Bearing Capacity And Optimization Design Of Composite Bucket Foundation For Offshore Wind Turbine

In the early90s, the bucket foundation firtly applied to the needs of the oilfielddevelopment. With the continuous development and expansion of wind powerindustry, the research on the bucket foundation is full swing in China. The bucketfoundation as a new type makes up for deficiencies of the traditional foundations,gravity and pile foundation. But due to the high cost and complexity of marinegeological and hodrolycal conditions, the bucket foundation should be further studied.This paper in-depth studied the bearing capacity and failure modes of compositebucket foundation, buckling of large diameter thin-walled steel bucket andoptimization type-selection of foundation. The main contents and innovations are asfollows.This paper in-depth studied the bearing capacity and failure modes of bucketfoundation under horizontal load, vertical load and moment by finite element,experimental and theoretical methods. A displacement control method for verticalbearing capacity and a safety factor control method for horizontal (including moment)bearing capacity ware proposed. The results showed that the foundation achievedvertical ultimate bearing capacity, when the vertical displacement reached0.06D (D isthe diameter of bucket); the horizontal (including moment) ultimate bearing capacitywas obtained by control the safety factor against overturning and sliding. Thedifferences between this research and previous studies were that the cap of bucket wasthe main bearing part and separation beween bucket and the internal soil wasconsidered under horizontal load and moment. The gap between the cap of bucket andthe internal soil was less than25%considering the vertical loading, but the gap wasmore than50%not considering the vertical loading. The failure mode underhotizontal (including moment) load showed the rotational point varied with thechange of the length-diameter ratio (L/D). When L/D≤0.5, the rotational point ofbucket foundation was about0.5L under the seabed, and it was about0.7L under theseabed, when L/D>0.5. The angle of inclination for the wide-shallow bucketfoundation was about3°which was larger than narrow-deep foundation. The resultsalso showed that the traditional method (m method) and p-y cure method are notapplicable to the earth pressure calculation, and Rankine’s earth pressure theory is more suiltable for the wide-shallow bucket foundation. Finally, a series of reseachesfor prototype bucket foundation showed that all conclusions obtained from modelfoundation can apply equally to a prototype foundation, and have practicalapplications.A bucket foundation which is subjected to large axial load and negative pressureis the thin-walled steel cylinder structure with height-diameter ratio (H/D) less than1.0and large radius-diameter ratio. Unreasonable designed of the cylinder wall caneasily lead to buckling. The difference between the bucket foundation and previousthin-walled steel cylinders under negative pressure is that in addition to the radialpressure, the negative pressure on the cap of bucket has a great influence on thebuckling. This paper studied the buckling by numerical method, and the resultsshowed that the traditional method to calculate the buckling critical strss is too largedue to the difference between assumption and actual situation. A method to calculatethe buckling critical stress which was suitable for the large-diameter steel cylinderwas proposed and the coefficient should be0.061that was similar to the result fromSteel pressure vessels (GB150-1998) code. The subdivision plate can improve thebuckling capacity by50%under axial load. The buckling capacity under negativepressure was discussed in two kinds of conditions: first, the subdivision plate onlyimproved the buckling capacity by10%not considering the negative pressure on capof bucket, and the bucket foundation was safe under0.2times of an atmosphere.Second, the subdivision plate had a great influence on the buckling capacityconsidering the negative pressure on cap of bucket. When H/D≤0.3, the bucklingcapacity was about10times than that not considering subdivision plate, and thebucket could bear two atmospheres. The buckling critical stress under thecombination of axial load and negative pressure from finite element method wascompared with that from commom specification methods. The results showed that thebuckling critical stress calculated by EC3code method where the thin-walled steelcylinder bears large pressure was similar to the actual situation, and the main reasonfor difference was that the height-diameter ratio is not considered.The difficutly in optimization design of offshore wind turbines is that the forceconducting way of the bucket foundation is not clear, and internal force iscomplicated to be calculated. Meanwhile, prestress reinforcement and inclinedsupport increase optimizing variables and the difficulty of optimization design. Theoptimal force transferring path was obtained by topological optimization method. A prestressed steel-concrete composite bucket foundation with inclined support was putforward, and its simplified computational model was proposed based on the theorystructural mechanics and equivalence principle. A bucket foundation for6MW windturbine was designed by mathematical optimization method combined with finiteelement method (FEM). The influence of the inclined support on the wave load wasstudied. The important conclusions were: the smaller the diameter of bucket is, thewell the stress condition of reinforced concrete is, and the larger the whole rigidity is.Meanwhile the gradient of the peak of bucket foundation decreases. The inclinedsupport not only improves the safety of structure, but almost no effect on the waveload, which saves construction costs.