Low Frequency Drift Motions |
 
|
Low Frequency Drift Motions |
|
Vessel reference point low frequency or drift motions can be defined in two ways, as sinusoids or via timetrace data read from file, but the actual motions are handled in the same way regardless of which specification you are using. In the most general case you input or define three drift translations and three rotations. The handling of the three displacements is discussed first.
The drift translations can be input in either the global axes or a so-called local system. The handling of drift displacements in the global axes is straight-forward. If on the other hand you input drift displacements in local axes, Flexcom assumes these are relative to the initial orientation of the vessel axes, as defined in the Initial Orientation of Vessel Axes figure.
The significance of the three drift rotations is very similar to the handling of the static offset rotations. Specifically the so-called yaw drift at any time, which we denote γD(t), represents a further rotation of the vessel and vessel axes about the global X direction. So at any time, the orientation of the vessel 
axis relative to global Y is simply the sum of the initial yaw θ, the static yaw Ψ and the instantaneous yaw drift γD(t). See the figure below. The roll and pitch drift rotations are then defined relative to this instantaneous yawed axis system. Again these two rotations define two components of a standard Flexcom rotation vector. They are combined using standard Flexcom techniques with the static roll and pitch, which are themselves continuously defined (or redefined) relative to the instantaneous 
axis system.
Instantaneous Vessel Axes – Large Angles
The handling of the three drift displacements is likewise unaffected by the choice of large or small angles, which affects only vessel rotations.
The significance of the three drift rotations in the small angle case is very similar to the handling of the small angle static rotations. Specifically the drift yaw, roll and pitch represent further rotations of the vessel about the undisplaced vessel axes. The combined effect of offset and drift rotations at any time is found by simply adding yaw components to get combined yaw, roll components to get combined roll, and pitch components to get combined drift.
•*VESSEL,INTEGRATED is used to specify all information pertaining to a vessel or vessels. Specifically, the ANGLES= input is used to specify the theory used to combine vessel rotations.
•*DRIFT is used to define vessel drift motions.