Reference points for the hydrodynamic data of the multibody structure in the potential-flow solution

Wind & Water Computer-Aided Engineering Software Tools
1

Hello!!!
I am now using WAMIT to output the potential flow solution for OpenFAST v.2.6. It is multibody calculation. Here is my question. Should I set the reference points of WAMIT output forces and moments to the SWL (Location where x=0, y= 0, z= 0)? I checked the NREL semisub model WAMIT model and found that NREL set both XBODY(3) and VCG to zero which mean that the forces and moments from WAMIT will be at/about the sea surface (x=0,y=0,z=0).

2

Dear @Danupon.Subanapong,

You mention a multibody calculation, so, presumably you have multiple WAMIT bodies you want to use in HydroDyn and you are representing the substructure flexibly using SubDyn. The hydrodynamic bodies represented in WAMIT are likely large volume and thus structurally stiff (represented as rigid links in SubDyn) and I would expect these are interconnected by small volume members (represented as beam elements in SubDyn and strip-theory members in HydroDyn). In that case, you’d want to set the reference point of each WAMIT body to be located in such a way that the loads from each WAMIT body get mapped to the rigid links in SubDyn, e.g., at the centerline of each large volume body.

Best regards,

3

Thank you so much for your reply. It is very clear.

4

Dear Dr. Jonkman

I just finished the WAMIT multiple bodies calculation (There are two bodies. The first body is floating body and second one is submerged in the water). The geometry of each body is described by separate body coordinate system (BCS). The BCS of each body is located at the centerline of each body. After the WAMIT calculation was done, I checked the WAMIT output and found that the hydrostatic stiffness matrix seems to be strange for the 1st body. The hydrostatic stiffness of the first body due to heave motion (C33) is negative. Also, another hydrostatic stiffness such as C44 and C55 are around 4 times larger than when I do WAMIT calculation by assigning BCS for both body at the sea water level (Since I have 2 bodies, the hydrostatic stiffness is 12x12 matrix).

Could you please kindly explain?? Is my WAMIT calculation strange??

5

Dear @Danupon.Subanapong,

Indeed that sounds strange and likely a better question for Chang-Ho Lee of WAMIT Inc.

HydroDyn also allows multiple WAMIT bodies to specified neglecting coupling between the bodies (NBodyMod = 2 or 3), which allows you to use separate WAMIT solutions for each body, so in your case, two sets of 6x6 matrices are used instead of 12x12 matrices. Perhaps that feature is of use to you?

Best regards,