Integral design of a floating wind turbine support structure

Due to the strongly coupled dynamics, the design of a floating wind turbine is very challenging. As earlier work indicated feasibility of floating wind turbines, this project focussed on the challenges for design of a semi-submersible wind turbine support structure, with the following three main activities: 1) development of a coupled time domain method for numerical simulation, 2) integral design of the support structure and 3) wave basin model tests.

To enable coupled time domain simulation of the complete floating wind turbine, the aero-elastic wind turbine simulation tool Phatas (initially developed at TNO, currently owned and maintained by LM Wind Power) has been coupled to both the hydrodynamic codes aNySIM (from MARIN) and Ansys-AQWA (used by GustoMSC). Extensive verification of the code indicated correct implementation of the coupling. The validation of aNySIMPhatas has been performed by TNO with model test results and benchmark against other numerical methods in the IEA task 30.

Design of the floating support has several stages with increased complexity, from stability checks to detailed structural design. To cope with the strongly coupled dynamic behavior, the consortium created an integral design approach with several iteration loops. As part of this design task, TNO developed a novel wind turbine controller for floating wind turbines. Due to the interaction between wind turbine controller and floating platform motion (apparent wind results in axial loading of the rotor), the system can become instable. It has been shown that the dedicated controller ensures both stable operation with sufficient generator power and speed regulation and reduces loads, which had a positive effect on the motions, dimensions and weight of the floating platform.

Wave basin model test is common practise in the design of floating offshore structures, to check the numerical simulation model and unmodelled effects like wave runup. A scaled wind turbine and wind generator have been developed at MARIN, that provides typical Froude scaled loading on the floating platform. To investigate the influence of the wind turbine controller, the scaled model allows for active blade pitch control. TNO developed a scaled version of the floating wind turbine controller for the test campaign.

The integral design approach, including mooring and control, led to a cost effective design with a low kg/MW ratio. Please contact us if you are interested to hear how this could benefit your floating wind turbine design.