Coupling Soil–Fluid–Structure Domains by Localized Lagrange Multipliers Mixed Formulation (u, p) for Modeling Offshore Wind Turbine Vibration

Abstract

Modeling is frequently used to design structures in a large range of engineering applications. Coupled models can be solved by several numerical procedures. This paper presents a soil–fluid–structure coupling analysis applied to offshore wind turbines design. The goal of this work is to develop a coupled structural finite element procedure using Localized Lagrange Multipliers (LLM) at idealized offshore wind turbines with poroelastic soil foundation. The poroelastic media theory of Biot is used to represent the soil domain as two-phase material which is considered to be fully saturated with water. The numerical model is validated through a fully coupled model at classical problems results. In this work, the mixed formulation (u,p) is used to model the interface frames between the domains. The interface domain behaves as fictitious porous material generating a nonsymmetrical coupling between elastic solid and potential fluid. The momentum equilibrium and mass continuity equations are solved by algebraic equation system imposed by Lagrange multipliers methodology. In order to fully model the coupled system, aerodynamic decoupling effects are implemented to separate the tower structure, the soil foundation and the rotor-nacelle assembly. This procedure is based on the blade aerodynamic characteristics. The force and moment vectors are assembled considering the aerodynamic damping coupling between inplane and outplane rotor motions. Finally, the equations of the tower-nacelle structure, ocean fluid and poroelastic soil are obtained by the classical finite element method. In addition, their interaction is modeled using LLM. The numerical model for monopile and jacked offshore wind turbine is developed in terms of time dynamic response which is evaluated for a range of physical parameters including the wind nature and soil type.