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.