OrcaFlex Manual - Orcina

System Modelling: Data and Results, 6D Buoys
Hydrodynamic Loads
See the Added Mass and Damping and Drag pages on the buoy data form.
6.9.9 Spar Buoy and Towed Fish Drag & Slam
Munk Moment Coefficient
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Slender bodies in near-axial flow experience a destabilising moment called the Munk moment. This effect can be
modelled by specifying a non-zero Munk moment coefficient.
Normal drag area calculated from geometry
If this option is checked then the normal drag area, for each cylinder, is calculated directly from the cylinder
geometry by multiplying the outer diameter by the cylinder length.
If this option is not checked then the normal drag area, for each cylinder, must be specified by the user.
Drag Forces and Moments
Drag loads are the hydrodynamic loads that are proportional to the square of fluid velocity relative to the cylinder.
For details of the drag load formulae see Spar Buoy and Towed Fish Theory. For information when modelling a
SPAR or CALM buoy see Modelling a Surface-Piercing Buoy.
The drag forces are calculated on each cylinder using the "cross flow" assumption. That is, the relative velocity of the
sea past the cylinder is split into its normal and axial components and these components are used, together with the
specified drag areas and coefficients, to calculate the normal and axial components of the drag force.
The drag forces are specified by giving separate Drag Area and Drag Coefficient values for flow in the normal
direction (local x and y directions) and in the axial direction (local z direction). The Drag Area is a reference area
that is multiplied by the Drag Coefficient in the drag force formula. You can therefore use any positive Drag Area that
suits your need, but you then need to give a Drag Coefficient that corresponds to that specified reference area.
The Drag moments are specified and calculated in a similar way to the drag forces, except that the reference drag
area is replaced by a reference Area Moment. This and the Drag Coefficient are multiplied together in the drag
moment formula, so again you can use any positive Area Moment that suits your need, providing you then specify a
Drag Coefficient that corresponds to the specified Area Moment.
There are two alternative methods that you can adopt when specifying the drag data. The first method is to set the
OrcaFlex data to get best possible match with real measured results for the buoy (e.g. from model tests or full scale
measurements). This is the most accurate method, and we recommend it for CALM and discus buoys – see Modelling
a Surface-Piercing Buoy for details. Because the Drag Area and Drag Coefficient data are simply multiplied together,
you can calibrate the model to the real results by fixing one of these two data items and then adjusting the other. For
example, you could set the axial Drag Coefficient to 1 and adjust the axial Drag Area until the heave response decay
rate in the OrcaFlex model best matches the model test results. Or, you could set the axial Drag Area to a fixed value
(e.g. 1 or some appropriate reference area) and then adjust the axial Drag Coefficient until the heave response decay
rate in OrcaFlex best matches the model test results.
The second method is to set the drag data using theoretical values or given in the literature. It is less accurate but
can be used if you cannot get any real buoy results against which to calibrate. To use this method, set the data as
follows.
Set the Drag Areas to the projected surface area that is exposed to drag in that direction and then set the Drag Force
Coefficients based on values given in the literature (see Barltrop & Adams, 1991, Hoerner,1965 and DNV-RP-C205).
Note that the drag area specified should be the total projected area exposed to drag when the buoy is fully
submerged, since OrcaFlex allows for the proportion wet in the drag force formula. For a simple cylinder of
diameter D and length L the total projected drag area is D.L for the normal direction and (π.D 2 )/4 for the axial
direction, but if the buoy has attachments that will experience drag then their areas must also be included.
Set the Drag Area Moments to the 3rd absolute moments of projected area exposed to drag in the direction
concerned; see Drag Area Moments for details. And then set the Drag Moment Coefficients based on values given in
the literature.
Slam Force Coefficients
The slam force, as the buoy enters or exits the water through the surface, can be modelled by specifying non-zero
Slam Coefficients. Separate coefficients are specified for water entry and water exit. For spar buoys and towed fish
the slam area is not specified by the user – it is set to the instantaneous water plane area. If the Slam Coeffcient is