damping at the site, the methods for obtaining this information are still in question. The present study reports non-dimensional frameworks for determining the impedance functions for soil-foundation system at the load reference point along with the results of performance measure parameters that track the response of suction caissons harmonically oscillating on homogeneous soil. The equations are expressed in terms of logarithmic and polynomial models based on a statistical analysis of existing cyclic test results for small-scale Aalborg University Instrumented Suction Caissons, 300mm in diameter and 300mm in skirt length.The variables used in the equations for normalized secant stiffness are load characteristics (ξb and ξc) and number of load cycles. The present study also identifies critical new findings regarding the local densification and the plastic shake-down at the caisson-soil interface, which are all important factors to consider in a performance based-framework for designing offshore structures.

PREDICTION OF UPLIFT CAPACITY OF SUCTION CAISSON IN CLAY USING FUNCTIONAL NETWORK AND MULTIVARIATE ADAPTIVE REGRESSION SPLINE

Suction caissons are extensively used as anchors for offshore foundation structures. The uplift capacity of suction caisson is an important factor from effective design point of view. In this paper, two recently developed AI techniques, functional network (FN) and multivariate adaptive regression spline (MARS), have been used to predict the uplift capacity of suction caisson in clay. The performances of the developed models are compared with other AI techniques; artificial neural network, support vector machine, relevance vector machine, genetic programming, extreme learning machine and group method of data handling with harmony search (GMDH-HS). The model inputs include the aspect ratio of the caisson, undrained shear strength of soil at the depth of the caisson tip, relative depth of the lug at which the caisson force is applied, load inclination angle and load rate parameter. Comparative analyses are made with the results of the above AI techniques, using different statistical performances criteria; correlation coefficient (R), root mean square error, Nash-Sutcliffe coefficient of efficiency, log-normal distribution of ratio of predicted to observed load capacity, with a ranking system to find out the best predictive model. The FN and MARS models are found to be comparably efficient and they outperform other AI techniques.

Investigation of the load-bearing capacity of suction caissons used for offshore wind turbines

This paper presents the results of three-dimensional finite element analyses of the suction bucket foundation used for offshore wind turbines. The behavior of the bucket and the response of soil supporting the bucket in dense and medium dense sandy soils subjected to static horizontal load are investigated. Field tests results and a centrifuge model test are used to validate the numerical model. Dimensionless horizontal load-displacement and overturning moment-rotation relationships are derived utilizing the Power law and Buckingham’s theorem. The results show good agreement between the numerical analysis results and the straight lines obtained from the Power law until a specific value of horizontal load and overturning moment. Regarding stress behavior of soil supporting the bucket, due to soil densification and bucket movement, maximum stresses are seen near the bucket tip at the right inside of the bucket. The major part of the applied load is transferred by the bucket skirt. Numerical analysis modeling results show that the bucket rotation and displacement are highly dependent on the bucket geometry and soil properties in addition to loading conditions. Normalized equations and figures for the ultimate horizontal load and overturning-moment capacities are presented and can be used for the preliminary design of the bucket foundations in sandy soils.

Using the Elastic Vertical Vibration of a Rigid Caisson at Low Frequencies to Stabilize the Foundation of Coastal Engineering Structures

ABSTRACT He, R.; Zhang, J., and Chen, W., 2017. Using the elastic vertical vibration of a rigid caisson at low frequencies to stabilize the foundation of coastal engineering structures. A suction caisson or a bucket foundation offers a suitable and economic foundation type for many coastal structures (such as breakwaters and wind turbines). Because of the relative stiffness of a caisson foundation compared with the surrounding soil is high, the caisson is treated as a rigid cylindrical shell in this article. Seawater and seabed are treated as a coupled seawater–seabed half-space. In that way, the elastic, vertical vibration of a caisson can be modeled as the vertical vibration of a rigid shell embedded in a fully saturated, porous seabed. The solution to the problem is obtained with the ring-load Green's function and proper boundary conditions at the caisson–soil interface. With integral equations, the governing equations are transformed to Fredholm equations and are solved numerically. Numerical results ...

Analysis of pullout load capacity of suction caissons in clay by a three-dimensional displacement approach

ABSTRACTA study was made to present analytical solutions of pullout load capacity for a suction caisson subjected to inclined tension in clay. The inclined tension on the skirt of the suction caisson is transformed into an equivalent system comprised of the vertical, horizontal, and moment load applied on the center of the lid. The vertical and horizontal stiffness coefficients along the skirt of the suction caisson in clay are presented by three-dimensional elastic solutions considering the nonhomogeneous and nonlinear property of clay. The vertical, horizontal, and rocking stiffness coefficient of the suction caisson on the base are presented considering the solutions of a hollow rigid cylindrical punch acting on the surface of clay. The envelopes of the horizontal and vertical ultimate load capacity for clay are presented. The yield, pullout, and failure for clay are taken into consideration. The effects of load inclination, loading depth, and aspect ratio on the pullout load capacity are shown. Behavior of the suction caisson in clay up to failure is investigated using the relationship between tensile load and displacement and that between depth, vertical, and horizontal pressure.

Serviceability of suction caisson founded offshore structures

Suction caissons have recently been considered as a cost-effective alternative to conventional foundations for offshore met masts and wind turbines. Such foundation arrangements are suitable for applications within water up to 20 m deep. Most offshore structures have stringent serviceability limits imposed on their design dictating the allowable structural deflections and accumulated rotations throughout its operational life. This paper summarises the findings from a series of scale model tests and identifies key factors which influence the serviceability performance of an offshore structure founded on a single caisson. Tests were conducted using representative caisson models in loose sand under single-g conditions, replicating a fully drained prototype state. These experiments recorded the rotational foundation stiffness (soil–structure interaction), the evolution of foundation stiffness under cyclic loading and the accumulation of structural rotation with loading cycles. It was discovered that the caisson stiffness was dependent on the local soil strain, and under cyclic loading would increase in a logarithmic manner. Further it was found that a caisson system will retain and accumulate structural rotation under cyclic loading, following a power relationship. From these observations it was possible to produce an analytical model describing the changing serviceability state of a prototype structure with loading cycles.

Dynamic lateral response of suction caissons

Deeper water installations of offshore wind turbines may be supported by jacket structures. This study investigates the dynamic response of suction caissons for jackets by analysing 3D finite element models in the frequency domain. The numerical modelling was firstly validated by analytical solutions for pile foundations. Groups of crucial dimensionless parameters related to the soil profile and the foundation geometry are identified and their effects on the response of suction caissons are studied. Static stiffness coefficients are presented in a form of mathematical formulas obtained by fitting the numerical results, pertaining foundations with different slenderness ratios and embedded in different soil profiles.Sensitivity of the dynamic impedances of suction caissons on the skirt length was showed in this study. Moreover, the results for the suction caissons indicated that the overall dynamic response is profoundly affected by the relative thickness of the soil layer and by the variation of soil stiffness with depth.

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.

Failure mode and capacity of suction caisson under inclined short-term static and one-way cyclic loadings

ABSTRACTIn addition to the loading angle at the pad eye, the types of applied forces will affect the failure mode and the capacity of suction caisson in a soft clay seabed. In this study, a specially designed platform was used to perform model tests with different loading angles and cyclic parameters. First, the short-term static capacities and the failure modes for the typical inclined loading angles were studied, and then the influences of cyclic amplitude, number of cycles and cyclic period on the subsequent failure mode, and caisson capacity were investigated by applying a series of one-way cyclic loadings. The experimental results showed that when the loading angle decreases from 50.2° to 32.0°, the model caisson’s failure mode changes to a combined mode consisting vertical motion, horizontal motion, and anticlockwise rotation, and its corresponding capacity noticeably increases. However, the increases in amplitude, number, and frequency of the previous cyclic loadings would turn this combined failure into a dominant vertical pullout and reduce its capacity to some degree. Therefore, the influence of the previous cyclic loadings should be taken into account to accurately estimate the holding capacity of the suction caisson.

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.

Fuzzy modeling of holding capacity of offshore suction caisson anchors

SummaryA fuzzy algorithm, the Takagi–Sugeno model, is implemented to develop a fuzzy inference system for predicting the holding capacity of suction caisson foundations for offshore platforms. The premise parameters of the fuzzy model are optimized by using a subtractive clustering algorithm. The consequent parameters are optimally determined via a weighted least square estimation. The input variables used for training the fuzzy model include the aspect ratio of the caisson, the undrained shear strength of the clay, and the angle that the chain force forms with the horizontal. The output of the proposed fuzzy model is the capacity of the suction caisson anchor. To demonstrate the effectiveness of the fuzzy modeling framework, the results of extensive finite element analyses are investigated. Comparisons of the trained fuzzy model with the data demonstrate that the proposed modeling framework is an effective method to estimate the holding capacity of offshore suction caisson systems. Moreover, the performance of the fuzzy model is robust against higher levels of input data uncertainties. Copyright © 2017 John Wiley & Sons, Ltd.

The Pull-out Capacity of Suction Caissons in Model Investigations

Abstract A small-scale model experiment on the pull-out resistance of suction caissons is described. The pull-out force and suction developed within the caisson in the extraction process were recorded during the experiment. A simple breakout model, together with an elementary static formulae, is applied to predict the results obtained experimentally. There is a reasonably good agreement between the experimental results and predictions. An extensive discussion of the approach applied is included. The analysis presented in this paper is original, as it differs from other approaches mentioned in this paper, and leads to acceptable predictions. At the end, the results are also compared with another approach for predicting the capacity of suction caissons.

H-M bearing capacity of a modified suction caisson determined by using load-/displacement-controlled methods

This paper presents a series of monotonically combined lateral loading tests to investigate the bearing capacity of the MSCs (modified suction caissons) in the saturated marine fine sand. The lateral loads were applied under load- and displacement-controlled methods at the loading eccentricity ratios of 1.5, 2.0 and 2.5. Results show that, in the displacement-controlled test, the deflection-softening behavior of load-deflection curves for MSCs was observed, and the softening degree of the load-deflection response increased with the increasing external skirt length or the decreasing loading eccentricity. It was also found that the rotation center of the MSC at failure determined by the load-controlled method is slightly lower than that by the displacement-controlled method. The calculated MSC capacity based on the rotation center position in serviceability limit state is relatively conservative, compared with the calculated capacity based on the rotation center position in the ultimate limit state. In the limit state, the passive earth pressures opposite the loading direction under load- and displacement-controlled methods decrease by 46% and 74% corresponding to peak values, respectively; however, the passive earth pressures in the loading direction at failure only decrease by approximately 3% and 7%, compared with their peak values.

Interactions in offshore foundation design

This paper presents some examples of design problems for offshore foundations, drawn from the jack-up industry and the wind turbine industry. The examples are chosen to illustrate some general points about foundation design, geotechnical engineering and its interaction with other disciplines. The first example is drawn from the assessment of the safety of installation of jack-up units (large mobile offshore drilling rigs). It illustrates how more rational approaches can be achieved through a deeper use of probabilistic methods in both the prediction of performance and the assessment of field observations. The second example also comes from jack-up practice, but has wider application too: it addresses the classical problem of the performance of foundations under combined loading, and how this can be understood in a simple theoretical and practical framework based on plasticity theory. The final example comes from the renewables sector, where the rapidly expanding offshore wind industry poses new foundation challenges for geotechnical engineers. Practical and economic foundation solutions are required if the UK is to meet its ambitious plans to exploit larger turbines in deeper waters. Both conventional (monopile) and novel solutions (suction caissons, screw piles) to the foundation problem are discussed. The paper also demonstrates how interactions with other disciplines can enrich geotechnical engineering, illustrated by specific practical examples from the author's experience.

Embedment Effects on Vertical Bearing Capacity of Offshore Bucket Foundations on Cohesionless Soil

AbstractThis paper presents the results from a series of physical modeling and three-dimensional finite-element (FE) analyses in which the authors examined the uniaxial vertical capacity of suction caissons for offshore wind turbines. The experiments were carried out in quartz sand and involved monotonic application of vertical load. It was found that the drained capacity of suction caissons is dependent on embedment ratio. In contrast, predictions from conventional semiempirical depth factors were found to somewhat underestimate when applied to rough foundations. On the basis of the tests and FE analyses, new expressions for the depth factor of shallow foundations were validated for embedment ratios (aspect ratios) up to unity, calibrating the fitting parameters by using data from a range of soil profiles.

Large-scale experimental investigation of the installation of suction caissons in silt sand

Offshore wind power is a rapidly growing area of electricity in China. In the present paper, interaction mechanisms between the caisson for wind turbines and saturated silt sand are investigated with laboratory tests based on two different installation methods, jacking installation and suction installation. For the jacking installation process, the results indicate that the soil pressures inner and outer the skirt of the caisson vary with a similar feature and the magnitudes of the two are nearly balanced. The tip resistance plays a key role in the total jacking installation resistance. This paper examines the predictive performance of qc method and API approach for jacking installation resistance. It is demonstrated that the qc method provides better predictions. The resistance coefficients are recommended. For the case of suction installation, however, the changes of soil pressures inner and outer the skirt are contrasting. Specifically, the inner pressure and tip resistance fall dramatically, but the outer pressure increases when suction is applied. Seepage effect is found to be an important mechanism for the installation of suction caisson. The reduction ratios of the inner friction and tip resistance follow a power-function with the normalized suction. Based on the test results, a prediction method for the required suction has been developed and evaluated.

Undrained pullout capacity of cylindrical suction caissons by finite element limit analysis

Finite element limit analysis with axisymmetric condition is employed to determine plasticity solutions of undrained pullout capacity of cylindrical suction caissons in clay with linear increase of strength with depth and zero strength at the seabed. A closed-form approximation of numerical solutions of a suction caisson is proposed from nonlinear regression analysis. The proposed equation is used to check the accuracy and validity of the total pullout force and reverse end bearing force by conventional methods. The three-dimensional effect of suction caissons between axisymmetric and plane strain conditions is also deduced to evaluate the validity of conventionally assumed shape factor.