Turbine-soil interaction. Influence on the mode shapes.

Wind & Water Structural Analysis
61

Dear Christos,

From my brief look at your results, the solution does not look numerically unstable. Rather, it looks like the tower/platform motion is not restrained at all, as if the distributed springs are not functioning. Are you sure you are using the version of FAST you’ve compiled with distributed springs and that you’ve enabled the call to UserTwrLd when running the simulation?

Best regards,

62

Dear mr Jonkman,
thank you for all your support and your advice. I figured out that there was a careless mistake in the input files, resulting to the simulation crashing. The problem is now solved. I am really grateful for your assistance.

Best regards,
Christos Lathourakis

63

Dear mr Jonkman,

I tried to read most of forum’s topics before posting, but I still have some questions.
I am a new user of FAST v7 and I am currently trying to use it for my master thesis. I try to study a new 5MW OWT, so I need to figure out my turbine’s properties in order for it’s frequency to be between 1P-3P. I tried to use nrel’s 5MW-offshore baseline as a guide and modify my turbine to fit the site I study. Here are my known properties:

Depth: 15m
Monopile above MSL: 10m
Tower height: 77.6m
Penetration length: 10D-3D=60-18m (Many lengths in order to study the difference on the final natural frequency of the turbine)

Using BModes, I calculate the natural frequency of the tower and monopile above mudline as a cantilever beam.
So, now, I need to calculate the natural frequency of my OWT using a coupled springs model (CS) or even a distributed springs model (DS).
I want to know which BModes program should I use and which .bmi file.
I found [url]National Wind Technology Center's Information Portal | Wind Research | NREL, so I guess this is what I need to use, but my problem is on the inputs of the .bmi file and tower_secs file.

  1. In the tower_secs file I model the tower and monopile above mudline (just like I did before, 15+10+77.6=102.6m).
  2. In the .bmi file what should I modify and where is the soil’s stiffnes matrix imported?
  3. For the hydro_m and hydro_k matrices should I use HydroDyn separately?
  4. Since, afterwards, I am going to use FAST_v7.02.00d-bjj_AeroDyn_v13.00.02a-bjj_BladedDLLInterface that recalles HydroDyn routines on it’s own is it ok to use the natural frequency I calculated above?

For the soil’s stiffnes matrices, I will use Lpile to find the p-y curves for every penetration length and load case I want to study.

I am deeply sorry, if I posted and the answer is already written.

Best regards,
Thanasis Petridis

64

Dear Thanasis,

Yes, you should use the BModes found in my public directory on wind.nrel.gov (called BModes_JJ in several forum posts).

Here are my answers to your direction questions:

  1. Yes, I agree for the CS model. For the DS model, you should define the distributed mass and stiffness of the tower + monopile including the pile penetrating the seabed in the tower_secs file.
  2. Assuming you’re starting with CS_Monopile.bmi, you should set radius = 10+77.6 = 87.6 m and draft = 15 m for the CS model; you should change draft to 15 + penetration depth for the DS models.
  3. You don’t need HydroDyn to set your BModes input file. You should set mooring_K equal to the 6x6 coupled springs stiffness matrix for the CS model and hydroM = hydro_K equal to a 6x6 matrix of zeros. For the DS model you should set all three 6x6 matrices to zero, and instead, you should set the distributed springs linear stiffness using the section of the BModes input file titled, “Distributed elastic stiffness per unit length along a flexible portion of the tower length”.
  4. I don’t really understand your fourth question.

I hope that helps.

Best regards,

65

Dear mr Jonkman,

thank you for your immediate response. You have made everything clear to me.
My last question doesn’t really matter now that I don’t need to use HydroDyn.
Again thank you very much, you have been really helpful.

Best regards,

Thanasis Petridis