Suction Caissons Research Articles

Numerical studies on soil-skirt roughness and sand plug impact in bearing capacity prediction of caisson foundations subjected to the uniaxial loads

Skirted foundations denoted as suction caissons are becoming an increasingly prevalent offshore foundation solution for either the oil and gas industry or renewable energy infrastructure. Their response to combined vertical, horizontal, and moment loading must be found to ensure their stability under harsh environmental conditions. As part of this process, knowledge of uniaxial capacities is required. Previous studies have neglected the effect of deformable ground by assuming that the soil within skirts behaves rigid during drained loading, but this assumption needs rigorous studies. A series of 3-D finite element analyses has been conducted to investigate directly how the skirt geometry, soil–skirt interface roughness, and deformable plug within the skirt compartment affect the drained skirted foundation capacity and depth factors under uniaxial loading. The results show that the foundation embedment, interface friction, and soil plug placed within the skirt significantly influence the accompanying mechanisms occurring at failure and therefore the uniaxial capacities. The finite element analyses under uniaxial loads are performed for the rough and smooth interface assumptions, to show how the roughness interface influences the ultimate bearing capacities and depth factors and the corresponding failure mechanisms in different soil profiles.

Determination of Uplift Capacity of Suction Caisson Using Gaussian Process Regression, Minimax Probability Machine Regression and Extreme Learning Machine

This article employs Gaussian process regression (GPR), minimax probability machine regression (MPMR) and extreme learning machine (ELM) for prediction of uplift capacity (–) of suction caisson. This study uses GPR, MPMR and ELM as regression techniques. L/d (L is the embedded length of the caisson and d is the diameter of caisson), undrained shear strength of soil at the depth of the caisson tip (Su), D/L (D is the depth of the load application point from the soil surface), inclined angle (θ) and load rate parameter (Tk) have been adopted as inputs of GPR, MPMR and ELM models. The output of GPR, MPMR and ELM is P. The results of GPR, MPMR and ELM have been compared with the artificial neural network (ANN) model. The developed models have also been used to determine the effect of each input on P. This study shows that the developed GPR, MPMR and ELM are robust models for prediction of P of suction caisson.

Lateral Bearing Capacity of Modified Suction Caissons Determined by Using the Limit Equilibrium Method

The modified suction caisson (MSC) adds a short-skirted structure around the regular suction caissons to increase the lateral bearing capacity and limit the deflection. The MSC is suitable for acting as the offshore wind turbine foundation subjected to larger lateral loads compared with the imposed vertical loads. Determination of the lateral bearing capacity is a key issue for the MSC design. The formula estimating the lateral bearing capacity of the MSC was proposed in terms of the limit equilibrium method and was verified by the test results. Parametric studies on the lateral bearing capacity were also carried out. It was found that the lateral bearing capacity of the MSC increases with the increasing length and radius of the external skirt, and the lateral bearing capacity increases linearly with the increasing coefficient of subgrade reaction. The maximum lateral bearing capacity of the MSC is attained when the ratio of the radii of the internal compartment to the external skirt equals 0.82 and the ratio of the lengths of the external skirt to the internal compartment equals 0.48, provided that the steel usage of the MSC is kept constant.

Design of suction foundations

Suction foundations have been deployed in the last three decades in a growing number of offshore developments, for bottom-fixed and floating structures, in shallow and deep waters, many of them successfully. Suction foundations, traditionally used as anchors and jacket foundations in the oil and gas industry, are now also used in the offshore renewable (wind) industry, e.g. for monopods, tripods, and jackets. When technically feasible, suction foundations are often cheaper than pile foundations. Additionally, their installation is relatively noise-free and, by applying overpressure, they can be removed during decommissioning. This paper focuses on the more complex design issues and some pitfalls related to suction foundation design. Additionally design practices and recommendations for suction caisson design, including installation and extraction feasibility, foundation resistance, settlements and response in sand, clay and layered soil profiles, will be presented for basic understanding.

Closed form solution for the first natural frequency of offshore wind turbine jackets supported on multiple foundations incorporating soil-structure interaction

Offshore Wind Turbines (OWTs) are dynamically sensitive structures and as a result estimating the natural frequency of the whole system taking into effect the flexibility of the foundation is one of the major design considerations. The natural frequency is necessary to predict the long-term performance as well as the fatigue life. Currently, jackets supported on multiple foundations (such as piles or suction caissons) are being considered to support WTG (Wind Turbine Generators) for deeper water developments. This paper presents a practical method to compute the natural frequency of a jacket supporting WTG by incorporating Soil-Structure-Interaction (SSI) based on closed form solutions. The formulation presented can be easily programmed in a spreadsheet type program and can serve as a convenient way to obtain the natural frequency with the least amount of input. The basis of this method is the Euler-Bernoulli beam theory where foundations are idealized with a set of linear springs. In this method, a 3-Dimensional jacket is first converted into a 2-Dimensional problem along the orthogonal planes of vibration which are essentially the principal axes of the foundation geometry. Subsequently, the jacket is converted into an equivalent beam representing it's stiffness and a formulation is presented to find an equivalent beam for the entire tower-jacket system. Using energy methods, an equivalent mass of the RNA (Rotor Nacelle Assembly)-tower-jacket system is also calculated and the fixed base frequency of the jacket is estimated. To consider the flexibility effects of the foundation, a formulation for an equivalent rotational spring of the foundation is developed. A method to incorporate the mass of the transition piece is also presented. Finally, a step-by-step application of the methodology is presented by taking example problems from literature which also serves the purpose of validation and verification.

INSTALLATION CONSTRAINTS OF SUCTION ASSISTED FOUNDATIONS AND ANCHORS FOR OFFSHORE ENERGY DEVELOPMENT

The application of suction caissons has increased over the last two decades for offshore energy developments. Their installation challenges in different seabed types affect their execution process and load-bearing capacity. Hence, the identification of these challenges and understanding their root causes are highly important. As such, this paper aims to review the recorded installation constraints due to different seabed conditions, discuss the various factors related to each of these constraints, and finally provide some suggestions to rectify each constraint and/or its relevant factors. To do so, the approach is to evaluate the geological (geophysical and geotechnical) conditions in multiple case studies and analyze the stability of suction caisson installation in different soil types. Results show that some factors such as plug heave contributes about 29% of the installation issues in both homogeneous clay soils and layered clay soils, soil piping and bottom resistance failure contribute about 16% and 10% respectively of the installation issues in both sand and layered sand soils, while high penetration resistance contributes about 23% of installation issues in layered soils. Also, the uncertainty of soil parameters or behavior is a complementary factor which adds more complexities to the above-mentioned factors. Therefore, a good understanding of the seabed conditions and soil parameters before and during installation, as well as constant monitoring of the induced suction created during penetration with respect to the penetration depth is essential to mitigate the likely issues.

Integrated optimal design of jackets and foundations

The article proposes a method for integrated design of jackets and foundations using numerical structural optimization. Both piles and suction caissons are examined in both clayey and sandy soil, and several design procedures are taken into account. The optimal design problem enables an automatic design process which minimizes the primary steel mass of the jacket and the foundations. Both leg distance and soil stiffness are found to have a significant influence on the total mass as well as the first natural frequency of the full offshore wind turbine structure. The results indicate that an integrated design approach is valuable in the conceptual design phase. Firstly, the soil characteristics and foundation type have a significant influence on the optimal leg distance for the jacket. Secondly, the jacket mass has a significant influence on the optimal foundation type and foundation design.

Behavior of soil heave inside stiffened caissons being installed in clay

The length of suction caisson anchors has been increasing to support increasing dimensions and weight of floating facilities, which necessitates employing horizontal ring stiffeners at intervals along the inner wall of the thin skirt of caissons to ensure structural integrity. The addition of these stiffeners has created significant uncertainties regarding soil flow mechanisms, in particular soil heave inside the caisson, which may reduce the caisson final penetration depth and influence the process of installation due to the need to avoid inside soil suction in the pumping equipment. This paper reports results of large-deformation finite element (LDFE) analyses investigating soil heave inside stiffened caissons during installation in nonhomogeneous clay deposits, with the corresponding evolution of soil flow mechanisms and penetration resistance profiles reported by Zhou et al. in 2016. The LDFE analyses have simulated continuous penetration of stiffened caissons from the seabed surface. A detailed parametric study has been undertaken, exploring the relevant range of soil strength nonhomogeneity and normalized strength, stiffened caisson geometry, soil effective unit weight, and caisson roughness. Of particular interest is the influence of stiffeners on soil heave and potential penetration refusal. The results have been validated against previously published centrifuge test data in terms of soil heave and penetration resistance profile, with good agreement obtained. It is shown that the soil normalized strength at the mudline and its nonhomogeneity, caisson diameter relative to the sum of skirt thickness and stiffener width, and caisson penetration depth have significant influence on the inner soil heave and its profile across the caisson radius. An expression, based on the LDFE results is proposed to predict the maximum inner soil heave during installation of stiffened caissons in the field.

Theoretical studies on penetration resistance of suction caissons in clay

ABSTRACTThe suction caisson is commonly a top-closed cylindrical steel structure with large diameter, short length and much thinner skirt wall thickness. The resistance to penetrating is calculated as the sum of the tip bearing capacity and the adhesion on the both sides of the skirt wall. Since the thickness of the skirt wall is very small, the downward adhesion produced by the skirt wall will cause the additional vertical stress and shear stress in the soil at the skirt tip level, increasing the skirt tip resistance. However, the increase in skirt tip resistance caused by the additional vertical stress rather than shear stress in soil at the skirt tip level was only considered, this may lead to an inaccurate estimation for the tip bearing capacity and the suction required. Thus, a modified slip-line field is put forward in this study to estimate the tip resistance. The expression of obtaining the minimum suction to install the suction caisson in clay is derived in terms of the force equilibrium. Results from calculations of the minimum suction have been proved to be in a good agreement with the measured data.

Simplified load estimation and sizing of suction anchors for spar buoy type floating offshore wind turbines

Floating offshore wind turbines are complex dynamic structures, and detailed analysis of their loads require coupled aero-servo-hydro-elasto-dynamic simulations. However, time domain approach used for such analysis is slow, computationally expensive and requires detailed data about the wind turbine. Therefore, simplified approaches are necessary for feasibility studies, front-end engineering design (FEED) and the early phases of detailed design. This paper aims to provide a methodology with which the designer of the anchors can easily and quickly assess the expected ultimate loads on the foundations. For this purpose, a combination of a quasi-static wind load analysis and Morison's equation for wave load estimation using Airy waves is employed. Dynamic amplification is also considered and design load cases are established for ultimate limit state (ULS) design. A simple procedure is also presented for sizing suction caisson anchors. All steps are demonstrated through an example problem and the Hywind case study is considered for such purpose.

Response of suction caissons for tidal current turbine applications in soft clay to monotonic and cyclic vertical loading

In soft marine clays, suction caissons provide a foundation system for tidal current turbines that further promote the sustainable nature of the system by allowing for their removal at the end of the structure’s design life. When configured as a multipod, the moment loads resulting from the horizontal flow of water will be transferred to the suction caissons as compression–uplift loads on opposing foundation legs. The behavior of a suction caisson in soft clay was investigated at aspect ratios of 1 and 2 under monotonic and cyclic vertical loading applicable to multipod-supported tidal current turbine design. Installation and solely monotonic vertical load tests indicated lower back-calculated adhesion factor, α, values and higher back-calculated bearing capacity factor, Nc, values than design standards recommend. The capacity and stiffness response of the foundation after undergoing cyclic loading was found to be largely dependent on the magnitude of displacement the foundation underwent during cyclic loading. Additionally, a threshold of elastic foundation response was observed during cyclic loading defined by a cyclic displacement amplitude. These results indicate serviceability constraints will be critical in the design of suction caisson foundations for tidal current turbine applications.

New design equation for undrained pullout capacity of suction caissons considering combined effects of caisson aspect ratio, adhesion factor at interface, and linearly increasing strength

Undrained pullout capacity of suction caisson in clays with homogeneous and linearly increasing strengths with depth was investigated by two-dimensional axisymmetric finite element analysis. Effects of the caisson aspect ratios, the complete range of adhesion factors at soil-caisson interface, and the full range of dimensionless strength gradients on the dimensionless pullout factor of suction caissons were comprehensively examined. Based on a nonlinear regression to the derived finite element solutions, a new design equation of the undrained pullout capacity of suction caissons was proposed, considering the practical ranges of caisson aspect ratios, the complete range of adhesion factors at soil-caisson interface, and a general profile of both homogeneous and linearly increasing strengths with depth. The proposed equation was validated through a comparison of existing solutions and published experimental data.

Experimental studies on sand plug formation in suction caisson during extraction

ABSTRACTA series of model tests were conducted on Perspex-made suction caissons in saturated dense marine sand to study the sand plug formation during extraction. Suction caissons were extracted by pullout loading or by pumping air into the suction caisson. Effects of the pullout rates, aspect ratios and loading ways (monotonic or sustained) on the pullout capacity, and plug formation were investigated. It was found that the ultimate pullout capacity of the suction caisson increases with increasing the pullout rate. The sand plug formation under the pullout loading is significantly influenced by the pullout rate and the loading way. When the suction caisson is extracted at a relatively slow rate, the general sand boiling through the sand plug along the inner caisson wall occurs. On the contrary, the local sand boiling will occur at the bottom of the suction caisson subjected to a rapid monotonic loading or a sustained loading. Test results of the suction caisson extracted by pumping air into the caisson show that the pressure in the suction caisson almost follows a linear relationship with the upward displacement. The maximum pressures for suction caissons with aspect ratios of 1.0 and 2.0 during extraction by pumping air into the caisson are 1.70 and 2.27 times the maximum suction required to penetrate the suction caisson into sand. It was found that the sand plug moves downward during extraction by pumping air into the caisson and the variation in the sand plug height is mainly caused by the outflow of the sand particles from the inside of the suction caisson to the outside. When the suction caisson model is extracted under the pullout rate of 2 mm/s (0.28 mm/s for the prototype), the hydraulic gradient along the suction caisson wall increases to the maximum value with increasing the penetration depth and then reduces to zero. On the contrary, when extracted under the pullout rate of 10 mm/s (1.4 mm/s for the prototype), the hydraulic gradient along the suction caisson wall increases with increasing the pullout displacement. When extracted by pumping air into the caisson, the hydraulic gradient reaches the critical value, and at the same time, the seepage failure occurs around the suction caisson tip.

Large deformation finite element analysis of the installation of suction caisson in clay

ABSTRACTThe suction caisson (or called suction anchor) which is considered as a relatively new type of foundation of offshore structures, has been extensively studied and applied for offshore wind turbines and oil platforms. The installation of the suction caisson is of great importance in the design and construction because it can bring about several issues and further influence the performance of holding capacity in safety service. In this paper, large deformation finite element (FE) analyses are performed to model the installation of suction caisson (SC) by suction and jacking in normally consolidated clay. The penetration of the suction caisson is modeled using an axisymmetric FE approach with the help of the Arbitrary Lagrangian–Eulerian (ALE) formulation which can satisfactorily solve the large deformation problem. The undrained shear strength of the clay and elastic modulus are varied with depth of soil through the subroutine VUFIELD. The numerical results allow quantification of the penetration resistance and its dependence on the installation method. The centrifuge test and theoretical solution are used for the FE model validation. After the validation, the penetration resistance, the soil plug heave, and the caisson wall friction have been examined through the FE model. Based on the numerical results, it is shown that the ALE technique can simulate the entire suction caisson penetration without mesh distortion problem. The installation method can play an important role on the penetration resistance, namely, the suction installation reduces the penetration resistance significantly compared to the purely jacked installation. With a further study on the suction case, it is found that as the final applied suction pressure increases, the soil plug heave increases, while the penetration resistance reduces with increase of the final suction pressure. The effect of the friction of internal caisson walls has been also investigated and a conclusion is drawn that internal wall friction has a significant contribution to the penetration resistance and it can be implicitly represented by varying coefficient of internal wall friction. As for the penetration resistance, both jacked and suction installation have great dependency on the internal wall friction.

Investigation of scour effects on lateral behaviors of suction caisson

ABSTRACTThe use of suction caissons can reduce the development costs of offshore wind energy and has broad application prospects. However scour around marine foundations is inevitable, it gravely affects the stability of marine engineering. Therefore, there is an urgent need to study the weakening effects of scour on suction caisson. In this study, the variation trends of remaining soil parameters (the effective unit weight and the peak effective friction angle) after scour are examined with consideration of the dilatancy and stress history of sandy silt. It is found that the parameters of shallow soil change considerably after scour, and the larger the scour depth, the greater is the change in the parameters. However, the deep soil is less affected. On the basis of these findings, scour effects on the ultimate moment capacity of suction caisson are studied. The ultimate moment capacity is found to greatly reduce under scour, and its calculated value is larger than the actual value when the effects of dilatancy and stress history are ignored. To simplify calculation, it is feasible to replace the ultimate moment capacity when both dilatancy and stress history are considered with that when only dilatancy is considered.

Impedance functions for rigid skirted caissons supporting offshore wind turbines

Large diameter caissons are being considered as plausible foundations for supporting offshore wind turbines (OWTs) where reductions in overall cost and environmentally friendly installation methods are expected. The design calculations required for optimization of dimensions/sizing of such caissons are critically dependent on the foundation stiffness as it is necessary for SLS (Serviceability Limit State), FLS (Fatigue Limit State), and natural frequency predictions. This paper derives closed form expressions for the 3 stiffness terms (Lateral stiffness KL, Rotational Stiffness KR and Cross-Coupling term KLR) for suction caissons having aspect ratio between 0.5 and 2 (i.e. 0.5 < L/D < 2) which are based on extensive finite element analysis followed by non-linear regression. The derived stiffness terms are then validated and verified using studies available in literature. An example problem is taken to demonstrate the application of the methodology.

Undrained pullout capacity of modified suction caisson in clay by finite element limit analysis

ABSTRACTThe modified suction caisson (MSC) adds a short-skirted structure around the regular suction caisson to increase the bearing capacity and limit deflection. In this study, the undrained pullout capacity of MSC in nonhomogeneous clay with shear strength increasing linearly with depth was investigated using the finite element limit analysis. An implicit dimensionless pullout bearing capacity factor was proposed by considering the effects of adhesion factor α of the soil–suction caisson interface, embedment ratio d/l, diameter ratio D/d, and embedded length ratio L/l. Results indicate that the MSC can effectively provide a higher undrained pullout capacity. The tends to increase with increasing the embedment ratio d/l, diameter ratio D/d, and embedded length ratio L/l, and to decrease with increasing the adhesion factor α of the soil–MSC interface. A theoretical method of calculating the pullout capacity of the MSC was also proposed in terms of the vertical static force equilibrium.

Seismic analysis of a suction caisson foundation for offshore wind turbines

Seismic analysis of a suction caisson foundation for offshore wind turbines

The response of suction caissons to long-term lateral cyclic loading in single-layer and layered seabeds

Suction caissons are being increasingly considered as an alternative foundation type to monopiles for offshore wind turbines. Single caisson foundations (or monopods) for offshore wind turbines are subjected to lateral cyclic loading from wind and waves acting on the structure. Recent studies have considered the response of suction caissons to such loading in sand, but have generally been limited to a few thousand cycles, whereas offshore wind turbines will generally experience millions of loading cycles over their lifetime. This paper presents the results from a programme of caisson tests in sand, clay and sand over clay seabed profiles, where each test involved about one million cycles of lateral load. The capacity and rotation response is shown to approach that measured in the sand seabed when the sand–clay interface is located at or beneath the caisson skirt tip. In contrast to previously published studies in sand, one-way cyclic loading is the most onerous loading symmetry for a layered seabed with a sand thickness equal to half the skirt length. However, the rotation for this seabed profile is essentially identical if the load is sustained or cyclic, provided that the cyclic loading remains one way. Lateral cyclic loading was seen to increase caisson capacity by up to 30% – with a bias towards clay-dominated seabed profiles – and stiffness by up to 50%. Such stiffness increases need to be considered when assessing the system dynamics for the offshore wind turbine, as demonstrated in the paper.

Determination of Maximum Penetration Depth of Suction Caissons in Sand

The suction caisson is a large top-closed cylindrical steel structure in diameter, short in length and much thinner in skirt wall thickness. The total resistance of the suction caisson during installation consists of the tip resistance and the skirt wall friction. However, since the thickness of the skirt wall is very small, the skirt wall friction may produce additional vertical stress and shear stress in soil at the skirt tip level, and this additional vertical stress and shear stress will contribute to the increase in the skirt tip resistance. At the same time, seepage induced by suction also causes the tip resistance to reduce significantly. A modified slip-line field is proposed in this study estimating the tip resistance in terms of the slip-line theory. The expression obtaining the minimum suction to install the suction caisson is also proposed in terms of the force equilibrium. In addition, the critical suction is determined based on the mechanism of sand piping. Thus, the maximum penetration depth of the suction caisson can be reached when the critical suction equals the minimum suction. Results from calculations of the minimum suction and the maximum penetration depth have been proved to be in a good agreement with the measured data.