Linear Stiffness Matrix for VolturnUS Platform Mooring Modeling and Platform QTFs

Wind & Water Computer-Aided Engineering Software Tools
1

Hello,

I am working on an in-house coupled model of the VolturnUS semi-sub and IEA 15 MW RWT. I have some questions about modeling the platform mooring system and also the QTFs that are available in the VolturnUS definition document.

  1. It says that the QTFs are calculated from the RAOs and a linear stiffness matrix which models the mooring system. I can’t find the stiffness matrix in the document or online. Could anyone point me towards where to find that information, or further information on how it was derived?

  2. The RAOs presented in the definition document say that they are obtained via time domain simulation in HydroDyn and include the mooring system. Is the mooring system also modeled as a linear stiffness matrix for the RAO calculation? Are those RAOs the same ones being used in the QTF calculation later on, or are the RAOs used for the QTF computation based only on the added mass, damping, platform hydrostatic stiffness, and mass matrices?

If anyone is able to help me with these topics it would be super helpful!

best,
Jackson

2

Dear @Jackson.Hardin,

I was not the one who made this model, but I do see that the linear stiffness matrix (WAMIT input ISTIF) is set in the set of 2nd-order WAMIT input files for the IEA Wind 15-MW RWT atop VolturnUS semisubmersible repository: IEA-15-240-RWT/OpenFAST/IEA-15-240-RWT-UMaineSemi/HydroData/wamit_inputs_2ndOrder at master · IEAWindTask37/IEA-15-240-RWT · GitHub (see IEA-15-240-RWT_2nd.frc).

Typically when using WAMIT to generate QTFs, WAMIT will internally calculate the RAO based on the linearized external mass, damping, and stiffness matrices provided. Typically when using time-domain simulations in OpenFAST to derive RAOs, the full mooring system model in OpenFAST (MAP++ or MoorDyn) will be used.

Best regards,

3

Hi Jason,

Thank you so much that is very helpful. Just to clarify, do you know if the IEA-15-240-RWT_2nd.frc you pointed to is the stiffness matrix for only the additional stiffness, added mass, and damping provided by the mooring system? or is it a sum of that and 2nd order terms?

best,
Jackson

4

Dear @Jackson.Hardin,

According to the document describing the VolturnUS semisubmersible (https://www.nrel.gov/docs/fy20osti/76773.pdf), the QTF was derived based on the linearized stiffness of the mooring system, but I was not the one to do this work and can’t comment on the details.

Best regards,

1 Like
5

Okay thank you very much for the help!