Installation Of Caissons Research Articles

Numerical Simulation of the Behavior of Caisson Based on Physical Modeling

Stiffened caissons are a new kind of offshore platform foundation which has been widely used in recent years. Stiffeners are employed to avoid buckling during the installation process. However, they also create a significant challenge in terms of understating the soil-flow patterns and corresponding installation resistance prediction. Although centrifuge and in situ tests can simulate the caisson installation process very well, their high costs prevent their widespread application. Model tests have been widely used in research on caisson behavior during installation, as they are convenient and cost less compared to centrifuge and prototype tests. However, the quantitative conclusions of the resulting predictions of installation resistance have some uncertainties because it is quite hard to strictly follow the similarity principle in 1 g model tests. Therefore, it is important to establish a method to calibrate the data from model tests, providing better estimates of caisson behavior in field tests. In our research, large deformation finite element (LDFE) analyses were conducted to provide insights into differences in the outcomes of caisson installation approaches between prototype tests and 1 g model tests. Prior to carrying out parametric studies, validations were conducted with good results. The results show that normalized soil strength significantly influences the behavior of caissons of various dimensions in 1 g model tests. In uniform clay, caissons exhibit consistent installation behavior; otherwise, they show significant differences. Based on systematic research, this paper reveals the mechanisms of the difference between model tests and prototype tests with different sizes of caissons and identifies the factors influencing these differences.

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.

The effect of shear rate and overconsolidation ratios on undrained clay–suction caisson interface behaviors

The interface resistance during installation is crucial for the stability and safety of suction caisson in offshore geotechnical engineering, which is strongly affected by the penetration rate and soil–structure interface mechanical properties. This research conducts a series of clay–structure interface shear tests using modified direct simple shear device to fully study the mechanical behavior of clay–suction caisson interface. The effect of shear rate, over consolidation ratios (OCRs), interface boundary conditions, stress levels, and interface roughness were considered. Results show that as the OCR increases, the strength of both the clay and interface increase but show distinct patterns under constant volume (CV) and constant normal load (CNL) boundary condition. It was found that the interface strength is positively related to interface roughness and shear rate impact both the clay and corresponding interface strength. Under CNL conditions, the strength of normally consolidated (NC) clay decreases with rising shear rate, while the over consolidated (OC) clay demonstrate a opposite trend. In contrast, the effect of shear rate on interface behavior gets complicated owing to the combination of roughness, stress levels, and OCRs. Under CV conditions, the shear strength of clay and interface exhibits a logarithmic growth relationship with shear rates. The result of this work can provide a basis for interface resistance evaluation for suction caisson installation in clay.

Penetration resistance of stiffened caissons in uniform and soft-over-stiff clays

The prediction of the penetration resistance of stiffened caisson involves uncertainties due to the complex soil failure mechanisms, which makes an obstacle for its application. In this paper, stiffened caisson installations in single-layer uniform and double-layer soft-over-stiff clays are investigated. Large deformation finite-element (LDFE) analyses are employed to simulate the caisson installation process. Soil flow mechanisms, especially inside the caisson along the inner stiffeners, and the caisson installation resistance are studied. Along the inner stiffeners, both soil/soil sliding and soil/skirt sliding mechanisms are observed based on the soil strength and caisson geometry, especially the stiffener geometry and spacing. During the caisson installation in soft-over-stiff clays, the soil flow mechanisms observed in the single layer uniform clay can be replicated in the top soft clay layer before the caisson penetrates into the bottom stiff clay. Caisson full installation resistance can be predicted based on the combined soil-flow mechanisms revealed in single- and double-layer uniform clays.

Experimental and numerical investigation on the impact of MS2AF on local scour protection performance

Suction anchor foundations are widely used in the offshore wind power industry. The installation of suction caissons alters the near-bed flow field and leads to local scour. This study proposes a novel integrated design for the scour protection of suction anchor foundations, known as the multihole sector suction anchor foundation (MS2AF). Through indoor flume experiments and numerical flume simulations, the local scour protection performances of MS2AFs with different design parameters under the action of steady flow were investigated. The results showed that the scour evolution of MS2AFs can be divided into three stages: slow scour, rapid scour, and scour equilibrium. The MS2AF reduced the flow shear stress on the sediment bed, thereby decreasing the depth and range of the local scour. The maximum scour depth, width, and length of the scour hole exhibited negative linear relationships with the width and thickness of the sector and decreasing-then-increasing relationships with the diameter and area ratio of the holes. Among these factors, the sector's thickness had the most significant impact. There existed an optimal diameter and area ratio that maximized the stratification effect and deceleration effect of the multihole sector.

Numerical study of regular wave-induced oscillatory soil response during the caisson installation

In this study, the PORO-FSSI-FOAM model is applied to investigate the wave-induced oscillatory soil response during the caisson installation, where the Reynolds averaged Navier-stokes equation is used to simulate the flow motions, and the poro-elastic equation is used to govern the soil dynamic response around the caisson. Unlike previous studies, the caisson installation process is considered when investigating the wave-induced oscillatory soil response in this paper. Firstly, the accuracy of the PORO-FSSI-FOAM model in simulating the wave-seabed interaction is validated. Then, the effects of the submerged depth, the geometrical dimensions and the installation sequence of the caisson on the wave-induced oscillatory pore pressure response are examined, and the momentary soil liquefaction during the caisson installation is explored. Finally, a simple formula that describes the relationship between the maximum soil liquefaction depth and the caisson submerged depth is proposed. Numerical results indicate that the caisson dimensions can greatly influence the wave-induced pore pressure response, and the relationship between the oscillatory pore pressure response and the caisson submerged depth is inconsistent at different locations surrounding the structure. Moreover, the installation sequence can significantly affect the momentary soil liquefaction and the simple formula proposed in this study can provide reference for engineers to assess the seabed stability during the caisson installation.

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.

Installation design of a large tubular caisson in the spatially varying seabed

The installation of large steel tubular caissons, used as protection for underwater production systems, is still a challenge. To evaluate the reliability of installation, soil spatial variability is accounted for in this study through two-dimensional random field establishments and Monte Carlo simulation. As vertical random parameters are identified by field CPT data, the effect of the horizontal scale of fluctuation (δh) is explored, which becomes significant due to the extremely large diameter of the caisson. Probability distributions of self-weight penetration, inclination angle, and the ultimate soil resistance with various δh are derived from conventional installation analyses with spatially varying soil domains. It shows that increasing δh reduces the likelihood of over-tilt, yet raises the possibility of vibratory drivability refusal (failure probability). The effect of stiffeners during caisson installation is also studied, including an increase in penetration resistance and failure probability. This analysis sheds light on the impact of the soil correlation distance as well as caisson stiffeners and is useful to the reliable installation design of the large tubular caisson, particularly in layered soil.

Numerical Investigation of Ground Settlements Induced by Installation of Large Diameter Deeply-Buried Caissons in Undrained Clays

Numerical Investigation of Ground Settlements Induced by Installation of Large Diameter Deeply-Buried Caissons in Undrained Clays

Interaction Effect between Caissons by Installation of New Caisson on Existing Caisson Breakwater in Second Order Stokes Wave Condition

In order to increase the structural stability of existing caisson breakwater, the design and the construction is carried out by installation of new caissons on the back or the front of old caissons. In this study, we use the ANSYS AQWA program to analyze the wave forces acting on individual caisson according to effects of wave structure interaction when new caissons are additionally installed on existing caisson breakwater. Firstly, the wave force characteristics acting on the individual caisson were analyzed for each period (frequency) in the frequency domain. In time domain analysis, the dynamic wave force characteristics were strongly influenced by the distance between caissons on the frequency at which the unusual distribution of wave forces occurs.

Inner soil heave of stiffened caisson during installation in soft-over-stiff clay

The identification of soil heaves inside a caisson during installation is important for the caisson’s design length, since the inner soil heave during the caisson installation affects the final effective caisson penetration depth, hence caisson capacity. This paper reports an extensive parametric study of stiffened caissons penetrating in double-layer of soft-over-stiff clay using large deformation finite element (LDFE) analyses. The LDFE analysis was first validated against the existing results from centrifuge tests and numerical analyses. Then the parametric study was carried out to explore the inner soil movements during stiffened caisson installations with various influencing factors, such as caisson diameter, caisson stiffener width, top-layer soil strength, double-layer soil strength ratio, and roughness of soil-caisson interface. It is found that the caisson diameter, stiffener width, and top-layer soil strength have significant influences on the inner soil behaviors. Besides, the movements of both soil surface and soil layer interface were also investigated. Based on the LDFE results, design formulas are proposed to estimate the inner soil heaves for soft-over-stiff clay. A design formula to estimate the movement of the soil layer interface for soft-over-stiff clays is also reported.

Solid-Fluid Coupled Numerical Analysis of Suction Caisson Installation in Sand

Suction caissons are widely used foundations in offshore engineering. The change in excess pore pressure and seepage field caused by penetration and suction significantly affects the soil resistance around the caisson wall and tip, and also affects the deformation of the soil within and adjacent to the caisson. This study uses Arbitrary Lagrangian–Eulerian (ALE) large deformation solid-fluid coupled FEM to investigate the changes in suction pressure and the seepage field during the process of the suction caisson installation in sand. A nonlinear Drucker-Prager model is used to model soil, while Coulomb friction is applied at the soil-caisson interface. The ALE solid-fluid coupled FEM is shown to be able to successfully simulate both jacked penetration and suction penetration caisson installation processes in sand observed in centrifuge tests. The difference in penetration resistance for jacked and suction installation is found to be caused by the seepage and excess pore pressure generated during the suction caisson installation, highlighting the importance of using solid-fluid coupled effective stress-based analysis to consider seepage in the evaluation of suction caisson penetration.

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.

Ground movements induced by installation of twin large diameter deeply-buried caissons: 3D numerical modeling

The assessment and control of ground movements during the installation of large diameter deeply-buried (LDDB) caissons are critically important to maintain the stability of surrounding infrastructures. However, for twin LDDB caissons which have been installed worldwide, no well-documented guidelines for assessing the induced ground movements are available due to the complexities of caisson–soil interaction. To this end, considering the mechanical boundaries of caissons and mechanized installation process, this paper presents a simple kinematic mechanical model balancing both computational cost and accuracy, which can be easily incorporated in commercial finite-element (FE) programs. Based on a project of twin LDDB caissons alternately installed employing a newly developed installation technology in wet ground with stiff clays in Zhenjiang, China, a three-dimensional (3D) numerical model is developed to capture the ground movements in terms of surface settlements and radial displacements induced by the installation of twin LDDB caissons. Moreover, hardening soil model with small-strain stiffness (HSSmall model) conceptually capable of capturing the nonlinear soil stiffness from very small to large strain levels is used to simulate undrained ground. The validations against field observations, empirical predictions and centrifuge test data are carried out to demonstrate the accuracy and validity of the developed FE model. Subsequently, the comparisons of ground movements numerically obtained in three frequently used installation schemes (i.e., synchronous, asynchronous and alternating installation) are conducted for installation sequence optimization of twin caissons. It is found that synchronous installation is the optimal scheme for limiting ground movements. Parametric studies considering the effects of horizontal spacing between twin caissons, staged penetration depth, inner diameter, controllable soil-plugging height, frictional coefficient between caisson–soil interface, as well as cutting edge gradient are thus performed in synchronous installation scheme. Based on an artificial data set generated through FE calculation, the multivariate adaptive regression splines (MARS) model capable of accurately capturing the nonlinear relationships between a set of input variables and output variables in multi-dimensions is used to analyze the sensitivity of caisson design parameters. Finally, the MARS mathematical equations for predicting the maximum surface settlement and radial displacement used in preliminary caisson design are proposed.

Characteristics of Wave by Additional Installation of Porous Dual Circular Caissons on the Existing Breakwater

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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.

Wave Structure Interaction by Installation of New Circular Caissons on Old Circular Caisson Breakwater

Wave Structure Interaction by Installation of New Circular Caissons on Old Circular Caisson Breakwater

Investigation into the influence of caisson installation process on its capacities in clay

The installation process of caisson foundations causes the seabed soil heaving as well as the soil strength remoulding. This paper investigates these two effects by examining the installation process of caissons through large deformation finite element analysis. The short-term undrained bearing capacity of caisson foundations in clay after installation, which is a critical engineering scenario because the long term foundation capacity increases with time due to seabed soil consolidation, is subsequently evaluated. Expressed in terms of bearing capacity envelopes under combined loading, the short-term bearing capacity exhibits significant reduction affected by the two effects (i.e. soil heaving and soil remoulding) during installation. The analysis results show that the caisson foundation capacity under uniaxial vertical load reduces by up to 40.95% when the ratio of caisson skirt length (d) to diameter of the caisson (D) is d/D = 4 while the uniaxial moment capacity reduces by up to 42.21% when d/D = 1. Meanwhile, all failure envelopes shrink due to the influence of installation. The maximum capacity degradation factors of all H-M failure envelopes are always greater than those of the uniaxial vertical, horizontal or moment capacity. The influence of the installation effects on the capacity of caisson post installation is more obviously when V ≤ 0.5Vult.

Investigation on the Behavior of Stiffened Caisson Installation in Uniform Clay from Large Deformation Modeling

Abstract Suction caissons, widely applied in offshore engineering, suffer from significant uncertainties about the flow mechanism of the surrounding soil during installation, especially for 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.