Foundation Stiffness and damping in FAST v8

Discuss theory and modeling of wind-turbine structures.

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VeronicaLiverud.Krathe
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Foundation Stiffness and damping in FAST v8

Postby VeronicaLiverud.Krathe » Fri Jan 23, 2015 7:24 am

Hi,

I am new to FAST and I am trying to figure out whether to use FAST v7 or FAST v8 for my dynamic analysis.

I need to be able to control and vary the stiffness and damping of the soil/foundation in my analyses. As far as I have learned this can be done in FAST v7 by creating a subroutine, UserPtfmLd(). Is this correct? And is it possible to do the same in FAST v8, and if so, could I please get some hints on how to do it?

Thank you!

Veronica

Jason.Jonkman
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Re: Foundation Stiffness and damping in FAST v8

Postby Jason.Jonkman » Fri Jan 23, 2015 8:58 am

Dear Veronica,

I encourage you (and everyone) to upgrade to FAST v8 whenever possible.

You can model soil stiffness/damping in FAST v7 for land-based or offshore turbines through the user-written UserPtfmLd() routine. However, if you are modeling an offshore wind turbine, the offshore capabilities in FAST v8 are vastly superior to those of FAST v7.

The UserPtfmLd() routine is not yet available in FAST v8 (although you could customize the code yourself), but you could quite easily set a linear platform stiffness and damping matrix within HydroDyn for a land-based turbine foundation (disabling the hydrodynamic options). For a fixed-bottom offshore turbine, presumably you'd be using SubDyn in FAST v8. The SubDyn model is currently limited to rigid connections between the bottom of the substructure and the seabed; however, a common way to mimic a flexible foundation, which is possible in SubDyn, is to use the "apparent fixity" approach and define beam elements at the base of the substructure to mimic the flexibility that the real foundation would experience.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

VeronicaLiverud.Krathe
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Re: Foundation Stiffness and damping in FAST v8

Postby VeronicaLiverud.Krathe » Mon Jan 26, 2015 3:12 am

Thank you for your answer.

Is it possible to mimic the flexibility of the foundation using a Distributed Springs Model, as described in the paper by Bush and Manuel: http://enu.kz/repository/2009/AIAA-2009-1037.pdf?

And is there any possibility to use HydroDyn to set a linear platform stiffness and damping matrix for a fixed-bottom offshore turbine?

Best regards,
Veronica

Jason.Jonkman
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Re: Foundation Stiffness and damping in FAST v8

Postby Jason.Jonkman » Mon Jan 26, 2015 6:45 am

Dear Veronica,

In FAST v8 currently, it is not possible--without customizing the source code--to include a distributed-springs model or a linear stiffness and damping matrix to mimic a flexible foundation for a fixed-bottom offshore wind turbine. This is because, as I said in my previous post, the SubDyn module is currently limited to rigid connections between the bottom of the substructure and the seabed.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Arash.HemmatiTopkanloo
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Re: Foundation Stiffness and damping in FAST v8

Postby Arash.HemmatiTopkanloo » Mon Nov 16, 2015 2:02 pm

Dear Jason,

Thank you for your support here.

I need to model soil pile interaction (by any means) for offshore wind turbinbes in FAST v8. I read this forum and as you said I need to recompile of FAST with UserPtfmLd. Could you please explain it more?

Where can i find it? and how? do i need to create superlement stiffness matrix and include in the source code?
Any word about that would be highly appreciated.

Thank you again,

Regards,

Arash,

Jason.Jonkman
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Re: Foundation Stiffness and damping in FAST v8

Postby Jason.Jonkman » Mon Nov 23, 2015 10:51 am

Dear Arash,

As I've said, the UserPtfmLd() routine is not yet available in FAST v8, but the bigger problem is that SubDyn is currently limited to a rigid connection between the bottom of the substructure and the seabed. You'd have to change this boundary condition in SubDyn before adding a pile stiffness and damping matrix. However, I'm not able to provide detailed guidance for making this customization at this time. I suggest you start by reviewing the SubDyn theory basis from section 6 of the draft SubDyn User's Guide and Theory Manual: https://wind.nrel.gov/nwtc/docs/SubDyn_Manual.pdf.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Arash.HemmatiTopkanloo
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Re: Foundation Stiffness and damping in FAST v8

Postby Arash.HemmatiTopkanloo » Fri Nov 04, 2016 9:19 am

Dear Jason,

I am wondering if I could add viscous damping (dashpots) into the model to represent soil damping?

Thank you,

Best Regards,
Arash,

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Re: Foundation Stiffness and damping in FAST v8

Postby Jason.Jonkman » Fri Nov 04, 2016 12:04 pm

Dear Arash,

Based on the above discussion, how are treating the soil currently in FAST v8? Have you customized the source code to add this yourself? Or are you using the apparent fixity approach within SubDyn?

If you've already customized the code to add a stiffness matrix or distributed springs to represent the soil conditions, you could also and damping in the same place. If you are not customizing the source code, and you are using SubDyn, the only real damping option available in SubDyn is through structural damping of the Craig-Bampton modes (through input JDampings). The platform additional damping feature available in HydroDyn (through inputs AddBLin and AddBQuad) is another option, but this damping is applied at the intersection of the tower centerline and the mean sea level.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Patrick.Quasten
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Re: Foundation Stiffness and damping in FAST v8

Postby Patrick.Quasten » Fri Nov 25, 2016 9:43 am

Dear all,

I´m trying to study soil-structure interactions of land-based wind turbine with FAST v8. Following the advice of Mr. Jonkman given in this topic I use as a starting point the files in Test18 (land-based Wind Turbine) and I am trying to implement linear Stiffness and Damping matrices of several real soil types (silt till rock) in HydroDyn in the Platform Additional Stiffness and Damping section (with disabled hydrodynamic options), but FAST aborts the calculations. I am not using SubDyn in this calculation.
How would I have to change Geometry Data in the HydroDyn Input file so only my given matrices K and C interact with the whole system? Am I overseeing something?
Could anybody please help me and give me a rough list of steps to run such a simulation? This would be very pleasant and I thank in advance!

Best regards,
Patrick Quasten

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Re: Foundation Stiffness and damping in FAST v8

Postby Jason.Jonkman » Fri Nov 25, 2016 2:44 pm

Dear Patrick,

What do you mean that FAST aborts; what error is FAST giving you?

You should be able to use a HydroDyn file with no waves, no current, no potential flow, and no strip-theory joints or members. All that's left is then the platform additional stiffness and damping.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Patrick.Quasten
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Re: Foundation Stiffness and damping in FAST v8

Postby Patrick.Quasten » Sun Nov 27, 2016 11:45 am

Dear Jason,

at first I thank you for your prompt reply!
Yes, I still disabled all hydrodynamics but I was not sure if I have as well to disable the members because in tzhe end in both ways (disabling members YES or NO) FAST aborted. After your confirmation of members' deletion, at first I concentrated to try neglecting the members incl. the relating joints in a way that only the section "PLATFORM ADDITIONAL STIFFNESS AND DAMPING" and acc. to the HydroDyn manual the six true flags in the section above ("FLOATING PLATFORM FORCE FLAGS") remain with the aim of no failur. It no matters what initial HydroDyn file I use (e.g. ITI Barge, OC3 Tripod, ...) because the same error message orccurs always which is written as

"FAST_InitializeAll:HydroDyn_Init: Both meshes must be committed before they can be mapped.
HydroDyn_Init: Both meshes must be committed before they can be mapped."

Are there two the same failures? Could you please give me a hint what the reason is or could be?

Best regards,
Patrick

Jason.Jonkman
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Re: Foundation Stiffness and damping in FAST v8

Postby Jason.Jonkman » Mon Nov 28, 2016 11:11 am

Dear Patrick,

I was able to reproduce your error when I set up a similar model myself. It appears that there is a bug in HydroDyn preventing it from functioning properly when a model has no strip-theory nodes (NJoints = NMembers = 0). We'll fix this in a future release of HydroDyn. In the meantime, I suggest that you set up your HydroDyn module with two joints and one member, but with all member properties and coefficients set to zero.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Patrick.Quasten
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Re: Foundation Stiffness and damping in FAST v8

Postby Patrick.Quasten » Tue Dec 13, 2016 7:33 am

Dear Jason,

I apologize for my late reply!
After applying your tip FAST runs. I thank you for your hints!
With foresight I`ll check the results of several simulations about their plausibility in matters of the SSI.

Best regards,

Patrick

Patrick.Quasten
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Re: Foundation Stiffness and damping in FAST v8

Postby Patrick.Quasten » Thu Dec 22, 2016 10:21 am

Dear Jason,

now I check several soil types regarding the linear stiffnes and linear damping matrices in HydroDyn, taken from Test19.
I apply Test18 as basic and in HydroDyn I changed the member and joints incl. their properties and coefficents, like you recommanded before, and FAST runs. This happens with a middle strong soil type like sand, but now when I change the matrices I get several failure messages. One failing soil type is between the running ones and the other failing types are with very high K and C values. The several messages are basically the same (only different in the values) and sound like this:

FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 5, blade 2):BEMT_UnCoupledSolve:DeterminePhiBounds:There is no valid value of phi for these operating condition! Vx = 6.8481, Vy = -30.858, rlocal = 11.77, theta = 0.23225

The problem is that I cannot understand why this happens. Do you have an idea what could be the reason?

Best regards,

Patrick Quasten

Jason.Jonkman
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Re: Foundation Stiffness and damping in FAST v8

Postby Jason.Jonkman » Thu Dec 22, 2016 12:01 pm

Dear Patrick,

I'm not sure I understand everything in your post, but in the error message you are reporting, a value of Vy = -30.858 m/s sounds unrealistic. Perhaps the model is going numerically unstable in some way? It could be that the introduction of the foundation stiffness and damping necessitates a smaller FAST time step. I suggest dropping the time step (DT) to see if the solution is sensitive to the value.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov


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