Precise position and motion control of offshore vessels is often challenging, especially in harsh environment due to highly nonlinear dynamic loads from free-surface ocean waves and currents. In addition, coupled nonlinear effects of risers and mooring cables connected to the vessel can lead to unexpected responses, thus justifying the significance of modeling these nonlinear coupled effects for safer and cost-effective design and operation of offshore structures. In this study, a fully coupled multi-field fluid-structure-interaction (FSI) solver is developed to simulate the wave- and flow-induced vibration of the flexible multibody system with constraints (viz., vessel-riser system) in a turbulent flow. The structural domain with multibody systems is solved using nonlinear co-rotational finite element method, whereas the fluid domain is solved using Petrov-Galerkin finite element method for moving boundary Navier-Stokes solutions. A partitioned iterative scheme based on non-linear interface force corrections is employed for coupling of the turbulent fluid-flexible multibody system with nonmatching interface meshes. Delayed Detached Eddy Simulation (DDES) via the Positivity Preserving Variational (PPV) method is employed for modeling turbulence effects at high Reynolds number. The free-surface ocean waves are modeled by the Allen-Cahn based phase-field method. We address two key challenges in the present variational coupled formulation. Firstly, the coupling of the incompressible turbulent flow with a system of nonlinear elastic bodies described in a co-rotated frame. Secondly, the two-phase coupling based on the phase-field approach to model the air-water interface. We then present the dynamics of coupled vessel-riser system studied in harsh environmental conditions with a view of developing a robust station keeping system. The proposed fully-integrated methodology based on the first principles of variational continuum mechanics removes many assumptions and empirically assigned parameters (e.g. drag and inertia coefficients) for modeling the surrounding fluid flow at high Reynolds number.

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