Verification and Validation Study of CFD Simulations of Semi-Submersible Platform for Floating Offshore Wind Turbine

Abstract

In the proposed study, Computational Fluid Dynamic (CFD) code ReFRESCO is utilized to analyze the hydrodynamic behaviour of a Floating Offshore Wind Turbine with a semi-submersible platform. An in-house nonlinear Finite Element Method (FEM) mooring analysis module MOORING3D is coupled with ReFRESCO to count in the hydrodynamic effect brought by the mooring system. Four load conditions are simulated using the coupled CFD-FEM code. They are respectively pitch free decay, regular waves, bichromatic waves and irregular waves. In the simulations of pitch free decay condition, Reynolds Average Navier Stokes (RANS) equations are used as governing equations closed by KSKL turbulence model. While k-ω SST-IDDES turbulence model is used in irregular wave simulation. In the remaining simulations, laminar flow assumption is adopted. Volume of Fluid (VOF) method is utilized to model the free surfaces. A six degree-of-freedoms (DOFs) motion solver is used to interact with both flow solver and mooring solver.

A systematic verification and validation study is applied to selected metrics obtained in the first three load conditions. Numerical uncertainties embedded in the signal processing, iterative method, grid and time step combinations are quantified. The respective uncertainties are namely statistical uncertainty, iterative uncertainty and discretization uncertainty. The statistical uncertainty is quantified for periodic simulations with waves using Transient Scanning Technique (TST). The iterative uncertainty is quantified using least square method towards iterative errors obtained from repeated simulations with different convergence tolerance. The discretization uncertainty is quantified by applying the similar method towards solutions obtained by repeated simulations with different combination of grid sizes and time step sizes. Validation uncertainties are computed by summing the numerical, input and experimental uncertainties. The simulation results are validated against the experimental measurements with the uncertainty range. A standard procedure of verification and validation study is generalized from this study for applications of hydrodynamic analysis of FOWT using CFD simulations.