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In this research, the seismic response of offshore wind turbines, considering the interaction of saturated soil-pile-structure, has been investigated using numerical method of finite element. OpenSees software has been used to consider the conditions of soil saturation and pore water pressure changes. Due to the existence of soil constitutive model in OpenSees, such as the PDMY model and coupled u-p elements, it has a good ability to model saturated soil and pore water pressure changes.For numerical verification, a centrifuge test carried out by Yu et al. was used. This test was carried out on an offshore wind turbine with tripod foundation, with a height of 13 meters and three piles, 0.5 in diameter and 3 meters in length with a triangular arrangement, and the response of the turbine tower and pore water pressure variations under the earthquake load have been investigated. In this experiment, blades, hub and Nacelle were simplified as a rigid mass on top of wind turbine tower and so large moment caused by the earthquake load was modeled on the foundation. For simulation and creating numerical model, only one half of the system was modeled using symmetry boundary condition. Soil 3D continuous medium was modeled through coupled u-p formulation correlated to saturated porous medium using PDMY constitutive model that has the ability to simulate sandy soil behaviour under cyclic loadings in drained and undrained condition. The model consisted of 14288 nodes and 12420 coupled u-p 3D elements for saturated soil part. Nonlinear beam-column elements were used for pile parts. For simulating actual size of pile cross-section, rigid beam elements perpendicular to the longitudinal axis of the piles were used. Actually these rigid elements were beam-column type that their stiffness is 10000 times larger than the stiffness of pile elements. One node of this elements was connected to the pile and the other node was tied to the soil node with same location through equal DOf constraint. Each pile with 3 meter in length consisted of 12 nonlinear beam-column elements. For half pile 65 rigid elements and for full pile 104 rigid elements was used to simulate actual size of pile cross-section. Wind load on tower is estimated by equation provided in DNV standard. Also the thrust force (force applied by the wind on the rotor of turbine) is calculated through the previous study (Leite) and using Manwell equation. Wave load is calculated by Morison equation and the kinematics of water particles are simulated by Airy wave theory (linear wave theory). After passing the verification stage, through the parametric study, the effect of other environmental loads (wind and wave load) and peak ground acceleration (PGA) on the seismic response of the offshore wind turbine are investigated. The results showed that in seismic analysis of offshore wind turbines, the interaction of environmental loads should be considered, and the Superposition Principle can not be easily applied. It was also found that the relationship between the peak ground acceleration and the turbine tower response is nonlinear. On the other hand, by increasing the PGA, the effect of soil-piles interaction on the r_u ratio increases.

Keywords: Offshore wind turbine, OpenSees, PDMY constitutive model, soil-pile-Structure, wind and wave load

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