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Axial Added Mass Coefficient for Substructure Modelling
Hi Mr. David
Hope you are well and healthy. My questions are—
- In the attched image, it is a substructure file for an offshore floating platform. I need to assign axial Ca for the stabilizers/heave plates (marked inside blue circles). The diameter of the stabilizers are 35 meter. The water density is 1025 kg/m^3. I am to assign Ca only on one side of the stabilizer. Now for an axial Ca=1, what would be the added mass?
I have already tried to calculate it. In the Qblade guide, it is stated that “The reference volume for added mass and Froude-Krylov forces is evaluated as the volume of a semisphere, using the hydrodynamic member diameter: V=((dia.^3)/12)*pie. By this formula, the reference mass= ((dia.^3)/12)*pie*1025 = 11505291.88 kg. Now, Added mass coefficient, Ca=(added mass/reference mass). So, if we assign Ca=1, then the added mass should be 11505291.88 kg. Am I correct about the calculation?
2. From the model test results of the platform, the heave and pitch period are ~31 sec and ~42 sec respectively. But Qblade is showing the same period for both heave and pitch (It was a Morrison simulation only). I guess the axial added mass coefficient would different for heave and pitch. Am I correct? If I am, then is there any option to simulate both heave and pitch response correctly in a single model? Or, do I need to seperately model for each case with different axial Ca value?
Best Regards
S.H.M. Labib Mahmud
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Hello Mahmud,
Regarding your first question: yes, your interpretation is correct. For a circular member end, the axial added mass in QBlade is based on the volume of a hemisphere computed from the member diameter, multiplied by the water density and the axial added mass coefficient at that end joint. Therefore, for Ca = 1, the resulting axial added mass (acting only during acceleration along the member’s axis) corresponds exactly to this reference mass.
Concerning the heave and pitch periods: QBlade is fully capable of reproducing distinct measured heave and pitch natural periods. We have built multiple floating platform models that accurately match decay test results. There is no need to assign different axial Ca values depending on whether the decay test is heave or pitch.
The natural periods depend not only on the axial added mass, but also on the total structural mass, the platform’s rotational inertia, and the distributed buoyancy forces acting on the system. With appropriate tuning of these parameters, you should be able to reproduce both measured periods within a single, consistent model.
Best regards,
David
