Questions about the OC4_DeepCwind orcaflex model

Wind & Water Computer-Aided Engineering Software Tools
1

Dear Jason
I’ve downloaded the Sample models for FAST and OrcaFlexInterface (nwtc.nrel.gov/OrcaFlexInterface) and tried to modify the OC4_DeepCwind model so I can use it in the second order hydrodynamis analysis. But there are some settings in the Orca model that confused me and hope you may help me out. Here are the questions.
1. In the orca model, there is a applied loads acting on the vessel whose value is 140E6, what is that for?
As far as I know, the applied load is equal to the gross gravity of the platform+tower+nacelle, so I assume that is for the gravitional equilibrium. But when I try to do a free decay analysis with orcaflex alone, I set the platform primary motion to “calculated(6 DOF)” and input the mass and inertia data in the vessel type data. But the vessel won’t get balanced and it only does when I uncheck the applied loads. So I am confused because if it is to balance the gravity, isn’t it supposed to get balance when i input the value in the vessel mass and inertia data?

2. When I try to do a free decay analysis in orca alone, it seems like that I have to change the negative values in the hysrostatic stiffness matrix to positive so I can do the analysis. Is that a result of conventional differences between the two tools or just I get it the wrong way? The hydrostatic stiffness matrix you give in the document Definition of the Semisubmersible Floating System for Phase II of OC4 also does have the negative value, but isn’t all the values in a stiffness matrix supposed to be positive?

3.Is it possible that you may offer me the natural periods of the DeepCwind so I can check whether the model I modified is correct?

Best wishes.

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Dear Sean,

Your first question is probably better answered by Orcina.

The hydrostatic stiffness matrix provided in the OC4-DeepCwind semisubmersible specifications report you reference only includes the contribution of buoyancy, not body weight (gravitational restoring), so, the pitch/roll stiffness terms are negative-valued. The body weight (gravitational restoring) term was separated out because it is often calculated by the structural code (as it is in FAST). Perhaps OrcaFlex treats this term differently?–again, a question probably better answered by Orcina.

The natural periods of the OC4-DeepCwind semisubmersible are documented in the OC4 Phase II results–see Figure 3 of the following report: nrel.gov/docs/fy14osti/61154.pdf.

Best regards,

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Dear Jason：
I see. Thanks for your fast reply, that’s a lot of help to me.
Best regards.
Sean.Wu