Seismic Motions in OpenFAST

Dear @Jason.Jonkman Sir,

I am trying to mimic seismic motions at the base of the support platform(OC3) using the following reference using Stc Module.

But the output motions (PtfmTAxi) do not give me satisfactory values. In addition, there a controller related issues as well. I use uniform wind of 12 m/s along with a still water situation.

Kindly advise for the same.


Dear @Kashyap.Subham,

It looks like you also sent me files via e-mail, but I don’t see that you are setting up the model as you describe. The reaction joint is locked at JointID = 1 and SubDyn and I don’t see that large mass has been set in SubDyn. Can you clarify?

Best regards,

Yes sir, I had sent summary files over email as I didn’t find a suitable option to attach files here.
I am completely unaware of how to achieve the desired setup. I don’t fairly comprehend the required changes to be made to the subdyn input file to achieve the required ground motions. I am having a hard time transitioning to OpenFast.

But in totality, I locked the elastodyn rotational dofs of the platform. Further I increased the platform mass to almost 10 times the orginal mass. This was done with the intention to have dominant inertial effect to achieve the ground motion using StC defined at 0,0,-20 in global coordinates. Though I am not sure if this right. I use the DISCON dll for the servodynamics.

But I observed unexpected blade pitch oscillations despite steady wind of 12 m/s. I had even set the initial blade pitch angles as 2.5 deg as observed mean from the plot. But the oscillations prevailed.

Also, it would help if you can tell what changes need to be made to subdyn to achieve the ground motion.

MXddot + CXdot + KX = Ft = Mxgddot is the equation i assume. So by putting a large M the damping force and stiffness force would be negligible. And Xddot = Xgddot shall be achieved. Atleast I hope so.

Please advise. Thanks.

Dear Sir,

I have just gone through the user manual of SubDyn and checked the input file. I have the following queries regarding seismic simulations.

  1. How are dofs in ElastoDyn different from the nodal constraints in SubDyn? If I understand correctly, the ElastoDyn dofs for the platform shall be superseded by the constraints defined in the SubDyn input file.
  2. Does setting “RctTDXss” flag to 0 will allow the forces to be transferred as defined using an StC force time series?
  3. The method described in the previous case, thus, ensures the coupling of seismic and aeroservodynamic effects as the platform inertia is manipulated to replicate a rigid body motion. Based on this, am I correct in claiming that the platform base and the tower base essentially have the same motions?

Eagerly waiting for your reply.

Dear @Kashyap.Subham,

I’ve never tried to simulate seismic excitation in OpenFAST via StC with SubDyn enabled, and I don’t think this is possible following the approach used by the Seismic module of FAST v7 without changing the source code because the user-specified force time history in StC must be predefined and can’t be written based on the difference between the desired and actual displacement and velocity at the reaction joint.

To answer your specific SubDyn-related questions, but I would refer to the SubDyn documentation (4.2.5. SubDyn User Guide and Theory Manual — OpenFAST v3.5.0 documentation) for more detailed answers:

  1. ElastoDyn and SubDyn are coupled with the motions (position/orientation, velocity, and acceleration) sent from ElastoDyn to SubDyn and the loads (forces and moments) sent from SubDyn to ElastoDyn. SubDyn consists of Guyan modes, Craig-Bampton modes, and static improvement modes. The motion of the Guyan modes involves motion of the interface joint, as controlled by the platform degrees of freedom in ElastoDyn. The Craig-Bampton and static improvement modes are solved within SubDyn based on the interface joint being fixed. To ensure proper coupling between ElastoDyn and SubDyn, the SubDyn modeling guidance recommends keeping all six ElastoDyn platform DOFs enabled when SubDyn is enabled.
  2. Setting RctTDXss = 0 means that the translation displacement in the X direction of the reaction joint will be free to move as a result of loads applied.
  3. I’m not sure I fully understand what you are asking, but in ElastoDyn, the tower base is called the platform. When SubDyn is enabled, the motion of the platform is the motion of the Guyan modes and determined by the loads applied to the structure.

Best regards,

Dear Sir,

What can be a possible way forward for implementing seismic in OpenFAST? Can you please list the options that I have?

Also, I believe the use of super-elements is an option. But that would also mean including seismic capability in subdyn.

Dear @Kashyap.Subham,

I cannot provide detailed guidance because NREL has not yet been funded to add seismic functionality to OpenFAST. However, I would think we would move forward to replicate the FAST v7 seismic functionality in OpenFAST by starting with and modifying the SoilDyn module that has not yet been merged into OpenFAST: SoilDyn -- soil dynamics module by andrew-platt · Pull Request #986 · OpenFAST/openfast · GitHub. Adding seismic functionality to SoilDyn would enable it to apply to OpenFAST models both with and without SubDyn enabled.

Best regards,