Modal analysis of FOWT tower with shell elements

Dear engineers,
I am performing a modal analysis in Homer2 (Bureau Veritas) for a tower with parameters as in article https://www.nrel.gov/docs/fy20osti/76773.pdf. Could you, please, answer my theoretical questions?

  1. For simulating the load of RNA at the top of my tower, I have added an element, associated with mass matrix (mass + moments of inertia) and made a “rigid link” with all the nodes on the surface of the tower. Is it more plausible to apply the load only for the contour of the upper surface?

  2. Geometry with an upper surface entails non-physical behavior in modal shapes(the upper surface is strangely deformed, although we expect only flexible modes (when the tower tilts) or expansion modes (when some parts of the tower expand or “collapse”). How can I get rid of modes that deform the upper surface? Is it more plausible to perform the analysis without an upper surface, since there is actually no plane between the tower and the RNA?

  3. In the article tower is described as a set of blocks with different thickness. The lowest block is fixed. What is the difference from the point of view of physical meaning, if the lowest block is fixed or if we delete the lowest block and fix only the bottom surface of the tower?

Thank you in advance