This data category is used to input the added mass and damping at drift frequency, and the non-linear drag forces due to the yaw motion of the structure at drift frequency, for each structure.

The coefficients are applicable to the following programs:

Aqwa-Fer: Used only to calculate the drift frequency motions in the frequency domain. They are not applicable when using Aqwa-Fer for the calculation of wave frequency motions.

Aqwa-Drift: Used to calculate the drift frequency motions in the time domain.

Aqwa-Line: May be input for scaling (see Change Geometric/Mass Characteristics - GMCH (Data Category 16L)).

Aqwa-Librium: Used to calculate the dynamic stability of the drift frequency motions. They are not applicable when using Aqwa-Librium to calculate the static equilibrium position.

Aqwa-Naut: Although Aqwa-Naut does not include any drift forces, frequency independent added mass and damping input using the FIDA and FIDD data records is used.

The Yaw-Rate Drag Forces

In general, a structure with yaw velocity, in the presence of a current, experiences non-linear drag forces additional to those present when the structure is stationary. These forces are referred to as yaw-rate drag forces and are zero when there is no yaw velocity. The forces act only in the surge, sway and yaw (translational X,Y and rotational Z) directions and are a function of the local water velocity and the geometry of the structure, for a fully-developed wake (i.e. they are only applicable at drift frequency).

The user is required to specify an average drag force coefficient, and a length along which the local drag force is integrated, to give the total forces on the structure.

A linearized form of the force equation is used in the calculations of motions (Aqwa-Fer, if LDRG option is on) and, in general, the yaw rate drag forces do not strongly influence the results.