Penetration and Pullout Capacity of Low-Skirted Suction Caissons

The bearing capacity of suction caissons is the key to the design of offshore structures. A new type of cross-shaped low-skirted suction caisson is invented to effectively improve the bearing capacity, considering inevitable “soil plug” phenomenon. The behaviors of penetration and pullout for new low-skirted suction caisson are investigated by performing model tests. A new formula for calculating the penetration resistance is suggested based on the limit equilibrium theory and the test data, which can consider the change of the lateral area of the suction caisson during penetration. The behaviors of low-skirted suction caisson under inclined loading are analyzed by carrying out finite element simulation. The effects of loading angles and loading positions on the ultimate bearing capacity and failure mechanism of low-skirted suction caissons are discussed. The research results can provide a reference for the design of suction bucket foundation for offshore structures.

Numerical Study on the Behavior of Square Stiffened Caissons Penetrating into Normally Consolidated Clay

Significant difference between predicted and measured installation resistance of stiffened suction caissons was identified due to the existing uncertainty regarding the mobilized soil flow mechanisms. This paper describes an extensive investigation of square stiffened caisson penetration in nonhomogeneous clays undertaken through large deformation FE (LDFE) analysis to identify the soil flow mechanisms around and between lateral ring stiffeners. A detailed parametric study has been carried out, exploring a range of nondimensional parameters related to stiffened caisson geometry, caisson roughness, and soil strength. The LDFE results were compared with centrifuge test data in terms of soil flow mechanisms, with good agreement obtained. Two interesting features of soil flow inside the caisson were observed including soil backflow into the gaps between the embedded stiffeners and soil heaving at the surface. It shows that the cavity depth can reach ∼5 m. Finally, simple expressions were proposed for estimating the critical depths of soil backflow and cavity formation.

Uplift Capacity of Suction Caissons in Sand for General Conditions Of Drainage

Suction caissons have emerged as a viable solution for the foundations of offshore wind turbines, which are gaining momentum worldwide as an alternate energy source. When used in a multi-bucket jacket system, the system capacity is often governed by the uplift capacity of the windward bucket foundation. Seabed conditions at offshore windfarm sites often comprise dense sand where the soil response may be drained, partially drained or undrained depending on the loading regime, the foundation dimensions and the soil conditions. Given the large difference in uplift capacity of caissons for these different drainage conditions, predicting the behavior of a suction caisson under a range of drainage conditions becomes a paramount concern. Consequently, this paper presents the findings of a coupled finite element investigation of the monotonic uplift response of the windward caisson of a multi-bucket jacket system in a typical dense silica sand for a range of drainage conditions. The study adopts a Hypoplastic soil constitutive model capable of simulating the stress-strain-strength behavior of dense sand. This choice is justified by conducting a comparative study with other soil models — namely the Mohr Coulomb and bounding surface sand models — to determine the most efficient soil failure model to capture the complex undrained behavior of dense sand. The numerical predictions made in this study are verified by recreating the test conditions adopted in centrifuge tests previously conducted at the University of Western Australia, and demonstrating that the capacity from numerical analysis is consistent with the test results. The Hypoplastic soil constitutive model also provides an efficient method to produce accurate load capacity transition curves from an undrained to a drained soil state.

Suction Caissons as Multipod OWT Foundations in Clay: Investigation of Loading Rate Effects

The paper elaborates on the bearing mechanics of suction caissons serving as the foundation system of offshore wind turbines (OWTs) supported on jacket structures. As axial loading governs foundation design in this configuration, the study proceeds to a numerical investigation of the tensile load carrying capacity of a suction caisson in clay through a series of coupled pore fluid diffusion — effective stress analyses with a hypoplastic constitutive model. The latter allows for the quantification of loading rate effects on the amount of negative excess pore pressures (passive suction) generated within the confined soil plug and their contribution to uplift caisson resistance. Based on these results, a simplified uncoupled procedure is employed to account for loading rate effects in the performance-based design of an 8MW OWT founded on a Suction Bucket Jacket (SBJ).

Limit Equilibrium Solutions to Anti-overturning Bearing Capacity of Suction Caissons in Uniform and Linearly Increasing Strength Clays

Suction caissons supporting offshore wind turbines are exposed to great horizontal loading above the soil surface, which may lead to overturning failure of the caisson. This paper presents a modified three-dimensional failure mechanism to analyze the anti-overturning bearing capacity of suction caissons. The modified failure mechanism is composed of meniscus-conical wedge having meniscus shape at the soil surface and scooped shape. The analytical solution to the anti-overturning bearing capacity of suction caisson is deduced in terms of the limit equilibrium method, following by a parametric study of wedge depth ratio (c) to optimize the critical failure mechanism that satisfies both the force and moment equilibriums. Thus, the methodologies are relatively easy to implement in traditional spreadsheets and the analyses tend to perform very fast. Meanwhile, the effects of gap formation at the rear side of the caisson, loading eccentricity and adhesion factor at caisson-soil interface on anti-overturning bearing capacity are investigated. Comparing with finite element limit analysis results, experimental data and existing theoretical solutions, it is proved that the presented limit equilibrium analysis can satisfactorily predict the anti-overturning bearing capacity of suction caissons with low aspect ratios for offshore wind turbines in uniform and linearly increasing strength clays.