Can subdyn use apparent fixity method?

Dear developers,

Recently I read papers using apparent fixity method to model soil-pile interaction. In my understanding, apparent fixity method extend the pile from seabed surface to a certain length. This certain length is derived in a way that can equivalently represent the soil-pile interaction effect.

Now I want to adopt apparent fixity method and I have derived this equivalent length of pile. When I create pile in subdyn, is it the correct way to create several pile nodes below the original seabed surface of an equivalent length?
Would it result in problems in terms of interpolation of hydrodynamic loads to subdyn pile?

Regards,
Ran

Dear @Ran.Tu,

That is correct. You can model the foundation in SubDyn via the apparent fixity method as you propose. HydroDyn will only consider hydrodynamic loads down to the seabed, even if the SubDyn model extends below the seabed.

Best regards,

Thank you Jason.

One more question.
I want to apply external force at pile nodes in subdyn varying based on displacement/rotation of relevant pile nodes. This external force can be caluculated by mathematical formula. Is there a subroutine that I can apply such force formula mathematically in subdyn?

If there is no available subroutine, I come up with a possible way to realized it. I assume hydrodynamic loads are interplolatd to subdyn nodes and undated every time increment. So maybe I could add my external force at the same position in the code where hydrodynamic loads are introduced to subdyn.

Regards,

Dear @Ran.Tu,

SubDyn itself supports an apparent fixity (AF) representation of the foundation, as well as a coupled springs (CS) representation of the foundation; with the later available through the `SSIfile`. Are these options sufficient for your needs?

The new SoilDyn module of OpenFAST (to be released soon through the following OpenFAST pull request: SoilDyn -- soil dynamics module by andrew-platt · Pull Request #986 · OpenFAST/openfast · GitHub) has a placeholder for future implementation of a distributed springs (DS) representation of the foundation.

Best regards,

Dear Jason,

I have seen this new module. But it seems that openfast v4.0 still needs some time to be released. I am not sure if soildyn can be developed now before the release of openfast v4.0.

Regards,

Dear @Jason.Jonkman ,

I am reading the source code of subdyn. And I am using OpenFAST V3.4.1. I found that in this version, subdyn actually has remained SoilStiffness and SoilMesh in type SD_InitInputType. It said that soildyn would offer these two variables for subdyn. Given than soildyn is still under develped, I am considering to program several codes in subdyn, to make subdyn directly read SoilStiffness and SoilMesh in subdyn input files.

The aim of this work is to achieve distributed linear spring model in subdyn. Subdyn could read SoilStiffness on SoilMesh, and then insert SoilStiffness to the global stiffness matrix. I believe subdyn has already had such insertation subroutine.

Could I ask for your comments on this coding work? Would it offer any contribution to future openfast, or it is just redundant if soildyn has the same functionality.

Regards,

Dear @Ran.Tu,

SubDyn already supports multiple reaction joints with a user-specified linear soil stiffness matrix by setting `NReact` > 1; no source code change is necessary for that. You could use that feature to support a linear distributed spring model (where the distributed springs are lumped at SubDyn analysis nodes).

Our intent is for any soil properties beyond a linear soil stiffness matrix to be accounted for in SoilDyn. This is because SubDyn is inherently linear and cannot support nonlinearities, whereas SoilDyn can support nonlinearities.

Best regards,

Thank you Jason. It is good to hear that reaction joints can solve my problem. Great!

Regards,