Abstract
Representative models of the nonlinear behavior of floating platforms are essential for their successful design, especially in the emerging field of wave energy conversion where nonlinear dynamics can have substantially detrimental effects on the converter efficiency. The spar buoy, commonly used for deep-water drilling, oil and natural gas extraction and storage, as well as offshore wind and wave energy generation, is known to be prone to experience parametric resonance. In the vast majority of cases, parametric resonance is studied by means of simplified analytical models, considering only two degrees of freedom (DoFs) of archetypical geometries, while neglecting collateral complexity of ancillary systems. On the contrary, this paper implements a representative 7-DoF nonlinear hydrodynamic model of the full complexity of a realistic spar buoy wave energy converter, which is used to verify the likelihood of parametric instability, quantify the severity of the parametrically excited response and evaluate its consequences on power conversion efficiency. It is found that the numerical model agrees with expected conditions for parametric instability from simplified analytical models. The model is then used as a design tool to determine the best ballast configuration, limiting detrimental effects of parametric resonance while maximizing power conversion efficiency.
Highlights
Spar floating platforms are axisymmetric thin and long structures that became established solutions for deep-water drilling, oil and gas extraction and storage, and, more recently, for hosting offshore wind turbines [6,8,27,41]
The objective of this paper is to provide a comprehensive and computationally accessible nonlinear model, able to articulate parametric resonance due to nonlinear time-variations of the parameters of the system, for a realistic device, comprising complex viscous losses, power take-off (PTO), and realistic mooring system
Parametric resonance produces a roll response in the vicinity of Tn,4/2, with the instability region widening as the wave height increases, with a consequent increase in the amplitude of oscillation
Summary
Spar floating platforms are axisymmetric thin and long structures that became established solutions for deep-water drilling, oil and gas extraction and storage, and, more recently, for hosting offshore wind turbines [6,8,27,41]. In such applications, correct operational conditions require the floating structure to be as stable as possible, i.e., unresponsive to the wave excitation. Only computationally fast models are eligible to be used for extensive simulations required to inform the design and control tasks
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