OrcaFlex Manual - Orcina

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System Modelling: Data and Results, Vessels
One common situation where the wave drift loads can matter is with a moored vessel. The vessel's natural
frequencies in surge, sway and yaw are typically quite low and so the low frequency wave drift loads can generate
large slow drift excursions in these directions.
Options for Modelling Slow Drift
In OrcaFlex you can model slow drift motion in broadly 2 ways. First, you can calculate the vessel slow drift motion
outside OrcaFlex and then impose that motion on the vessel. This can either be done using time history or
externally-calculated options for primary motion, or done using the time history or harmonic motion options for
superimposed motion. Wave frequency motion can also then be superimposed on top of that slow drift motion, by
using displacement RAOs or a time history superimposed motion.
Alternatively OrcaFlex can calculate and apply the slow drift motion for you. To do this you need to do the following:
� On the vessel form, select 6 DOF for the static analysis. The OrcaFlex static analysis will then calculate the
equilibrium position allowing for the mean wave drift load. And set the primary motion to Calculated (6 DOF).
The OrcaFlex simulation will then calculate the vessel's resulting dynamic motion.
� On the Structure page on the vessel type form, specify the vessel centre of gravity, mass and moments of inertia
data for the appropriate draught. And on the Stiffness, Added Mass and Damping page, specify the stiffness and
hydrostatic equilibrium position, added mass and damping matrices and the reference origin to which they
apply. And include Added Mass and Damping in the vessel's Included Effects. (The hydrostatic stiffness is
always included.)
� Specify QTF data on the wave drift page of the vessel type form (the wave drift loads are calculated based on
this data), and include Wave Drift Load (2 nd Order) in the vessel's Included Effects. This tells OrcaFlex to apply
the mean wave drift load to the vessel during the static analysis, and then in the dynamic analysis to apply the
mean and time varying wave drift load.
� Optionally, include wave drift damping in the vessel's Included Effects. OrcaFlex will then include the damping
effect due to the way the wave drift load varies with vessel low frequency velocity and with current.
� Optionally, include Manoeuvring Load in the vessel's Included Effects. OrcaFlex will then include the low
frequency 2 nd order potential theory manoeuvring load in the analysis.
� Optionally (usually needed), include Current Load in the vessel's Included Effects and specify appropriate data
for current load and yaw rate drag.
� Optionally, include wind load on vessels (on Wind page of the environment data form) and include Wind Load
in this vessel's Included Effects. And then specify the wind data on the environment data form, and wind load
data on the vessel type data form.
� Optionally (e.g. to model thruster loads), include Applied Load in the vessel's Included Effects and specify
appropriate applied load data.
� OrcaFlex will automatically include loads from any lines or other objects that are connected to the vessel.
� If you have wave load RAO data available, then we recommend that you specify that data in the vessel type's
Load RAOs and include Wave Load (1 st order) in the vessel's Included Effects. Then set the vessel
superimposed motion to None, so that the first order vessel motion is fully calculated and takes into account
coupling effects between the wave frequency and low frequency response.
� If you do not have the wave load RAO data, then you should not include Wave Load (1 st order) in the vessel's
Included Effects, and you can instead instead model the wave frequency response using displacement RAOs, by
setting the vessel's superimposed motion to Displacement RAOs + Harmonic Motion but with no harmonic
motion specified (on the Superimposed Motion page of the vessel data form). This will superimpose the wavefrequency
motion, defined by the displacement RAOs, on top of the calculated low-frequency primary motion.
This method will not include coupling effects between the two parts of the motion. Note also that this
combination of calculated and superimposed motion is not compatible with implicit integration: in this case you
will have to use explicit integration.
Finally, you should set Primary Motion is Treated As to either Low frequency or to Both low and wave frequency.
The former is appropriate if you are using superimposed displacement RAOs to model the wave frequency motion.
But if all the motion is being modelled as primary motion, e.g. using wave load RAOs as the excitation for this
motion, then you should treat the primary motion as Both low and wave frequency and specify a suitable Dividing
Period for OrcaFlex to use to filter the primary motion into its low and wave frequency parts. See Vessel Modelling
Overview for further information.