OrcaFlex Manual - Orcina

VIV Toolbox, Time Domain Models
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modes) or axial (the tensile modes). It is therefore clear cut as to which modes to export to SHEAR7, i.e. the
transverse modes, and OrcaFlex selects these for you.
U-shape Catenary, In-plane Current
For this case the transverse direction is the out-of-plane direction, so the transverse modes are the out-of-plane
modes. These typically have virtually 100% of their power in the transverse direction, whereas the remaining
modes have very little power in the transverse direction, so again it is clear cut as to which modes to export to
SHEAR7.
U-shape Catenary, Out-of-plane Current
For this case the transverse direction is in-plane and normal to the line axis. It therefore varies along the line and so
the transverse modes are some, but not all, of the in-plane modes.
The lowest in-plane mode is typically the in-plane fundamental 'swinging' mode. In the parts of the line that are
nearly vertical this mode is transverse, but near the bottom of the U the motion is near axial. This mode is therefore
often displayed as Mostly Transverse. OrcaFlex removes the axial components of the modes when exporting to
SHEAR7 (see Values Exported) so it is reasonable to export this mode.
Most of the remaining in-plane modes are bending modes in which the nodes oscillate laterally, with the wavelength
decreasing as the frequency increases. These are predominantly in the transverse direction and so are suitable for
export to SHEAR7.
However there are also some tensile in-plane modes present, in which the nodes oscillate in the axial direction,
causing alternating tension and compression in the line. These tend to be in amongst the higher frequency modes,
due to the typically high axial stiffness of a line.
U-shape Catenary, Oblique Current
If the current is at 10°, say, to the plane of the catenary, then the transverse direction is at 80° to the plane. None of
the modes will be purely in this direction, but the out-of-plane modes are nearest to this direction, so they are the
best ones to choose.
SHEAR7 will assume that each exported mode is purely transverse, so an approximation is involved. This
approximation gets worse as the angle of the current to the plane increases up to 45°. The approximation is worst
for the low modes. For the higher modes the out-of-plane modes and the in-plane lateral modes tend to have quite
similar frequencies and shapes, so the approximation is less of a problem.
9.2 TIME DOMAIN MODELS
There are four time domain models, two being wake oscillator models and two being vortex tracking models.
With all the time domain models, the vortex force applied in the static analysis is the standard Morison drag force.
Then, during the build-up stage of the simulation, the ramping function is used to smoothly change to the vortex
force given by the VIV model.
Note: This ramping is only applied for the components of vortex force which are calculated by the VIV
model. For example, the wake oscillator models only provide transverse vortex force. So, for the
wake oscillators, the ramping is done for the transverse component of force, but the inline
component of force is calculated using the standard Morison drag formulation.
The data described below are common to all the time domain models.
Outer Time Step
For all the time domain models, it is important that the outer time step (on the General Data form) is set to a value
that is small compared with the Strouhal Period. Assuming a Strouhal number of 0.2, then the Strouhal period is
given by 5D/V where D is the line diameter and V is the relative flow velocity. The outer time step needs to be set to
a fraction of this Strouhal period, the fraction to use depending on which model is being used, as follows.
Wake oscillator models
The wake oscillator calculations are done every outer time step and experience so far suggests that the integration
of the wake oscillator loses accuracy if this time step is greater than about 1/200th of the Strouhal period. At the
start of the simulation OrcaFlex checks and warns if the outer time step exceeds this limit at any node on lines that
use a wake oscillator. Note that this check is against the Strouhal period for the flow velocity that applies in the
static analysis, so it does not take into account changes in Strouhal period during the simulation.