Hydrodynamic Characteristics of Three-Bucket Jacket Foundation for Offshore Wind Turbines During the Lowering Process

The three-bucket jacket foundation is a new type of foundation for offshore wind turbine that has the advantages of fast construction speed and suitability for deep water. The study of the hoisting and launching process is of great significance to ensure construction safety in actual projects. In this paper, a new launching technology is proposed that is based on the foundation of the three-bucket jacket for offshore wind turbine. A complete time domain simulation of the launching process of three-bucket jacket foundation is carried out by a theoretical analysis combined with hydrodynamic software Moses. At the same time, the effects of different initial air storage and sea conditions on the motion response of the structure and the hoisting cable tension are studied. The results show that the motion response of the structure is the highest when it is lowered to 1.5 times the bucket height. The natural period of each degree of freedom of the structure increases with the increase of the lowering depth. The structural motion response and the hoisting cable tension vary greatly in the early phases of Stages Ⅰ and Ⅲ, smaller in Stage Ⅱ, and gradually stabilize in the middle and late phases of Stage Ⅲ.

Shaking Table Tests of Four-Bucket Jacket Foundation for Offshore Wind Turbines

As the offshore wind turbine foundation,the four-bucket jacket foundation has a large stiffness and the structure is difficult to be damaged under seismic load.Nevertheless,the saturated subsoil of the four-bucket jacket foundation tends to be liquefied under earthquake,which greatly affects the safety of offshore wind turbine.Therefore,the seismic performance of four-bucket jacket foundation is mainly reflected in the anti-liquefaction capacity of foundation soil.In this paper,the liquefaction resistance of sandy soil of four-bucket jacket foundation for offshore wind turbine is studied.The liquefaction and dynamic response of sandy soil foundation of four-bucket jacket foundation under seismic load are obtained by carrying out the shaking table test,and the influence mechanism of four-bucket jacket foundation on the liquefaction resistance of sandy soil foundation is analyzed.

Experimental Study on Tilt Adjustment Technique of Tripod Bucket Jacket Foundation for Offshore Wind Turbine in Sand

For the tripod bucket jacket foundations used in offshore wind turbines, the probable critical tilt angles should be avoidedduring tilt adjustment operation. Thus, these critical values must be identified by engineers, and remedial techniques mustbe established prior to the occurrence of the problem. Model tests were carried out for typical tilting conditions of tripodbucket foundations, which were allowed to tilt freely at various penetration depths without interruption by manualoperation. After the foundation ceased its tilting, some measures, such as water pumping, water injection, air injection, or acombination of the above methods, were enabled for adjustment. The research results showed two critical values in thetilting state of the tripod bucket jacket foundation, namely the terminal and allowable angles. In the installation condition,the terminal angle was negatively correlated with the initial penetration depth, but the opposite was observed with theremoval condition. The allowable angle was less than or equal to the terminal angle. The allowable angle in the installationwas related to the terminal angle. The critical angles all varied linearly with the initial penetration depth. When tiltingduring installation, adjustment measures can be used in the order of high drum pumping, low drum water injection, highdrum pumping and low drum water injection, air injection, and exhaust. When tilting during removal, the sequential use oflow drum water injection, air, and exhaust was applied. For buckets that were sensitive to angle changes, adjustmentmeasures of the “point injection” mode can be selected.

Shaking Table Tests and Seismic Response of Three-Bucket Jacket Foundations for Offshore Wind Turbines

The seismic response characteristics of three-bucket jacket foundations for offshore wind turbines(OWTs)and the liquefaction of the surrounding soil are particularly important for the development and application of this type of structure for offshore use.Using the shaking table test and three-dimensional finite element analysis,different magnitudes of simulated earthquake waves were used as inputs to the shaking table to model seismic excitations.The resulting changes in the excess pore water pressure and acceleration response of the soil under horizontal earthquake are compared in this paper.Calculations of the anti-liquefaction shear stress and equivalent shearing stress during the earthquake,determination of the areas prone to liquefaction,and identification of the effect of the three-bucket jacket foundation on the soil liquefaction resistance were conducted by developing a soil-structure finite element model.The development law of the soil’s amplification effect on seismic acceleration and the seismic response of the foundation soil under various magnitude earthquake waves were also discussed.Results indicate that liquefying the soil inside the bucket of the foundation is more difficult than that outside the bucket during the excitation of seismic waves due to the large upper load and the restraint of the surrounding hoop.This finding confirms the advantages of the three-bucket jacket foundations in improving the liquefaction resistance of the soil inside the bucket.However,the confinement has a barely noticeable impact on the nearby soil outside the skirt.The phenomenon of soil liquefaction at the bottom of the skirt occurred earlier than that in other positions during the seismic excitation,and the excess pore water pressure slowly dissipated.The acceleration amplification coefficient of the sand outside the bucket increases with depth,but that of the sand inside the bucket is substantially inhibited in the height range of the bucket foundation.This result proves the inhibition effects of the three-bucket jacket foundations on the seismic responses of soils.The liquefied soil layer has a significant effect in absorbing a certain amount of seismic wave energy and reducing the amplification effect.The numerical simulation results are consistent with the phenomenon and data measured during the shaking table test.The current study also verifies the feasibility of the excess pore water pressure ratio and the anti-liquefaction shear stress method for judging soil liquefaction.

砂土中长径比对三筒基础水平承载特性的影响

The tripod foundation(TF)is a prevalent foundation configuration in contemporary engineering practices.In comparison to a single pile,TF comprised interconnected individual piles,resulting in enhanced bearing capacity and stability.A physical model test was conducted within a sandy soil foundation,systematically varying the length-to-diameter ratio of the TF.The investigation aimed to comprehend the impact of altering the height of the central bucket on the historical horizontal bearing capacity of the foundation in saturated sand.Additionally,the study scrutinized the historical consequences of soil pressure and pore water pressure surrounding the bucket throughout the loading process.The historical findings revealed a significant enhancement in the horizontal bearing capacity of the TF under undrained conditions.When subjected to a historical horizontal loading angle of 0°for a single pile,the multi-bucket foundation exhibited superior historical bearing capacity compared to a single-pile foundation experiencing a historical loading angle of 180°under pulling conditions.With each historical increment in bucket height from 150 mm to 350 mm in 100 mm intervals,the historical horizontal bearing capacity of the TF exhibited an approximately 75%increase relative to the 150 mm bucket height,indicating a proportional relationship.Importantly,the historical internal pore water pressure within the bucket foundation remained unaffected by drainage conditions during loading.Conversely,undrained conditions led to a historical elevation in pore water pressure at the lower side of the pressure bucket.Consequently,in practical engineering applications,the optimization of the historical bearing efficacy of the TF necessitated the historical closure of the valve atop the foundation to sustain internal negative pressure within the bucket.This historical measure served to augment the historical horizontal bearing capacity.Simultaneously,historical external loads,such as wind,waves,and currents,were directed towards any individual bucket within the TF for optimal historical performance.

海上风电螺旋桩侧向承载能力分析

The rapid development of offshore wind power and the need to move to deeper sea areas while reducing costs per kilowatt necessitate the employment of a new jacket and helical pile combination.This new combination combines the advantages of both jacket structures and helical piles and provides a superior bearing capacity and installation efficiency compared to conventional pile foundations.Foundations account for 25%-34%of the overall cost of construction,but the use of this new foundation would be highly significant for the further development of offshore wind power.This study presents numerical results for the horizontal bearing capacity when horizontal displacement is applied,focusing on the bearing capacity and characteristics of the helical pile jacket foundation as well as the differences between the bearing mechanisms and failure modes of normal pile and helical pile types.ABAQUS model parameters are obtained through trial calculations based on actual engineering data,and the finite element model(FEM)is validated using data from a model experiment.Subsequently,different FEMs are established,and numerical results are compared and presented.Through a comparison between a normal pile jacket foundation and a helical pile jacket foundation with different helical blade numbers,the differences in the bearing mechanisms and failure modes are revealed.The failure of the normal pile jacket foundation is instantaneous and sudden,whereas that of the helical pile foundation is incremental and accumulative.These data highlight the most significant contributions and vulnerabilities of the one-pile side of the foundation and suggest that the addition of blades on the one-pile side is the most effective way of improving the foundation’s bearing performance.In addition,the interaction between the compression side and tension side is analyzed in relation to differing the relative magnitudes of their bearing capacities.