How should CoG be defined in AQWA when generating WAMIT files for OpenFAST?

[catz0781]

Dear All,

I am working with the VolturnUS-S semi-submersible platform and using OpenFAST and ANSYS AQWA to simulate the platform response under specific environmental conditions.
To generate hydrodynamic coefficients for OpenFAST, I exported results from AQWA in WAMIT format. Before running my own cases, I am trying to validate hydrodynamic coefficients using the IEA 15-MW reference turbine with the VolturnUS-S platform (200 m water depth) files available on GitHub.

In my AQWA model, I kept all platform dimensions consistent with the definition document. However, I noticed that:
• In the original WAMIT files, the centre of gravity (CoG) is defined as [0, 0, 0].
• In the definition document, the vertical CoG of the semi-submersible platform is −14.94 m.
In a previous NREL forum discussion on the OC4-DeepCwind semi-submersible WAMIT files, it was mentioned that “the CoG in the first-order WAMIT solution used by OpenFAST is set to zero because the body weight terms are accounted for in the structural modules of OpenFAST”. (https://forums.nlr.gov/t/oc4-deepcwind-semisubmersible-wamit-files/2203)

Since the non-zero vertical CoG may influence the hydrodynamic coefficients, particularly the added mass and radiation damping in roll and pitch.
I would like to ask:

1. How the effect of the actual platform CoG (z = −14.94 m) is accounted for in OpenFAST when the WAMIT CoG is [0, 0, 0]?
2. When generating hydrodynamic coefficients in AQWA for use with OpenFAST, should the CoG in the hydrodynamic model always be set to [0, 0, 0], or should the physical CoG (−14.94 m) from the definition document be used?

Many thanks for your help.

Best regards,
Lidan

[jjonkman]

Dear @catz0781,

Here are my answers to your questions:

1. The structural weights are directly considered in the structural modules of OpenFAST and will change with displacement, so, e.g., the hydrostatic restoring associated with body weight is already accounted for and should not be double counted in HydroDyn. It is better to include these effects in the structural modules anyway due to the structural flexibility, where the CoG can change with elastic displacement.
2. When generating the first-order potential-flow solution, yes. For the second-order potential-flow solution, which depends on first-order response amplitude operators (RAOs), the RAOs require accurate setting of the CoG, but this should only affect the QTFs.

FYI: The platform CoG should not impact the added mass or radiation damping matrices about (0,0,0), which should only depend on the mode of motion and wetted geometry. The platform CoG will have an affect the hydrostatic restoring; setting it to zero in the potential-flow solution will avoid the terms from being added to the hyrostatic restoring matrix.

Best regards,

[catz0781]

Dear @jjonkman,

Thank you for your reply. I have a follow-up question regarding hydrodynamic data preparation.

I am considering generating hydrodynamic results in AQWA for two cases: one with a zero centre of gravity (CoG) and another with a non-zero CoG. I would then use BEMRosetta to convert both sets of results into WAMIT format files.

In OpenFAST, I would use the first order hydrodynamic data (with extensions .1, .3, .hst) from the zero CoG case, and the second order data (with extensions .7, .8, .9, .10d, .11d, .12d, .10s, .11s, .12s) from the non-zero CoG case.

My question is whether this mixed-CoG approach would improve the overall accuracy of OpenFAST simulations compared to using hydrodynamic data generated with a single CoG setting, or whether it could introduce inconsistencies in the hydrodynamic modelling.

Thank you very much for your help.

Best wishes,
Lidan

[jjonkman]

Dear @catz0781,

Yes, I agree with your approach, which is what I was trying to explain in the second bullet of my previous reply.

The QTFs depends not only on the second-order potential, but also on the quadratic interaction of first-order terms that depend on proper first-order motion RAOs. For the latter, you need to set the system CoG properly when generating the QTFs.

Best regards,