Suction Bucket Research Articles

Seismic responses analysis of suction bucket foundation for offshore wind turbine in clays

Suction buckets are widely used as foundations for offshore wind turbines because of its many advantages such as simple structure, convenient installation, low construction cost and reusable. It is very essential to perform the seismic response analysis of suction bucket foundations for the design of offshore wind turbines. However, the related research is still relatively rare. In this research, three-dimensional finite element analysis is performed to predict seismic responses of suction buckets for offshore wind turbines in clays by using a simplified kinematic hardening constitutive model available in the Abaqus library. The seismic bearing mechanism of bucket foundation in clays is investigated, and various factors affecting seismic responses of suction bucket are analyzed, including the soil strength, seismic intensity and frequency, and bucket geometry. The research results can provide a reference for the seismic design of suction bucket foundations for offshore wind turbines.

Suction Bucket Foundations under Cyclic Tensile Loading—Physical and Numerical Modeling

Abstract Although suction buckets are believed to be a promising alternative for multipod foundations of offshore wind turbines, the economic and ecological advantages are still limited because of a lack of knowledge regarding the tensile bearing behavior. Major uncertainties exist in terms of the response under cyclic loading and the assessment of the partially drained loading condition, where negative differential pressure beneath the suction bucket’s lid contributes to the total tensile resistance. This article presents findings from 1-g model tests of suction buckets in sand subjected to various cyclic tensile loads. The suction bucket was installed via negative differential pressure (suction). The results show a strong effect of the applied load magnitude on the evolution of displacement, plug heave, and suction induced. Greater displacement accumulation is observed for the smaller considered frequency. Monotonic tests with varying displacement rates supplement the cyclic tests and serve for the verification of a hydraulic-mechanic coupled finite element model, which is afterward utilized for the back-calculation of cyclic model tests. Although a rather simple elasto-plastic material law was used for the sand, good agreement was found, indicating that the bearing behavior is mainly governed by the hydraulic conditions and only subordinately by the soil mechanics.

Effect of partial drainage on the pullout behaviour of a suction bucket foundation

Suction-bucket-supported jacket foundations are being used increasingly for offshore wind turbines, the advantages being large bearing capacity and efficient installation. For a large-capacity wind turbine, the pullout capacity and displacement of the bucket in the tripod or tetrapod jacket can be critical to the safety of the structure. The loading rate, which is commonly dominated by the offshore environmental conditions, has a significant effect on the ultimate capacity and stiffness of the bucket foundation. In this paper, a numerical model is developed to investigate the response of a bucket foundation under vertical pullout loading at different rates. A bounding-surface model based on critical state soil mechanics is introduced to account for the soil behaviour under different stress states and drainage conditions. The model is modified and calibrated against a comprehensive set of laboratory tests, including both triaxial compression, triaxial extension and direct simple shear tests in drained and undrained conditions. How partial drainage affects the load–displacement response and pore pressure field are thus presented. A parametric study on the selection of soil parameters is also conducted to shed light on practical foundation design.

Winkler springs for axial response of suction bucket foundations in cohesionless soil

Jacket structures mounted on suction buckets carry potential as cost-efficient foundations for next-generation 10 MW+ offshore wind turbines located in transitional water depths. Given this foundation method, resistance to overturning moment relies mainly on the axial response of the buckets. In practice, suction bucket foundations can be modelled as “Beams on Non-Linear Winkler Foundation” where soil is idealised as non-linear springs. This paper describes the derivation process of static load-transfer or t–z curves for suction buckets installed in cohesionless soil. The mathematical formulation of the curves is based on regression analysis of data obtained from 100 axisymmetric numerical models in a medium characterised by the Hardening Soil model for representing the stress–strain relationships for Frederikshavn sand, which is a typical offshore sand. Various bucket dimensions, soil properties and drainage conditions were simulated considering tension and compression, in order to describe frictional behaviour at the skirt–soil interface. The non-linear springs' properties are therefore linked to foundation diameter, friction angle and vertical overburden pressure. By superimposing the effects of all springs, load–displacement curves are generated and compared with results from available experimental and numerical studies on suction buckets, revealing reasonable agreement. It is shown that the existing t–z formulations for piles are inapplicable to large-diameter suction buckets.

Seismic response of a novel hybrid foundation for offshore wind turbine by geotechnical centrifuge modeling

Offshore wind farms are built in coastal areas prone to have earthquakes. When offshore wind turbines (OWTs) erected on cohesionless sediments are subjected to earthquakes, liquefaction might occur in the shallow deposits that might lead to severe overturning and settlement of the foundation structure. A novel hybrid foundation structure is proposed in this paper consisting of three major components: monopile, friction wheel, and suction bucket. This paper focuses on the performance of the hybrid foundation under earthquake loads. Hybrid foundation models with different dimensions were installed on both dry and saturated loose sand deposits and subjected to dynamic geotechnical centrifuge tests. Pore-water transducers were embedded in the soil to monitor its susceptibility to liquefaction. The dynamic responses of the superstructure were also monitored by linear variable differential transducers (LVDTs) and accelerometers. The experimental results indicated that the new hybrid foundation improved the ground liquefaction resistance. The extent of improvements was more significant in saturated sand than in dry sand. In saturated sand, it was observed that the hybrid foundation with larger bucket depth densified the surrounding soil and helped to mitigate the liquefaction. Consequently, the hybrid system showed responses of a stiffer foundation where the lateral displacement and settlement were reduced significantly.

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 model tests and effective stress analyses of offshore wind turbine systems with a suction bucket foundation subject to seismic load

The seismic behavior of a suction bucket foundation for offshore wind turbine systems is studied using centrifuge model tests with a scaling factor of 1/100 and an effective stress analysis based on the strain space multiple mechanism model implemented in the FLIP computer code (Finite element analysis program of Liquefaction Process). The primary dimensional parameters of the prototype scale suction bucket foundations selected for this study are as follows: skirt length L = 8 m, diameter D = 19 m, L = 9 m and D = 12 m. Placed on firmly compacted saturated sand, suction bucket foundations supporting an idealized wind turbine tower 100 m in height are subject to a recorded earthquake motion with a peak acceleration of 0.25g.Results of the model tests and the analyses indicate that the constitutive model used in this study is capable of capturing the essential features of the seismic behavior of offshore wind turbine systems supported by suction bucket foundation. In particular, this model is capable of evaluating the confining effect of a suction bucket on the increase in excess pore water pressure inside the bucket in response to the vertical confining pressure applied from the tower and shaft structure of the wind turbine system. This model is also capable of evaluating the inertia effects of the tower and shaft structure above the foundation affecting the driving mudline moment and shear, which leads to the deformation of the suction bucket in terms of rotation. The residual inclination of the tower is less than 0.001 rad, which satisfies the design criteria, and has a rotation angle of 0.005 rad.The strain space multiple mechanism model is applied to a general combination of static and cyclic loads in storm conditions. The computed results are consistent with those of a proposed cyclic load response diagram based on 1 g model tests at partially drained conditions in an earlier study by Nielsen et al. (2017), suggesting the applicability of this model to general load conditions is reasonable.

Construction and Preliminary Analysis of Three-dimensional Model of Offshore Wind Power Suction Bucket

As a high-rise structure, offshore wind turbines are subjected to huge horizontal loads. Compared with the traditional tubular foundation for offshore wind power, the large-scale tubular foundation with a load-bearing top has a larger load-bearing capacity due to its unique broad-shallow tubular top load-bearing mode. For reasons of towing stability, construction subsidence, fine leveling and structural strength, a subdivision plate is arranged in the concrete cylinder foundation cylinder. We established solid and structural elements through numerical methods, and took into account the tube-soil contact, so as to finely construct a real three-dimensional complex model of this new type of tube-based foundation and soil, which plays an important role in analyzing its deformation and bearing characteristics, as well as the safety and stability of the foundation structure design and practical application.

Improved load capacity of a hybrid bucket foundation for offshore structures in sand

As offshore structures grow larger with increasing depth and distance from shore, their foundations must increase in strength owing to the larger supporting capacity needed. This requires new approaches to ensure economic and reliable infrastructure. Suction bucket foundations (SBFs) are viable options in offshore settings owing to their easy installation and affordability; however, they suffer from performance limitations. Recently, a hybrid bucket foundation (HBF) design has been proposed in which a suction bucket is connected to an underlying mat to increase load capacity. However, the behavior of this design under combined vertical, horizontal, and/or moment loads is not yet well understood. In this study, we used centrifuge models to investigate the behavior of HBFs and conventional SBFs under different load conditions. The results demonstrated the feasibility and superior performance of HBFs. In particular, the bearing capacities of HBFs under vertical and combined loads were ~1.91 and ~1.82 times higher, respectively, than those of SBFs within the testing ranges. These findings indicate that the mat component attached on top of the hybrid bucket unit significantly improves the bearing capacity.

Modelling of suction bucket foundation in clay: From finite element analyses to macro-elements

This study aims at modelling the suction bucket foundation in clay from full finite element analyses to simplified macro-element. To ensure the reliability of finite element simulation results as much as possible, two well-known constitutive models, the Modified Cam-Clay model (MCC) and the Hardening Soil model (HS), are adopted respectively for the foundation soil which is normally consolidated clay with parameters directly determined from laboratory tests instead of inverse analysis or fitting using simple models in previous studies. The finite element analyses using both models are validated by comparing centrifugal tests and further extended to study the failure patterns in V–H–M (Vertical force–Horizontal force–Moment) space through radial displacement tests. Based on all results, two new alternative analytical formulations based on both models are then proposed to describe the three-dimensional (3D) failure envelope in V–H–M space. Finally, two new alternative macro-element design models of suction bucket foundation in clay under the same framework of hypoplasticity for both monotonic and cyclic loading conditions are proposed. By comparing with experimental results, the effectiveness and efficiency of both macro-element models are verified applicable and the one based on HS has slightly better performance than the one of MCC.

Suction Bucket Pile–Soil–Structure Interactions of Offshore Wind Turbine Jacket Foundations Using Coupled Dynamic Analysis

This paper presents a procedure for the coupled dynamic analysis of offshore wind turbine–jacket foundation-suction bucket piles and compares the American Petroleum Institute (API) standard method and Jeanjean’s methods used to model the piles. Nonlinear springs were used to represent soil lateral, axial, and tip resistances through the P–Y, T–Z, and Q–Z curves obtained by either API’s or Jeanjean’s methods. Rotational springs with a stiffness equated to the tangent or secant modulus characterized soil resistance to acentric loads. The procedure was implemented in X-SEA program. Analyses of a laterally loaded single pile in a soft clay soil performed in both the X-SEA and Structural Analysis Computer System (SACS) programs showed good agreements. The behaviors of a five MW offshore wind turbine system in South Korea were examined by considering waves, current, wind effects, and marine growth. In a free vibration analysis done with soil stiffness through the API method, the piles were found to bend in their first mode and to twist in the second and third modes, whereas the first three modes using Jeanjean’s method were all found to twist. The natural frequencies resulting from Jeanjean’s method were higher than those from the API method. In a forced vibration analysis, the system responses were significantly influenced by soil spring stiffness type. The procedure was found to be computationally expensive due to spring nonlinearities introduced.

Scour Risk Assessment of Offshore Wind Turbine with Suction Bucket Foundation

Kim, D.-H. and Kim, Y.-J., 2020. Scour risk assessment of offshore wind turbine suction bucket foundation. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 237–241. Coconut Creek (Florida), ISSN 0749-0208.Risk assessment procedure for offshore wind turbine support structures induced by scour was proposed. Probabilistic distribution of scour depth, a hazard curve, was calculated by using the C.S.U formula with parametric uncertainties. To get fragility curves of support structure, dynamic analysis was done with a certain assumed scour depth, given significant wave height and return period. Scour induced failure probability of support structure for a given wave height then can be found by integrating the multiplication of hazard and fragility. It is repeated for possible range of scour depths to form a risk curve. In numerical example, scour induced structural risk of 3MW offshore wind turbine supported by suction bucket foundation is evaluated. The proposed approach can be used in determining wind farm location where scour is significant.

Dynamic Characteristics of an Offshore Wind Turbine with Tripod Suction Buckets via Full-Scale Testing

The dynamic characteristics of an offshore wind turbine with tripod suction buckets are investigated through finite element analysis and full-scale experiments. In finite element analysis, an integrated framework is suggested to create a simple yet accurate high fidelity model. The integrated framework accounts for not only the strain dependency of the soil but also for all dynamics in the seabed, including those of the soil, suction bucket skirt, and cap. Hence, the model accurately describes the coupling effect of translational and rotational motions of the seabed. The prediction results are compared to the experimental results obtained via full-scale testing in four stages during construction and in several operational conditions. The comparison shows that the stiffness of the suction bucket cap and strain dependency of the soil play a significant role in predicting natural frequency, suggesting that these two factors should be considered in finite element analysis for the accurate prediction of dynamic responses of an offshore wind conversion system. Moreover, dynamic analysis of the strain and acceleration measured during operational conditions shows that strain is more robust than acceleration with regard to the characterization of the overall dynamics of an offshore wind conversion system because the natural frequency of an offshore wind turbine is very low. It can be inferred that the measurement of strain is a more effective way to monitor the long-term evolution of dynamic characteristics. The suggested integrated framework and measurement campaign are useful not only to avoid conservatism that may incur additional costs during load calculation and design phases but also to establish an intelligent operation and maintenance strategy with a novel sensing technique.

Monitoring of suction bucket jackets for offshore wind turbines: Dynamic load bearing behaviour and modelling

Suction bucket foundations for offshore wind turbines (OWT) have considerable advantages compared to conventional foundation types: Due to the installation process with dead weight and applied negative pressure, noise from pile driving can be completely avoided. In addition, the installation process of the whole substructure, consisting of the buckets connected to the jacket, can be carried out in one work step, which increases efficiency. A prototype of the suction bucket jacket was installed in the wind park ‘Borkum Riffgrund 1’ (North Sea) in August 2014. Due to the pre-installed and comprehensive measuring system, it was possible to monitor all installation and operating phases. The data analysis of a storm event show an amplitude and frequency-dependent behaviour of the soil stiffness and the suction bucket foundation without wind turbine. In the frequency range of the first and second eigenfrequency (0.2 Hz < f < 5 Hz), the system behaves linearly. Here, the Frequency Domain Decomposition is used for identification and monitoring. For the lower frequency band (0.05 Hz < f < 0.2 Hz) where higher forces and displacements occur, a non-linear multilayer perceptron is chosen to model the non-linear relations between measurements. By applying two mathematical models for the relevant frequency ranges, all the information from the measurement data can be used for system identification and novelty detection under varying environmental conditions.

Observations of the Effects of a Clay Layer on Suction Bucket Installation in Sand

AbstractSuction buckets are becoming established as a viable foundation solution for offshore wind turbines. In sand, suction-induced seepage flow reduces effective stresses at the skirt tips, whic...

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.

Soil plug heave induced by suction bucket installation on sand via centrifuge model tests

Suction pressure created during suction bucket installation may disturb the surrounding soil and affect its properties. Soil heaving can also occur owing to upward seepage inside the bucket embedded in sand, which hinders soil penetration; however, this mechanism is not well understood. Centrifuge model tests were performed to investigate this soil behaviour and its effect on the installation. Suction pressure, seepage flow, and soil plug heave were monitored under various installation conditions. Bender element and cone penetration tests were also conducted to evaluate the effect of soil plug loosening indirectly. The results illustrate the effects of upward seepage on soil plug heave during suction bucket installation. This study highlights the changes in the soil properties inside the bucket owing to the suction installation. This is an important design consideration for bucket installation, which also affects the bearing capacity of the suction bucket foundation.

Observations during suction bucket installation in sand

Suction buckets represent a viable solution as foundations for offshore wind turbines. Installation in sand is relatively straightforward, albeit with limited understanding of the resulting changes in soil state. This paper describes an experimental methodology that allows for visualisation and quantification of changes in soil state during suction bucket installation, validated in sand. Insights obtained from particle image velocimetry analyses, performed on images of a half-bucket installing against a Perspex window taken in a geotechnical centrifuge are discussed. Compared with the initial self-weight penetration, the deformation mechanism governing the suction-assisted phase shows a preference for the soil below the skirt tips to move inwards and upwards inside the bucket. The installation process is responsible for changes in relative density and permeability within the bucket. In these experiments, the majority of the soil plug heave can be attributed to the soil displaced inwards by the advancing skirts, with a minor contribution caused by dilation. The confidence in the experimental methodology provided through the results of suction bucket installation in sand discussed herein now enables suction bucket installation in more complex seabeds to be investigated.

The centrifuge experiments on influence of inclination on bearing capacity of the suction bucket

To study the mechanism of bearing capacity of a suction bucket, a series of centrifugal tests were designed to investigate the uplift bearing capacity, and the entire dynamic process of pore water pressure were monitored as well. The results show that with the increase of inclined degree of suction foundation, the bearing capacity of suction foundation decreases continuously, and the influence of inclination on the foundation bearing capacity is obvious.

On the responses of soil layers surrounding a suction bucket under cyclic loads

In this study, we performed laboratory experiments and numerical modeling to investigate the responses of soil layers surrounding a suction bucket under cyclic loads. Specially designed centrifugal experiments were conducted to obtain preliminary results. Numerical modeling using FLAC3D was then conducted for a sensitivity analysis of the controlling factors on the responses, for which the distribution characteristics of deformation and excess pore pressure are the main focuses. Our studies indicate that the excess pore pressure and the deformation develop gradually to a stable state, while the peak and fluctuation amplitude of those responses decrease with the increase in the bucket’s radius, elastic modulus and cohesion and with the decrease in the load amplitude and frequency. Along the depth of the bucket, there is an inflection point of the displacement and excess pore pressure.