I am currently working on a comparison between an OrcaFlex-Fast model and a simplified OrcaFlex standalone model. In order to understand the platform motions and to identify easily the potential problems in my models, I started with a very simple load case: a uniform wind without waves.
I used classical calculation parameters: a quasi-steady BEM axial-induction model with the Beddoes-Leishman dynamic-stall model, and with an axial-induction model. There is no tower shadow, and all the turbine is rigid. The turbine is the 5MW NREL turbine. The floater is a semisubmersible platform and the control file is the ITI Barge controller.
I did 4 runs, in order to test different wind speeds: 5.5 m/s, 11.4 m/s, 13 m/s and 25 m/s. Results can be found on figure 1 and show clearly that the 13m/s-run is problematic. I have identified the divergence of the platform motion as a negative damping issue.
In order to correct this behaviour, I tried to calibrate my blade-pitch controller as described in the part 7.2.3 of Jonkman’s PhD Thesis Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine. Indeed, the pitch natural frequency of my platform is lower than the pitch frequency of the ITI Barge. So I adjusted my proportional and integrated gains. It is definitively better but still not perfect: as shown on the following figure 2, motions are still diverging.
So (if someone is still reading here), here are my questions:
- Is it possible to adapt the control in order to avoid such negative damping, and how? Is there some other modification depending on the platform design, such as the previous calibration, required in order to get rid of the negative damping?
- If not, should I consider that the damping terms B[size=85]viscous[/size] and B[size=85]radiation[/size] (equation (7-4) of Jonkman’s Thesis) are too small and then that the design of my platform is not valid?
I agree that the pitch-controller-induced negative damping appears to be the cause of your problem. Perhaps you need to further reduce the PI gains to fully eliminate the negative damping issue. There are now many papers in the literature that describe more advanced methods to eliminate the negative damping problem through modifications of the controller without resorting simply to reducing gains, which should solve the negative damping problem, but may lead to excessive rotor-speed excursions.
Another thing to consider is whether the hydrodynamic damping in pitch is specified properly for your semisubmersible. Have you been enable to compare e.g. pitch free-decay tests or pitch Response-Amplitude Operators (RAOs) against experimental data for your semisubmsersible to ensure that your model is behaving and is damping as expected?
Thank you very much for your reply.
Regarding the hydrodynamic damping in pitch, I’ve followed your advice and compared my values with experimental data. Unfortunately, they are already quite overestimated in my numerical model! So I cannot really find an issue on this way.
Regarding your answer for the controller, I am relieved to learn that there are other methods to eliminate the negative damping problem. I am definitely not an expert in this area; that is why I used the ITI-barge controller in first place. Do you have some advice on these “more advanced methods”, such as the title of a recommended paper or an example of *.dll file?
Thank you again for your support.
Here is a link to some of the NREL papers on this topic: nrel-primo.hosted.exlibrisgroup. … mit=Search. But there are many others in the literature.
Thank you very much for your help and for your very interesting recommendations. I found all the information I needed in the very clear article Evaluating methods for control of an offshore floating turbine. (Fleming, P. A., Pineda, I., Rossetti, M., Wright, A. D., & Arora, D.).