This paper presents a new type of wide-shallow composite bucket foundation (WSCBF) for offshore wind turbines, which can be adapted to the characteristics and development needs of offshore wind farms ...due its special structural form. Several field tests on the horizontal bearing capacity of WSCBF were carried out in saturated clay off the coast of Jiangsu. The deformation mechanism, the soil–structure interaction and the ultimate bearing capacity of the WSCBF are determined depending on the tests. Based on the position of the rotation center, analytical expressions of soil pressure and ultimate bearing capacity for the WSCBF are presented. The accuracy of the test results and analytical expressions is validated and supplemented by numerical simulation with a finite element method. Moreover, by means of numerical simulation, the envelope curve of ultimate bearing capacity of the WSCBF is described in H–M load space, clarifying the load-bearing characteristics of the WSCBF under combined loads.
●The position of the instantaneous rotation centers moves with increasing horizontal loads.●The soil pressure on the lower surface of the lid in the passive zone increases substantially due to the horizontal restraint of the bulkheads and the wall.●Analytical expressions of soil pressure and the ultimate bearing capacity of the bucket foundation are presented based on the rotation center.●The failure envelope of the WSCBF is almost straight and can be divided into three sections which have different meaning.
A composite bucket foundation (CBF) with seven compartments arranged in a similar honeycomb structure is a relatively new type of structure for offshore wind turbines. Field tests were conducted at a ...natural water pool with a water depth of 0.4 m in saturated silty sand off the coast of Jiangsu to estimate the horizontal bearing capacity of the CBF. The distribution of soil pressures along the skirt of the CBF with different loading steps helps in explaining the soil-bucket interaction due to the deformation of the CBF. The results show that an increased load will result in a significant increase in the soil pressure in passive zones outside the bucket skirt whereas the soil pressure on the skirt wall in the corresponding active zone remains largely the same. Moreover, the suction pressure inside the separation between the bucket lid and the soil results in a potential bearing capacity with larger loading application, though it will dissipate with further displacement because seepage is observed in the silty sand. The load-displacement relationships obtained using a FEM simulation are almost in same trend with the test results when the elasticity modulus of the soils in the FEM model is set approximately four or five times the compression modulus obtained from the laboratory test. The rotation center of the CBF obtained from the test and FEM results is approximately 0.8 skirt height below the soil surface around the bulkhead along the loading direction under the ultimate horizontal loading.
•The deformation mechanism of the CBF with different loading steps.•The rotation center of the CBF is approximately (0.45R, 0.8H) under the ultimate horizontal loading.
The wide-shallow composite bucket foundation (WSCBF) is a new type of offshore wind power foundation that can be built on land and rapidly installed offshore, thereby effectively reducing the ...construction time and costs of offshore wind power foundation. In this study, the horizontal bearing capacity is calculated by finite element simulation and compared with test results to verify the validity of results. In this process, the vertical load and bending load are respectively calculated by the finite element simulation. Under the vertical load effect, the bucket foundation and the soil inside are regarded as a whole, and the corresponding buckling failure mode is obtained. The ultimate vertical bearing capacity is calculated using empirical and theoretical formulas; the theoretical formula is also revised by finite element results. Under bending load, the rotational center of the composite bucket foundation (in a region close to the bucket bottom) gradually moves from the left of the central axis (reverse to loading direction) to the nearby compartment boards along the loading direction. The H–M envelope line shows a linear relationship, and it is determined that the vertical and bending ultimate bearing capacities can be improved by an appropriate vertical load.
In this paper, the influential design thctors of wide-shallow composite bucket foundation for 3 MW off- shore wind turbine are systematically studied by numerical simulation. The results show that ...the bucket diameter is larger than 27 m in generak and the range of 7--12 m is appropriate for cylinder height. In particular the bucket foun- dation with diameter of 30 m and cylinder height of 10 m is suitable for most soils. Under ultimate loads, the bucket diameter and elasticity modulus of soil have major effects on the deibrmability of bucket foundation, while the influ- ence of friction coefficient between the bucket and soil is relatively slight.
Two large-scale sinking tests of the hybrid bucket foundation were revealed in typical saturated silty clay of China offshore wind farms. The experimental foundation has an outer diameter of 3.5m and ...a clear wall height of 0.9m with seven rooms divided inside the HBF by steel bulkheads, which are arranged in a honeycomb structure. The different pressure inside the seven compartments can control the levelness of the HBF during suction installation. During the sinking process of the bucket foundation, real-time monitoring was conducted of the produced underpressure, the soil pressure and the pore pressure inside and outside the bucket wall, which reflected the sinking state of the bucket foundation. Test results show that suction can be combined with air pumping to reasonably control the sinking speed of the bucket foundation and the levelness at each stage. In the cases where the soil surface at the sinking location for the bucket foundation was relatively horizontal, the levelness could be ensured to meet the standard and design requirements after the sinking of the bucket foundation was completed.
•The penetration testing of the composite bucket foundation with 7 compartments.•The sinking technique of sinking–adjusting–stabilizing–sinking process.•The tilt adjusting technique by applying suction/positive pressure and intermittent pumping among the 7 compartments.
A series of model experiments of bucket foundations concerning suction installation and negative pressure consolidation in saturated silt were carried out in a cube steel bin at Tianjin University. ...The experimental results show that the silt inside the bucket has been strengthened by negative pressure, and the strengthening effect decreases with the increase of the distance from the bucket. A three-dimensional numerical model of the experiments was built by means of finite element software ABAQUS with fluid-solid coupling method. The results show that the bearing capacity of the silt inside the bucket foundation increases significantly at the former stage of negative pressure consolidation, while the increasing trend slows down over time. The rotation centers of the bucket foundation and the inner soil region tend to be closer to each other based on the consolidation. The bearing capacity of the bucket foundation is improved effectively with the increase of soil strength. The effects of negative pressure consolidation on the bearing capacity of bucket foundation were also illustrated by an actual offshore wind power project case.
As an appropriate type of foundation for offshore wind turbines (OWTs), wide-shallow composite bucket foundation (WSCBF) is cost-competitive, and it has a unique and special substructure that ...comprises seven internal rooms arranged in a honeycomb-like structure. In this study, the cyclic behavior of WSCBF for OWTs embedded in saturated clay was investigated using a large-scale model subjected to lateral cyclic loading. The responses of foundation under constant- and various-amplitude cyclic loadings were recorded in terms of displacements, rotations, and bending moments. The variations in stiffness and damping were obtained, and a collaborative bearing mechanical model between the bucket and soil was considered, which was beneficial for improving the stiffness of the whole structure. Accumulative deformation was found to have little effect on the bearing capacity of the foundation. Dynamic analysis in frequency domain was further performed on both moment and rotation data, and the complex, frequency-dependent impedance was also studied.
A series of model experiments of bucket foundations concerning suction installation and negative pressureconsolidation in saturated silt were carried out in a cube steel bin at Tianjin University. ...The experimental resultsshow that the silt inside the bucket has been strengthened by negative pressure, and the strengthening effectdecreases with the increase of the distance from the bucket. A three-dimensional numerical model of the experimentswas built by means of finite element software ABAQUS with fluid-solid coupling method. The results showthat the bearing capacity of the silt inside the bucket foundation increases significantly at the former stage of negativepressure consolidation, while the increasing trend slows down over time. The rotation centers of the bucketfoundation and the inner soil region tend to be closer to each other based on the consolidation. The bearing capacityof the bucket foundation is improved effectively with the increase of soil strength. The effects of negative pressureconsolidation on the bearing capacity of bucket foundation were also illustrated by an actual offshore wind powerproject case.
