Stress analyses of the offshore wind turbine structures subjected to ocean waves

Two common offshore wind turbine structures, the monopile type and the jacket type, subjected to ocean wave load were analyzed using finite element simulations. The FEM models were built in ABAQUS, and applied the load combinations consisting of the structural weight, the weight of wind turbine, the wind force, and the ocean wave force. In the analyses, we especially focused on the connection between the tower and the supporting structures, because wind force and ocean wave force may cause the damage to occur at the connections. In the model of the monopile structure, the grout connection to tie the upper steel tower and the lower steel pile is modeled by 3D cohesive elements. In the model of the jacket structure, the connection is a joint welded inclined members to tie the upper tower and the lower sleeves. Ocean wave forces acting on the structures are calculated based on Morison's equation, using the wave velocity and acceleration estimated by linear wave theorem and the site conditions of the wind farm in Taiwan. Both static and dynamic analyses with the distributed ocean wave force were carried out. Its effects on the stresses and deformation of two wind turbine structures were then investigated. We also discussed the possibility of using duplex stainless steel instead of mild carbon steel in the connection part of the jacket structure, because duplex steel has high yielding strength and excellent corrosion resistivity.