Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Comparative study of coupled and uncoupled analysis of semi-submersible floating offshore wind turbines
Shan Gao1,2, Dezhi Wang2, Yu Lei3,4, Kanmin Shen1,2, Bin Wang1,2, Jinzhong Chen2, Yunze Xu5
1. Key Laboratory of Far-shore Wind Power Technology of Zhejiang Province, Hangzhou Zhejiang China
2. Powerchina Huadong Engineering Corporation Limited, Hangzhou Zhejiang China
3. China Huaneng Clean Energy Research Institute, Beijing, China
4. Institute for Ocean Engineering, Tsinghua Shenzhen International Graduate School, Shenzhen Guangdong China
5. Dalian University of Technology, Dalian Liaoning, China

ABSTRACT

carry out the fully aero-hydro-servo-elastic coupled analysis of aFOWT, and they are widely used and validated. Haid et al. (Haid et al.,2013) examined the appropriate length of a FOWT simulation withFAST. The results indicated that the loads did not increase for longer
simulations and the fatigue analysis was more sensitive to theprocedure used for counting half cycles than the simulation length itself.

In the current wind industry, wind turbines and foundations are usuallydesigned by different companies and there is a general problem ofsharing wind turbine structure and controller details. As a result, theuncoupled analysis method, sometimes with iterations, has beendeveloped. Nevertheless, whether such a method can be applied to theanalysis of floating offshore wind turbine (FOWT) and whether its
results are conservative or nonconservative should be studied. In thiswork, the OO-Star floater with a 12MW wind turbine is selected as anexample to make a comparison of the fully-coupled method and theuncoupled analysis. The results show that, if the loads of the windturbine (which is considered as the external loads) are calculated basedon the first-order motion of the floater, the results would give obviousdeviation in response spectrum of the motion compared to the fullycoupled analysis, although the second-order wave loads are used in theun-coupled method.

Bae et al. (Bae et al., 2012) integrated CHARM3D with FAST to study
the effects of the elastic tower on the dynamic response of a TLP typeFOWT. It has been found that the elasticity of the tower candramatically affect the platform pitch motion. Coulling et al. (Coullinget al., 2013) presented the validation of a FAST numerical model of theDeepCwind semi-submersible FOWT using 1:50 scale model test data.
Their study indicated that FAST performed well in predicting theresponse of the DeepCwind semi-submersible FOWT with anappropriate mooring model and hydrodynamic model. Shi et al. (Shi etal., 2018) implemented a numerical ice-structure interaction model toHAWC2 to investigate the ice load effect on monopile and spar
offshore wind turbines. It is observed that the varying ice thickness
field will increase the dynamic effect of the ice loads. Besides, alimited number of works on validation of FOWT’s design tools withfull-scale field data have also been carried out. The comparative study
by Skaare et al. (Skaare et al., 2015) on the Hywind Demo showed thatin an overall sense good agreement was reached between FAST
simulations and full-scale measurements. Tanaka et al. (Tanaka et al.,2020) also compared the simulated dynamic responses of a 2MW spartype FOWT under typhoon environments with the measured data. As
the time-series records of the site environmental conditions were usedas the input in the simulations, a fairly good agreement was obtained
after accounting for the coherence of wind speed. Takata et al. (Takataet al., 2021) carried out tank experiments to clarify the floater elastic
response and its influence on the floater motion using a flexible multicolumn FOWT. The test results are compared with the simulations ofNK-UTWind and WAMIT codes which indicated that NK-UTWind codegave better agreement with the test results as the elasticity of the floater
was included.

KEY WORDS: floating wind turbine; fully-coupled and uncoupled
methods; first -order motion; second-order wave.

INTRODUCTION
In recent years, the exploration of offshore wind has moved fromshallow waters to deep waters, and the floating offshore wind turbine(FOWT) is considered a competitive candidate to harvest wind energy
in waters with a depth of more than 50m (Henderson and Witcher,2010). FOWT is a kind of complex engineering equipment consistingof a multi-subsystem, such as wind turbine, tower, floating foundationand mooring system. It is subjected to a variety of environmental loads
during service including wind, wave and current loads. Besides, thefloating foundation has rigid body motions. These loads and motions
are coupled and nonlinear so that a fully-coupled dynamic analysis ispreferable.
For the simulation of the dynamic responses of a FOWT, a number ofcodes have been successfully developed, such as FAST (Jonkman andBuhl Jr, 2005), HAWC2 (Larsen and Hansen, 2007). Both of them canFurthermore, the traditional offshore dynamic analysis software, such254