OrcaFlex Manual - Orcina

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Obtaining the data
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System Modelling: Data and Results, Vessels
All of the above data can generally be obtained from the results of a diffraction program. OrcaFlex can import these
data from the output files of some specific programs (AQWA and WAMIT) and from generic text files with OrcaFlexspecific
markers added. There are two different ways to do this import.
The easiest and most reliable way is to import all the hydrodynamic data using the Import Hydrodynamic Data
button on the Vessel Types data form. Alternatively, you may use the Import Matrices button to import the
frequency-dependent added mass and damping matrices.
Other Damping
There are various sources of drag or damping (the terms are often used interchangeably) on vessel motion. OrcaFlex
models most of these explicitly using data on the vessel type data form for each form of damping: current and wind
loads, wave radiation damping, wave drift damping. In addition to these, OrcaFlex also offers wave frequency Other
Damping, as a way to incorporate other sources of damping which do not fall into any of these categories. Viscous
roll damping, for example, is a wave-frequency effect which is not covered by any of these specific damping forms
and so would be modelled using Other Damping.
Other Damping is specified using the following data on the vessel type data form, all of which are automatically
Froude scaled to the vessel length if it differs from that of the vessel type. It will only be calculated if Other Damping
is specified as included on the vessel data form.
Reference Origin
The point on the vessel to which the Damping Coefficients refer. The relative velocity used to calculate the Other
Damping load is the value at this reference origin, and the Other Damping load is applied at this point. It is specified
by giving its coordinates relative to vessel axes.
Note: The Other Damping data only accepts coefficients for the diagonal terms of the damping matrix, so
no coupling effects (surge-pitch, sway-roll, etc.) are applied at the Reference Origin, and coupling
effects will only be included due to the offset of the reference origin from the vessel origin. The
Reference Origin should therefore be close to the overall centre of damping load, where such
coupling effects are small.
Damping Coefficients
Damping coefficients are specified for all six vessel degrees of freedom and are given relative to the vessel axes. Two
sets of coefficents are available, one for linear damping and one for quadratic. The details of how they are applied
are given in Other Damping Theory.
Note: The vessel type Symmetry convention affects how the quadratic coefficients are used. If the
symmetry is set to Circular, then the coefficients are used with a cross-flow drag model, with a
vertical axis. For other symmetry settings, the quadratic coefficients are used with a drag model
that treats each direction of motion independently.
For details of the damping force model see Other Damping Theory.
Current and Wind Loads
Current and wind drag loads on a vessel are loads due to the relative velocity of the fluid past the vessel. They can be
modelled using the data on the Current Load and Wind Load pages on the vessel type data form. If the length of the
vessel differs from that of the vessel type then the vessel type data will be scaled accordingly.
These loads are an important source of damping when modelling vessel slow drift. For a discussion of the various
damping sources see Damping Effects on Vessel Slow Drift.
The velocity used to calculate the drag loads is the relative low-frequency velocity of the fluid past the vessel. This
includes any current or wind velocity and the vessel velocity due to any low-frequency primary motion. The drag
forces and moments due to translational motion are modelled using the standard OCIMF method. The drag forces
and moments due to any vessel rate of yaw are modelled using yaw rate drag load factors. For details of how the
loads are calculated, see Vessel Theory: Current and Wind Loads.
Warning: The current and wind loads are based on theory for surface vessels and are not suitable for
submerged vessels.