Dear all,
I am writing you to ask some advice about the mooring system of the OC4 floating platform.
My desire is to run FAST with this support platform, for a water depth different from the default one (200 m).
How can I change then the mooring system (depth is 55 m for my case)?
I was thinking to keep the mooring line mass and structural properties (axial stiffness, diameter, mass per unit length), and then to find the anchor position and line unstretched length which give the same vertical force and line angle at the fairlead, that the system has for a depth of 200 m, at rest (in absence of waves and wind).
Honestly, I am not sure whether this approach is correct or not.
I am looking forward to receive your suggestions and opinions.
All the best,
Dear Michele,
I’m not sure that the change you propose would work because the pretension of a catenary mooring system is dictated by the weight of mooring system hanging in the water, which would change if you kept the mooring properties identical, changing only the line lengths, water depth, and anchor positions.
My guess is for a such a drastic change in water depth a complete redesign of the mooring system would be warranted–that is, selection of mooring line cross sections, lengths, and anchor positions to obtain the desired response characteristics and stay within the load envelope.
Best regards,
Hi
I want to have the extreme load envelope for the full-scale test of the wind turbine blades. How can these loads be obtained through the FAST software?
Dear @Ali.Rouhbakhsh,
I’m not sure I really understand your question. Are you referring to IEC design load cases or something else?
Best regards,
Yes both IEC and DNV-GL 0376 extreme loads for full-scale test of wind turbine blade
Dear @Ali.Rouhbakhsh,
Sorry, but I’m still not understanding. I’m not familiar with DNV-GL 0376. Are you referring to the static strength (pull) test of a standalone blade in laboratory?
Best regards,
I want to get a graph like the second image from FAST software
Dear @Ali.Rouhbakhsh,
Are you trying to get the plot of “design load cases from load simulation”? This appears to be a plot of the minimum and maximum flapwise bending moment and the associated edgewise bending moment derived from various load case simulations, as well as plots of the minimum and maximum edgewise bending moment and the associated flapwise bending moment derived from various load case simulations, plotted together on the same graph. You can certainly derive such a plot via post-processing of FAST/OpenFAST results based on the extreme event table data derived from MExtremes or the like.
Best regards,
Dear Jason sir,
I was trying to simulate a condition where NREL baseline controller perform worse (or unstable ) for the MIT/TLP turbine, to implement my algorithm upon the same baseline controller to make it work stably. I tried to change the water depth of the turbine from 200 m to 70 m which gave an error and I changed the “UnstrLen” in MAP.dat file corresponding to the difference in depth and simulation worked without errors. With this change I found that baseline controller is giving resonant oscillations in platform motion and generator speed, for turbulent wind and irregular wave conditions simulations . Is my approach justified if my aim is only to simulate an unstable baseline controller condition?
Thanks and regards
Dear @Nitin.Sivakumar,
I’m not quite following. If you decrease the depth of the MIT/NREL TLP, I would expect the surge frequency to increase, which I would not expect to cause an issue with the baseline controller.
Best regards,
Dear Jason sir,
What I mean is that, if the vessel height is kept intact as 200 m and the mooring line length is decreased corresponding to the difference in water depth. Will that cause an unstable baseline controller and excessive platform motion?
Thanks and regards
Dear @Nitin.Sivakumar,
I’m not sure what you mean by “if the vessel height is kept intact at 200 m”, but I would not expect changing the water depth and corresponding mooring line length to result in an unstable baseline controller. Whether the platform motion as a result will increase will depend on what the new floating system natural frequencies are relative to the wind and wave excitation frequencies, in case possible resonance could occur.
Best regards,
Dear Jason sir,
I had a mistake in what I said, to be clear. The new vessel height is - 47m, water depth is 70 m and unstretched mooring length is 23 m. Is this configuration okay for the TLP floating turbine?
That sounds reasonable to me.
Dear Jason sir,
I compared the performance of the Nrel baseline controller to MIT/TLP OpenFast model with water depth set as 200m and 70m and all land modes + only platform surge mode active. I did the NTM 16 m/s turb C wind along with 2 different wave condition. A low wave height and spectral peak of 1.5m and 6.6s as first case, and a significant height of 4.55 m and spectral period of 13.33s as case 2. I have attached the time plots of pitch
angle, generator speed, platform surge and towerFA displacement comparison of the 200m and 70m in each plot marked in legend . Here for higher wave condition the controller is going unstable. but for lower waves it is okay. What can I infer from this as the reason for unstable operation? Can you give some insight. Thanks and best regards
Dear @Nitin.Sivakumar,
It sounds like you believe the controller to be unstable, but I’m guessing this is more of a resonance phenomena. When you change the water depth, how do the natural frequencies in surge and pitch change, and do these overlap with excitation frequencies from wind or waves?
Best regards,
Dear Jason sir,
Q1. Is it appropriate to say then, that controller is not able to mitigate the resonance when the exitation frequencies of wind or wave overlap with the natural frequencies of surge and pitch?
Q2. By pitch you mean platform pitch or pitch angle sir?
Q3. And another question I have is how do I confirm that, this overlap has happened using the simulation data?
Q4. Also if I want to simulate a resonance condition for the turbine without changing the water depth, what wind or wave exitation parameters should I give and how do I pre-evaluate or calculate those before simulation?
Please excuse me if my question is trivial, as I am not from the background of structural and mechanical design.
Thanks and best regards
