Numerical Analysis of Fluid-Structure Interaction (FSI) for Semi-Submersible FOWTs

XI International Conference on Computational Methods in Marine Engineering (MARINE 2025)
Sang-seok Han 1*, Saishuai Dai 1, Momchil Terziev 1, Hae-chan Yun 2, Soon-seok Song 2
1 Department of Naval Architecture, Ocean and Marine Engineering, University of Strathclyde, the United Kingdom
2 Department of Naval Architecture and Ocean Engineering, Inha University, Republic of Korea

Abstract

This study presents a computational fluid–structure interaction (FSI) analysis of a semi-submersible floating offshore wind turbine (FOWT) using a computationally efficient CFD-based framework. The method enables structural flexibility without requiring external finite element solvers. The simulation model is based on the OC6 Phase Ia semi-submersible platform at 1:50 scale and is implemented in STAR-CCM+ using the DFBI Morphing feature combined with a multi-overset mesh strategy.

Structural deformation is handled entirely within the CFD environment, offering a practical and self-contained FSI simulation approach. Model validation is conducted via free decay and regular wave RAO tests, showing good agreement with experimental data. Comparative analyses between rigid-body and FSI-enabled models reveal that structural flexibility increases surge and heave responses while slightly reducing pitch amplitudes. Additionally, internal stress analysis identifies critical loading regions under wave excitation, emphasising the importance of structural compliance in early-stage FOWT design. These results demonstrate the feasibility of integrated CFD–FSI analysis using commercial tools and provide a foundation for extending the framework to more realistic ocean environments, including irregular and extreme wave conditions.

Conference paper (2025)
XI International Conference on Computational Methods in Marine Engineering (MARINE 2025)
DOI: 10.23967/marine.2025.141
Selected Figures
Double-overset mesh configuration for the FSI simulation of the OC6 semi-submersible platform
Figure 1. Mesh configuration for FSI simulation using a double-overset strategy, enabling structural deformation and stress evaluation.
The FSI simulations employ a double-overset configuration in which the first overset region resolves the interaction between the platform and the surrounding fluid, while a second region linked to the morphing mesh captures structural deformation and internal stress. The resulting setup contains approximately 4.6 million cells and is intended to improve coupling fidelity between hydrodynamic loading and structural response.
Time histories of surge, heave, and pitch for rigid-body, steel FSI, and aluminium FSI models under regular waves
Figure 2. Time history of platform motions (surge, heave, and pitch) for FSI (Steel, Al) and Non-FSI models under regular wave condition (H = 0.148 m, T = 1.697 s).
The motion histories compare the conventional rigid-body model with deformable-body steel and aluminium configurations under identical regular-wave forcing. The results indicate that fluid–structure interaction effects are dependent on both the motion mode and material properties, with distinct changes observed in surge, heave, and pitch response.
Von Mises and mean stress contours on the semi-submersible FOWT platform under regular wave loading
Figure 3. Contour plot of Von Mises stress (left) and Mean Stress (right) in the semi-submersible platform under regular wave condition (H = 0.148 m).
The structural stress fields identify localised loading regions within the deformable-body model. Von Mises stress concentrations of approximately 16 kPa appear near the outer column bases, pontoon connections, and central column joints, while the mean-stress distribution indicates compression around the lower column–pontoon regions and tension near the column tops. These patterns provide a basis for preliminary structural and fatigue assessment under wave excitation.