Vessel data

Name

Used to refer to the vessel.

Type

Specifies the vessel type. The vessel types button allows you to view and edit the vessel type data.

Draught

Which draught of the specified vessel type to use. Each vessel type can have data for a number of different draughts. The draught cannot be changed during a simulation.

Length

The length of this vessel. The default value '~' signifies that this value is the same as the vessel type length. If the length differs from the vessel type length, then OrcaFlex will scale all the vessel type's data to allow for the scaling factor VesselLength/VesselTypeLength. This is useful if you have data for a 70m ship, for example, but want to use a 50m ship that is otherwise very similar.

The scaling is done using Froude scaling (see Rawson and Tupper). Froude scaling scales each item of data by a factor that depends on the units of that item. If $R$ is the ratio of vessel length to vessel type length, then the scaling factors are follows:

Data with other units are scaled by writing the units in terms of the fundamental units of mass, length and time, and then applying these factors. For example, force data have units Mass x Length / Time2 so are scaled by $R^3R/(R^{1/2})^2 = R^3$. Dimensionless items such as translational RAOs (surge, sway, heave) and phase angles are unchanged by the scaling process, but note that Froude scaling does apply to the periods (or frequencies) specified for RAO and QTF data. Note also that rotational RAOs (roll, pitch, yaw) may be given in dimensional terms (e.g. degrees per metre) and these also scale.

These scaling rules are the same as those used in deriving full scale ship performance from physical model tests, and are correct if the vessel is a perfect scaled replica of the vessel type in all respects.

Warning: Unless the vessel scales uniformly in all dimensions, then this type of scaling introduces errors and should not normally be used, and OrcaFlex issues a warning to this effect. Instead, accurate data specific to this vessel should be obtained.

Connection

A vessel can be free, fixed, anchored or connected to another object. Typically, a vessel will be free and will have the full range of calculation data available. A vessel that is not free will either be fixed in position or connected as a child to a parent object, either of which will restrict the vessel's calculation options.

Note: A vessel can only be included in frequency domain dynamics if its connection is free.

Initial position and orientation

The initial position is specified in terms of $x$, $y$ and $z$ connection coordinates of the vessel origin. Its initial orientation is specified by giving 3 angles, heading, trim and heel, which are successive rotations that define the orientation of the vessel axes, $\Vxyz$, relative to the connection axes, $\Cxyz$, as follows: first align $\Vxyz$ with $\Cxyz$; then apply the heading rotation about $V_\mathrm{z}$, then the trim rotation about the new $V_\mathrm{y}$ direction and finally the heel rotation about the new (and final) $V_\mathrm{x}$ direction.

If a free vessel is not included in the static analysis, then the initial position and orientation are taken to be the static position and orientation of the vessel. If the vessel is included in the static analysis, then the initial position and orientation are used as an initial estimate of the vessel position and orientation. However, the static analysis will translate and rotate the vessel from this position until an equilibrium position is found.

Note: The vessel z coordinate of a free vessel can only be changed by editing the vessel data form. Dragging in the z-direction with the mouse is prevented.
Warning: If you have included any vessel 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.

Characteristic scales

For some models it may be desirable to explicitly set a characteristic length and characteristic force for the vessel. These characteristic scales directly affect the convergence criteria of the iterative solvers employed in the analysis. The data does not appear on the vessel data form but can be found on the all objects data form.

Further vessel data

The remaining vessel data are specified on separate pages of the vessel data form. To save space, some of these pages will be hidden if they are not relevant: this depends on the choices you make on the calculation page for primary and superimposed motion (in the case of time domain analysis) or low frequency and wave frequency motion (frequency domain analysis), as well as the included effects.