OrcaFlex Manual - Orcina

System Modelling: Data and Results, Vessels
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If the vessel is not included in the static analysis then this Initial Position is taken to be the static position of the
vessel. If the vessel is included in the static analysis, then this Initial Position is used as an initial estimate of the
vessel position and the statics calculation will move the vessel from this position iteratively until an equilibrium
position is found.
Note: The vessel Z coordinate can only be changed by editing on the vessel data form. Dragging in the Z
direction with the mouse is prevented.
Warning: If you have included any harmonic motion on the vessel (see Harmonic Motion) then the phases of
the harmonic motions will normally depend on the vessel Initial Position, so if you change the
Initial Position you may need to change the harmonic motion phases accordingly.
Further Vessel Data
The remaining vessel data is specified on the following separate pages of the data form:
� Calculation page specifies how the vessel motion is modelled.
� Primary Motion page specifies any further data for any Primary Motion. This is only used if Prescribed, Time
History or Externally Calculated primary motion are used.
� Superimposed Motion page specifies any further data for any Superimposed Motion. This is only used if
Harmonic Motion or Time History superimposed motion are used.
� Multiple Statics page allows you to specify a grid of different positions of the vessel, for a series of static
analyses.
� Drawing and Shaded Drawing pages specify how the vessel will be drawn in the 3D view.
Calculation Data
The following settings (on the Calculation page on the vessel data form) control how the vessel's static position and
dynamic motion are determined. For an overview see Vessel Modelling Overview
Included in Static Analysis
You can control whether the OrcaFlex static analysis calculates the static equilibrium position of the vessel, or
simply places the vessel in the user-specified initial position.
OrcaFlex first places the vessel at the initial position and orientation specified by the user. If Included in Static
Analysis is set to None then OrcaFlex leaves the vessel in this user-specified position. This is not necessarily an
equilibrium position.
If Included in Static Analysis is set to 3 DOF then OrcaFlex starts from the user-specified position and adjusts the
vessel's X, Y and Heading until an equilibrium position is reached. Note that only these 3 free degrees of freedom of
the vessel (X, Y and Heading) are included in the calculation. The other three degrees of freedom (Z, Heel and Trim)
are assumed to be constrained and so are left at the values specified by the user. This means that the Z component of
resultant force may be non-zero in the equilibrium position. Likewise the resultant Heel and Trim moments may be
non-zero in the equilibrium position.
If Included in Static Analysis is set to 6 DOF then OrcaFlex starts from the user-specified position and adjusts all
six degrees of freedom until an equilibrium position is reached. In this case, all of the X, Y, Z forces and Heel, Trim,
Heading moments will be zero at the equilibrium position.
The loads on the vessel to be taken into account in calculating the equilibrium position are specified by the Included
Effects. In the static analysis many of these (first order wave load, sum frequency load, added mass & damping load,
manoeuvring load and other damping load) will be zero; and any included wave drift load will only include the mean
drift load (allowing for wave drift damping, if included), not any slowly varying contribution to wave drift load.
Note: If multiple statics are being performed on the vessel then no equilibrium calculation is performed
on the vessel and its placement is determined by the multiple statics data. Other vessels in the
model are included in the static analysis as specified by their own data.
Dynamic Analysis
OrcaFlex provides two vessel motions, Primary and Superimposed, each of which is optional. When both are
present they are applied concurrently, with the latter being superimposed on the former. See Vessel Modelling
Overview for an introduction and examples of how these options can be used.