Suction Caissons Research Articles

Centrifuge experiment and numerical simulation of suction caisson installation in sand

<p indent="0mm">Suction caissons are widely used foundations in offshore engineering. Compared with traditional forms of foundations, a significant advantage of the suction caisson is its cost-effectiveness owing to its simple installation equipment requirement, short installation time, convenience in transportation, and recycle and reuse potential. Changes in excess pore pressure and seepage field caused by penetration and suction have a significant impact on soil resistance around the caisson wall and tip. The centrifuge test and Arbitrary Lagrangian-Eulerian (ALE) large deformation solid-fluid coupled FEM are used in this study to investigate the changes in suction pressure and the seepage field during the suction caisson installation in the sand. The ALE solid-fluid coupled FEM successfully simulates suction caisson installation processes in sand observed in centrifuge tests. The difference in suction penetration resistance is found to be caused by seepage and excess pore pressure generated during suction caisson installation. Therefore, the solid-fluid effective stress analysis is needed in the evaluation of suction caisson penetration to consider the importance of seepage.

Upper-bound solutions for inclined capacity of suction caissons in a trenched seabed

Large seabed trenches have recently been found adjacent to suction caissons with taut and semi-taut moorings. Seabed trenching leads to loss of soil support and additional caisson rotation, and can therefore significantly decrease geotechnical capacity. In this paper, upper-bound solutions are presented that enable calculation of the reduction in ultimate load-carrying capacity of a suction caisson in a trenched seabed compared to optimal capacity in an intact seabed. Failure mechanisms for caissons embedded in an intact seabed are augmented to incorporate a kinematically admissible mechanism for a trench extending from the mudline. The rate of work dissipation is calculated to determine the optimised solution of the inclined capacity of suction caissons. Comparisons of the results of the upper-bound solutions with finite-element analyses show that the failure mechanisms and the calculated load-carrying capacities derived from the two methods agree well. Therefore, the analytical upper-bound method presented in this paper can be employed for efficient routine calculations of suction caisson capacity under inclined load considering seabed trenches, which are increasingly encountered in engineering practice.

Sidewall Shell Contribution to the Lateral Capacity of Offshore Suction Caissons in Clay

The response of suction caisson (bucket) foundations subjected to horizontal head displacement, or head rotation, is parametrically studied by means of three-dimensional finite-element analyses. The undrained clay stratum is either homogeneous or linearly inhomogeneous (Gibson soil). The effect of the nature of the interface between foundation and surrounding soil is explored with two different scenarios: an ideal case of a fully bonded contact (FBC), and a possibly more realistic tensionless and sliding interface of limited shear resistance (TSI). The latter scenario results in a significant reduction of failure loads. Closed-form expressions are developed for the maximum horizontal and moment capacity for a range of bucket aspect ratios in both soil types, and with both interface conditions. They are validated through comparisons with available results from the literature. The contribution of the suction caisson’s cylindrical sidewall shell to the overall response of the system is investigated. It is shown that for an embedment depth exceeding the caisson’s radius the shell provides practically the same lateral bearing capacity as the whole suction caisson, in contrast with the vertical (axial) loading of a caisson with sliding interface in which the role of the shell is much less significant.

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.

Real-time prediction of the mechanical behavior of suction caisson during installation process using GA-BP neural network

Due to the short construction period, convenient transportation and strong geological adaptability, the suction caisson has received more and more attention in offshore geotechnical foundation engineering associated with oil and gas production platforms. A good understanding of the mechanical behaviors of soils, such as real-time effective stress and settlement, around the suction caisson during its installation is important for analyzing the stability and loading capacity of the platforms in deep water. The objective of this research is to develop a numerical model for predicting the mechanical behavior of soils around the suction caisson during installation based on the data including real-time settlement and effective stress produced from the centrifuge tests in the literature by using the genetic algorithm-back propagation (GA-BP) neural network method. Furthermore, the nonlinear relationship among the settlement, penetration pressure, effective stress, and suction required during the installation is thoroughly investigated. It is found that, compared with the experimental results, the relative error of the prediction results from the proposed model is −0.3%∼1.4% for settlement, 0.8%∼2.3% for effective lateral stress in a single clay layer, and −2.5%∼4.5% for normalized required suction in multilayered soils at the final stage. It can be concluded that the GA-BP neural network method can be used to predict the real-time hydromechanical behavior of the caisson foundation during the installation process in terms of efficiency and accuracy.

Use of suction caissons for seawall construction

With rising sea level and demand for more nearshore infrastructures, the current methods for seawall construction may no longer be sustainable. Alternative seawall construction methods need to be developed. When seawall or other coastal defense systems are constructed on soft marine deposits, suction caissons are feasible to be installed into marine clay layer as foundations. Using suction caissons as part of the foundation for seawall might offer a more economical solution when the water depth is relatively deep. A conceptual design and related construction procedure for the seawall constructed on suction caisson foundation is proposed in this paper for a land reclamation project. The design issues for the seawall constructed on soft marine deposits are discussed. An analytical method for analyzing the stability of the seawall on suction caisson foundation is also proposed based on a limit equilibrium method. The proposed analytical method is verified using Plaxis 2D finite element analysis (FEA) which has been verified using model tests of suction caisson in soft clay beforehand. The results from the finite element simulation are consistent with those from the analytical method. The horizontal deflection of seawall is about 1/75 of the seawall height.

Seepage induced displacements of suction caissons: FE analysis

Finite element analyses of suction caissons are performed under seepage flow in elastic soil. Poroelastic water elements are used underneath the caisson lid allowing for modelling of suction and inducing seepage flow into the caisson compartment. A detailed parametric study is undertaken exploring the effects of caisson aspect ratio, soil-caisson interface roughness, caisson inner ring stiffeners, and potential gap formation between caisson outer wall and soil, and soil permeability on caisson upward displacement. The results indicate that caisson roughness and stiffeners affect caisson displacement behaviour. A rough caisson causes earlier dissipation of suction hence caisson upward displacements are minimal for a rough caisson compared to a smooth caisson. A steady state seepage condition is only achieved for a smooth caisson with no stiffeners. Presence of stiffeners and surface roughness leads to a halt in caisson upward displacement. Any potential gap along the outer skirt interface enhances the upward displacement for a smooth non-stiffened caisson, but not for a caisson with stiffeners or a rough caisson. For the smooth non-stiffened caissons, unified equations are proposed to estimate seepage length and caisson displacement for soils with different permeabilities.

Bearing capacity and soil deformation around modified suction caisson in clay under monotonic horizontal loading

Suction caissons for offshore wind turbine (OWT) are required to withstand larger horizontal load resulting from wind and wave. In order to increase the horizontal bearing capacity, the modified suction caisson (MSC) was proposed. Experimental studies were conducted to investigate the bearing behavior and soil deformation around MSC in clay under horizontal loading. Influence factors such as the undrained shear strength and installation methods were studied. Results show that the horizontal bearing capacity of the MSC increases by 20–40% compared with the regular suction caisson (RSC). The horizontal bearing capacity of the MSC increases with increasing the undrained shear strength. In addition, the horizontal bearing capacity of the MSC installed by jacked force is higher than that installed by applied suction. The rotation center was found to move downwards with the increase of the suction caisson deflection. In the limit state, upward deformation of clay and cracks within the deformed clay were observed in the loading direction. However, a gap between clay and the suction caisson edge opposite the loading direction was found. The deformation range around the MSC is larger than that around the RSC, indicating that the MSC can mobilize more soil to resist the horizontal load.

Numerical modelling on effect of loading rate on uplift behavior of suction caissons

This paper details the development of a hydro-mechanical (HM) coupling model in ABAQUS to mimic the seepage and separation on the interface between clayey seabed and suction caissons subjected to uplift loading. This was achieved through introducing triple-node pore-water cohesive interface elements to the caisson-soil interface, thus enabling normal and tangential flow on the interface to be modelled properly. The model was validated against centrifuge tests conducted at different loading rates as well as companion numerical results, and then used to examine the uplift behavior of suction caissons under different drainage conditions. The results showed that the gradual transition of a localized failure mechanism to a global one, with the increase in loading rate, can be successfully captured. Besides, the development of negative pore water pressure inside the caisson illustrated strong sensitivity to the loading rate, when the dimensionless velocity group vD/cv fell within the range that denoted a partially drained condition. The model also proves to be capable of reproducing the separation of the soil plug from the inner wall and the necking behavior. A detailed parametric study discussed the influence of caisson wall roughness, caisson aspect ratio and typical clay properties on the uplift behavior of suction caissons.

Failure surface considering the ultimate tensile and compressive capacities of suction caissons in clay

One of the important issues for suction caissons is to demonstrate the effect of the contact between the base and the soil on the ultimate capacities of vertical (V), lateral (H), and moment (M) loads. It has not yet been analytically revealed how the ultimate capacities of suction caissons are dependent on the contact conditions of tension and no-tension interfaces under combined loads including vertical compressive and tensile loads. In this article, an analytical investigation into the effect of interfaces on the ultimate vertical, lateral, and moment capacities of suction caissons in nonhomogeneous clay is made using a traditional bearing capacity theory with tension and no-tension interfaces. For failure envelopes of suction caissons in clay subjected to lateral and moment loads under vertical compressive and tensile loads, the results predicted by the present method are compared with those obtained from the finite element analysis. In order to compare failure envelopes obtained from a tension interface and those obtained from a no-tension interface under general loading, failure envelopes in the H-V, M-V, and H-M planes and in H-M-V space for suction caissons in clay are investigated.

Soil failure mechanisms during installation and inclined pullout of stiffened suction caisson in calcareous silt

Soil failure mechanisms of stiffened suction caisson during installation and inclined pullout in calcareous silt have been investigated through centrifuge model tests and small strain finite element analyses (FEA). In the tests, a model half-caisson was installed followed by loading at a 45° inclination. Digital images were captured and subsequently analysed using particle image velocimetry (PIV), allowing soil failure mechanisms to be observed. The FEA were carried out to study the effects of pullout angles for caissons with and without internal suction. The PIV results revealed that during suction assisted installation, localised shear strains develop surrounding the skirt. As the stiffeners penetrated into the soil, the particles filled up the gaps between the inside wall and soil plug. During inclined pullout, a double wedge type failure was observed at the front and rear sides that extended to 1.5 caisson diameter at the soil surface. A pure shear type failure was observed around the skirt tip. The FEA results showed that, for caissons without internal suction, a wedge failure mechanism was prevalent. The size of the wedge decreased with increasing pullout angle. For caissons with internal suction, a rotational failure mechanism was apparent.

The seepage study of transient caisson pull-out in sand

Under transient pull-out, caisson foundations will exhibit resistance from considerable suction, the magnitude of which is identified in the literature to be mainly the result of a complex interaction between the caisson dimensions, loading rate, ambient pressure, soil type, and load history. This paper investigates the seepage conditions developed around suction caissons in cohesionless soils under transient pull-out. Validations with benchmark tests are performed, using an advanced numerical simulation in Abaqus with coupled pore-fluid-diffusion/stress analysis, and employing the SANISAND constitutive model. The pore fluid response is evaluated by parametric studies on soil permeability, pull-out rate, and foundation aspect ratio, based on which a simplified expression is proposed for estimating the peak suction, with modification factors given for varying normalized pull-out rates and aspect ratios. It was identified that exponentially larger suctions can be anticipated when adopting faster pull-out rates, lower permeabilities, and higher aspect ratios. Qualitative assessment of the seepage regime indicates that higher suction developments are noticeably linked to the shortened drainage paths that pass around the skirt tip, as the pressure loss contours align closer together around the tip of skirts. Further, the distribution of mobilised friction angles inside and around the bucket is demonstrated as contributions of the outer wall friction and a bulb representing a zone of mobilised friction formed beneath the caisson during pull-out.

A SIMPLIFIED ASSESSMENT OF THE LOAD BEARING CAPACITY OF SUCTION CAISSON FOR OFFSHORE WIND TURBINES BASED ON FINITE ELEMENT ANALYSIS

Suction caisson is widely used for offshore wind turbine applications. Its loadbearing capacity depends on the bucket geometry and its embedded soil properties. This paper presents a simplified assessment of the loadbearing capacity of suction caisson based on finite element analysis using the Plaxis 2D program. The load-bearing capacity of the suction caisson is determined based on the resulting load-displacement curve via the tangent intersection method. In addition, this study developed an equivalent equation exploring the relationship between the load-bearing capacity of the suction caisson and the surface foundation. The findingsin the study showed that the geometry of the suction has a significant influence on its loadbearing capacity. The suction caissons whose aspect ratios are larger resulted in higher loadbearing capacities. Besides, the equivalent equation in this study could be applied to effectively estimate the load-bearing capacity of suctioncaisson based on its geometry. The finite element program and the soil ground model analyzed in this study was only an assumption. In the future, experimental studies should investigate the loadbearing capacity of a suction caisson related to its geometry and the embedded soil profile using centrifuge models and large-scale models.

Analytical Study on the Pullout Resistance of Suction Caisson in Clay

Abstract The pullout resistance is a design consideration for suction caisson-supported offshore structures, such as offshore platforms and wind turbines. An analytical method is proposed in this paper to calculate the pullout resistance of a single caisson foundation embedded in clay. Comparisons with laboratory model tests were also made to verify the accuracy of the proposed analytical method. Parametric studies were conducted to investigate the effects of key factors on the pullout resistance of caisson. This study found the pullout force nonlinearly increases with the increase of caisson diameter squared but linearly increases with the increase of caisson–soil frictions. For caisson wall thickness over height ratio ranging from 0.00125 to 0.00325 and caisson diameter over height ratio ranging from 0.25 to 0.75, the normal stress decreases nonlinearly along with the depth of the caisson skirt wall and is heavily influenced by the skirt wall thickness but slightly influenced by caisson diameter.

Experimental studies on suction-assisted installation of the modified suction caisson in dense sand

The modified suction caisson (MSC) consists of an internal compartment (IC) and an external structure (ES), being an innovative foundation for offshore structures. Penetration of the MSC into the desired position under the combination of the self-weight and applied suction is an issue for design. Model tests were performed on the MSC in saturated dense fine sand to explore the installation behavior of the MSC. Results show that the optimal method of penetrating the MSC (i.e., the maximum penetration is obtained) is only pumping out the air and water encased in the IC. During suction-assisted installation, subsidence regions around the IC and ES were found. The sand particles in the subsidence regions were found to move downwards along the external walls of the IC and ES. The length of sand particles migration path along the IC is higher than that along the ES, indicating that seepage effect in sand outside the IC is stronger than that outside the ES. Installation mechanisms under various installation methods were elaborated by analyzing the seepage distribution in sand around the MSC. Based on the method proposed by Houlsby and Byrne, modified equations of estimating the required suction of MSC were proposed. The research can provide guidance for the MSC installation design.

Experimental and Numerical Investigation of the Performance of Piles and Suction Caissons Subjected to Inclined Cyclic Loading in Cohesive Soils

Cyclic loading on caisson and pile anchors can be a significant design issue, particularly with respect to cumulative vertical displacements that can lead to reduction in embedment depth and load capacity of the anchor. Most existing design methodologies are based on monotonic load capacity, often accounting for potential soil strength degradation due to cyclic loading. Because the cyclic strength criterion is often based on cumulative strains, these approaches arguably implicitly consider cumulative deformations. However, cumulative plastic deformations under storm conditions should be investigated. Piles and caissons serving as anchors for floating offshore structures experience inclined cyclic loading. This paper presents (1) monotonic and cyclic loading tests in the vertical and lateral directions, and (2) a finite-element (FE) investigation of different aspect ratios (L/D=4, 5, and 6) of caissons subjected to different load angles (ψ=0° to 45° from the horizontal). Additionally, it presents a precise method to calibrate the constitutive model (nonlinear kinematic hardening model), where the calibrated model was match to monotonic and cyclic lateral and vertical loading tests. Nonuniform load amplitudes were used in the FE investigation. The primary finding from the investigation was that the influence of the cumulative displacements increases when increasing the load amplitudes, especially for the load inclinations of 0°–20° for the lateral displacements and 20°–45° for the vertical displacements.

Spatial characterization of seabed environmental conditions and geotechnical properties for the development of marine renewable energy in Sweden

Although knowledge of seabed properties is of high importance in selecting sites and determining technical designs and solutions for renewable energy offshore installations, it is often overlooked in marine spatial planning, owing to the absence of appropriate spatial analysis of these conditions. Identification and quantification of seabed conditions and geotechnical properties in finding safe and environmentally sustainable areas for installations of offshore renewable energy are therefore presented, using information produced in marine geological mapping. Six seabed environmental and 13 geotechnical parameters, which can be extracted from existing marine geological information and are of importance in analysing environmental conditions and planning designs are identified and presented, in addition to the suitability of various installation techniques for different areas on the Swedish seabed. Geographical information systems (GIS) are used to geospatially evaluate the different parameters in finding suitable locations and cable routes for a wave energy plant with gravity and/or suction caisson foundations. The presented categories and ranges of the environmental and geotechnical values for the various parameters have the possibility to be improved as new data are produced from future mapping. The parameters identified and presented here are valuable as they can be incorporated into multi-parameter evaluations for optimal site selection of different offshore installations. Supplementary material: The open geological information is available at http://www.sgu.se

Prediction of Uplift Capacity of Cylindrical Caissons in Anisotropic and Inhomogeneous Clays Using Multivariate Adaptive Regression Splines

The uplift capacity factor of cylindrical suction caisson in anisotropic and inhomogeneous clays considering the adhesion factor at the interface is investigated in this paper. The finite element limit analysis based on lower bound and upper bound analyses is used for analyzing purposes. The anisotropic undrained shear model is employed to describe the anisotropic and inhomogeneous clay. The impact of these dimensionless parameters on the ratio of inhomogeneity or strength gradient ratio, the adhesion factor, the ratio of depth over diameter, and the ratio of anisotropic undrained shear strengths on the uplift resistance and the collapse mechanisms of suction caisson foundations are determined. The multivariate adaptive regression splines technique is employed to access the sensitivity of all considered dimensionless parameters on the uplift capacity factor and to propose an empirical design equation as an effective tool for predicting the uplift capacity factor. The results presented in this paper can be guidance for the preliminary design of suction caissons in anisotropic and non-homogeneous clays that are useful for engineering practitioners.

Simplified method for performance-based seismic design of suction caissons supporting jacket offshore wind turbines

The paper studies the performance of Offshore Wind Turbines (OWTs) founded on Suction Bucket Jackets (SBJs) in clay under combined wind and seismic loading. A detailed 3D FE model of the soil–foundation–structure (SFS) system is developed and used as a benchmark to assess the efficiency of an enhanced Winkler-based "Caisson-on-Winkler-Soil" (CWS) model, where the soil is replaced by nonlinear hysteretic elements. The proposed CWS model captures residual deformations and hysteresis and offers physical coupling between vertical and moment loading. It allows excellent prediction of the H – M failure envelope in the most relevant first quadrant of H – M space, where the loads act in the same direction. Despite its successful application for the simulation of inertial loading, the CWS model fails to reproduce the dual shearing mechanism that develops at the caisson shaft during shaking, stemming from the combination of kinematic soil shearing due to the vertically propagating shear waves, and shearing due to the superstructure inertial response, thus underpredicting the co-seismic caisson settlements. For the prediction of the latter, the research utilizes spectrum compatible input motions and 3D FE models of varying geometric and material properties to derive linear regression equations that correlate the co-seismic dimensionless settlement of caissons ( w E / D ) with characteristic dimensional variables of the problem under investigation and the Arias Intensity ( I A ) of the surface ground motion. As a final step, the paper proposes a hybrid method for performance-based assessment of SBJ OWTs. The proposed method employs the simplified CWS model to calculate the VHM loads and approximately estimate horizontal displacements and rotations at the jacket legs, followed by a preliminary assessment of caisson settlements using the correlations of w E / D with I A , on the basis of spectrum-compatible input motions. • A performance-based seismic assessment method of Suction Bucket Jackets is proposed. • A Winkler-type model is developed, capturing residual deformations and hysteresis. • Permanent caisson deformation under combined wind and seismic loading is explored. • The deficiency of decoupled methods to capture residual settlements is explained. • Caisson settlement is correlated with statistically significant Intensity Measures.

Mechanical Characteristics of Prestressed Concrete Curved Transition Section of Composite Bucket Foundations for Offshore Wind Turbines

The composite bucket foundation (CBF) consists of a concrete curved transition section, a concrete beam-slab system, and a suction caisson and is increasingly used as the foundation for offshore wind turbines. The curved transition section transmits the upper load from the tower to the foundation, and its force and transmission characteristics are related to the safety of the entire wind turbine structure. The arced transition section has the characteristics of complex geometry, load conditions, and large curvature. It is difficult to determine its bearing characteristics and force transmission system. In this paper, the boundary conditions and loading device of the CBF model test are designed, and three 1:20 arced transition section model specimens are made. The mechanical characteristic experiments of CBF are used to analyze the failure process, failure characteristics, and seismic performance of the structure. Results show that the cracking effect of the arced transition section after prestressing is obviously better than that of a reinforced concrete arced transition section structure. The arced transition section specimens equipped with prestressed tendons can increase the structural cracking load ratio by about 35% for reinforced concrete members. The energy dissipation capacity of the specimens has been significantly improved, and the material properties can be fully utilized. The failure mode of the arced transition section structure under horizontal reciprocating load shows the characteristics of bending and shear failure.