Tripod Suction Bucket Research Articles

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.

Seismic response of tripod suction bucket foundation for offshore wind turbine in sands

Tripod suction bucket foundation is increasingly used to support offshore wind turbines (OWTs) due to its convenient and low-cost installation and high overturning resistance. However, the seismic response characteristics of OWTs of tripod bucket foundations in sands is not well covered in the literature. Therefore, this study investigates the response of tripod bucket foundations installed in sands and subjected to seismic loads employing advanced three-dimensional finite difference analysis. The variation of pore pressure around suction bucket is evaluated, and the failure mechanism of the tripod bucket foundation in sands is revealed. In addition, the displacement response of the tower, and the acceleration amplification effect of the OWT system are elucidated. It is found that the earthquake magnitude, and sand relative density and liquefaction greatly affect the foundation seismic behavior. The results revealed that the overall rotation of the tripod bucket foundation is primarily due to progressively increasing differential vertical settlement between the foundation sides of ‘single bucket’ and ‘double bucket’ associated with loading direction. It was also found that the sand liquefaction under strong shaking may lead to excessive rotation of the foundation that exceeds the service limit state. The findings from this study can provide guidance for the design of OWT tripod bucket foundations in sands.

On the cyclic stability diagram of offshore wind turbine supported on multiple foundations in saturated dense sand

This study explored the issue of cyclic loading on a tripod suction bucket foundation system in silty sand through three-dimensional (3D) finite element (FE) models by developing a cyclic stability diagram, in which the response is presented in terms of the average load ratio (ALR) and the cyclic load ratio (CLR), and three regions are identified: (1) no cyclic load effects, (2) cyclic load effects, and (3) severe load effects after approximately 1000 one-way load cycles. The simulations, based on dynamic FE analysis, enabled the identification of the ‘’self-healing’’ phenomenon due to enhanced stiffness degradation of a foundation in tension by incorporating the correct estimate of the soil-foundation interface and geometric nonlinearities. In addition, an approximating expression for the calculation of the accumulated rotation of offshore wind turbine (OWT) supported on multiple foundations is provided as a function of ALR and CLR utilizing modified least-squares method (MLSM). The proposed expression leads to notable improvements in the prediction of induced rotation of OWTs such that the coefficient of determination, R2, was obtained ranging from minimum value of 0.89 to a maximum value of 0.99. This study suggests that the salient features of multiple foundations must be considered in the design process of offshore wind turbines.

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.

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.

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.

Simplified estimation of rotational stiffness of tripod foundation for offshore wind turbine under cyclic loadings

As the importance of safe and economical foundation design of offshore wind turbines has increased, suction bucket foundations have become increasingly common. A tripod suction bucket foundation consists of three individual buckets arranged in a triangular shape. The cyclic behaviour and soil-foundation-structure interaction of single suction bucket foundations has been a subject of active research. However, it is difficult to predict the behaviour of tripod bucket foundation because it is structurally complex and requires consideration of cyclic load effects. This study focuses on the behaviour of tripod suction foundations under cyclic loading. The rotational stiffness of the tripod foundation is estimated by combining equations depending on compression, pullout, and rotational stiffness of the single suction bucket with a series of centrifuge model test results. By presenting the theoretical stiffness values, it is verified that the rotational stiffness can be estimated only based on the cross-section of the foundation and the shear modulus of the soil. In addition, it is possible to predict the long-term rotational stiffness of the tripod by applying the vertical stiffness obtained from the cyclic loading test of the single bucket to the simplified method.

Studies on cyclic behavior of tripod suction bucket foundation system supporting offshore wind turbine using centrifuge model test

AbstractTo be competitive in offshore wind energy production, safe and economical foundation design is essential. In recent years, tripod suction bucket foundations have been considered as an alternative to conventional foundations owing to their unique features suitable for offshore construction environments, economic installation, and high overturning resistance. However, it is difficult to accurately predict the behavior of tripod foundation because the load acting on the tripod is complex in HVM (i.e., horizontal, vertical, and moment loads) and the response varies depending on the size and direction of the load. Moreover, it is harder to analyze because the effects of cyclic loads must be considered in an offshore environment. This study, therefore, has investigated the behavior of the tripod suction bucket foundation under cyclic loadings. To analyze the complex responses of the tripod foundation in detail, the overall behavior of the tripod foundation system was observed based on the compression–pullout behavior of a single bucket. Moment–rotation responses, the cyclic stiffness, and permanent displacements of the tripod foundation are evaluated by analyzing the vertical behavior of the single‐bucket foundations as well as the rotational behavior of the tripod foundation. A number of centrifuge model tests were carried out with different loading conditions (i.e., loading amplitudes and directions). It was confirmed that the cyclic behavior of the tripod bucket foundation is significantly affected by loading amplitudes and directions. Furthermore, this study emphasized the importance of considering load characteristics when designing the tripod foundation.

Centrifuge modelling of drained pullout and compression cyclic behaviour of suction bucket

In recent years, the tripod suction bucket foundation has been considered as an option to support offshore wind turbines. In an offshore environment, a wind turbine foundation should be designed considering cyclic loading, which can lead to the permanent deformation of structures, tilting and the degradation of soil stiffness. However, there are several uncertainties in the prediction of the cyclic behaviour of the tripod foundation, since the cyclic horizontal, vertical and moment loadings applied on each bucket lead to a complicated mechanism for each bucket. In this study, the behaviour of the tripod foundation under cyclic loadings was indirectly investigated through the vertical compression and pullout tests of the bucket because the overturning moment applied to the tripod foundation presumably acts as vertical compressive and pullout loads on each bucket. A series of centrifuge model tests were performed by applying one-way (i.e. vertical compression and pullout) and two-way cyclic loadings to the single bucket. Additionally, the effects of cyclic loading level were investigated. It was confirmed that the cyclic behaviour of the single bucket is significantly affected by loading direction and level. This study highlights the importance of considering load characteristics when predicting the cyclic behaviour of the tripod.

Investigating the monotonic behaviour of hybrid tripod suction bucket foundations for offshore wind towers in sand

Existing tripod suction bucket foundations, utilised for offshore wind turbines, are required to resist significant lateral loads and overturning moments generated by wind and currents. This paper presents an innovative type of tripod bucket foundation, ‘hybrid tripod bucket foundation’, for foundations of offshore wind turbines, which has the ability to provide a larger overturning capacity compared with conventional tripod buckets. The proposed foundation consists of a conventional tripod bucket combined with three large circular mats attached to each bucket. A series of experiments were conducted on small-scale models of the proposed foundation subjected to overturning moment under 1g conditions in loose sand. Different circular mat diameter sizes with various bucket spacings were considered and the results were compared with conventional tripod bucket foundation. Finite element models of the proposed foundation were developed and validated using experimental results and were used to conduct a parametric study to understand the behaviour of the hybrid tripod bucket foundation. The results showed that there is a significant increase in overturning capacity provided by the novel foundation. The results of this work can significantly improve lowering the costs associated with installation of foundations to support offshore wind turbines.

Cyclic behavior of unit bucket for tripod foundation system supporting offshore wind turbine via model tests

AbstractOffshore wind energy has been growing up as a promising renewable energy source. Recently, tripod suction bucket foundation is rapidly expanding as a foundation system supporting the offshore wind turbine. In offshore environment, wind turbine foundation structures should be designed considering cyclic loading which can lead to permanent deformation of structure, tilting problem, and overall degradation of soil stiffness. However, it is technically difficult to predict the cyclic behavior of the tripod accurately because the cyclic behaviors of the tripod bucket can be inferred from vertical pullout and compression behaviors of each single bucket elements. In this paper, a series of model tests was performed by applying cyclic vertical compression and extension loadings to a single bucket element that is one element of the tripod foundation. Loading directions, level, and rate were controlled for investigating of cyclic behavior of tripod foundation. On the basis of testing results, the permanent deformation and cyclic stiffness response of tripod suction caisson were discussed. Based on the test results, it was confirmed that the cyclic behavior of the single bucket is affected by the load level and rate. In addition, the behavior showed quite different trends with the loading directions: compression, pullout, and two‐way.

Investigation of Monotonic and Cyclic Behavior of Tripod Suction Bucket Foundations for Offshore Wind Towers Using Centrifuge Modeling

This paper performed centrifuge load tests of a tripod bucket foundation preliminarily designed as a supporting system of wind turbines and compared the results to those obtained from a test of a monopod bucket foundation. The tripod foundation prototype studied in this study has three bucket caissons, each of which is 6.5 m in diameter and 8.0 m in length. The center-to-center distance between the buckets was 26.9 m. The site is composed of an 11-m thick layer of dense silty sand overlying a thick sandy silt layer. The horizontal load was applied at a height of 33 m from the seabed floor according to the design load condition, and the vertical load was simulated by the self-weight of the model. The moment-rotation angle curves for the tripod foundations were compared with that of the monopod bucket foundation. The moment-rotation curve of the tripod was nearly bilinear, whereas that of the monopod showed a gradual decrease in slope. The yield moment for the tripod foundation was half that of the monopod, but the rotation angle for the yield moment was only 20% that of the monopod. The behavior of the tripod foundation under a cyclic load with respect to the accumulated plastic deformation has also been examined in this study. When the resultant moment of cyclic loading was smaller than the monotonic yield moment, negligible accumulated plastic deformation was observed for both one-way and two-way loading. However, when the resultant moment was higher than the monotonic yield moment, significant cumulated deformation resulted.