Hi, I want to use the hydrodynamic pressure extension for a simple Spar(vertical cylinder). I define hull and ballast masses in Aqwa and then I create a structural model(3D element Solid not surface type!) and Because mass properties in Ansys Structural is automatically calculated by itself, so just I add the ballast mass and I adjust the masses to be the same both in Aqwa and Structural. Then when I run the extension it says " Ansys cannot determine mass...." like the attached scrnshots. 
I dont know what to do. I tried many ways adding and deleting point masses, but it keeps showing this error. Please help!