Validation of drift motions for a semi-submersible floating wind turbine and associated challenges

Abstract

In the EU H2020 project LIFES50+, a 1:36 scaled model test campaign was carried out for the NAUTILUS-DTU10 semi-submersible floating offshore wind turbine with active ballast. This paper concentrates on the modelling capabilities of a state-of-the-art time domain simulation model FAST8 for floating offshore wind turbines and specific challenges associated with the validation process. For the modelling, the platform is considered as a rigid body, and the frequency dependant radiation damping, added mass, and wave excitation are evaluated with a panel code using potential theory. The results from the scaled model are compared to the simulations, looking into the effects on the platform when the first order radiation-diffraction hydrodynamics through the Cummins equation, the mean drift coefficients from the nearfield solution for Newman's approximation of the second-order difference-frequency wave forces, and full quadratic transfer function (QTF) are taken into account. Moreover, viscous forces on the floating platform are modelled through Morison elements with coefficients of drag. Frequency domain analysis of the motions showed good agreement after modifications of the coefficients of drag of the Morison elements for tests performed with a pink wave spectrum. On the other hand, the extreme wave test showed large discrepancies between results and simulations, which could not be overcome by the inclusion of the QTFs or tuning of the coefficients of drag which model the viscous forces. This is followed by a discussion of the challenges in the modelling approach, and other validation techniques are proposed for future research. The main goal is to define the Morison elements for the floater, and tune the drag coefficients to enable the numerical tool to capture the floater's motions. The effect of the change of the coefficients on the simulation outputs are shown.