PO087

Extreme wave loads on a floating wind turbine system using a high-fidelity approach within the FLOATGEN demo project

Friedemann Borisade 1, Thomas Choisnet2, Po Wen Cheng1
1Stuttgart Wind Energy, University of Stuttgart, Stuttgart, Germany, 2IDEOL, La Ciotat, France

Abstract

The EU-FP7 project FLOATGEN has been initated to demonstrate and benchmark a floating wind turbine system for power generation in the Atlantic Ocean. The FLOATGEN demo project will deploy a two MW floating offshore wind turbine at the SEM-REV test site in 2017 located twelve nautical miles from the French Atlantic coast.
The environmental conditions of a floating wind turbine system are dominated by a combination of turbulent winds, non-linear waves and currents. Furthermore, the motions, loads and deformations of the floating wind turbine is affected by the wind turbine controller. For design optimisation realistic and detailed load estimates are needed. Hydrodynamics of the offshore structures are commonly modelled using Morison equation, a semi-empirical approach, and potential flow theory. However, the simple methods neglect higher-order effects as flow physics are often non-linear and highly complex, especially for non-slender and non-cylindrical floating foundations like IDEOL's ring-shaped concept.
During the design process of a floating foundation the effects of extreme wave loads with different return periods are investigated. These events are usually tested in a wave tank model test. In this study a high-fidelity numerical approach is applied which is able to analyse the floating wind turbine system with respect to the non-linear dynamics and loads. The work is performed within the FLOATGEN demo project and supported by the floater designer IDEOL.

Method

This study is based on a high-fidelity simulation approach which has been validated against measurement data of a wave tank model test of the IDEOL floating foundation. The simulation applies a coupling between the multi-body system (MBS) tool Simpack and the computational fluid dynamics (CFD) solver Ansys CFX. Each time step is solved implicitly and loads and motion information is exchanged between the solvers. The hydrodynamics are modelled in CFD while the structural dynamics and the mooring system are modelled within MBS. The environmental conditions applied here are based on the SEM-REV test site where the FLOATGEN demo will be installed. Extreme waves are generated numerically via the wave focusing technique. Aerodynamic loads on blades and tower are included in the wind turbine aerodynamic model.

Results

Results presented in this study are based on numerical simulations as described before. Both aspects, the loads on the floating foundation and the resulting dynamics are analysed due to their impact on the floater design. The high-fidelity approach allows a detailed study of the flow physics like vortex formation, spatial distribution of wave induced pressure loads and green water on the deck, resulting accelerations at the tower top, wave run-up on the hull and tower etc. to name a few. The structural loads at the tower bottom and top are analysed with special emphasis on the incident wave event.

Conclusions

The assessment of the effect of extreme waves on the loads and dynamics of the floating offshore wind turbine systems is an important task in the floating wind turbine system design process. The presented high-fidelity numerical approach is an extension of the wave tank model tests of floating offshore wind turbines that can be used to reduce further the uncertainties in the extreme loads. Wave induced loads on the floater hull can be used as input for further structural design optimisations. With this high-fidelity coupling, the floater dynamics at extreme wave events can be evaluated with higher confidence with respect to maximum displacements, accelerations and loads.

Objectives

Conference delegates will learn about current progress within the EU-FP7 demonstration project FLOATGEN. As the floating wind turbine system will be installed in 2017 this presentation will give a very update to date overview of the project. Additionally, the audience will understand the benefits of the new high-fidelity simulation technologies for assessment of wave loads on floating foundations. Especially in this presentation the importance of the extreme wave events are demonstrated and the results on loads and floater dynamics are discussed. The delegates acquire the knowledge to discuss the progress in high-fidelity simulation technology and its pros and cons for detailed loads simulation in comparison to the wave tank tests.