Suction Caisson Foundations Research Articles

Study on Cyclic Cumulative Deformation Characteristics and the Equivalent Cyclic Creep Model of Soft Clay

Suction caisson foundations can be used to anchor tension leg platforms. The soil at the bottom of the caisson undergoes both unloading and cyclic loading under wind and wave loads. However, the problem of cyclic cumulative deformation of soft clay under unloading has rarely been addressed. So, the strain cumulative deformation and strain softening characteristics of soft clay are studied by cyclic triaxial tests. The test results show that under low static deviator stress ratios and dynamic deviator stress ratios, the soil has a low level of strain accumulation and softening. As the dynamic deviator stress ratios increase, the cumulative cyclic deformation gradually increases, which rapidly develops in the early stage and tends to stabilize in the later stage. Moreover, the softening index gradually increases and is linearly related to the logarithm of the number of cycles. The cyclic cumulative deformation of the soil increases with increases in unloading stress and dynamic deviator stress, showing a creep characteristic of attenuation and then stabilization. Based on the tests, an equivalent cyclic creep model is established to describe the strain accumulation and softening of soil and verified through comparison with the test results. Then, the model is extended to a three-dimension model, and a finite element subroutine is developed for studying the strain cumulative deformation and strain softening characteristics of soft clay.

Simple approach for including foundation–soil–foundation interaction in the static stiffnesses of multi-element shallow foundations

In common engineering practice, foundation–soil–foundation interaction of shallow foundations is frequently ignored. This is presumably for cost/benefit reasons, since computationally demanding finite-element and/or boundary element models are required for that purpose, and its effects are usually assumed to be negligible. In this sense, the present paper provides a simple and inexpensive way of incorporating foundation–soil–foundation interaction through a numerically explicit stiffness matrix formulation. The necessary ingredients for homogeneous and non-homogeneous (shear modulus power-law variation with depth) half-spaces are given. The proposed approach is then applied to offshore wind turbines’ multiple suction caisson foundations (tripod and tetrapod), where it is observed that the foundation–soil–foundation interaction is significant. Its range of validity is also established, and valuable ready-to-use closed-form formulas for the correction factors of the stiffnesses of tripod and tetrapod groups are also derived. The methodology is applicable as long as the spacing between foundations is somewhat greater than the foundation depth.

A Winkler model for suction caisson foundations in homogeneous and non-homogeneous linear elastic soil

Suction caisson foundations provide options for new foundation systems for offshore structures, particularly for wind turbine applications. During the foundation design process, it is necessary to make reliable predictions of the stiffness of the foundation, since this has an important influence on the dynamic performance of the overall support structure. The dynamic characteristics of the structure, in turn, influence its fatigue life. This paper describes a thermodynamically consistent Winkler model, called OxCaisson, that delivers computationally efficient estimates of foundation stiffness for caissons installed in homogeneous and non-homogeneous linear elastic soil, for general six degrees-of-freedom loading. OxCaisson is capable of delivering stiffness predictions that are comparable to those computed with three-dimensional finite-element analysis, but at a much lower computational cost. Therefore, the proposed model is suited to design applications where both speed and accuracy are essential, such as large-scale fatigue assessments of offshore wind farm structures. The paper demonstrates that the OxCaisson model can also be applied to short, rigid monopile foundations.

Effects of Relative Roughness and Particle Size on the Interface Behavior of Concrete Suction Caisson Foundation for Offshore Wind Turbines

The interface behavior between a caisson and the surrounding soil plays an important role in the installation of suction caissons as foundations for offshore wind turbines. A series of shear tests were carried out using a modified direct shear apparatus to study the interface shear behavior between sand and concrete. Sand samples with three particle size ranges (0.63–1.25 mm, 1.25–2.5 mm, 2.5–5.0 mm) and concrete plates with different relative roughness were used to explore the influence of the relative roughness parameter (Rn) and mean particle size (D50) on shear behavior. The responses from the pure sand shear test are also discussed for comparison. Test results show that the higher the relative roughness (Rn), the greater the maximum shear stress (τmax) appeared. The interface shear stress was weaker than that of the pure sand test. Furthermore, the interface friction angle (φ) of sand–concrete was closely related to the relative roughness of the concrete surface. Under the same conditions, the interface friction angle (φ) increased with relative roughness due to the effect of sand particles breakage and redistribution. By contrast, the effect of the mean particle size (D50) on the interface friction angle (φ) was less significant. However, for the pure sand shear test, the friction angle (φ′) obtained from the traditional shear test apparently increased with D50, indicating that the friction angle was more affected by D50 in the pure sand test than in the interface shear test.

Case studies on suction caisson foundations in offshore wind farms in China

ABSTRACT Suction caissons are cylindrical-shaped foundation elements closed at the top and open at the bottom which are almost completely embedded with their cylindrical part in the soil. The main advantage of this foundation compared to other commonly used foundation concepts is its fast, ease, and almost noiseless installation, as well as the effortless removal at the end of the lifetime. Suction caissons supporting three- or four-legged jackets have been employed successfully in several recent offshore windfarm projects around the world. The study presented herein investigates the general suitability of suction caissons in the south china sea at three representative sites. The results show that suction caisson foundations are technically feasible. Sizes can be large and may vary between 13 m to 15 m in diameter and 10 m to 13 m in length, which, however, may reduce noticeably when considering load redistribution in the design, typically done in a FEED and detailed design. The main challenges of suction caisson application are the very soft soil conditions and possible liquefaction potential due to large earthquake events. Although technically feasible, the economic viability of suction caissons needs to be investigated on project-specific basis. For that purpose, the authors provide an overview of general aspects which need to be considered in such an evaluation. This study gives realistic case studies for research and industry in offshore wind farm development.

A method to predict the torsional resistance of suction caisson with anti-rotation fins in clay

Suction caisson foundations have been increasingly employed as a primary solution to support the offshore fixed- or mobile-structures. Due to harsh environment and complex force transferring of offshore structures, they are still being developed as to satisfy increasing requirements in strict working scenarios. One of emerging challenges is a torsion-governing failure, which has been observed in the oil and gas industry (i.e. significant multiple-inline-force-induced torsion) and in the renewable energy field (i.e. non-coplanar tensile force induced torsion). This paper introduces a novel suction caisson foundation, with anti-rotational fins assembled on the outer skin of caisson. By a comprehensive numerical study, the evolution from local to global failure as the fin numbers from single to multiple, is examined in the clayey soil deposit with effects of soil strength heterogeneity, fins dimension, installation process and foundation-soil interface considered. Based on these, a set of methods to estimate the ultimate torsional capacity of such novel caisson is proposed, which starts from the gain in capacity for a single fin, and evaluate the changes of gains in capacity as fin numbers, then identify the optimised anti-rotational capacity. Finally, three key parameters (i.e. the required fin numbers, the available anti-rotational capacity and the optimised anti-rotational capacity) with some critical considerations, recommendations and implications have been concluded for design practice.

Design considerations of suction caisson foundations for offshore wind turbines in Southern China

Suction caisson foundations are increasingly considered as a foundation solution for offshore wind farm development in China. This paper outlines the design considerations for developing a suction caisson supported jacket solution for an offshore wind farm in Southern China. Geotechnical analyses for four major aspects, ultimate limit state (ULS), serviceability limit state (SLS), fatigue limit state (FLS) and installation are discussed. The design challenges encountered in the project due to soft seabed and metocean characteristics (severe typhoon loading and dominant wind directions) and technical approaches adopted by the project are presented. Areas for further studies are identified and discussed. The purpose of this paper is to disseminate this knowledge and raise awareness of several important aspects.

The influence of the drainage regime on the installation and the response to vertical cyclic loading of suction caissons in dense sand

Offshore wind turbines are considered as an integral part of renewable energy. Dense sand often characterises the sea bed and suction caisson foundations are increasingly used to support offshore wind turbines at North Sea sites. Significant experience with this technology exists in the oil and gas industry, although these platforms are typically located in deeper water with predominantly clay soils. Therefore, research is required to transfer the existing knowledge. This paper discusses a series of centrifuge tests that investigate the installation and the load transfer mechanisms governing the response of suction caissons embedded in dense sand subjected to vertical cyclic loading. The effect of potential loosening of the internal soil plug during suction installation on the in-service foundation performance is evaluated through cone penetration tests inside the caisson following suction installation at different pumping flow rates. In accompanying tests, the load displacement behaviour in response to vertical cyclic loading is investigated, including significant excursions into tension. The results identify the pore fluid behaviour as the key factor affecting the response, with the pumping flow rate applied during installation being less influential. Based on the findings from the experiments a simplified approach for the prediction of suction caisson behaviour under vertical cyclic loading is presented.

Bearing capacity of a side-rounded suction caisson foundation under general loading in clay

Abstract A novel side-rounded suction caisson foundation is proposed in this study. Compared to a conventional circular shape in plan, it has a rectangular middle section inserted in between the two circular halves for increased moment capacity. This paper presents an investigation into the bearing performance of this novel foundation in clay under uniaxial and combined loading by means of an extensive finite element parametric analysis. It is found that by adopting a dimensionless equivalent embedment ratio and a dimensionless equivalent strength heterogeneity ratio, which account for the side-rounded shape of the proposed foundation, its design approach can be unified with an existing framework established for conventional skirted circular footings. The advantage of the proposed foundation and the application of the proposed design method is demonstrated through an example application.

Characteristic Test Study on Bearing Capacity of Suction Caisson Foundation Under Vertical Load

Suction caisson foundations are often subjected to vertical uplift loads, but there are still no wide and spread engineering specifications on design and calculation method for uplift bearing capacity of suction caisson foundation. So it is important to establish an uplift failure criterion. In order to study the uplift bearing mechanism and failure mode of suction caisson foundation, a series of model tests were carried out considering the effects of aspect ratio, soil permeability and loading mode. Test results indicate that the residual negative pressure at the top of caisson is beneficial to enhance uplift bearing capacity. The smaller the permeability coefficient is, the higher the residual negative pressure will be. And the residual negative pressure is approximately equal to the water head that causes seepage in the caisson. When the load reaches the ultimate bearing capacity, both the top and bottom negative pressures are smaller than Su and both the top and bottom reverse bearing capacity factors are smaller than 1.0 in soft clay. Combined the uplift bearing characteristics of caisson in sandy soil and soft clay, the bearing capacity composition and the calculation method are proposed. It can provide a reference for the engineering design of suction caisson foundation under vertical load.

Assessment of numerical procedures for determining shallow foundation failure envelopes

The failure envelope approach is commonly used to assess the capacity of shallow foundations under combined loading, but there is limited published work that compares the performance of various numerical procedures for determining failure envelopes. This paper addresses this issue by carrying out a detailed numerical study to evaluate the accuracy, computational efficiency and resolution of these numerical procedures. The procedures evaluated are the displacement probe test, the load probe test, the swipe test (referred to in this paper as the single swipe test) and a less widely used procedure called the sequential swipe test. Each procedure is used to determine failure envelopes for a circular surface foundation and a circular suction caisson foundation under planar vertical, horizontal and moment (VHM) loading for a linear elastic, perfectly plastic von Mises soil. The calculations use conventional, incremental-iterative finite-element analysis (FEA) except for the load probe tests, which are performed using finite-element limit analysis (FELA). The results demonstrate that the procedures are similarly accurate, except for the single swipe test, which gives a load path that under-predicts the failure envelope in many of the examples considered. For determining a complete VHM failure envelope, the FEA-based sequential swipe test is shown to be more efficient and to provide better resolution than the displacement probe test, while the FELA-based load probe test is found to offer a good balance of efficiency and accuracy.

Centrifuge modelling of uplift response of suction caisson groups in soft clay

This paper describes a program of centrifuge model tests on the uplift behaviour of suction caisson foundations. The parameters considered were the loading rate, caisson diameter (D), soil strength profile, and type of footing (i.e., mono-caisson and tetra-caissons group). The loading responses were examined in terms of total uplift resistance, suction beneath the caisson lid, and the vertical displacements of the caisson and at the soil surface. There exists a critical uplift displacement, approximately 0.02D and 0.01D for the mono-caisson and the tetra-caissons groups, respectively, at which a turning point can be identified in the load–displacement curve. This was found to be attributed to the adhesion on the caisson–soil interface reaching a peak response and then dropping. Of interest is that the tetra-caissons group exhibits much greater normalized uplift resistance than the mono-caisson group before reaching an uplift displacement of about 0.02D, suggesting superiority of the former in term of serviceability. However, a reversed trend was observed at greater displacement, and accordingly an empirical model was derived to quantify the shadowing effect of caisson groups. Regarding the cyclic response, several cycles of large-amplitude loading are sufficient to reduce the ultimate bearing capacity of caisson(s) to below the self-weight of the inner soil plug(s), indicating a transition of failure mechanism.

Upper Bound Solutions for Uplift Ultimate Bearing Capacity of Suction Caisson Foundation

Suction caisson foundation derives most of their uplift resistance from passive suction developed during the pullout movement. It was observed that the passive suction generated in soil at the bottom of the caisson and the failure mode of suction caisson foundation subjecting pullout loading behaves as a reverse compression failure mechanism. The upper bound theorems have been proved to be a powerful method to find the critical failure mechanism and critical load associated with foundations, buried caissons and other geotechnical structures. However, limited attempts have been reported to estimate the uplift bearing capacity of the suction caisson foundation using the upper bound solution. In this paper, both reverse failure mechanisms from Prandtl and Hill were adopted as the failure mechanisms for the computation of the uplift bearing capacity of the suction caisson. New equations were proposed based on both failure mechanisms to estimate the pullout capacity of the suction caisson. The proposed equations were verified by the test results and experimental data from published literature. And the two solutions agree reasonably well with the other test results. It can be proved that both failure mechanisms are reasonably and more consistent with the actual force condition.

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.

Effect of foundation type and modelling on dynamic response and fatigue of offshore wind turbines

AbstractThis paper presents dynamic response and fatigue analyses of several bottom‐mounted offshore wind turbine (OWT) models, simulated in the aero‐hydro‐servo‐elastic simulation tool FAST. The distinction between the models is the foundations, which are modelled with different methods, concepts, and dimensions. The US National Renewable Energy Laboratory has developed a 5‐MW reference turbine supported on a monopile, the NREL 5MW, which was used as a reference model in this paper. The paper presents the implementation and comparison of two different foundation modeling methods, referred to as the simplified apparent fixity method and the improved apparent fixity method. Furthermore, sensitivity analyses of different monopile dimensions were performed, followed by sensitivity analyses of suction caisson foundations of different dimensions. The final part of the paper presents fatigue analyses for the foundation models considered in this study subjected to 17 load cases. Fatigue damage, fatigue life, and damage equivalent loads were calculated, as well as the relative fatigue contribution from each load case.

Model tests on performance of offshore wind turbine with suction caisson foundation in sand

The performance of steel caisson during and after installation with different penetration velocities in medium dense sand is presented. The applied jacking forces, the amount of formed soil heave and bearing capacity were measured in the model tests. The influence of penetration velocities on jacking forces, soil heave and bearing capacity were also discussed in detail. The results indicated that the jacking forces for caisson in medium dense sands were significantly affected by the penetration velocity. The larger the penetration velocity, the more soil flowed into the caisson cavity during installation. This will lead to larger inner shaft resistance and in turn more jacking forces required for the same penetration depth. The height of soil heave during installation increases with penetration velocity. The m value calculated by the ratio of the volumes of the soil heave to that of the penetrated caisson wall can be used to evaluate the soil heave. The larger the applied velocity, the larger the m value and larger bearing capacity of caisson after installation. The relationship between the m value and penetration velocity can be used to control the soil heave for a steel caisson with a wall thickness to external diameter ratio of 4.2% in medium dense sand by jacking method.

3D Finite element analyses of suction caisson foundations for offshore wind turbines in drained sand

ABSTRACT In the present study, three-dimensional finite element analyses are performed to assess load-bearing capacity of suction caisson foundations under drained condition for offshore wind turbines. The ultimate capacity for the Ultimate Limit State and the Serviceability Limit State are addressed. A Hardening Soil Model that considers the stress dependence of the soil stiffness is adopted in the numerical model, which is calibrated with published field test results. It is found that the caisson foundation experiences upward, downward and rotation movement under combined large loading, which leads to gap formation between the caisson lid and the plugged soil. Based on a numerical parametric study, the dependence of the ultimate capacity on the caisson size and the relative density of the sand is demonstrated. Finally, normalized interaction diagrams and normalized equations for ultimate capacity predictions are derived from the numerical simulation that can be used within a scope of preliminary design.

Seismic Behavior of Suction Caisson Foundations

Seismic Behavior of Suction Caisson Foundations

Elastic Stiffnesses of a Rigid Suction Caisson and Its Cylindrical Sidewall Shell

AbstractA finite-element study is presented for the linear-elastic behavior of rigid suction caisson (bucket) foundations installed in either a homogeneous or nonhomogeneous Gibson soil stratum. Cl...

A parametric study on the vertical pullout capacity of suction caisson foundation in cohesive soil

The pullout capacity of suction caisson foundation plays a vital role in its field performance. This study presents numerical investigation on the vertical pullout capacity of suction caisson foundation in cohesive soil under both drained and undrained conditions. The influence of soil cohesion, internal friction angle and caisson aspect ratio on the ultimate vertical pullout capacity of suction caisson foundation has been investigated. It is noted that the upper limit of pullout capacity is governed by undrained condition and the lower limit by drained condition. The pullout capacity increases with increasing soil cohesion, friction angle and caisson aspect ratio when caisson diameter is kept constant, whereas it decreases with increasing caisson aspect ratio when caisson length is kept constant. Mathematical models have been developed for both drained and undrained pullout capacity. The pullout capacity values have been compared with those of available analytical and simplified relationships, and it has been found that the developed models can accurately predict the vertical pullout capacity of suction caisson foundation.