Flexcom > Theory > Model Building > Vessels and Vessel Motions > Combined High and Low Frequency Motion Timetraces

Introduction

Combined low and high frequency motions are specified in the form of vessel motion timetraces or time histories.

Vessel motions are normally due to wave excitation. If you apply a time history of vessel motion, then you must ensure that you also specify the corresponding wave excitation which generated those vessel motions. This is typically specified via a Time History of Water Surface Elevation.

Large Angle Theory

The handling of these motions is similar in some respects to the low frequency drift motions. Like the drift translations, combined translations can be specified in global or so-called local axes. Local translations if specified are assumed to be relative to the initial vessel axes (shown in the Initial Orientation of Vessel Axes figure).

The first rotation read from the combined motions timetrace file is assumed to define a yaw rotation which rotates vessel and axes about the vertical axis. And like in the case of drift rotations, the remaining rotations are assumed to define roll and pitch respectively relative to this instantaneous yawed axis system, whose rotation from global Y is the sum of initial rotation θ, the yaw offset ψ and the combined yaw read from the timetrace file. Likewise vessel headings are calculated with respect to the instantaneous yawed axis system.

Small Angle Theory

The handling of these motions is very similar to that described earlier for RAO motions. Translations specified in local axes define motions relative to the undisplaced vessel axes. Likewise rotations define yaw, roll and pitch relative to the initial vessel orientation, and are simply added to corresponding static offset rotations if these are non-zero.

Relevant Keywords

•*VESSEL TIMETRACE is used to specify that the combined high and low frequency motions of a vessel are to be read from an ASCII timetrace data file.