OrcaFlex Manual - Orcina

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Primary Motion
257
System Modelling: Data and Results, Vessels
The Primary motion determines what OrcaFlex refers to as the primary position of the vessel. It can be one of the
following options:
� None. In this option there is no primary motion and the primary position of the vessel remains fixed at the
position determined by the static analysis.
� Prescribed. This option allows you to drive the vessel around the sea surface, for example to model the vessel
moving station during the simulation. The vessel's speed and course are specified by the data on the Primary
Motion page.
� Calculated (3 DOF). In this option OrcaFlex calculates the vessel primary motion in only 3 degrees of freedom
(surge, sway, yaw) based on the included loads plus loads from any lines or other objects that are attached to
the vessel. There is no primary motion in the other 3 degrees of freedom (heave, roll, pitch). The data for all the
included loads must be specified.
� Calculated (6 DOF). Here OrcaFlex calculates the vessel motion in all 6 degrees of freedom, based on the
included loads, plus loads from any lines or other objects that are attached to the vessel. The added mass,
damping, stiffness and equilibrium position of the vessel type must be specified, plus the data for all the
included loads.
� Time History. For this option the user specifies the primary motion in a time history file that defines, as a
function of time, the vessel Primary X, Primary Y, Primary Z, Primary Rotation 1, Primary Rotation 2 and
Primary Rotation 3. See the Time History data on the Primary Motion page.
� Externally Calculated. The primary motion is defined by an external function that is specified on the Primary
Motion page.
Superimposed Motion
The Superimposed motion is applied as an offset from the position given by the primary motion. It can be one of the
following options:
� None. There is no offset and the vessel position is equal to the primary position at all times.
� Displacement RAOs + Harmonic. The vessel's position oscillates harmonically about the primary position. The
harmonically varying offset comes from two sources. Firstly, if waves are present and you specify non-zero
displacement RAOs for the vessel type, then the offset will include the wave-generated harmonic motions
specified by those RAOs. Secondly, the vessel's superimposed offset also includes any harmonic motions that
you specify on the Superimposed Motion page on the vessel data form.
� Time History. The user specifies the offset in a time history file that defines the vessel Surge, Sway, Heave, Roll,
Pitch and Yaw as a function of time. See the Time History data on the Superimposed Motion page.
Included Effects
For each of the following vessel load effects, you can choose whether they are included in the analysis by ticking the
corresponding checkboxes on the calculation page of the vessel data form:
� Applied Loads: Data for these are on the vessel form Applied Loads page.
� Wave Load (1 st order): Data are on the vessel type form Load RAOs page.
� Wave Drift Load (2 nd order): Data are on the vessel type form Wave Drift QTFs page. See also Theory.
� Wave Drift Damping: Has no separate data. For theory see Wave Drift Damping. Requires that Wave Load (2 nd
order) is also included.
� Sum Frequency Load (2 nd order): Data are on the vessel type form Sum Frequency QTFs page. See also
Theory.
� Added Mass and Damping: Data are on the vessel type form Stiffness, Added Mass, Damping page. See also
Theory.
� Manoeuvring Load: Has no separate data. See Manoeuvring Load Theory.
� Other Damping: Data are on the vessel type form Other Damping page. See also Theory.
� Current Load: Data are on the vessel type form Current Load page. See also Theory.