Efficient multibody modeling of offshore wind turbines with flexible substructures

Offshore wind turbines, especially floating wind turbines, are often simulated assuming rigid substructures to obtain computationally efficient simulation models for preliminary parameter variation studies. This causes large errors in the determination of coupled natural frequencies and internal loads, particularly with increasing turbine sizes. Finite Element models for flexible substructures were developed by several researchers, often resulting in a high simulation effort. In this paper, a modally reduced Finite Element model, precomputed by the SubDyn module of OpenFAST, is directly included in the generalized Equation of Motion of the Simplified Low Order Wind turbine model SLOW. The approach was tested with the DTU10MW reference wind turbine mounted on a flexible monopile. It shows a high agreement with the former beam-based Multibody System in the calculated coupled natural frequencies and steady state results both for the linear and nonlinear model. A basis has been established to integrate flexible bodies of any shape even into computational efficient Multibody Systems of reduced order, such as SLOW, without coupling of two modules as in OpenFAST. This might improve numerical stability due to unified equations of motion.