OrcaFlex Manual - Orcina

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Theory, Vessel Theory
Notes: In these tables, the translational amplitudes are non-dimensionalised against wave amplitude and
the rotational amplitudes are non-dimensionalised against maximum wave slope.
The phases given are lags relative to the wave crest so that +90° means that the maximum positive
motion occurs 90° after the wave crest passes the vessel. In these tables we use the conventions of
positive surge is forward, positive sway is to port, positive heave is up, positive roll is starboard
down, positive pitch is bow down and positive yaw is bow to port.
When the amplitude is zero the phase value is irrelevant; this is indicated in the tables by '~' .
You can check RAOs in two ways. First, OrcaFlex provides RAO graphs that help spot errors, see Checking RAOs.
Second, you can run quick simulations with only the vessel in the model and then check that the motions you see are
sensible.
Consider a ship in waves coming from ahead. Set up a simple OrcaFlex model with the vessel only – nothing else –
set the vessel's primary motion to None, secondary motion to RAOs + Harmonic, and run a short simulation (say 10
seconds build up plus 2 wave periods). Use a large wave height (20m) and long time step – say 0.05 seconds for both
inner and outer time steps. When the run is finished (a few seconds only for such a trivial case) replay the last wave
period and watch to see whether the motion of the ship is realistic. The best view direction is horizontal, normal to
the direction of travel of the waves. With the waves coming from the right on screen, then in the wave crest the ship
should be at maximum heave up and moving to the left, and vice versa in the trough. At the point of maximum wave
slope as the crest approaches, the ship should be at maximum surge forwards into the wave and maximum pitch
angle with the bow up. If the phase convention has been misunderstood (e.g. leads have been read as lags) then the
motion is obviously wrong and you should go back and re-examine the data, or confirm your interpretation with the
data source.
This is an excellent check for phases, which are usually the most troublesome to get right. It is not quite so good for
amplitudes, but it is nevertheless worth pursuing. If the wave is very long compared to the ship, then the ship should
move like a small particle in the water surface. Heave amplitude should be equal to wave amplitude and pitch
motion should keep the deck of the ship parallel to the water surface. Surge amplitude should also be equal to wave
amplitude in deep water, but will be greater in shallow water in which the wave particle orbits are elliptical.
The check can be extended to other wave directions. Broadly speaking, we may expect the motion to be
predominantly in the wave direction, with the phasing of surge and sway such that the components in the wave
direction reinforce each other. Similarly, roll and pitch phasing should be such that the components of rotation
about an axis normal to the wave direction reinforce each other. Yaw phasing for a ship in seas off the bow should
be such that the ship yaws towards the broadside on position as the wave crest passes: this is easiest to see in a
near-plan view. Generally speaking, if it looks right in long waves, it probably is right. If not, then think again!
Wave Load RAOs
Since we can relate wave load RAOs to displacement RAOs, we can similarly determine the long-wave limit for wave
load RAOs. OrcaFlex does this for the RAO graphs to facilitate checking wave load RAOs.
The simple model-building exercise described above also works well for wave load RAOS: just set primary motion to
Calculated (6 DOF), superimposed motion to None.
Note: You may need to run a longer simulation, with smaller time steps, for calculated vessel motion than
for displacement RAOs, to allow the model to 'settle down'.
5.11.4 Current and Wind Loads
These loads are an important source of damping when modelling vessel slow drift. For a discussion of the various
damping sources see Damping Effects on Vessel Slow Drift.
The current and wind drag loads on a vessel are calculated using the data specified on the Current and Wind Load
pages on the vessel type data form. The loads are split into those due to translational relative velocity and yaw rate.
Drag Loads due to Translational Relative Velocity
The drag loads due to translational velocity of the sea and air past the vessel are calculated using the standard
OCIMF method, which is outlined below. For further details see Oil Companies International Marine Forum, 1994.
The OCIMF method calculates the surge, sway and yaw drag loads on a stationary vessel. OrcaFlex extends this to a
moving vessel by replacing the current (or wind) velocity used in the OCIMF method with the relative translational
velocity of the current (or wind) past the vessel.