The calculation approach on the stability at a large angle of inclination of a freely floating structure with internal tanks is similar to the concept discussed in Large Angle Stability. We assume that a free-floating body rotates a specified angle about a prescribed horizontal hinge axis and shifts upwards or downwards from its initial equilibrium position to ensure that the total external fluid displacement remains constant.
It is assumed that the combined COG defined in Equation 3–25 is "frozen" on the structure and will rigidly move with the structure.
At a known inclination angle rotated about a given horizontal hinge axis from the original
equilibrium state of a free-floating body with internal tanks, Aqwa first calculates the
vertical shifting position of the combined COG to keep the external fluid displacement
unchanged. Then, at the body's new orientation and position state, the external wetted body
surface and the wetted internal tank surfaces can be determined numerically. Denoting the
directional angle between the horizontal hinge axis and the global X-axis as 
, the inclination angle about the hinge axis from the initial equilibrium
position as 
, the combined COG (LSA origin) as (Xg
, Yg
, Zg
) and the corresponding wetted body surface and wetted internal tank surfaces as
, the righting moments about the horizontal axes of the FRA are similar to
those defined in Equation 3–16 but with the additional moments due to
the structure gravity force to the combined COG,
 | (3–45) |
The righting moment about the hinge direction and its perpendicular direction are given in the same form of Equation 3–17.
The conventional 
value, which is indicated in Figure 3.2: Large Angle Stability for a
free-floating structure with internal tanks, is given in Equation 3–18.