Elimination of freedoms at the center of gravity is used to remove one or more of the six degrees of freedom at the center of gravity. For example, if the analysis is one- or two-dimensional, then the user may eliminate the appropriate five or three degrees of freedom respectively. This facility is extremely useful when data for a full three-dimensional analysis is unavailable, or when a simple model is being analyzed. This is achieved by the use of the deactivated freedom function in Aqwa.

If an offshore structure system consists of M structures and the general matrix form of the equations of motion in the time domain is expressed as

(8–14)

where is a 6M×6M structural mass matrix and is the external force 6M×1 matrix, then the deactivation of the m-th degree of freedom in the whole structure system leads to the corresponding coefficients and force components in Equation 8–14 to be adjusted as

(8–15)

With these corrections, the acceleration response of the m-th degree of freedom is guaranteed to be zero in a time domain analysis procedure.

It should be noted that the deactivated degree of freedom refers to motion of the center of gravity in the fixed reference axes (FRA), not the local structure axes (LSA) described in Local Structure Axes.