,I read your comment somewhere where you said that not having any support inside the transient analysis will be okay and not cause any solver pivot error (or in other words, rigid body motion error) since the equations involves the damping and acceleration terms within them. So not having any supports means that the stiffness matrix term will go away, while the damping and acceleration terms may remain. In the static structural analysis, I don't think it makes use of the damping and acceleration terms within it. Because if it does, then whats gonna be the difference between the transient and static structural then? Only the stiffness matrix is involved, and having a singular stiffness matrix may cause the solution to undergo a pivot error.
I will also have a detailed look at the Inertia Relief theory inside the theory manual.