Modelling Entire Support Structure in SubDyn

Dear all

I have a project where I am trying to implement soil damping at the bottom of an offshore jacket. From my understanding, this damping is best implemented as SubDyn modal damping. However, these modes only apply to the jacket, but I am reliant on damping the modes of the entire support structure (tower + jacket). This is so I can target and apply damping at the modes that give large response at the piles.

In short: I need the to get the mass normalized mode shapes and frequencies of the combined tower-substructure so that I can apply modal damping to these modes.

A potential solution to this that I have studied is modelling the tower in SubDyn (as explained in Substructure Tower/Turbine Coupling and Using SubDyn To Model Full lattice towers up to Yaw Bearing). I am aware that SubDyn does not account for geometric nonlinearities, but I am willing to overlook this as the goal is to create a model that can compare damping values.

  1. Is this a reasonable approach to achieve my goals in the project? Are there any better modelling techniques give the desired result?
  2. To get accurate mode shapes one would need to account for the RNA mass and inertia. How can I add this contribution to SubDyn?

Best regards,
Magnus Saeland

Dear Magnus,

You can get the full-system mode shapes through the linearization functionality of OpenFAST, followed by eigenanalysis of the state matrix A. The resulting eigenvectors contain the full-system mode shapes, but these are not mass normalized. In fact, it is not really possible to get mass normalized mode shapes from OpenFAST when a combination of SubDyn and ElastoDyn are enabled to model the structure because the full system mass matrix is never formed in that case.

It is challenging to specify the damping of the support structure in OpenFAST given the combination of Guyan and Craig-Bampton modes in SubDyn and the tower mode shapes in ElastoDyn. (The Guyan modes in SubDyn involve motion of the transition piece, the Craig-Bampton modes in SubDyn assume a fixed transition piece, and the tower modes in ElastoDyn are influenced by the compliant substructure.) This challenge of specifying support structure damping has been discussed briefly in the following forum topic: Subdyn with Full FE option - #5 by George.Chan.

Modeling the full support structure in SubDyn and neglecting the tower model in SubDyn simplifies the process a bit, but does not fully resolve the issue, because in that case, the support structure is still represented as a combination of Guyan and Craig-Bampton modes. (In that case, the Guyan modes involve motions of the tower tower and the Craig-Bampton modes involve a constrained tower top.)

SubDyn can be used standalone (without ElastoDyn, uncoupled from OpenFAST). In that case, you can represent the RNA as a lumped mass and inertia through the “Joint Additional Concentrated Masses” option in SubDyn.

Best regards,