<p><i>Current-Induced Motion (CIM) is a complex phenomenon which may occur with offshore multi-column floating platforms. Recently, CIM or FIM (Flow -Induced Motion), as usually known, has been observed from towing tank tests with a small-scale model of a Floating Offshore Wind Turbine (FOWT), the OC4 Phase II floater, a 3+1 columned platform. The present paper proposes a reduced order mathematical model (ROM) to assess FIM of a FOWT. The ROM is developed on the horizontal plane, including yaw motions and nonlinear mooring stiffness. Current forces are represented through ‘wake variables’, adapting phenomenological models firstly used for Vortex-Induced Motion (VIM) of floating mono-column platforms. The ROM is built upon a set of eleven generalized coordinates, three for the rigid body motion on the horizontal plane and a pair of wake variables for each column, resulting in a system of eleven nonlinear second-order ODEs. The pairs of wake variables obey van der Pol equations, applied to each of the columns, using hydrodynamic coefficients and parameters obtained from previous experiments with small draught cylinders. Hydrodynamic interferences among columns are not considered, for simplicity. The validity of the proposed model is assessed using the small-scale experimental campaign as a case study. Numerical simulations are performed in MATLAB® environment. The simulations are done for three different current incidence angles, 0, 90 and 180 degrees, spanning a large range of reduced velocities. Despite ignoring possible vortex wake interferences among columns, the simulations reproduce well the oscillations observed in the experimental tests. A good agreement in transverse and in-line oscillations is found including lock-in regions. Moreover, the simulations depict an important phenomenon: a resonant yaw motion emerged at high reduced velocities.</i></p>

A Reduced-Order Mathematical Model for the Current-Induced Motion of a Floating Offshore Wind Turbine

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/IOWTC2021/volume/84768

ASME 2021 International Offshore Wind Technical Conference (IOWTC2021)

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