Comparison Of Second-Order Loads On A Semisubmersible Floating Wind Turbine
Author S├ębastien Gueydon (MARIN), Tiago Duarte (Instituto Superior T├ęcnico), Jason Jonkman (National Renewable Energy Laboratory)
Title Comparison Of Second-Order Loads On A Semisubmersible Floating Wind Turbine
Conference/Journal OMAE ASME 33rd International Conference on Ocean, Offshore and Arctic Engineering, San Francisco, CA
Year 2014

Abstract
As offshore wind projects move to deeper waters, floating platforms become the most feasible solution for supporting the turbines. The oil and gas industry has gained experience with floating platforms that can be applied to offshore wind projects. This paper focuses on the analysis of second-order wave loading on semisubmersible platforms. Semisubmersibles, which are being chosen for different floating offshore wind concepts, are particularly prone to slow-drift motions. The slack catenary moorings usually result in large natural periods for surge and sway motions (more than 100 s), which are in the range of the second-order difference-frequency excitation force.
Modeling these complex structures requires coupled design codes. Codes have been developed that include turbine aerodynamics, hydrodynamic forces on the platform, restoring forces from the mooring lines, flexibility of the turbine, and the influence of the turbine control system. In this paper two different codes are employed: FAST, which was developed by the National Renewable Energy Laboratory, and aNySIM, which was developed by the Maritime Research Institute Netherlands. The hydrodynamic loads are based on potential-flow theory, up to the second order. Hydrodynamic coefficients for wave excitation, radiation, and hydrostatic forces are obtained with two different panel codes, WAMIT (developed by the Massachusetts Institute of Technology) and DIFFRAC (developed by MARIN).

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