Dear @Pietro.Bortolotti,
I am looking to model the IEA 22MW reference turbine with a stiff-stiff tower configuration. In most references I find online, there is a clear soft-soft, soft-stiff and stiff-stiff design region for the tower design (see left side of the figure below). However, the IEA 22MW reference turbine large rpm range (1.81-7.06 rpm <~> 0.03-0.12 Hz) means that the 1P, 3P, and 6P regions overlap (see right side of the figure below). In other words, no such “safe” soft-stiff and stiff-stiff designs are possible.
Would it be possible to tell me why such a large rpm range was chosen and suggest how to define the soft-stiff and stiff-stiff regions? I assume the next best thing is to use the rated rpm as boundaries for the 1P, 3P and 6P regions, but then there will always be resonance somewhere between the cut-in and rated wind speed.
Kind regards,
Victor Rappe
Hi @Victor.Rappe ,
Thanks for the timely question. This PR is updating the design of the floating tower of the 22-MW reference wind turbine Revise floating tower with MIT-Univ Porto design by gbarter · Pull Request #164 · IEAWindSystems/IEA-22-280-RWT · GitHub. The new design, driven by fatigue, is a stiff-stiff design. It is my understanding that these ultra large rotors spin too slowly to leave enough room for clean frequency avoidance, and rely on controllers to avoid resonance phenomena. However our design was not of sufficiently high fidelity to investigate these trade-offs. Also, the DLCs that we’ve run have not shown problems so far.
Does this help?
Best regards,
Pietro
Hi @Pietro.Bortolotti,
Thank you for the quick reply. I have found some 20+MW designs that do have a clear frequency band (e.g. Design and optimisation of a 20 MW offshore wind turbine blade - ScienceDirect , https://www.innwind.eu/-/media/sites/innwind/news/36729101_innwind_v4.pdf), so I am not quite sure if it is something inherent to these ultra large wind turbines.
However, I have read the preprint of the paper of Ribeiro et al. ( [2601.01657] FLOAT: Fatigue-Aware Design Optimization of Floating Offshore Wind Turbine Towers ) describing the design process of their stiff-stiff design and they indeed do not seem to have any problems with resonance.
On another note, this fatigue-optimised stiff-stiff design has a maximum OD of 12m. The original design has a 10m maximum OD, so I was wondering if this would give issues with tower strike? While openFAST does give error messages when a tower strike is detected, no explicit tower diameters or blade geometry are specified in the input files. I assume this means that a “tower strike” is defined when the underlying beam models collide.
Is this assumption correct? And if yes, do you have a rule of thumb for a safe distance between the blade tip and the tower?
Kind regards,
Victor
Dear @Victor.Rappe,
Regarding how OpenFAST calculates blade tip to tower clearance, see a similar question asked and and answered in the following forum topic: Better understanding of TipClrnc and non-interference validation.
Best regards,
Hi @Victor.Rappe , you are correct that the new floating tower might be slightly violating the minimum tower clearance constraint due to the larger outer diameter. At the same time a growth of 2m is relatively small in a 284m rotor diameter characterized by large cone and tilt angles and large overhang, so I don’t expect tower strike events. Also, the rotor was designed in a fixed bottom configuration and I don’t know if the tower clearance constraint was over or underspecified for the floating configuration. In a real setting the controller will likely go through some final tuning to make sure all constraints are checked.
I also heard back from DTU expert Thanasis Barlas who confirmed that in a real installation a resonant-speed-avoidance (RSA) control would likely be applied to avoid tower resonance together with additional dampers.
Dear @Pietro.Bortolotti and @Jason.Jonkman,
Thank you both for the information and the time you have put in this thread. With a quick analysis with a rigid tower and using rated wind conditions with corresponding wave height and spectral period, I get a minimum clearance of 11.0 m - tower radius. So it goes from a clearance of 6m (OD = 10m) to a clearance of 5m (OD=12m).
From GL: Guideline for the Certification of Wind Turbines (2010), there should be a minimum clearance of 30% compared to the unloaded configuration. Under no wave and no wind conditions, I get a clearance of 38.0 m - tower radius. This corresponds to a clearance of 33.0 m (OD=10m) and clearance of 32.0 m (OD=12m). The two tower designs have clearance percentages of 18.2% and 15.6% for OD=10m and OD=12m, respectively. In other words, both tower designs don’t pass this requirement.
As both towers have similar clearances (and it also isn’t really the focus of my work), I think it’s okay for me to continue with this new stiff-stiff tower.
Thanks again,
Victor
Hi Victor, I have not run checks myself, but I don’t think your numbers are correct. The original fixed-bottom design was checked aeroelastically, and minimum clearance was within limits. I would suggest to check things again…
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I did a quick check. In six seeds of DLC 1.1 I see a maximum blade tip deflection of 25 meters. Undeflected clearance is 40.6 meters. Although more DLCs are definitely needed for a more thorough conclusion, these two numbers match the minimum clearance.
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Hello @Pietro.Bortolotti,
Does your undeflected clearance consider the tower geometry or is this the one reported from the OpenFAST output (TipClrnc)?
I ran a few simulations on the IEA-22-280-RWT-semi folder from the IEA github repository. I have not changed anything in the input files except of disabling the DOFs of the floating platform and changing the windspeed to 11m/s. You can see the results for the tip displacement and tip clearance of blade 1 on the figure below.
The blade tip displacement (bottom graph) has a maximum value of 24.4m. I think this is reasonably close to your value of 25m, considering that I did not model any turbulence. However at the moment of minimum tip clearance (top graph), a tip displacement of 21.2 m is observed. Together with the minimum tip clearance of 17.3m, this gives an undeflected clearance of 38.5m. This value corresponds to the minimum clearance I get when disabling the aerodynamic loads.
Given the fact that the tip displacement is quite similar and I haven’t changed anything in the input files I don’t know what is going wrong.
Kind regards,
Victor
Victor, I’m sorry but I don’t think I follow. This script WISDEM/wisdem/commonse/turbine_constraints.py at f4519dc59a6ac1d32c761182dfdc1c9d2cc63a19 · NLRWindSystems/WISDEM · GitHub can be used to estimate the undeflected clearance. For the 22MW I obtain 40.6 meters. This value accounts for the tower diameter of 10m, so clearance would drop to 39.6 for the new floating tower with an outer diameter of 12 m. This clearance successfully accommodates tip deflections of ~25m
