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Suction caissons have been extensively used in oil and gas industry. Recently bucket foundations are considered an innovative foundation solution to offshore wind turbines. Skirted foundations are increasingly used in the design of offshore wind turbine structures and facilities to withstand combined vertical, horizontal and moment loading conditions. In this study, three-dimensional finite element models of bucket foundations in medium dense sand were analyzed under combined loading. Numerical models were validated by comparing the results of finite element analysis. Sensitivity analyses were performed at different length-to-diameter aspect ratios under combined load with different load eccentricities. The responses of combined load, from pure horizontal load to overturning moments with varying tower heights, to pure moments were compared. Results showed how combined bearing capacity of foundations would depend on aspect ratio and loading conditions. Ultimate limit states were represented as normalized capacities, and the expressions were derived from the results of analysis. Offshore applications of shallow foundations are not limited to the oil and gas industry. In fact, they are recently being used as a new solution to wind turbines. Offshore wind turbines are relatively sensitive to deformations, particularly tilting. In order to withstand against tilting due to wind and wave loads and large lateral and overturning forces, the foundations of offshore wind turbines are setup with peripheral steel skirts which transmit the seabed, trapping a soil plug. Foundations of offshore structures require carrying a very small vertical load, but large horizontal loads and overturning moments. Suction buckets are circular shallow foundations with large diameter, closed at top and open at bottom. Bucket foundations penetrate into the seabed by its self-weight to provide a seal between the skirt tip and the soil and then penetrate by applying an under-pressure inside the skirt compartment until full contact with the soil is obtained. The bearing capacity of the bucket foundations under combined loads increases due to embedment by comparison with a surface foundation (L/D = 0). The bearing capacity behavior of the bucket foundations can be expressed through failure envelopes that expand with increasing aspect ratio. In the literature, it is not clearly and quantitatively clarified how the bearing capacity depends on bucket embedment length but can be stated with increasing aspect ratio, failure mechanisms are forced deeper within the soil mass. In November 2002 the first Suction bucket foundation for using of the wind energy converter was installed at the large scale test facility in Frederikshavn. The project was at the time being the largest wind turbine in Denmark. Its total height of turbine was equal 125 m with a bucket diameter of 12 m and skirt length of 6 m (i.e. L/D =0.5) In this report, the effect of aspect ratio on combined bearing capacity of bucket foundations installed in saturated medium dense sand was evaluated. Load-deformation behavior of suction buckets under loads was investigated and compared. The analysis was carried out by loading bucket foundations with a load that allowed the bucket to move freely. When considering the design of the foundations of offshore wind turbines, it is important to understand their elastoplastic deformation behavior as well as ultimate capacities.

Keywords: Bucket foundation, aspect ratio, finite element analysis, bearing capacity, medium dense sand

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