Abstract

The choice of drag and inertia coefficients are critical for the evaluation of hydrodynamic loads in slender cylinders using either Morison’s equation or an approach where viscous forces are simply added to the results of potential theory. Many studies available in the literature have considered fixed cylinders under the action of (two dimensional) sinusoidal currents, showing that the average values of drag and added mass coefficients can be correlated with the Keulegan-Carpenter and Reynolds numbers. However, when the semi-empirical models are used for the analysis of Floating Offshore Wind Turbines (FOWTs), many other aspects of the flow may play an important role, such as the spatial variations of the wave flow over the hull, three-dimensional flow effects associated with the floater motions, the presence of heave plates, among others. The present work is based on a case-study involving a simplified version of the floater of a semi-submersible FOWT and deals with cases where the incoming flow is composed of more than one frequency and body motions are a combination of periodic components with very different frequencies (wave frequency and slow-drift motions). In this case, the choice of proper coefficients for the Morison’s approach becomes somewhat puzzling, to say the least. The objective is to understand how the more complex flow and the coexistence of different frequencies affect the hydrodynamic forces and whether proper values of force coefficients can indeed be obtained from simplified model tests performed in the absence of incoming waves, such as forced oscillations and decay tests. For doing so, the paper analyses the results of an experimental campaign performed with the model scale floater (1:80) composed of four vertical circular columns. Three sets of tests are taken into account: forced oscillations of the hull, free decays of the moored model, and motions under the action of waves (monochromatic and bichromatic). The first two are used to assess the values of added mass and drag coefficients (and also for obtaining linearized damping levels), while the third group of tests helps to evaluate the accuracy of the motions predicted when using these coefficients in frequency-domain computations.

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