A floating offshore wind turbine (FOWT) is a complex system, whose motions are strongly coupled. Motions are affected by many different components and design aspects. First, there is the wave- and current loading and the floater hydrodynamics. Another important contribution is found in the aerodynamic forces on the wind turbine rotor. The controller that determines the turbine’s blade pitch angle responds to both the incoming turbulent wind speed, and to the motions of the platform. Thereby, the blade pitch controller couples directly the wave- and wind loads. Finally, the motions are affected by the mooring system restoring forces. As all these factors interact with each other, it is then clear that a FOWT system must be evaluated in an integrated manner, with all system components represented. Typically, studies are performed using numerical models. As these models entail a simplification of reality and contain several assumptions, scale model testing is often used to validate the results found from the numerical model.
For physical model scale testing, a challenge is found in scaling all the loads. Hydrodynamic loads are scaled following Froude scaling laws. Here, the velocity decreases when scaling down a system for testing. Aerodynamic forces, on the other hand, follow Reynolds scaling. In Reynolds scaling, the velocity increases when scaling down. As a result, a Froude-scaled wind turbine model would experience a unrealistically low Reynolds number, and thus wrongly modelled aerodynamic forces. Different strategies exist to overcome this challenge. In a ‘full approach’, the wind turbine rotor geometry can be re-designed to correctly model the Froude-scaled forces, when subjected to a Froude-scaled wind speed and rotor speed, making all scaling consistent again. An alternative is found in a ‘simplified approach’, where the wind turbine is not modelled physically, but numerically. This form of testing is called software-in-the-loop (SiL), denoting the presence of a numerical wind turbine model in an otherwise physical test environment.
In SiL testing, there are two environments. The first environment is the physical domain of the wave basin. Here, a physical floater and mooring system are subjected to waves and current. This results in motions of the model. These motions are measured, translated to full-scale values, and communicated to the second environment, which is the numerical domain. The numerical model contains the wind turbine, blade pitch controller, and the wind environment. It is a full-scale model, removing the Reynolds scaling challenge. The aerodynamic forces, resulting from the model motions and wind speed, are calculated, and sent back to the wave basin. The forces are scaled to model-scale values, and applied to the model using actuators. These actuators can be for example winches or on-board fans. The wind loads add to the excitation of waves and current, and affects the model motions. Running this loop at a high update frequency closes the loop, and subjects the FOWT system to the full environment of waves, current, and wind.
In the AFLOWT project, model scale testing of a generic, 22MW semi-submersible FOWT platform has been performed using a SiL modelling approach for the wind turbine. This allowed for cost-effective testing for early stage development. Moreover, the SiL set-up also allow for more freedom in testing. As the rotor, blade pitch controller, and wind environment are all numerical, they can be varied easily between tests. In the AFLOWT project, using an external, black box wind turbine model prepared by Fraunhofer in the numerical model of MARIN has also been demonstrated. A good agreement was found in motion results from the basin test and from a numerical model. Furthermore, a good match was seen between MARIN’s in-house aerodynamic model, and the aerodynamic model of MoWiT (www.mowit.info) contained in Fraunhofer IWES’s FMU, demonstrating a successful integration. This functionality allows to include proprietary wind turbine models in the model test campaign. A sensitivity study performed by Fraunhofer demonstrated the importance of modelling structural flexibility, which may also be included in a numerical (FMU) model. A scaling challenge remains, however, in the numerical speed of the model. For a Froude-scaled model, time moves faster. This means that the numerical model has to run faster than real-time to keep up with the wave basin. For the model scale 60 that was used in the AFLOWT model tests, the numerical model must run approximately 9 times faster than real-time.
Finally, care must still be taken when testing with a SiL system. The inclusion of a numerical model and a set of actuators necessitates extensive system identification to ensure adequate system performance, both on the software and the hardware. Furthermore, it cannot be considered a fully independent validation of a numerical model, as part of the numerical model (the wind turbine) itself is used in the wave basin tests. Nonetheless, SiL testing is demonstrated to a be a very capable tool, which can be used in the integrated development and testing of FOWT systems.
