Bucket Foundation Research Articles

Numerical analysis on the mechanical response of grouted connection for pile-bucket foundation

As a new foundation type for offshore wind turbine (OWT), pile-bucket foundation, which consists of a monopile and a bucket foundation, has good prospect in engineering application and has received wide attention. However, most of the current research ignored the function of grouted connection (GC) and assumed that the pile and bucket were rigidly connected, which may lead to unrealistic estimation of the loading response of pile-bucket foundation. In this study, Finite Element Analysis (FEA) is conducted to study the influence of grout connection on the bearing characteristics of pile-bucket foundation, with a special focus on the mechanical response of the grout connection itself. The concrete damage plasticity (CDP) model is used to simulate the mechanical response of the grout material. After validation of the numerical model, twelve simulations are conducted by changing the grout thickness, grout length and stiffener arrangements. Though the variance of grout thickness and grout length has little impact on the load-displacement curves of the pile-bucket foundation, the optimal grout thickness and grout length could be determined according to the development of contact pressure, stress and damage of the grout connection. On the other hand, the employment of stiffener leads to notable stress concentration and fluctuation, however, it also improves the overall capacity of the pile-bucket foundation and effectively controls the damage of the grouted connection. The numerical result is expected to provide some reference value for the structural design of grouted connection for pile-bucket foundation.

Analysis of bucket foundation installation in clay considering soil large deformation

Bucket foundations are increasingly used to support offshore wind turbines. Its bearing performance has been extensively investigated, but its installation process has been addressed in a few investigations. This study investigates the penetration resistance during the installation of bucket foundations in clay through finite element models that consider large soil deformations. The proper finite element modeling scheme is assessed first, then a validated numerical model is employed to investigate the effects of length-diameter ratio and soil properties on the soil plug and penetration resistance. The results indicate that a minimum element dimension and penetration velocity should be set to 1/6th of skirt thickness and 0.3 m/s to ensure a good balance between computational accuracy and efficiency. An increase in the bucket length-to-diameter ratio increases the height of soil heave, while soil nonuniformity reduces it. In addition, the height of soil heave is slightly influenced by the friction coefficient and undrained shear strength of the soil. On the other hand, the penetration resistance is strongly affected by the soil undrained shear strength and bucket length-diameter ratio, but slightly affected by the friction coefficient.

Bearing Characteristics of Tripod Bucket Foundation under Horizontal and Moment Load in Sand

Based on a series of physical model tests and numerical simulations, the bearing characteristics of a tripod-bucket foundation are investigated. It is found that with the decrease in aspect ratio (L/D), the rotation center of the foundation will decrease, and the displacement model change from rotation to uplift. Characteristics of earth pressure on the buckets from both finite element analysis and model tests are studied, which is used to explain the failure mechanism for tripod-bucket foundations with different L/D. A revised method is proposed to estimate the moment bearing capacity of the tripod bucket foundation under horizontal and moment load. This method is thought to be more convenient and applicable in the practice.

Experimental study of local scour around bucket foundation in sand

Laboratory flume experiments were conducted to study scour characteristics around bucket foundation under current only condition and current-wave condition. Scour topography from the experiments shows that the evolution of scour can be divided into three stages, rapid scouring stage, slow scouring stage, and scouring equilibrium stage. Semicircular scour pit occurs in front of the bucket and herringbone siltation appears behind the bucket. Scour depth of the mono-bucket was compared with that of the unsubmerged monopile and submerged monopile. Slope of the scour pit is found to be smaller than the predicted value of the monopile. Maximum scour depth of the tripod-bucket foundation is shown to be affected by the factors including the ratio of exposed height to diameter H/D, the relative bucket spacing G/D, and the incoming flow angle α. The maximum scour depth increases with the increase of H/D. Scour depth of the inner side of the tripod-bucket foundation is shown to be significantly affected by G/D due to the jet effect. The rear bucket may be influenced by shielding effect or vortex shedding. And the incoming angle has influence on the scour development around tripod-bucket foundation.

Study on hydrodynamic characteristics of a hybrid wind-wave energy system combing a composite bucket foundation and wave energy converter

In this paper, a new type of combined wind-wave system is proposed, that is, composite bucket foundation-oscillating buoy (CBF-OB) combined device. A three-dimensional numerical wave flume is established by using the renormalization group k–ε model. The hydrodynamic characteristics around the wave and the combined power generation device are studied. The relationship between wave parameters and wave run-up and wave pressure is analyzed. The absorption efficiency performance of the combined power generation device is evaluated. The results show that in the combined power generation system, the wave run-up and wave pressure at 0°–135° around the CBF are smaller than those in the presence of only CBF, but near 180° the ones are larger than those in the presence of only CBF. In the rear side of the combined power generation system, the smaller the scattering parameters, the more obvious the phenomenon of the second wave peak, and the stronger the nonlinearity of the wave and CBF-OB combined power generation system. The proposed CBF-OB combined power generation device can significantly improve the absorption efficiency of the buoy, which can be increased by about 1.5–4.0 times compared with the absorption efficiency under the action of only the buoy alone. There is an optimal power takeoff parameter, that is, when the damping parameter is 500 N·s/m, to maximize the absorption efficiency of the combined power generation device.

The offshore prefabrication and semi-wet towing of a bucket foundation for offshore wind turbines

In recent years, the bucket foundation as a new form of supporting structure of offshore wind turbines (OWTs) has attracted increasing attention with the rapid development of offshore wind power. A synthetical prefabrication and transportation (P&T) technique is proposed in this paper to enable a broader application of bucket foundations efficiently and economically in the offshore wind market. The novel P&T technology consists of two key auxiliary pieces of equipment, i.e., an offshore prefabrication platform (OPP) and a U–K barge. The process of the offshore prefabrication is elaborated, and the requirement that need to be met for the monopod-bucket foundation with subdivisions (MBFS) to achieve offshore prefabrication utilizing the OPP is analyzed. Then, the concept of the semi-wet towing with a U–K barge is proposed, which can provide sufficient buoyancy and stability to ensure the safety of the MBFS during the towing operation. To investigate the seakeeping of the combined semi-wet system in moored and towing scenarios, a series of experiments were carried out to prove the reliability of the U–K barge design. Sensitivity analysis regarding towing speed, wave height, and wave period for the disintegration risk is performed, and three methods are proposed to avoid the disintegration of the U–K barge and MBFS at extreme sea cases. The results show that the proposed offshore prefabrication is feasible, and it is suggested that as the MBFS floats up, the water depth of the construction site should not be less than 6 m. In addition, it is proved that the combined semi-wet towing system composed of the U–K barge and the MBFS has an exceptional seakeeping capability with regard to the pitching angles, accelerations, and towing resistances, and that the wave height and the towing speed have greater effects on the disintegration risk than the wave period.

A cyclic-softening macro element model for mono-bucket foundations supporting offshore wind turbines in clay

A cyclic-softening macro element model that represents mono-bucket foundation supporting offshore wind turbines in clay is presented in this paper. The model is formulated based on multi-surface plasticity with kinematic and isotropic hardening rule. The softening behaviour of the foundation subjected to a cyclic load sequence is captured real-timely in the macro element model through the contraction of yield surfaces. The model parameters are calibrated by uniaxial quasi-static load–displacement curves, which are obtained by finite element analyses with the cyclic behaviour of soil simulated via the cyclic contour diagram approach. The performance of the macro element model in representing the softening of a bucket foundation during a cyclic load sequence is demonstrated by comparing with parallel finite element analyses. A reasonable agreement has been achieved. The procedure to apply the model is suggested, considering different working scenarios with various cyclic to average load ratios. The macro element model developed is capable of capturing the real-time displacement of foundation in a load sequence, with high computational efficiency and accuracy.

Effect of Torsion on Failure Mechanism and Undrained Horizontal Capacity of Tripod Bucket Foundations in Clay

Effect of Torsion on Failure Mechanism and Undrained Horizontal Capacity of Tripod Bucket Foundations in Clay

Analysis of Regular Wave Floating Characteristics of Mono-Column Composite Bucket Foundation during Towing

The mono-column composite bucket foundation has gained practical application in offshore wind power due to its advantages of simple fabrication, fast construction, and low cost. To ensure the safe and stable transportation of this structure to its designated sinking position, this study focuses on producing a scaled model of the mono-column composite bucket foundation. Through model testing, the floating characteristics of the structure during towing in regular waves are examined. The conclusion of the study is as follows: a significant towing force is required to provide the initial velocity of the structure. As the wave period and height increase, the structure necessitates a larger towing force, experiences greater pitch and heave responses, and exhibits more noticeable fluctuations in internal air pressure. The paper aims at providing practical engineering insights.

Research on the calculation method of penetration resistance of bucket foundation for offshore wind turbines

The cost of foundations for offshore wind turbines constitutes approximately 35% of the total cost of an offshore wind farm. The bucket foundations show significant potential due to their superior transportation and installation efficiencies compared to pile foundations, leading to potential cost savings for the foundation of up to 30%. For a bucket foundation to be installed successfully, the penetration resistance must be predicted. However, there is currently a lack of clarity on how to select a suitable calculation method for penetration resistance based on known geological parameters to guide construction. In order to evaluate the current methods of calculation for bucket foundation penetration resistance, this study combines theoretical calculation methods with two sets of practical measurement data from the field. The calculation methods of penetration resistance for bucket foundation are first reviewed and categorized. The applicability range of each method and the parameters needed for calculation are given. Next, according to the measured data in the process of penetration of bucket foundation on site, the evolution of compartment pressure, tilt angle, resistance and required suction in the process of penetration are analyzed. Finally, the reviewed methods are compared to the results of two practical projects in order to analyze the differences between them and make recommendations for the calculation technique. The findings can be used as a guide for calculating the bucket foundation's penetration resistance in complex geological conditions.

Bearing characteristics of mono-column composite bucket foundation in sand for offshore wind turbines

Using a new steel compartment structure, the mono-column composite bucket foundation applied in the South China Sea deep-water project for the first time is based on the optimization of the steel-concrete composite bucket and is suitable for wind farms with a water depth of more than 30m. However, at this stage, there are few studies on large-diameter mono-tube multi-compartment bucket foundations, especially those related to the bearing characteristics of mono-column composite bucket foundations after long-term service. In this study, a cyclic load test is carried out on a mono-column composite bucket foundation in sand, and the influence of cyclic load factors on the cumulative rotation angle, cyclic stiffness, and ultimate bearing capacity after cyclic loading of the foundation is analyzed, and the bearing characteristics of the foundation are analyzed by finite element analysis. The results show that after the cyclic loading, the bearing capacity and stiffness have a certain increase compared with before the cyclic loading in sand. The ultimate bearing capacity calculation formula based on the Mohr-Coulomb strength criterion derived from the limit state of the foundation failure has higher reliability compared with the test results and can be used to predict the bearing capacity of the mono-column composite bucket foundation.

Lateral bearing behavior and failure mechanism of monopile-bucket hybrid foundations on sand deposit

With the development of OWT technology, the requirement for bearing capacity of OWT is becoming stricter. A new type of monopile-bucket hybrid foundation has been recently proposed. As the bearing behavior and failure mechanism of the hybrid foundation have not been investigated comprehensively, laboratory experiments and numerical analysis of the monopile-bucket hybrid foundation subjected to H-M-T combined loads in sand are finished in this paper. Firstly, the model tests were carried out to explore the lateral bearing performance of the hybrid foundation with different loading eccentricity, skirt heights and pre-torque loads. Then, based on the numerical analysis, a quantitative analysis is proceeded to find out the influence of different factors on the bearing performance of the hybrid foundation in detail. The results show that, the lateral bearing capacity of the hybrid foundation is greater than the sum of bucket foundation and monopile. It's also found that the torque load has a strong weakening effect on the lateral bearing capacity of the hybrid foundation. Finally, an empirical calculated formula for calculating the lateral bearing capacity of the hybrid foundation in sand is proposed, which may provide a valuable engineering application reference for monopile-bucket hybrid foundation in sand under H-M-T combined loads.

Optimize characteristic value of SPT for seismic design of offshore wind turbines in liquefiable soils by stochastic models

Dynamic numerical analyses on an offshore wind turbine (OWT) supported by a suction bucket foundation were performed to investigate the effects of soil spatial variability on the ground liquefaction and tilts of OWT, and to provide suggestions on selecting a representative percentile of the random distributions of a clean sand equivalent SPT blow count, (N1)60cs, for deterministic models. The material parameters of an elastoplastic constitutive model (Cyclic Mobility model) in the numerical model were derived from the (N1)60cs (spatial variability). Monte Carlo simulations were used to describe the seismic responses of the laterally loaded suction bucket foundation in spatially variable soil. Simulation results for deterministic models (uniform properties of soil) were compared to results for random models (spatially variable properties of soil). Tilts of OWT and liquefaction potential index of ground are chosen as criteria, and those obtained from the random models and deterministic models were compared to identify the representative (N1)60cs, based on the principle that the deterministic models produce reasonable consistency with the upper limit of the random model responses. The effects of site conditions, seismic conditions, and other factors were evaluated, and it is found that the coefficient of variation (COV), (N1)60cs, acceleration, and frequency have significant influence.

Hydrodynamic behaviour of composite bucket foundation with random waves

In real sea conditions, the effect of nonlinear phenomena, such as wave run-up and wave pressure, on the foundations of offshore wind structures may threaten the safety of the platforms. This study investigated the hydrodynamic behaviour of a composite bucket foundation (CBF) under random waves. A hydrodynamic simulation model of the interaction between random waves and the CBF was developed using the renormalisation group (RNG) k-ε model, which was verified through experiments. The relationship between wave run-up and wave parameters was studied, and the distributions of wave pressure around the CBF was described. The flow field and velocity vector distribution characteristics around the CBF were analysed. The results show that two types of wave fields appear around the CBF. The maximum run-up value of the CBF is 100%–150% higher than the minimum value, and the run-up value at the back side is 50%–70% higher than the minimum value. A larger wave steepness parameter leads to a smaller wave run-up value at the location of the smallest wave run-up value. The results can provide a reference for the practical application of a CBF.

Seismic responses of suction bucket foundation in liquefiable seabed considering spatial variability

The abundant wind energy happens to be distributed along the seismic regions with potential liquefaction. Offshore wind turbines have to be built in these regions for high power generation efficiency. The earthquake-induced liquefaction affected by the natural variability of the seabed makes the safety assessment of suction bucket foundations a challenge. The seismic responses of the laterally-loaded suction bucket foundation in spatially variable soil modeled by an advanced constitutive model were investigated using an FE- FD method. The SPT-(N1)60cs was considered a random variable and the seabed was modeled as a non-Gaussian random field by the Cholesky decomposition technique. The seismic responses were described by the Monte Carlo simulation and the FE-FD method. Statistics of the tilt and the liquefaction potential index influenced by the coefficient of variation and the scale of fluctuation were investigated. The stochastic model was compared with the deterministic model. The liquefaction mechanism and failure mechanism of the suction bucket foundation in random soil were investigated. The tilts and liquefaction may be underestimated without considering the soil spatial variability. Safety factors were introduced into the calculation of the failure probability of the suction bucket foundations. The simulations provide insight into the nonlinear behavior of suction bucket foundations considering the liquefaction of spatially variable soil.

Tests on the incline rectification of suction bucket foundation installation under local cyclic shock loading

ABSTRACT There are many problems with the vertical installation of suction buckets due to the complex ocean wave current and seabed geological conditions. In this paper, a set of tests were developed to study the mechanism of incline rectification of the suction bucket foundation based on resistance-reduced vibration on local side walls (RRV-LSW). In this experiment, the dynamic evolution process of pore water pressure around the suction barrel foundation was monitored from three angles of vibration frequency, initial inclination angle and load, and the influence of local cyclic vibration parameters on the slope velocity of the suction barrel foundation was quantified. The results indicate that a tilted suction caisson foundation can be effectively corrected through lateral wall vibration, and the correction effect is closely related to the vibration frequency, initial tilt angle, and static pressure load.

Study on penetration resistance for thin-walled bucket foundation in muddy clay

In view of the problem that the resistance reduction effect of ring stiffeners is not considered in the existing calculation methods of penetration resistance, based on the field penetration date of thin-walled bucket foundation with ring stiffeners and combined with the field installation situation, the resistance reduction effect of the angle iron ring stiffeners on the skin friction resistance was explored, the calculation method of the penetration resistance considering the resistance reduction coefficient β was proposed. The results show that because the resistance reduction effect of ring stiffeners is not considered, the calculation results of the standard and theoretical equation method are larger than the measured resistance. The end resistance increased by ring stiffeners is about 55%–65% of the end resistance calculated by the standard and theoretical equation method, the end resistance increased by ring stiffeners should be considered in actual project. It is proposed that the calculation method considering the resistance reduction coefficient β can accurately predict the penetration resistance and the required differential pressure of bucket foundation with ring stiffeners, the recommended resistance reduction coefficient is 0.52 for the static penetration method, 0.33 for the undrained shear strength method, and 0.31 for the c and φ parameter method.

Shake table testing and computational framework for seismic response of utility-scale bucket foundation offshore wind turbines

Shake table testing was conducted to document the seismic response of a bucket foundation offshore wind turbine (OWT) system. Salient response of the system's soil-structure interaction effects is presented and discussed. Among the observed response characteristics, excess pore pressure fluctuation within and around the soil-bucket domain is thoroughly addressed, including the strong tendency for the soil dilation excursions driven by the induced cyclic strains. The experimental data is used to calibrate a numerical model with dynamic soil response simulated by a coupled solid-fluid formulation. The calibrated model is extended to investigate seismic response of a prototype utility-scale OWT, with and without added wind loading effects. Overall, the research outcomes indicate that: i) excess pore pressure fluctuations in the vicinity of the bucket play an important role in dictating the extent of potential permanent base rotation, ii) consideration should be given to wind loading that might further exacerbate this base rotation, and iii) it is of importance to model the turbine tower as a system of discrete masses rather than the simplified proposed for practice equivalent top mass idealization.

Reliability analysis of bucket foundation breakwater considering water-level variations under complex natural conditions

Bucket foundation breakwater is a new type of structure suitable for deep water, strong waves, soft ground and other complex natural conditions. At present, the research on bucket foundation breakwater focuses on the failure mode and stability, and further research on the reliability is needed to promote the application of this structure. Based on the massive field monitoring data that lasted nearly seven years of bucket foundation breakwater in Xuwei Port Area of Lianyungang Port, China, the statistical analysis was carried out according to the bimodal distribution. The composite limit state equation of the structure was established by using the analytic hierarchy process. At the same time, three single failure modes of slip, settlement and overturning, as well as composite failure modes were simulated by Monte-Carlo method, and the influence of the water-level variations on structural reliability were further studied. The results showed that, with the increase of water-level, the rotation center of bucket foundation breakwater gradually moves to the harbor side. Furthermore, due to the increase of overturning moment and the decrease of structure weight, the reliability of bucket foundation breakwater decreases, but it still has a high reliability. The possibility of slip failure is greater than that of overturning failure, and the settlement failure can usually be ignored. It is recommended to use the designed average water-level, establish the composite limit state equation of the structure based on the analytic hierarchy process, and conduct the structural reliability analysis according to the composite failure mode by Monte-Carlo method to obtain reasonable and accurate results.

The Anti-Overturning Response of Tripod Bucket Foundation for Offshore Wind Turbines

The tripod bucket jacket foundation is proven to be a practicable solution for offshore wind turbines (OWTs) to withstand huge environmental loads in deep water. This paper presents model tests for a scaled tripod bucket jacket foundation with reference to a prototype applied in China to obtain its lateral load bearing behavior in medium-dense sands. Extended finite element analyses were conducted by ABAQUS to compare anti-overturning responses for the tripod bucket foundation in both sand and soft clay, and the influences of loading direction and aspect ratio were also taken into account. The results showed that the failure modes of the laterally loaded tripod bucket foundation are the pull-out of the windward bucket in sand and the settlement of the leeward bucket in soft clay, respectively. Thus, the unfavorable loading direction of the foundation changes with soil type. It is also shown that the bearing capacity for the foundation in soft clay will be enhanced more effectively as the bucket diameter increases. Instead of the rotational soil resistance resulting from the rotation of the bucket, the vertical soil resistance governs the anti-overturning bearing capacity of a tripod bucket foundation. As the tilt created by the overturning moment rises, the rotational stiffness of the foundation dramatically declines.