High-Fidelity Structural Analysis of a 10 MW Offshore Floating Wind Turbine Rotor Blade

Floating wind turbines are complicated multidisciplinary structures consisting of tower, blades, control systems, floater, and mooring systems. Computational modeling of different components of wind turbines is a challenging problem due to strong coupling between physical phenomena. Traditionally, each component is analyzed and designed separately using low-fidelity computational models however due to strong coupling this leads to sub-optimal design and negates the accuracy and efficiency of the wind turbine. In order to model wind turbines under aero-hydro-struct-control loads, accurate and efficient coupled multi-physics computational models are required.

Today, wind turbine analysis relies heavily on beam finite element and multibody aeroelastic models like OpenFAST, HAWC2, etc. While effective, simplified low-fidelity physics models are used to reduce computational cost by making permanent assumptions on the physics, restricting the applicable domain of the first-principles that were used to derive such models. For example, blade element momentum methods are used for aerodynamics, and potential flow theory is used for hydrodynamics. For traditional wind turbine configurations, especially onshore, such low-order models are often well justified. However, for unconventional, flexible, or more complex configurations such as downwind turbines with large coning, vertical axis wind turbines, and floating offshore platforms, the simplified models become either less accurate, or simply cannot be used.  Even for traditional designs some driving load cases are difficult to model accurately with traditional low-fidelity models. The nonlinearities and the transient/unsteady behavior in material, structural, and fluidic domains become more dominant in unconventional and/or flexible designs. Furthermore, design of the structure of the wind turbine, optimization of stress distribution, and the strength and fatigue life of materials used in construction heavily depends on the assumed “external” loads. The conditions that offshore turbines experience, ranging from currents, waves, winds, precipitation, temperatures, salinity, loads through the mooring system, and other effects, must be accurately accounted for. The optimization process, and ultimately the performance of the final design, strongly depends on these assumed load scenarios. Consequently, accurate knowledge of offshore ocean and air conditions is necessary.

This paper develops a high-fidelity structural dynamics modeling toolkit for accurate and efficient modeling of structural components in floating wind turbines. The open-source code TACS [1-2] is initially utilized for modeling dynamic mechanical fields in the context of finite element framework. This model is enhanced with high-fidelity description of the external loadings including the loading from air and water and internal loadings induced by servo actuators and electrical generator [3]. Correspondingly the boundary conditions, inertial, damping and stiffness properties of the system are developed to account for the resulting structural dynamic modal behavior, such as the pitching and rolling of the floating platform, the bending and twisting of the tower and the blades. This computational model is utilized to analyze the linearized aero-hydro-servo-structural dynamics of a 10 MW floating wind turbine in stochastic wind and waves. The findings of this study are compared with existing results in the literature, to evaluate the importance of high-fidelity structural dynamics modeling.

Keywords: Wind turbine, composites, structural dynamics, reduced order modeling, optimization.