FAST & HydroDyn questions


Firstly let me apologise for the length of my post and if any of the questions are very simple or have already been adressed. I would like to ask a few questions when it comes to modelling a fixed offshore monopile.

-In Hydrodyn manual it is stated that if simulating irregular waves WaveTmax is at least 1hr of simulation is required but this doesn’s have to match FAST simulation time.
So if my WaveTmax=1hr is there a way to determine how long I need to run FAST to get the Hydrodynamic loading on the platform? I guess matching the two sim. times is the safest choice but will it be sufficient if I run FAST for smaller Tmax?

-Can I get the loads exactly at the wave elevation height instead of HydroFxi etc which apply to MSL?

-Any stretching models available for FASTv8 soon?

-Platform additional stifness and damping. Should I change the matrix only for floating turbines?

-Validating FAST results. Except the verification of FAST v8 report is there anything available with regards to FAST validation? should I validate my results?

-In Test19 in subdyn’s input file under Member X section property, for PropSetID=4 I guess this corresponds to the interface. Should I keep these values as default everytime?
-Platform mass is set to 0 in Test19. Can you please explain that?


Dear Nick,

I’ll try to answers your questions:

Are you asking how long you need to run FAST to get statistical significance in the results? If so, the answer likely depends on the nature of the sea state and substructure. At a minimum, the IEC 61400-3 offshore wind turbine design standard requires 6 seeds of 10-minute simulations per bin for the simplest cases, but this is only a minimum.

I’m not sure I understand this question. HydroFxi is the total integrated hydrodynamic force along the xi direction. While the reference point is important for moments, it is irrelevant for forces.

NREL has not yet been budgeted to add a wave-stretching model to FAST v8. However, we are working to add a user-defined wave-kinematics option for the next release of HydroDyn that will allow wave kinematics to be prescribed up to the instantaneous water elevation, assuming the user has access to data that can be applied. Alternatively, you could modify the source code yourself (although implementing a general wave-stretching solution applicable to all substructures would not be trivial).

The platform additional stiffness and damping in HydroDyn can be used to tune the substructure natural frequencies and damping ratios of any offshore FAST model. When SubDyn is enabled, the SubDyn manual recommends at least setting a nonzero value for AddBLin(3,3).

NREL has validated the offshore capability of FAST within several projects. FAST results have been compared with measurements taken from offshore basin tests at MARIN under the DeepCwind consortium, involving combined wind-wave tests of a wind turbine atop a spar buoy, tension-leg platform (TLP), and semisubmersible, and with measurements taken from open-ocean tests for the SWAY, WindFloat, and Hywind prototypes. Further validation of FAST’s new offshore features is ongoing under the IEA Wind Task 30 Offshore Code Comparison Collaboration, Continued, with Correlation (OC5) project. That said, I would highly encourage you to validate your own results, especially if you have access to the appropriate data or if you’ve created a model that is different from one of the default models NREL has provided.

PropSetID = 4 within the SubDyn model of FAST Test19 is not actually used by any member. We used that property set while debugging a model, but didn’t remove it from the file before releasing FAST.

The platform mass (PtfmMass) in ElastoDyn for a fixed-bottom offshore wind model can be used to define the point mass of the transition piece. However, the details of the transition piece have not been specified for the OC3-monopile model of Test19. The distributed mass of the tower and substructure are specified in the ElastoDyn and SubDyn input files of Test19, respectively.

I hope that helps.

Best regards,


I am working on the modelization of a semisubmersible floating windturbine. I am just at the beginning: my starting point is the 0C4 DeepCWind, and I change some parameters to see their influence on the structure.
I’ve already changed the mooring system, the wind type and mean velocity, the nature of the sea (Hs and Tp), the angle between the directions of waves and wind…

I want now to add some sea current and study the impact of the angle between wind and current direction. My problem is : when I change the HydroDyn file to add current, it only works when it is aligned with the wind. Otherwise, some values are missing in the output files (replaced by NaN).

I’ve read in the HydroDyn manual that the current is not taken in account when it works in potential theory only, which is not my case.

Does this mean we can’t put an angle between current and wind ? If not, how can I fix this ?

Thank you,


Dear Laure,

You should be able to add current in arbitrary directions to a HydroDyn model. You are correct that current is not used in the potential-flow solution. For a strip-theory solution (or a hybrid strip-theory and potential-flow solution), the current induces hydrodynamic loads through the viscous-drag terms.

I’m not sure I know why are getting NaNs in your output file. I suggest simplifying the model a bit to debug.

Best regards,

Thank you for your answer, I’ll try to simplify.



Hello there,
I hope you are doing well.
I have been working on OC4 jacket platform using FAST v.8 and I have been able to get along with it gratefully.But something is rather vague in the OC4 jacket HydroDyn primary input file and that is about the current which has been implied as “none” by setting the currmod to -0-.Nevertheless,the strip theory is governed as a jacket type of substructure is being analysed.Moreover refering to project Upwind by stuttgart university for deep water site,there is some information about current expressing the subsurface and near surface current velocity at the sea surface with values 0.6 and 1.2 m/s for normal and extreme current respectively.
So,I would be pleased if you could reply the following two;
1.was the current unnecessary to be involved in the analysis of OC4 jacket?
2.If adding the current is optional,how would we able to set both values for normal and extreme current while there is only one value which is required for each of the 3 model of current in FAST?!

Dear Arsalan,

Here are my answers to your questions:

  1. I’m assuming you are referring to Test21 in the FAST v8 CertTest, which is a model of the OC4-jacket. This model is simply used to test features of FAST and provide an example for users. It is not meant to represent a specific design load case. You are of course free to add currents if that makes sense for the analysis you are doing.

  2. The input files of FAST should be changed between distinct load-case simulations. Normally one would run thousands of load-case simulations in the design of a structure to cover all reasonable operational and extreme conditions. Separate simulations would be run with normal and extreme current. See the IEC 61400-3 offshore wind turbine design standard for guidance on the load-case simulation requirements.

Best regards,

Dear Jason,
I realy appreciate you for the quick reply.
So,regarding to your answer the more condition we have the more times we have to run the simulation is required and it is logical.on the same way,when we tend to change the wave and current direction of propagation(in Hydrodyn input file) and the wind direction of propagation (in InflowWind) we have to run a new simulation with new implied direction.
Is it right ?
Kind regards,

Dear Arsalan,

Yes, your understanding is correct.

Best regards,

Dear Jason,
I really appreciate again for your answer and this useful forum here .
Best regards,