Quantifying the numerical uncertainty for CFD simulation of Floating Offshore Wind Turbines in pitch decay motion

Significance 

Compared with the in-situ experiment-based techniques, numerical tools have been widely used in predicting the responses of floating offshore wind turbines (FOWTs) owing to their low cost and higher accuracy levels. Recently, several engineering models with mid-fidelity tools have been cross-verified and validated with both numerical and experimental data. Unfortunately, these mid-fidelity tools proved ineffective as they significantly underpredicted the responses of semi-submersible FOWTs, especially under low-frequency nonlinear wave excitations. Lately, there has been a gradual development of high-fidelity CFD simulations capable of accurately capturing all the nonlinearities and viscous damping effects. Besides, it has been established that using CFD tools for cross-verification and validation could help improve the efficiency and accuracy of engineering-level tools.

The compatibility of accurate hydrodynamics prediction of floating offshore wind turbines with CFD tools has been extensively studied. Specifically, these tools have been used to evaluate the Hywind floater, a spar-type floater used as the primary model in many studies. CFD tools have proved beneficial in tunning the inputs of low-cost mid-fidelity tools and quantifying the viscous effects and nonlinearities that are difficult to quantify numerically. However, most of these studies have performed grid refinement studies and sometimes time-step studies, but almost never numerical uncertainty quantification. To accurately conduct the validation against experimental measurements, it is important to evaluate the total numerical uncertainties of numerical models. This is yet to be fully realized to date.

Herein, PhD candidate Yu Wang and Professor Hamn-Ching Chen from Texas A&M University in collaboration with Dr. Arjen Koop from Maritime Research Institute Netherlands and Dr. Guilherme Vaz from Blue Ocean Sustainable Solution performed an innovative verification and validation study to accurately estimate the numerical uncertainty of CFD simulations for a semi-submersible floating offshore wind turbine in pitch free-decay motion. The simulations were performed with both dynamic and linear mooring models to derive the quadratic and linear damping coefficients of the semi-submersible. A total of 20 simulations were performed. They started by estimating the spatial and temporal discretization uncertainty, followed by validations with uncertainties in pitch motions. Their work is published in the journal, Ocean Engineering.

The research team reported that the iterative uncertainty had little impact on the total numerical uncertainty and was neglected. The mean and maximum discretization uncertainty values for pitch motion were obtained as 0.236 and 0.531 degrees, respectively. Using a dynamic mooring model significantly improved the validation results. It accurately predicted the natural pitch period though it slightly under-predicted the quadratic damping coefficient. It was found that the total numerical uncertainty originated purely from the discretization uncertainty. Furthermore, a guideline was provided to guide future verification and validation studies of CFD tools for marine structures subjected to decay tests.

In summary, a detailed verification and simulation study was conducted towards CFD simulations of FOWT with a semi-submersible platform in free decay motion. The numerical uncertainties were successfully validated against the experimental measurements. Importantly, the predicted linear damping coefficient and pitch natural period were in good agreement with the experimental results. In a statement to Advances in Engineering, first author Yu Wang said the new study provided valuable insights that will advance free-decay dynamical analysis of floating marine structures.

Quantifying the numerical uncertainty for CFD simulation of Floating Offshore Wind Turbines in pitch decay motion

About the author

Dr. Ir. Arjen Koop
Team leader Waves & Motions, R&D Department, MARIN

Arjen Koop was born on June 1st, 1979, in Curacao, Dutch Antilles. He graduated at the University of Twente, in Enschede, Netherlands in 2003, obtaining the Applied Mathematics engineer degree. In 2008, he obtained his Ph.D. degree at the same University on CFD for the numerical simulation of cavitating flows. Since July 2008 he is employed by MARIN. First, he was Project Manager and team leader in the Offshore department and now he is working in the Research & Development Department as Team leader for Ocean Engineering and Sea-keeping. His main specializations are viscous flow CFD for Offshore hydrodynamic applications, Vortex Induced Motions utilizing model tests and CFD as well as CFD for Renewable Energy applications such as floating wind turbines and floating solar panels.

About the author

Dr. Hamn-Ching Chen is the Holder of the A.P. & Florence Wiley Professor I in the Zachry Department of Civil and Environmental Engineering and a Joint Professor in the Department of Ocean Engineering, Texas A&M University. He received his B.S. and M.S. degrees in Power Mechanical Engineering from National Tsing Hua University, Taiwan, in 1976 and 1978, respectively; and his Ph.D. in Mechanical Engineering from the University of Iowa in 1982. Dr. Chen is a Fellow of ASME and an Associate Fellow of AIAA. He is the originator and primary developer of the Finite-Analytic Navier-Stokes (FANS) and CHimera finite Analytic Method Potential-flow Solver (CHAMPS) codes which has been used extensively for simulations of fluid-structure interaction problems including violent free surface flows.

His research interests include ship berthing operations, passing ship effects, propeller wash in harbors, ship bow and stern impacts in random waves, vortex-induced vibrations of marine risers and offshore pipelines, vortex-induced motions of semisubmersible platforms, hurricane wave impact on offshore structures, floating offshore wind turbines, bridge scour, and internal cooling and film-cooling of turbine blades.

About the author

Dr. Ir. Guilherme Vaz
R&D Manager at blueOASIS [GV]

Finished Msc in Aerospace Engineering in IST, Technical University of Lisbon in 1999. Pursued his Phd studies on Cavitation of Marine Propellers at MARIN and at IST, having obtained the Doctor degree in 2005. Then he worked in the CFD Group of the Research & Development (R&D) department of MARIN, with specialization on CFD code development, High-Performance Computing (HPC), turbulence, grid-handling methods, propellers, cavitation, offshore and renewable energy systems. He was the founder of open-source community based CFD code ReFRESCO until August 2019. From September 2016 until August 2019 he was also the responsible for the technical coordination of the CFD developments at MARIN. Since 2017 he has been invited Lecturer at the University of Duisburg-Essen in Germany and visiting researcher at University of Southampton. In August 2019 he moved back to Portugal and became the CSO of Renewable Energy Institute WavEC, and team leader of the HPC and Data Science team, where he started to expand his expertise’s into the AI world. He is now the R&D Manager of blueOASIS (Blue Ocean Sustainable Solutions), a start-up dealing with Industry 4.0 tools applied to the Ocean Sustainability and World Decarbonization.

About the author

Yu Wang is a PhD student in the Department of Ocean Engineering, Texas A&M University. His research focuses on verification and validation study of CFD simulations for the hydrodynamic responses of Floating Offshore Wind Turbines.

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Reference

Wang, Y., Chen, H., Koop, A., & Vaz, G. (2021). Verification and validation of CFD simulations for semi-submersible floating offshore wind turbine under pitch free-decay motionOcean Engineering, 242, 109993.

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