Suction Bucket Foundation Research Articles

Lateral cyclic behavior of OWT tripod suction bucket foundation in clays

Tripod suction bucket foundations are increasingly used to support offshore wind turbines (OWTs) due to their economic advantages and high overturning resistance. In the harsh marine environment, OWT tripod bucket foundations are subjected to long-term lateral cyclic loads from wind, waves and current. However, the response of tripod bucket foundations in clay to cyclic lateral loads is not well covered in the literature. Therefore, a three-dimensional (3D) numerical method is developed for analyzing their lateral cyclic response based on a simplified bounding surface model of clay. The numerical model is validated by comparing its results with scaled model test results available in the literature. The validated model is then used to investigate the rotation mechanism of tripod suction bucket under lateral monotonic and cyclic loading. The ‘bonded contact’ and ‘separable contact’ were set between the soil plug and the inner wall to simulate the upper limit and the lower limit cases of the tensile capacity of the suction bucket. The influences of the bucket-soil contact condition, loading direction and cyclic loading mode (one-way and two-way) on the cyclic behavior were analyzed. It was found that these factors have a significant impact on the cyclic responses, permanent rotation angle and failure mechanism of the tripod bucket foundation. The findings from this study can provide guidance for the design of OWT tripod bucket foundations.

The optimisation of anti-overturning capacities for tripod suction bucket foundations in clay

The tripod bucket foundation that combines three single buckets within a triangular shape, is an emerging alternative foundation solution to support offshore wind turbine in deep water. Its performance of geotechnical capacity, especially under overturning loading, is increasingly concerned by offshore engineering. This paper reports an investigation to anti-overturning capacity of tripod bucket foundations in clay, with focus on the relatively larger foundation embedment. The kinematic mechanisms of the whole foundation system are systematically examined against increasing foundation spacing, considering the influence of foundation embedment. Based on these, an efficient method to predict the anti-overturning capacity is proposed, covering a wide range of embedment, spacing and soil strength heterogeneity. Finally, some recommendations of optimization for sizing foundation are offered, with the example application demonstrated.

Sensitivity Analysis of Factors Affecting the Bearing Capacity of Suction Bucket Foundation in Soft Clay

A suction bucket is the foundation for the development of offshore wind power technology in the deep sea, and its stability is crucial to the superstructure of the wind power generation system. Combined with soft clay soil strata along the Chinese coast, the bearing capacity of suction bucket foundations was studied using a numerical model. Sensitivity factors such as soil strength with random space distribution, dimensions of foundation, wind and wave loads in different directions, and cycle times were considered. The results show that the normalized foundation bearing capacity coefficient increases with the increase of the foundation length–diameter ratio. When the foundation length–diameter ratio is less than 1.0, the foundation bearing capacity coefficient is more sensitive to the soil non-uniformity coefficient than the length–diameter ratio. When the length–diameter ratio of the suction bucket is large enough, the influence of the soil non-uniformity coefficient on the bearing capacity of the suction bucket foundation gradually diminishes. When the direction of wind and wave loads is 15°, the bearing capacity of the suction bucket foundation is the weakest. Under the cyclic loads, the shallow soil strength weakens faster in the initial stage and the attenuation rate of soil strength slows down in the latter stage.

Assessing the life cycle risks of offshore wind turbines with suction bucket foundations

The risk assessment of offshore wind turbines is increasingly significant due to the increase in accidents. However, the life cycle risk assessment of suction bucket foundations is challenging owing to the limited number of installations and lack of relevant data. In this study, an improved fuzzy fault tree analysis method (IFFTA) based on the analytic hierarchy process (AHP) is proposed to evaluate the life cycle risks of offshore wind turbines (OWTs) with suction bucket foundations. First, a fault tree analysis (FTA) model was employed to establish a risk index system for the suction bucket foundation. Then, the fuzzy set theory was performed to obtain the crisp probabilities of basic events, and AHP was adopted to determine the importance weights of basic events. Finally, the risk occurrence probability and risk level of OWTs with suction bucket foundations were determined, and the critical risk factors were identified. A case study demonstrated the applicability of the proposed method. The results show that the risk occurrence probabilities of OWTs with suction bucket foundations in the construction, service, survey and design periods are 3.25e-03, 4.47e-03, and 5.48e-04, respectively, and the corresponding risk levels according to DNV-RP-F107, one of the most authoritative recommended practices in the field of ocean engineering, are high, high, and medium respectively. The proposed method overcomes the limitations of traditional methods and provides some risk reduction measures for engineering references. • Improved fuzzy fault tree analysis is developed for the life cycle risk assessment of offshore wind turbines. • The risk factors of offshore wind turbines with suction bucket foundations are identified. • The critical risk factors are identified, and several risk reduction measures are recommended. • The economic advantages of offshore wind turbines with suction bucket foundations are studied.

Centrifuge modeling and numerical analysis on lateral performance of mono-bucket foundation for offshore wind turbines

The lateral loading performance of the mono-bucket foundation for offshore wind turbines is investigated in this study through a series of centrifuge tests and numerical analysis. Six mono-bucket models are considered to assess the influences of aspect ratio under the ultimate condition. The FE model is validated against the centrifuge test results, and the failure mechanism of the foundation is revealed. The parametric study and sensitivity analysis of the soil parameters are performed in the numerical simulations. A modified theoretical calculation method is proposed to estimate the ultimate lateral capacity of the suction bucket foundation by solving the limit equilibrium equations. A correction term is added in the calculation using the “m” method. The position of the rotation center is calculated, demonstrating a better accuracy compared to the previously reported geometric method. The modified calculation method is verified against the field test, the centrifuge test, and the laboratory test. The method is proved to be applicable in estimating the lateral performance of the mono-bucket foundation in a general form. This study aims to provide well-documented data and design references for practical engineers.

Response of offshore wind turbine tripod suction bucket foundation to seismic and environmental loading

Tripod suction buckets offer many advantages as foundations for offshore wind turbines including fast and economic installation and high overturning resistance. In this application, tripod suction buckets are subjected to dynamic loads such as wind and waves as well as earthquakes. However, their dynamic and seismic performance characteristics in cohesive soils are not well covered in the literature. This study investigates the dynamic behavior of tripod foundation installed in clay employing advanced three-dimensional nonlinear finite element analysis. Its rotation mechanism is analyzed considering different load cases including environmental loads, seismic load or combined action of seismic and environmental loads. The dynamic response was evaluated considering ground motions with varying intensity and frequency content, and environmental loads with varying magnitudes. It was found that even though the OWTs system may not experience instability failure due to the combined action of seismic and environmental loads, it is likely to fail to meet the requirements of SLS due to large permanent rotation associated with large settlement of the leeward bucket compared to that of the windward bucket. It was found that the frequency content of the ground motion has a significant effect on dynamic response of tripod foundation; larger permanent rotation can occur when the seismic predominant frequency is close to the frequency ranges of 1p and 3p for the wind turbine structure. The rotation angle of wind turbine structure depends on both the predominant frequency and intensity of the ground motion. These observations should be considered in the seismic design of tripod foundations of OWTs.

Analysis of Bearing Performance of Monopile and Single Suction Bucket Foundation for Offshore Wind Power under Horizontal Load

In conjunction with the geological conditions of the East China Sea, the bearing performance of monopile and single-suction bucket foundations is compared and analyzed in shallow and deep-sea conditions under static horizontal loads. Furthermore, the statistical data of wind and wave from 2010 to 2020 in the East China Sea were tabulated into amplitude curves applied to the two foundations in the form of dynamic loads, and the bearing performances of the two foundations under dynamic loads were analyzed. The results show that the typical suction bucket foundation for a wind turbine currently designed in the shallow sea is destabilized under static horizontal loads, while the pile foundation is more stable; both foundations are stable in the deep-sea area. However, the suction bucket foundation displacement is less than the pile foundation. Under dynamic loading, the maximum displacement of monopile in the shallow sea was 127 mm. The maximum displacement of the suction bucket foundation was 434 mm, and the foundation was unstable. Both foundations are stable in deep-sea conditions, and the maximum displacement of the pile foundation is 1.4 times the maximum displacement of the suction bucket. Considering the difficulty construction in the deep sea, it is recommended to use suction bucket foundations.

Investigation of the Penetration Resistance Coefficients for the CPT-Based Method for Suction Bucket Foundation Installation in Sand

Investigation of the Penetration Resistance Coefficients for the CPT-Based Method for Suction Bucket Foundation Installation in Sand

Response of suction bucket foundation subjected to wind and earthquake loads on liquefiable sandy seabed

Offshore wind turbines (OWTs) are now being built in seismically active regions with the implementation of net-zero emissions of carbon dioxide. Safe operation of OWTs on suction bucket foundation is severely threatened by earthquake-induced liquefaction. This research investigated the performance of the suction bucket foundation supported the OWT in the liquefiable sand under wind and earthquake loads. A series of three-dimensional numerical models were established to perform the nonlinear dynamic analysis, in which the sandy seabed was described by an advanced elastic-plastic constitutive model. Various factors affecting seismic responses of the suction bucket foundation were analyzed, including the wind loads, seismic loads, aspect ratios of suction buckets, and sand densities. The responses were evaluated in terms of accelerations, excess pore water pressure ratios, horizontal displacements, settlements, and rotations of the OWT under wind and earthquake loads. The OWT may suffer a permanent tilt to exceed the serviceability limit state of the OWT due to wind loads, earthquakes, and liquefaction. The deformation mechanism of the suction bucket foundation under wind loads and earthquakes was revealed. The rotation-based proposed design charts were given for checking the design of seismic conditions. These research results can be used in a comparative sense for the design of the OWT in seismically active areas.

Seismic response of suction bucket foundation for offshore wind turbines: A parametric study

Offshore wind turbines are sensitive to tilt failures due to earthquake excitation. Soil liquefaction must be checked in seismically active areas. This study investigates the seismic response of suction bucket foundations in offshore wind turbines that are subjected to earthquake loadings. A series of shaking table tests are performed in a centrifuge, and a numerical analysis is performed in OpenSees and verified against centrifuge tests. The distribution law of the effective stress and excess pore water pressure ratio at the site is also analyzed. A parametric study is conducted by evaluating the influence of bucket configurations, overburden pressures, and seismic intensities. The soil inside and below the bucket foundation is strengthened and resists liquefaction more effectively. The bucket diameter and overburden pressure are key factors in enhancing seismic stability. The anti-liquefaction improvement ratio is assessed, and the distribution law of the anti-liquefaction performance of the soil in the area affected by the additional stress is described. A correction factor is proposed to modify the additional stress ratio by considering the distribution law of pore water pressure. This study provides a revised method to estimate the anti-liquefaction performance of the suction bucket foundation.

Formulas for Uniaxial Capacities of Tetrapod Bucket Foundations Considering Group Effects in Undrained Clay

Suction bucket foundation is a novel and cheaper foundation used in marine structures, such as offshore wind turbines, breakwater and oil platforms. Compared with a single bucket foundation, tetrapod bucket foundations can bear larger loads because of the group effects. However, the vertical, horizontal and moment capacity factors of tetrapod bucket foundations have not been presented in existing specifications. A series of three-dimensional finite-element analyses were conducted to investigate the group effects on uniaxial capacities and failure mechanisms of tetrapod bucket foundations in undrained clay considering various foundation separation distance ratios, embedment depth ratios, soil-strength heterogeneity indices and load direction angles. Generalized formulas for undrained uniaxial capacities of tetrapod bucket foundations were proposed in order to establish a bridge connecting the capacities of tetrapod bucket foundations and those of the single bucket foundation, which can provide a reference for industrial designs of capacities of tetrapod bucket foundations. The results show that the vertical group effect factor of tetrapod bucket foundations is basically not affected by the foundation separation distance ratio, embedment depth ratio, soil-strength heterogeneity index and load direction angle, which can adopted 0.9 based on a conservative estimation. The normalized horizontal and moment group effect factors of tetrapod bucket foundations are both affected by the separation distance ratio, embedment depth ratio and soil-strength heterogeneity index, but the moment group effect factor is also obviously affected by the load direction angle. The value of the horizontal and moment capacity factors of tetrapod bucket foundations are about 2.3 and 13.8 times that of a single bucket foundation, respectively, when the separation distance ratio is 3.5, embedment depth ratio is 1.0 and soil-strength heterogeneity index is 10, which have both been significantly enhanced. A value of 3.5 is suggested for the separation distance ratio to attain good capacities and a relatively high global stiffness for the tetrapod bucket foundations.

Seismic response of offshore wind turbines supported on Monopiles and Suction Buckets: Numerical modelling and soft computing study

In this study, a coupled numerical modeling and soft computing was carried out to investigate the seismic response and liquefaction resistance of two common types of foundations used for offshore wind turbines (OWT), namely the Monopile and Suction Bucket Foundations under combined loading of wave, wind and earthquake. A series of fully coupled non-linear dynamic analysis was performed using FLAC-3D to produce the database required in the soft computing procedure by evaluating the effect of different factors such as foundation's geometrical properties (foundation's type and its aspect ratio), soil properties (relative density and depth of liquefiable layer) and the input ground motion parameters (frequency and amplitude). Parameters of SANISAND constitutive model were calibrated using centrifuge studies performed on OWTs. Afterwards, genetic algorithm (GA) was used to develop prediction models for prediction the seismic response parameters, especially the maximum value of tower's rotation, which was verified against the results of centrifuge experiments.

Site investigation and soil parameters for offshore suction bucket design: A case study of Houhu wind turbine

The offshore wind farm industry in China has been growing exponentially in recent years, and suction bucket foundations are increasingly applied as an effective solution. This paper presents a comprehensive site investigation and outlines the key soil parameters for offshore suction bucket foundation design, by taking the Houhu wind farm in southern China as an example. The in-situ cone penetrometer (CPTU) tests and sampling borehole were carried out to investigate the geotechnical features of soils. The soil conditions are characterized by multilayers and complicated sediment components. The classification and mechanical properties, such as stress history, static and cyclic shear strengths, and shear modulus of soils were obtained and compared by CPTU interpretation and advanced laboratory tests. Then the probable foundation optimization design solution from the perspective of soil parameters in the light of set up effect and cyclic behavior was discussed and highlighted. Based on the experience of this project and other projects in offshore China, some common challenges were summarized and the relative recommendations were proposed finally. The main objective of this study is to provide more reliable guidance of geotechnical investigation for the rapid development of wind farms in offshore China.

A Numerical Analysis on Lateral Resistance of Pile–Bucket Foundation for Offshore Wind Turbines

The large-diameter single pile has been widely used in marine wind turbine foundations. In order to improve its lateral bearing capacity, the suction bucket foundation (BF) can be used as a reinforcement method, which surrounds the periphery of the monopile foundation (PF), forming a new wind turbine foundation, i.e., pile–bucket foundation (P–BF). By using the general finite element software ABAQUS, in this paper, several numerical models are established to investigate the influence of pile diameter and bucket diameter on the lateral bearing capacity of large-diameter monopiles. The numerical results show that the internal force distribution along the pile shaft for the case of P–BF is similar to the case of PF. Compared to bucket height, bucket diameter has more of an effect on lateral capacity and the p-y curve in the P–BF. In the combined P–BF, the buckets can provide lateral bearing capacity, and the piles can provide anti-overturning moments, resulting in higher lateral bearing capacity. In this paper, the p-y curve in the API specification is modified based on the results from the finite element simulation. The modified p-y curve fits well with the results of the finite element calculation and can be used as a reference for the design of the P–BF in actual engineering.

Integrated floating method based on four-bucket jacket foundation for offshore substations and converter stations

ABSTRACT The construction of deep-sea and offshore wind farms is key to the development of offshore wind power. Since the forms of bottom supporting structures and their installation methods greatly affect the overall cost of electrical platform, the latest developments in this filed were reviewed, based on which a novel integrated floating method was introduced. First, by incorporating the advantages of the integrated offshore wind turbine and the installation method for suction bucket foundation, the idea of integrated floating and quick installation based on a four-bucket jacket foundation was proposed. Second, the process of integrated wet towing of the four-bucket jacket foundation was described, which was further verified via indoor model experiments and numerical simulations. Results show that under 2 m wave height and level 5 sea conditions, the proposed four-bucket jacket foundation can be wet towed and floated at a speed of 5 knots under the optimum draft depth.

Seismic Fragility Assessment of a Novel Suction Bucket Foundation for Offshore Wind Turbine under Scour Condition

This study proposed a new suction bucket (SB) foundation model for offshore wind turbines (OWT) suitable for a shallow muddy seabed, using more than three single buckets through kinetic derivation. The performance of new optimal foundation was evaluated by its horizontal displacement capacity and compared with a conventional SB composed of three buckets. Under external loads such as earthquakes, wind, and the combination of the both, the stability of this novel SB foundation was verified. The seismic fragility curve was also evaluated at some scour depths. These results were compared with the response of a tripod suction bucket (TSB) foundation, which was also designed for a shallow muddy seabed. The results indicated that scour significantly changed the dynamic response of this novel SB foundation but it had a better bearing capacity than the TSB foundation, despite its smaller size and weight. The fragility of TSB is always higher than the developed foundation in the same environmental condition. With reasonable volume and size, this novel SB foundation has great potential for future industrialization and commercialization.

Investigation of Floating Transportation Equipment for the Offshore Wind Power

This paper presents a floating transportation equipment (FTE) for the negative pressure suction bucket foundation (NPSBF) of offshore wind turbines, and the basic design and main transportation means are introduced. The hydrodynamic characteristics of integrated FTE-NPSBF structure are comprehensively studied. The ability of FTE as a transportation aid to provide stability for the NPSBF is verified, and the vibration reduction measures under the condition of wave resonance during the floating transportation process are given.

A study on the behaviour of offshore suction bucket foundations under cyclic axial compressive loading

ABSTRACT Suction bucket foundations of offshore wind turbines (OWTs) are subjected to cyclic loads induced by wind and waves. The behaviour of buckets embedded in saturated sandy soil subjected to cyclic axial compression was studied using a finite element (FE) model. Accumulation of permanent soil deformations and excess pore water pressure in saturated sandy soil due to cyclic axial compression were taken into account by a fully coupled two-phase model combined with a hypoplastic constitutive model with intergranular strain. The Kozeny–Carman relationship was adopted for porosity–permeability dependence. The main focus was dedicated to the changes in load transfer via components of bucket with increasing number of load cycles. Loads that led to progressive failure, attenuation and shakedown behaviour were described. Effects of soil permeability and density, load frequency and amplitude, the bucket aspect ratio on the load transfer and bearing mechanism of the bucket were presented.

Centrifuge shaking table tests on offshore wind turbine bucket foundation in mildly inclined liquefiable seabed

Increasing number of offshore wind farm projects with wind turbines supported by suction bucket foundation in mildly sloping sandy seabed have being planned in earthquake-prone areas. Hence, it is crucial to evaluate the seismic performance of suction bucket foundations for offshore wind turbines in such liquefiable ground, especially considering the interaction between the bucket and both outside and inside bucket soil. Based on a pair of centrifuge shaking table tests, the seismic response of the bucket foundations and turbines in liquefiable ground is evaluated in this study. The dynamic response of soil, foundation, and turbine are presented. The test results indicate that the horizontal displacement of mildly sloping liquefiable seabed during earthquakes can results in excessive displacement and rotation of the bucket foundation. The influence of inside-bucket soil on foundation and turbine response, and especially on the kinematic and inertial interactions of the system are analyzed in detail. The analysis results indicate that inside-bucket soil plays an important role in stabilizing the foundation-turbine system during earthquakes, restricting the displacement and rotation of the foundation and the acceleration of the turbine.

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.