Bucket Foundation Research Articles

Sensitivity of Seabed Characteristics on the Seismic Performance of Suction Bucket-Supported Offshore Wind Turbines

The suction bucket foundation equipped for offshore wind turbines was a promising solution for sandy seabed locations. However, its typically short embedment depth presented additional challenges when installed in seismic zones. These challenges pertained not only to structural response but also to the seismic motion itself, which was strongly influenced by soil characteristics. This study examined the uncertainty of equivalent shear-wave velocities to explore the variability in input seismic motion characteristics and investigated their impact on the structural response in terms of tower-top displacement, mudline displacement, and acceleration amplification factor at the hub height of 3 MW and 5.5 MW suction bucket-supported offshore wind turbines (OWTs). Additionally, the influence of equivalent shear-wave velocities on the exceedance probabilities of various damage states, using fragility curves for tower-top and mudline displacement, was analyzed. The results indicated that equivalent shear velocities of soil significantly impacted the seismic performance of suction bucket-supported offshore wind turbines. These effects were closely related to the intensity of the seismic motion, highlighting the importance of carefully considering the correlation between site-specific shear velocities and earthquake intensities.

Large-scale penetration model tests of bucket foundations of different structural types in clay

Clarifying penetration characteristics and accurately predicting penetration resistance of bucket foundations is crucial to ensure their smooth penetration. This study performed large-scale model tests on nearshore seven-compartment (SC) bucket foundations and novel deep-sea five-connected (FC) bucket foundations to systematically explore the penetration attitude, development of soil plugs, flow rates, bucket-soil interactions, and required suction of these two structures in clay. The good penetrability was identified for the new FC bucket foundation. The development of soil plugs due to bucket wall replacement as well as the mechanism of drag reduction of the inner wall and bucket end were revealed. The experiments demonstrated that the penetration flow rate was almost equal to the volume of bucket body entering the soil and accounted for approximately 95% of the total flow rate. A formula for calculating penetration resistance in clay that considers the effect of suction on the reduction in resistance of the inner wall and bucket end was proposed and validated by field tests. The research results improve the safety of bucket foundation penetration in clay and provide significant guidance for the installation of deep-sea FC bucket foundations.

Dynamic behavior of kaolin clay under bidirectional cyclic loading for suction bucket foundation

The stability of offshore wind turbine (OWT) foundations is largely influenced by the dynamic characteristics of marine clay. In offshore engineering, marine clays may exposed to bidirectional cyclic stresses. To empirically examine the influence of different cyclic stress magnitudes on the dynamic characteristics of marine clay at different depth, a sequence of dynamic triaxial tests is conducted on marine kaolin clay, encompassing diverse values of the cyclic stress ratio (CSR) and effective confining pressure (p'0). Some valuable inferences have been drawn: At low cyclic stress ratios (CSR≤0.3), the axial strain (εa), stiffness (k), and excess pore water pressure (EPWP) of the specimen exhibit logarithmic increase, logarithmic decrease, and logarithmic accumulation, respectively. Under high cyclic stress ratio conditions (CSR＞0.3), the axial strain increases linearly, the stiffness deteriorates linearly, and the logarithmic excess pore water pressure accumulates. After analyzing the dynamic characteristics data of kaolin clay, the correlation between excess pore water pressure and axial strain was established. By analyzing the vertical cyclic mechanism of suction bucket foundation, a prediction model for suction bucket foundation under vertical cyclic loading was established based on the dynamic characteristics of marine clay and the model was validated. The comparison between the validation outcomes and the experimental outcomes shows good agreement. This study provides certain guidance for the design of suction bucket foundations for OWTs.

Optimization of spudcan penetration and influence on caisson foundation

The penetration of spudcans have adverse effects on pre-existing bucket foundations during the blade lifting process of offshore wind power systems, which brings uncertainties for the design and construction of bucket foundations. This study employs the Euler-Lagrange method to explore the influence of spudcan penetration on the adjacent caissons in homogeneous clay within a large deformation finite element (LDFE) model. The LDFE model is validated against centrifuge tests data, with good agreement obtained. Parametric study is then conducted to explore, (i) the influence of the angle between vessel and caissons on the movements of the caisson, and (ii) potential influence factors on the movements of the caisson with an optimal angle between the vessel and caissons, including soil properties, dimensions of caisson and spudcan, and the distance between spudcan and caissons. The results show that 60° is the optimal angle between the vessel and caissons that results in the smallest movement of the caisson, and the caisson foundation inclination angle ranges between 0° and 0.025°, when the pile shoe penetration progress ranges between 0.1 and 1.0. An empirical formula is proposed to predict the rotational angle of caisson with the optimal angle. The study establishes a quantitative approach to assess the influence of the spudcan penetration on existing three-caisson jacket foundations, which provides guidance for the design and construction of offshore wind turbine bucket foundations.

Experimental study on local scour for large-diameter mono-column composite bucket foundation for offshore wind turbines

The issue of seabed scour around offshore wind turbine foundations is a significant safety concern. In this study, local scour around a specific type of foundation, known as the large-diameter Mono-Column Composite Bucket Foundation (MCCBF) with six connectors, has been investigated through physical experiments. The study involved various flow conditions, including unidirectional flows, tidal flows, and combinations of regular waves and currents. It has been found that under similar flow intensity and orientation angles, unidirectional flow tends to cause deeper scour depths, but with a more limited extent compared to scour induced by tidal flows. The disparity in maximum scour depth between tidal and unidirectional flows decreases as flow velocity increases. Under regular wave-current interaction, both the scour depth and the scour extent are larger than those induced by unidirectional flow. The presence of lee-wake vortices and streamline contractions near the connectors results in larger scour depths on either side of the foundation, especially when the orientation angle is set to 0°. Different methods for predicting scour depth of the MCCBF were applied and discussed.

Research on semi-active vibration reduction of offshore wind turbine structure combined eddy current theory with tuned mass dampers

At present, the offshore wind power gradually shows a development trend towards the profound ocean with characteristic of larger capacity, higher tower and longer blades. Therefore, the offshore wind turbine (OWT) is more vulnerable to extreme loads and may lead to the negative influence of structural vibration safety. In this research, a new semi-active eddy current tuned mass damper (SEC-TMD) was proposed for the vibration control of OWT structures and its vibration reduction effect under multiple kinds of dynamic loads was also discussed. Firstly, taking one actual 3.0 MW OWT supported by bucket foundation as object, the SEC-TMD was designed based on the linear quadratic optimal control (LQR) algorithm and bounded Hrovat optimal control algorithm in order to obtain the optimal control force of structural vibration systems. The relationship between damping coefficient and magnetic conductivity spacing was further fitted to achieve variable damping control of SEC-TMD. Secondly, the theoretical model of 3.0 MW OWT structure installed with SEC-TMD was established and the effectiveness of the established theoretical model was verified through the comparison on modal analysis between the theoretical calculations and measured data identification. Finally, the vibration reduction effect of SEC-TMD on the measured OWT structure under wind-wave, seismic and vibration amplification conditions was analyzed and compared with the results calculated from the other passive dampers including EC-TMD and TLCD. It is shown that the maximum reduction percentage of peak value for vibration displacement and acceleration on the tower top can respectively reach 31.93 %, 44.11 %, 30.20 % and 54.57 %, 33.94 %, 31.75 % under three above conditions. Comparing with EC-TMD and TLCD, the better vibration reduction effect indicates that the proposed SEC-TMD has good engineering application value for the vibration suppression of OWT.

Seismic response of the composite bucket foundation supported offshore wind turbines under marine environmental loadings

Many offshore wind turbine (OWT) foundations are situated in earthquake-prone areas, where complex environmental loads intensified the seismic responses of the OWTs. In response to this situation, the composite bucket foundations is proposed and extensively applied due to its reliable installation and high seismic resistance. To analyze the seismic response of composite bucket foundations subjected to environmental loads, refined numerical models of the 6.45 MW OWT system are established, and parametric analyses are conducted, considering the influence of seismic frequency contents and intensities, the soil layer distributions and wind velocities. The results show that the composite bucket foundations undergo cumulative rotational deformation around a point biased toward the windward side of the foundation under the seismic and environmental loads. The increasing mean wind velocity significantly enlarges the displacement response envelope, while has limited influence of the acceleration response envelope of OWTs. Increasing the bucket diameter enhances the deformation resistance of the OWT, while the efficiency of this improvement diminishes as the diameter increases. Greater bucket skirt length also enhances its deformation resistance, but may lead to increased lateral acceleration response of the OWT.

Numerical Simulation of Vertical Cyclic Responses of a Bucket in Over-Consolidated Clay

Multi-bucket foundations have become an alternative for large offshore wind turbines, with the expansion of offshore wind energy into deeper waters. The vertical cyclic loading–displacement responses of the individual bucket of the tripod foundation are relevant to the deflection of multi-bucket foundations and crucial for serviceability design. Finite element analyses are used to investigate the responses of a bucket subjected to symmetric vertical cyclic loading in over-consolidated clay. The Undrained Cyclic Accumulation Model (UDCAM) is adopted to characterize the stress–strain properties of clay, the parameters of which are calibrated through monotonic and cyclic direct simple shear tests. The performance of the finite element (FE) model combined with UDCAM in simulating vertical displacement amplitudes is evaluated by comparison with existing centrifuge tests. Moreover, the impact of the bucket’s aspect ratio on vertical displacement amplitude is investigated through a parametric study. A predictive equation is proposed to estimate the vertical displacement amplitudes of bucket foundations with various aspect ratios, based on the cyclic displacement amplitude of a bucket with an aspect ratio of unity.

Approximate settlement influence factors for bucket foundations

ABSTRACT The response of bucket foundations for offshore structures has received significant attention from researchers but most of the reported studies focused on ultimate capacity. In this study, a comprehensive series of finite element analyses have been performed to investigate directly how the skirt geometry affects the elastic settlement of bucket foundations of arbitrary rigidity embedded in either a homogeneous or heterogeneous soil medium with finite layer thickness. The effect of soil drainage conditions is also explored. The results are graphically presented in the familiar form of settlement influence factors which can be used in hand calculations to predict the settlement of bucket foundations for a wide range of practical cases. Approximating expressions that capture the settlement influence factors well are developed. A worked example is given to illustrate the use of the graphical and approximating solutions.

Bearing characteristics of mono-column composite bucket foundations for offshore wind turbines in layered soil

The mono-column composite bucket foundation (MCCBF) is a new offshore wind turbine foundation suitable for shallow overburden geology. There are few studies on the bearing capacity and deformation of the new subdivided structure's all-steel MCCBF in layered soil under cyclic loading. This article conducts cyclic bearing characteristics tests on MCCBF in layered soil and shallow overburden layers and studies the ultimate bearing capacity, cumulative rotation angle development rules, and stiffness evolution mechanism. Combined with the finite element analysis results, a calculation method for the ultimate bearing capacity under layered soil is established. The results show that under bidirectional and multidirectional cyclic loading, the ultimate bearing capacity of pure sand changes little, but the strength of clay is reduced, and the ultimate bearing performance decreases. In the clay overlying sand, under high-amplitude cyclic loading, the strength of the sand at the bottom increases, which increases the cumulative angle stiffness and ultimate bearing capacity of the MCCBF. After cyclic loading, the shallow overburden layer will prevent the load from transferring to the deeper soil layer. The foundation will drive the soil in the compartment to slide on the surface of the shallow overburden layer, resulting in a decrease in the load-bearing performance. The initial stiffness of the foundation is increased so that the stiffness change during the cycle is not apparent. Finally, the accuracy of the calculation formula for the ultimate bearing capacity of MCCBF under layered soil is proposed and verified.

Numerical investigation of the performance of bucket foundation installed in sand under vertical loading

Numerical investigation of the performance of bucket foundation installed in sand under vertical loading

Effect of soil stiffness on end bearing resistance of foundations in clay from large deformation numerical modelling

Accurately predicting the installation resistance is of great benefit to a rational and economical design of foundations. This research performs a large-deformation finite-element analysis to investigate the effect of soil stiffness on the end bearing resistance in uniform clay. Different types of geotechnical structures and foundations, e.g., cone penetrometer, strip foundation, bucket foundation, and deep piled foundation are considered. Results show that soil stiffness has a negligible impact during shallow penetration but becomes pronounced at deep penetration. It is found that for strip footings, effect of soil stiffness is minimal, whereas the end bearing resistances of CPT and deep piled foundation with a partial plug are highly dependent on the soil stiffness. For a rough pile foundation, the resistance factor Nc,pf increases by approximately 30% as soil rigidity Ir increases from 50 to 500. For a bucket foundation, the effect of soil stiffness on the end bearing resistance falls between that of “shallow” and “deep” foundations. Based on the numerical results, empirical expressions are proposed to predict the end bearing resistance factor accounting for the effect of soil stiffness.

Excess pore water pressure response of soil inside the mini bucket embedded in saturated soil under seismic loading

This study investigates the behavior of suction bucket foundations during seismic loading induced soil liquefaction through shaking table experiments using both steel and 3D printed mini buckets. The pore water pressure response of the soil inside and around the mini buckets was monitored, and the results show that excess pore water pressure ratios greater than 1.0, indicating liquefaction occurred inside and around the buckets. In order to verify whether the soil plug liquefies under seismic loading, a floating ball experiments was specifically designed. The findings suggest that behavior of suction bucket foundations may differ from that of gravity-based foundations when experiencing soil liquefaction, owing to their unique semi-enclosed geometric structure and suction installation method. The appropriately weighted steel mini bucket has the advantage of being less prone settlement in the liquefied soil, making it suitable as a monitoring platform for soil liquefaction.

Penetration Characteristics of Mono-Column Composite Bucket Foundation for Offshore Wind Turbines

To address the issue of seepage and soil plugging during the sinking of mono-column composite bucket foundations (MCCBFs), experiments were conducted on the installation of foundations under a variety of complex geological conditions. The effect of negative pressure sinking mode on the foundation’s seepage field was analyzed, the formation mechanism of soil plugging in sand was explained, and an efficient method for calculating the height of soil plugging was proposed in conjunction with these investigations. The findings demonstrate that the finite element method simulation of the soil’s seepage field is the most accurate, that the pumping volume of the foundation during sinking through is high, that the formation of the soil plug height is high, and that clear depressions can be seen on the foundation manifold’s exterior. The equation presented in this study makes it easier to forecast the height of a foundation plug and can offer direction for engineering design.

Seismic response of tripod suction bucket foundation for offshore wind turbine considering multidirectional earthquake loads

Tripod suction buckets offer several advantages as offshore wind turbine (OWT) foundations, including cost-effective installation, high stability, and resistance to overturning. In seismically active regions, OWTs experience horizontal and vertical directions seismic excitations. This study employed an advanced three-dimensional nonlinear finite element analysis to investigate the influence of vertical seismic motions on the dynamic behavior of tripod bucket foundations installed in nonhomogeneous clay deposits. By adopting a simple kinematic hardening model to capture the foundation-soil nonlinear dynamic interaction, parametric analyses were conducted to study the effect of vertical seismic motions, considering several earthquake types and undrained shear strengths of the clay deposit. The results show that the influence of the vertical seismic motion on the dynamic response and deformation of the nacelle depends on the dominant frequency of the vertical seismic motion and the overturning resistance capacity of the OWT in different horizontal directions. The deformation mechanisms of an OWT vary with the strengths of the clay deposits. Based on these analyses, this study offers insights into the influence of vertical seismic motions on the dynamic response of tripod suction bucket foundations.

Foundation Types of Fixed Offshore Wind Turbine

Offshore wind turbines are supported by various foundations, each with its considerations in design and construction. Gravity, monopile, and suction bucket foundations encounter geotechnical issues, while jacket and tripod foundations face fatigue problems. Considering this, a gravity foundation based on a steel skirt was developed, and a monopile foundation was analyzed for Pile-Soil Interaction using the p–y curve and 3D finite element method (3D FEM). In addition, for suction bucket foundations, the effects of lateral and vertical loads were analyzed using 3D FEM and centrifuge tests. Fatigue analysis for jacket and tripod foundations was conducted using a hotspot stress approach. Some hybrid foundations and shape optimization techniques that change the shape to complement the problems of each foundation described above were assessed. Hybrid foundations could increase lateral resistance compared to existing foundations because of the combined appendages, and optimization techniques could reduce costs by maximizing the efficiency of the structure or by reducing costs and weight. This paper presents the characteristics and research directions of the foundation through various studies on the foundation. In addition, the optimal design method is presented by explaining the problems of the foundation and suggesting ways to supplement them.

On cyclic performance of suction bucket foundations in cohesionless soils

On cyclic performance of suction bucket foundations in cohesionless soils

Probabilistic failure envelopes of tripod bucket-supported offshore wind turbines in spatially variable clay based on a novel continuous method

Marine geologic conditions play key roles in offshore geotechnical preliminary design and construction decisions. Tripod bucket foundations are considered promising geotechnical foundations for offshore engineering. Their bearing characteristics are often determined based on tripod foundation system failure modes in uniform clay without considering spatially variable soil properties. Therefore, this study investigates the combined bearing capacity of tripod bucket foundations in spatially variable clay based on comprehensive finite element analysis with Monte Carlo simulation (MCS) using a novel simplified modified swipe (SMS) method considering the effects of bucket spacing and foundation skirt length. The SMS method can directly represent the ultimate combined bearing capacity of the whole space efficiently and accurately. The results show that a reduced bucket spacing or increased skirt length changes the failure mode of the foundation from independent caisson failure to global failure. Moreover, the spatial variation in soil leads to an asymmetric failure mode, which is more remarkable for foundations with small bucket group effects. An independent safety factor design approach is adopted to represent failure envelopes with different recurrence probabilities, which should be cost-effective, especially for foundations sensitive to soil variability. Finally, an application based on a convex polynomial optimization strategy for representing failure envelopes is demonstrated.

Undrained Capacity of a Bucket Foundation with a Limited-Tension Interface for Offshore Wind Turbines

Undrained Capacity of a Bucket Foundation with a Limited-Tension Interface for Offshore Wind Turbines

Tilt adjustment strategy and applicability of four-bucket jacket foundation for offshore wind turbines

With the rapid development of offshore wind turbine industry, the development of sea areas will gradually move from offshore to deep sea. In this process, multi-bucket jacket foundation will become a highly potential infrastructure solution in the water depth of about 50 m due to its unique advantages. The closed space is formed inside the bucket foundation during the penetration process, so the suction sinking method can be used for penetration, the accurate grasp of the critical suction during the penetration process of the four-bucket jacket foundation determines whether the penetration process can be carried out smoothly. At the same time, appropriate tilt adjustment strategies need to be adopted promptly after the inclination occurs during the penetration process. In this paper, the tilt adjustment technique in the penetration process of the four-bucket jacket foundation was taken as the research object. Firstly, the model test was used to study the critical suction of the foundation at three different penetration depths, and the test results are compared with the theoretical calculation results. Subsequently, foundation on the above research results, the applicability of the tilt adjustment strategy when the tilt angles of 1°, 2° and 3° occurred at three penetration depths was analyzed, which provides technical guidance for the development of practical projects.