Dear @Jason.Jonkman,
I am conducting analysis of the 5MW OC3 offshore wind turbine using OpenFAST , The output parameters M1N1FBxi, M1N1FByi, M1N1FBzi, M1N1MBxi, M1N1MByi, and M1N1MBzi represent distributed buoyancy forces/moments and I found their calculation formulas in “(https://webserver.nrel.gov/wind/nwtc/assets/downloads/HydroDyn/HydroDyn_Manual.pdf)”.
However:In the cylindrical section (constant radius R(z)), ∂z/∂Rshould theoretically be zero.The solver outputs M1N1MByi non-zero .I would greatly appreciate it if you could clarify why this occurs
-When modeling the platform as a rigid body, I would greatly appreciate clarification on how to calculate the forces and moments generated by dynamic pressure.
I would be appreciated it if you could reply. Thank you
Best regards,
Dear @Yingxin.Lv,
The equation you grabbed was from an older version of HydroDyn and is not used in newest version of OpenFAST.
More up-to-date theoretical details are provided here: 4.10. HydroDyn User Guide and Theory Manual — OpenFAST v4.0.2 documentation, for example, the buoyancy and dynamic pressure terms used by the strip-theory solution are documented here: https://openfast.readthedocs.io/en/main/_downloads/39aafbb61663ae16bd1a7e80f305c20b/HydroDyn_Plan_TCF_Morison.docx.
That said, I know there are updates to these documents, as well as additional implementation plans, that have not been uploaded yet to readthedocs.
Best regards,
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Dear @Jason.Jonkman,
I sincerely appreciate your earlier reply. However, I must admit there are still a few points where I’m feeling a bit confused, and I would greatly appreciate your further guidance if possible.
1.Regarding the first formula on the seventh page of the (https://openfast.readthedocs.io/en/main/_downloads/39aafbb61663ae16bd1a7e80f305c20b/HydroDyn_Plan_TCF_Morison.docx)", I have tried to derive it myself and found that there is a difference（m and cos） in one symbol in the result. I’m not sure where I went wrong and sincerely hope that you can help me solve this confusion.
2.Based on my understanding, I believe the calculated buoyancy ought to relate to the platform’s torsion, the water depth at the output point, and the platform’s radius in the following manner. I have made an attempt to derive this relationship, and although the curve shape of my result bears a resemblance to the output value M1N1FBxi, there exists a notable disparity in numerical values. It is unclear to me where the error might have occurred, and I would be extremely grateful if you could kindly take the time to point out any mistakes in my derivation process.
（Since the cross-sectional dimensions of the element being calculated remain unchanged, there are no terms involving (m_i) included.）
I would be appreciated it if you could reply
Best regards,
Hi @Yingxin.Lv,
Thank you for sharing your derivation; however, it is difficult for us to check and compare because the formulation you referenced is obsolete as Jason mentioned.
I would suggest referencing the subroutine getElementHstLds_Mod1 from openfast/modules/hydrodyn/src/Morison.f90 at main · OpenFAST/openfast · GitHub for the up-to-date formulation used in OpenFAST. Unfortunately, we don’t have a document that is ready to share on this new formulation right now.
Regarding the load comparison you showed, I just want to quickly mention that the output channel M1N1FBxi will give you the force per unit length of the member (N/m), not the force itself. Not sure if that makes a difference for your comparison.
Best,
Lu
Dear @ Lu.Wang,
I sincerely appreciate your thoughtful reply. The formula referenced in my inquiry originates from the new “up-to-date theoretical details” document kindly shared by Professor Jason in his response, as opposed to the older version of HydroDyn.
Best regards,
Dear @Yingxin.Lv,
I also said that there were updates to these documents. I was not aware that this part of the hydrostatics formulation had changed, but Lu corrected me.
Best regards,
