CFD-based Analysis of a 6MW Spar-type Floating Wind Turbine with Focus on Nonlinear Wave Loads

Spar-type floating wind turbines (FWTs) represent an important and promising means of utilizing wind energy over deep water. The slender 6MW spar-type floating platform investigated in this paper is a mini spar structure designed for medium water depth, for which important viscous effects and nonlinearities may not be properly modelled by the state-of-the-art design tools from hydrodynamic point of view.

A CFD-based, three-dimensional, numerical wave-tank (NWT) analysis is applied in this paper to study the hydrodynamic responses of the substructure of this 6MW spar-type FWT, with a stiffness-equivalent mooring system designed. CFD results agree well with data from a 1:65.3 model test and numerical simulation results from FAST.

INTRODUCTION Floating wind turbines (FWTs) generate renewable energy by utilizing wind energy over deep water. The design of a floating offshore wind turbine must accurately predict the critical loads it will encounter under various turbulent-wind and stochastic-wave conditions. Considering the high costs of FWT scale-model testing and the limitations inherent in simultaneously satisfying the essential scaling laws, numerical methods with unlimited full-scale capability have been developed.

However, accurately calculating the critical loads for an FWT is difficult, because of the complex multi-physical phenomena contributing to real conditions and the limitations of numerical tools. Therefore, numerical analysis methods must be carefully validated through code-to-code and code-to-data comparisons to ensure their reliability.

Among the various loads the FWT received, the hydrodynamic load is crucial for the prediction of dynamic responses of the FWT platform, especially in extreme wave conditions (Jonkman, 2009, Beyer, Arnold and Cheng, 2013, Bayati, Jonkman, Robertson and Platt, 2014). Currently, the hydrodynamic models used in the offshore industry are typically based on linear and second-order, weakly nonlinear, potential-flow theories and/or Morison's equation (Morison, Johnson and Schaaf, 1950).

These methods cannot directly consider the effect of viscous flow in hydrodynamic simulations (Jonkman, 2009), and an additional damping coefficient is usually applied for the accurate prediction of hydrodynamic loads. These approaches have been implemented in the HydroDyn module of the FAST (Fatigue, Aerodynamics, Structures, & Turbulence) code (Jonkman, Robertson and Hayman, 2014), which was developed by the United States National Renewable Energy Laboratory (NREL) and has been widely applied.

The HydroDyn module requires WAMIT (Lee and Newman, 1991), an external program based on potential-flow theory, to provide coefficients matrices for the hydrostatic restoring force, added mass, damping, and wave excitation loads for the supporting platform. However, viscous-flow separation and other higher-order nonlinear hydrodynamic effects are neglected in these models, despite being potentially crucial for predicting the global performance of FWTs, especially in extreme wave conditions.

By solving the Navier–Stokes equations with proper turbulence modeling, CFD analysis can capture these nonlinearities with some efforts (Chen and Yu, 2009, Ramirez, Frigaard, Andersen and Christensen, 2011). It is therefore of interest to apply CFD in investigating hydrodynamic responses of FWT platforms.

Based on CFD method, some previous studies investigated the decoupled only hydrodynamic responses of the floating wind turbines: