OrcaFlex Manual - Orcina

System Modelling: Data and Results, 3D Buoys
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Set the Drag Force Coefficient based on values given in the literature. For short simple cylinders fully immersed
there are standard values given in the literature (see Barltrop & Adams, 1991, Hoerner,1965 and DNV-RP-C205).
However, the standard book values do not include energy absorption by wave-making at the free surface. Strictly,
this is a linear term (forces directly proportional to velocity), but in OrcaFlex this must be done by adjusting the
drag coefficients of one or more cylinders.
The Unit Damping Force data can be set to zero. If you later find that the buoy shows persistent small amplitude
oscillations then you may wish to set a non-zero value to damp this out.
Set the Drag Area Moments, Drag Moment Coefficients and Unit Damping Moment data. For the normal direction
these data items can usually all be left as zero, providing you have subdivided the buoy into short enough cylinders
(since these terms involve a high power of L, the cylinder length). For the axial direction these data items model the
yaw drag and damping effects, so if this is important to you then set them to model the two main sources, namely
skin friction on the cylinder surface and form drag on any protuberances on the buoy.
Having set up this drag and damping data, it is well worth now running simulations of heave and pitch oscillations
and checking that their rate of decay is reasonable and consistent with any real data you have available.
Discus and CALM Buoys
These types of buoy require different treatment since they have little axial extension. Instead it is their radial
extension that most affects the buoy's pitch properties. As a result the axial discretisation of the buoy into cylinders
does not capture the important effects. For example the pitch damping is often mostly due to radiation damping, i.e.
surface wave generation; this is especially important for a CALM buoy with a skirt.
To deal with this OrcaFlex offers the rotational drag and damping data, but there is little information in the
literature to help in setting up this data. We therefore strongly recommend that you set the data up by calibration
against real test results from model or full scale tests. The easiest information to work with are time history graphs
of the buoy heave and pitch in still water, starting from a displaced position. This will give the heave and pitch
natural periods and the rates of decay and you can adjust the buoy's drag and damping data until you get a good
match with this measured behaviour.
Here is the approach we use:
� For the normal direction, set the Drag Area, Drag Force Coefficient and Unit Damping Force as described for
Spar buoys above.
� Then set the axial Unit Damping Force to zero and run a simulation that matches the conditions that existed in
the real heave time history results, i.e. with the same initial Z displacement.
� Then adjust the axial Drag Area and Drag Force Coefficients until the OrcaFlex buoy's Z time history matches the
real time history. These two data items are simply multiplied together when they are used to calculate the drag
force, so you can give one of the two data items a fixed positive value (e.g. 1) and then adjust the other.
� The match will probably be poor in the later parts of the time history, where the heave amplitude has decayed
to small values. This is because the square law drag term is insignificant at small amplitude and instead the
damping force takes over. Therefore we now adjust the axial Unit Damping Force to further improve the match
where the amplitude is small. You may find that this disturbs the match in the large amplitude part, in which
case you might need to readjust the drag data.
� For the axial direction, set the Drag Area Moment, Drag Moment Coefficient and Unit Damping Moment as
described for Spar buoys above.
� Then set the normal Drag Area Moment, Drag Moment Coefficient and Unit Damping Moment to best match the
real pitch time history, in a similar way to that used above to match the heave time history.
6.10 3D BUOYS
OrcaFlex 3D Buoys are simplified point elements with only 3 degrees of freedom: X, Y and Z. They do not rotate, but
remain aligned with the global axes. They therefore do not have rotational properties and moments on the buoy are
ignored. They should therefore be used only where these limitations are unimportant.
3D Buoys are able to float part-submerged at the surface, and may also be used independently, with no lines
attached. Although they are much less sophisticated than 6D Buoys, 3D Buoys are easier to use and are convenient
for modelling buoys at line junctions etc.