Session: 01-04-01 Design & Analysis-I

Paper Number: 133104

133104 - Aero-Hydro-Servo-Elastic Code-to-Code Comparison for a 10mw Floating Offshore Wind Turbine 

Floating offshore wind turbines(FOWTs) are exposed to various environmental loads such as wind, waves, and currents during their 20+ years of operation. To design a FOWT, it is necessary to accurately predict the loads and dynamic responses. This requires a deep understanding of multiple physical dynamics such as servo-controlled aerodynamics, hydrodynamics, mooring dynamics, and structural dynamics. Most software such as OpenFAST and Bladed originally aimed to predict the dynamic loads of earth-fixed turbines and later upgraded to be used for FOWTs. Meanwhile, OrcaFlex which has been popularly used for any floating body simulations was recently updated so that it can be used for the integrated load analyses (ILAs) of FOWTs. Even though hierarchy of OpenFAST and OrcaFlex is quite different, they are most often used for the load analyses of FOWTs.

This study aims to compare load analysis results obtained from OpenFAST and OrcaFlex. Through frequency response analysis for a 10 MW FOWT that was developed in Korea, the hydrodynamic coefficients of added mass, radiation damping coefficient, restoring stiffness, and 1^st/2^nd order wave excitation forces under a unit wave amplitude were derived. The viscous damping coefficients in heave, roll, and pitch motion components, which were obtained from free decay simulations tests based on computational fluid dynamics (CFD), were summed to the wave damping coefficients. One of OpenFAST module Turbsim was used to create wind velocity field in space- and time-domain. The ROSCO toolbox was used to obtain the control system constants. The motion response of the FOWT system and mooring tension response were compared and validated for eight independent load cases: Case 1) Free decay cases in the heave, roll, and pitch directions, Case 2) regular wave cases, Case 3) irregular wave cases, Case 4) irregular wave cases with 2^nd order wave force, Case 5) uniform wind cases, Case 6) turbulence wind cases, Case 7) cases of uniform wind speed and regular wave without controller, and Case 8) uniform wind and regular wave cases with controller.

Through fast Fourier transform of the floating body motion components, the motion spectra and natural frequencies were compared in the frequency domain. There was a good agreement in the natural frequencies between OpenFAST and OrcaFlex with significant differences in the amplitudes. The motion spectra in a very low frequency zone (drift frequency zone) were quite different each other, but motion response results were similar each other in the wave frequency zone. The FOWT motion responses for aerodynamic force-dominant cases (Case 5 and Case 6), were in high agreement. However, the motion and mooring tension results showed significant differences. The root cause of the result difference came from low frequency responses, hence this should be verified by other approaches of CFD.

Presenting Author: Hyeongsu Lee INHA university

Presenting Author Biography: Hyeongsu Lee is a Ph.D. student in the Department of Naval Architecture and Ocean Engineering at INHA university, South Korea.

He has been studying integrated load analysis of floating offshore wind turbine (FOWT). Today, he will present Aero-hydro-servo-elastic code-to-code comparison for a 10MW floating offshore wind turbine.

Authors:

Hyeongsu Lee INHA university
Joonmo Choung Inha university