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natural period of free decay motion

Hello, I want to know if QBlade can directly view the natural period of free decay motion? If you can check it, please let me know. I would be very grateful.

Hi Sun,

the easiest way to get the natural periods of a floating system is to start a time domain simulation in which the floater is initially displaced in the DoF’s of interest. When the simulation is complete you can check out the “All Data PSD Graph” in which you can directly read out the spectrum of any sensor.



Thank you very much, David

Hello, David, I want to ask a question about the OC4 platform structure file: How is the overall quality of the OC4 platform calculated? I can’t understand how the quality of ballast water is calculated in the document. According to the SUBMEMBERS table in the first picture, it seems that the density of water in the third picture ‘1025kg/m3′ is not used to calculate the quality of ballast water, but the data in the SUBMEMBERS table in the second picture, such as’ Upper ‘_ Column_ flooded_ 1 ‘, its RElmID is 2, which corresponds to the data’ 1.7668E+04kg/m ‘of ID2 in SUBELEMENTS. However, this part should be the mass of water. It seems that there is no water density for mass calculation.

Another problem is that the quality of the platform end caps does not seem to be defined, as shown in Figure 4.

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Hi Sun,

there are quite a few approaches when modeling a structure using the substructure file format.

The mass can either be modeled explicitly through the members or by assigning a 6×6 mass matrix (SUB_MASS). In the case of the OC4 platform file we define the complete mass using a 6×6 matrix. This mass also includes the weight contribution of the flooded members – thats why the members are not flooded explicitly in the SUBMEMBERS table.

The members that are defined in the SUBMEMBERS table of the OC4 file are rigid members (RElmID) defined in the SUBELEMENTSRIGID table, the definitions in SUBELEMENTS are not used in this setup. The rigid members are massless (neglible mass of 1kg per m length) and serve the purpose of generating hydrodynamic forces through the HYDROMEMBERCOEFF values assigned to them and to contribute to the buoyancy of the system. In the OC4 file buoyancy is calculated explicity through the members – another approach would be to assign a hydrodynamic stiffness matrix (SUB_HYDROSTIFFNESS).

The water density that would be used for the filled member calculation is the density that is defined in the simulation setup dialog. The value WATERDENSITY is only used to calculate masses in the turbine design module when no simulation has been defined yet and environmental parameters (water density, air density, etc…) are not available yet.



P.S. To keep the forum organized and to help others to find information more easily, could you please open up a new thread the next time when your question is not related to the thread topic. Thanks!

Thank you, David. Sorry, I will pay attention next time. I will do it according to your requirements.

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