OrcaFlex Manual - Orcina

VIV Toolbox, Time Domain Models
Filter RAO Amplitude
1.2
1
0.8
0.6
0.4
0.2
0
0 1 2 3 4 5
Period / FilterPeriod
462
Filter RAO Phase Lag (deg)
160
140
120
100
80
60
40
20
0
w
0 1 2 3 4 5
Period / FilterPeriod
Warning: Note also that with the Milan wake oscillator, if you set the filter period to Infinity or a very high
value, then the node mean position used by the wake oscillator will remain at the node's starting
position found by the static analysis. In that case it is important that the line uses Full statics, since
otherwise that starting position might not be a sensible mean position for the Milan wake
oscillator model to use.
Data for Each Section
The following data can be set for each section of the line. A node at the junction between two sections uses the value
for the higher numbered section.
VIV Diameter
The VIV diameter specifies the diameter used by the VIV model. Separate values can be specified for each section.
The value specified is used for all nodes in that section. For a node at the intersection of two sections the VIV
diameter of the following section is used. The VIV Diameter can be set to '~', which is taken to mean 'same as the
section normal drag diameter'.
VIV Enabled
You can control which sections of the line the VIV model is applied to. All the time domain models analyse what goes
on at a single specified point on the line, so OrcaFlex creates one instance of the model for each node in each of the
enabled sections.
You can use this switch to disable VIV modelling where you think VIV excitation is not significant. For example:
� You might disable VIV modelling if the normal component of flow velocity is not significant, or the incidence
angle is near to being tangential
� You might disable VIV modelling if there are VIV suppression devices (e.g. strakes, fairings) and you believe they
will be effective.
� The vortex tracking model requires a lot of computing, so you could improve the speed of simulation by
disabling VIV modelling where you believe VIV will not be significant.
Inline Drag Amplification Factor
Only available for the wake oscillator models. The inline component of drag is multiplied by this value. The data can
be specified as a variable data item which varies with Transverse A/D, that is the amplitude of transverse oscillation
divided by the VIV diameter.
This is not available for the vortex tracking models since they include the effect of inline drag amplification.
Vortex Force Factors
These factors allow you to scale the inline and transverse components of the vortex force. Set them both to 1 to use
the vortex force predicted by the VIV model, without adjustment.
If VIV suppression devices (e.g. strakes, fairings, shrouds) are fitted to a section of the line then you could allow for
their VIV-reduction effect by setting the transverse force factor to a value less than 1. The device is also likely to