OrcaFlex Manual - Orcina

System Modelling: Data and Results, Vessels
Checking RAOs
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The Check RAOs button on the vessel types form allows a visual check on the RAO data for either displacement
RAOs or wave load RAOs. For a given draught and wave direction, it displays graphs (one for each vessel degree of
freedom) showing how the RAO and phase vary with wave period.
There are 3 types of graph available:
� Complex Values
� Amplitude
� Phase
Amplitude and Phase Graphs
These graphs provide a straightforward graphical representation of the RAO data as input on the Vessel Types data
form. The amplitude or phase is plotted on the Y axis of the graph. For the X axis you have the choice of plotting
period, frequency in rad/s or frequency in Hz.
Complex Value Graphs
The graphs initially show the RAOs for the currently selected draught and direction. You can switch to other
draughts and directions, either by using the navigation buttons at the bottom of the form to step through the data,
else or by selecting from the drop-down lists. You can change the scale of the graphs (double click on the graph and
change the ranges of the axes). This is useful if the curve does not initially fit on the graph.
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Figure: Complex Value RAO Graph for Amplitude (R) and phase (φ)
The graph depicts the RAO data specified by the user for the specified RAO origin. The graph has two parts:
� A curve showing the RAO data specified by the user as a series of points joined in order of increasing period.
The curve starts from the 'short' wave response, which should have zero or very small amplitude, so the curve
should start from near the origin. Moving along the curve away from the origin corresponds to the wave period
increasing from zero. For surge, sway and heave, the other end of the curve is the 'long' wave RAO data specified
for period 'Infinity'. For roll, pitch and yaw, the RAO data for period 'Infinity' cannot (for technical reasons) be
included in the curve, so instead the other end of the curve is the RAO data for the largest finite period specified.
� A solid circle representing the expected long wave response limit for a freely floating vessel. See RAO Quality
Checks for details of the expected long wave RAOs.
Warning: The expected long wave response limits calculated by OrcaFlex only apply to free-floating vessels.
Also, the yaw response limit only applies to slender vessels (i.e. vessels that are long in the xdirection
and narrow in the y-direction).
The purpose of the graph is help you check your RAO data – the curve should normally be reasonably smooth and
tend towards the expected limit shown by the solid circle. See How to Check RAO Data for details.
The graph represents RAOs as points in polar coordinates (R,φ), where: