Three- and four-bucket offshore wind power foundations with a new form of force-transferring structure are proposed in this paper, and the integrated finite element model of foundation-soil-transition structure is established by using ABAQUS. The carrying capacity of the proposed foundations is studied under vertical load, horizontal load and bending moment. It can be seen that the vertical bearing capacity of multi-bucket foundation can be roughly estimated by the vertical bearing capacity of single-bucket; the horizontal bearing capacity of the three-bucket foundation scheme is controlled by displacement, while that of the four-bucket foundation scheme is controlled by the internal forces of soils. Moreover, the carrying capacity is provided by the overall structure formed by multi-bucket before soil failure. Compared with the conventional single-bucket foundation, there are mainly tension and pressure that are applied to the multi-bucket foundation, so that the carrying capacity of the foundation can be fully utilized. The probability of soil failure can be well reduced with the proposed multi-bucket foundation, and the stress transmission of force-transferring structure is more consistent through steel beams with variable cross-section.
Air-floating towing behaviors of multi-bucket foundation platform(MBFP)are investigated with the 1/20-scale model tests and hydrodynamic software MOSES.MOSES numerical model was validated by test results,and MOSES prototype model of MBFP can eliminate scale effect of model.The influences of towing factors of towing speed,water depth,freeboard,and wave direction on air-floating towing stability of MBFP were analyzed by model tests and validated MOSES prototype model.It is shown that the reduction of towing speed can effectively decrease the towing force and surge acceleration to improve towing stability.Water depth is another factor in towing stability.Obvious shallow water effect will appear in shallow water with small water depth-draft ratio and it will disappear gradually and air-floating towing becomes more stable with the increase of water depth.Accelerations of surge,sway and heave are small and they have modest changes when freeboard increases from 0.5 to 2 m.For MBFP,the freeboard is not suggested to be larger than 2 m in following wave.Wave direction has large influence on the towing stability,the surge acceleration and towing force are sensitive to the variation of wave direction,the surge acceleration and towing force in following wave(0)and counter wave(180)are much larger than that in transverse sea(90 and 270).
To investigate the natural frequencies and towing behaviors of a 3-bucket foundation platform at different drafts, the decay and towing experiments were carried out in a towing tank on a scale of 1:20. The air pressure inside the bucket foundations, the water pressure at the bottom of the bucket foundations, the acceleration of the platform and the towing force were determined in the test process. The time-history curves of the measured parameters were obtained, and the frequency responses of the parameters at different drafts were analyzed by means of fast Fourier transform(FFT). The results showed that the platform natural frequency of heave decreased slightly with the rise of draft. The natural frequencies of roll and pitch are much lower than that of heave, and they increased slightly with the increase of draft. When towing in the following sea, the maximum acceleration of surge, sway and heave has downward trends with the increase of draft, but the change range decreased gradually with the increase of draft. When the draft is 5.0 m(the ratio of draft to bucket height is 0.56), the towing dynamic responses achieve the maximum, which is not conducive to the towing of the platform. When the draft is 6.0 m(the ratio of draft to bucket height is 0.67), the towing dynamic responses are the most stable.
A hydraulics model is built in Moses to find the optimal internal skirt spacing for the maximum floating stability of the skirted foundation. The results show that the increase in the internal skirts' number can help improve the floating stability of the skirted foundation. However, with the increase in the internal skirts' number, the improvement of floating stability becomes more and more weak. In this study, an optimal number of four are found for the internal skirt spacing. Moreover, to testify the feasibility of internal skirt spacing, a practical project is modeled, which indicates that the optimal internal skirt spacing can satisfy the requirements of towing.
