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

A Coupled CFD and Dynamic Mooring Model for FOWT Hydrodynamics
Wenjie Zhong1, Nina Wang2, Decheng Wan1*
1

Computational Marine Hydrodynamics Lab (CMHL), School of Naval Architecture, Ocean and Civil Engineering,
Shanghai Jiao Tong University, Shanghai, China
2
Key Laboratory of Far-shore Wind Power Technology of Zhejiang Province, Huadong Engineering Corporation Limited, Hangzhou, China*
Corresponding author

is based on the computational fluid dynamics (CFD). The model-based
method uses empirical formula for the aerodynamic and hydrodynamic
loads, and possesses the feature of fast computation which is suitable
for the preliminary design purpose (Dai et al., 2018; Jonkman, 2009;
Nygaard et al., 2016; Robertson et al., 2014). While the low-ordermodels in aerodynamics and hydrodynamics affect the overall accuracy
and range of application of the model-based approach, the structuraldynamics can be directly obtained which can be used for ultimate andfatigue load evaluations (Kvittem and Moan, 2015; Luan et al., 2017).
The CFD approach has long been applied in simulating FOWTs (Chenget al., 2019; Huang et al., 2019; Liu et al., 2017; Tran and Kim, 2015,2016b; Yan et al., 2016). As real flow is modeled in CFD, the fluid
force is simply calculated by integration of pressure and viscous stresswith the accuracy dependent on the fidelity of the numerical model.
Particularly for the wave structure interaction problems, wave breaking,run-up and overtopping, vortex generation and shedding are inherentlyincluded in the nonlinear governing equations. It is to note that since
the structural parts are modeled as boundaries in CFD, the flexibility ofthe structure relies on external codes for fluid-structure interaction
simulations (Carrion et al., 2014; Lee et al., 2017; Liu et al., 2019).

ABSTRACTThe fast-growing floating offshore wind turbines (FOWTs) urge areliable tool for coupled analyses. While most studies utilize the quasistatic mooring model, the mooring dynamics has rarely been includedin the coupled computational fluid dynamics (CFD) analysis of FOWTs.
In the present paper, a finite element mooring dynamics code is coupled
to CFD for FOWT hydrodynamics. Free-decay tests and responses inregular waves of the DeepCwind semi-submersible platform moored bycatenary cables are simulated for validation purpose. Results show thatthe coupled model is accurate against the experiment data and exhibits
greater reliability than the model-based approach.

KEY WORDS: Floating offshore wind turbines; Computational fluid
dynamics; Dynamic mooring model; Coupled simulation; Free-decaytest; Regular wave.

INTRODUCTION
The development of floating offshore wind turbines (FOWTs)
along with the exploration of offshore wind energy has been oceanengineering trends for decades due to the demands for clean andrenewable energies. FOWTs operate in deep waters where more stablewind resources abound in comparison to onshore and nearshore sites,
and have become one of the research focuses among the wind energy
community. As the FOWT is a multi-body system containing the windturbine, the control sector, the tower, the floating platform and themooring system, challenges are directly faced when trying to assess thecoupling effects among the components (Liu et al., 2016). In specific,
the moving platform adds six degrees of freedom (DOFs) to themounted wind turbine, causing greater uncertainties in its aerodynamics
and power production (Liu et al., 2019; Tran and Kim, 2016a). On thecontrary, the hydrodynamics of the floating platform is modified due tothe wind-induced constant inclination and variational pitching motion(Antonutti et al., 2016). The restoration by dynamic mooring systems issignificant when the floating structure encounters severe sea conditions(Hall et al., 2014).

The distinct feature of FOWTs is the floating platform which hasits prototype in offshore oil and gas industry. Different from those inoffshore oil and gas productions, the mass and wave loading properties
of FOWT platforms are unique considering the interaction between theplatform and the mooring system. To include the role of mooring lines
into CFD simulations, a coupled CFD and mooring model is required.
Plenty of researches have tried using the quasi-static mooring model for
either coupled hydrodynamic-mooring (Lin et al., 2021; Huang et al.,2021; Tran and Kim, 2015) or coupled aerodynamic-hydrodynamicmooring (Cheng et al., 2019; Liu et al., 2017; Liu et al., 2019; Tran and
Kim, 2016b) analysis of FOWTs. The quasi-static model solves for themooring line profile and tension using the analytical equations with theassumption that the line is in static equilibrium. The hydrodynamic andinertial forces are neglected in the model which affects the prediction of
the motion response of the moored structure and the mooring loads.
A number of studies showed that the use of dynamic models is inmany cases necessary for accurately predicting mooring line loads inMethods for conducting coupled simulations of FOWTs generally
fall into two classes. One is based on engineering models and the other2577