Seakeeping Tests of a FOWT in Wind and Waves: An Analysis of Dynamic Coupling Effects and Their Impact on the Predictions of Pitch Motion Response

Giovanni Amaral,Edgard Malta,Pedro Mello,Lucas Do Carmo,Guilherme Franzini,Alexandre Simos,Izabela Alberto,Rodolfo Gonçalves,Hideyuki Suzuki

doi:10.3390/jmse9020179

Abstract

The present work highlights some of the dynamic couplings observed in a series of tests performed in a wave basin with a scaled-model of a Floating Offshore Wind Turbine (FOWT) with semi-submersible substructure. The model was moored by means of a conventional chain catenary system and an actively controlled fan was used for emulating the thrust loads during the tests. A set of wave tests was performed for concomitant effects of not aligned wave and wind. The experimental measurements illustrate the main coupling effects involved and how they affect the FOWT motions in waves, especially when the floater presents a non-negligible tilt angle. In addition, a frequency domain numerical analysis was performed in order to evaluate its ability to capture these effects properly. The influence of different modes of fan response, floater trim angles (changeable with ballast compensation) and variations in the mooring stiffness with the offsets were investigated in the analysis. Results attest that significant changes in the FOWT responses may indeed arise from coupling effects, thus indicating that caution must be taken when simplifying the hydrodynamic frequency-domain models often used as a basis for the simulation of FOWTs in waves and in optimization procedures for the design of the floater and mooring lines.

Highlights

Parametric optimization of floater and moorings is a procedure frequently employed in the first stages of the design of a new Floating Offshore Wind Turbine (FOWT)
Hydrodynamic coefficients derived from panel methods are the basis for seakeeping time-domain simulations that take into account the coupled dynamics of floater, rotor and mooring lines, considering the wave-frequency motions and the slow-motions induced by second-order effects
The model drifts were restrained by a set of three catenary mooring lines, with anchors placed at the basin floor

Summary

Introduction

Parametric optimization of floater and moorings is a procedure frequently employed in the first stages of the design of a new Floating Offshore Wind Turbine (FOWT). In this approach, simplified models are often used for predicting the dynamics of the system while ensuring that the process will meet the time constraints imposed by the design. A trade-off is established between the accuracy of force and motion predictions and the computational demand of an optimization procedure In this context, the predictions of motions in waves is generally based on pre-computed hydrodynamic loads or motion transfer functions obtained from radiation/diffraction codes [1,2,3]. Different approaches for emulating the rotor wind forces can be found in the literature, such as using Reynolds scaled wind [4], cables [5,6]

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