I believe I said the same thing as the User's Guide. The transverse force is indeed perpendicular to the plane formed by the velocity vector and the axis of the element.

For your example, if the pipe is in X, and the wave is in Z, the transverse force will be in Y. The force may be +Y or -Y, depending on the wave phase (which defines the plus/minus direction of the velocity in Z and how you defined the element's plus/minus direction.