OrcaFlex Manual - Orcina

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Theory, Shape Theory
Finally, if explicit integration is used then a reaction damping force D is also applied when the object is travelling
into the solid. This damping force is in the same outwards direction, and is given by:
where
D = 2λ(MKA) ½ Vn
λ = percentage of critical damping / 100
M = mass of the penetrating object
Vn = component of object velocity in direction into the solid.
The damping force is only applied when the object is travelling into the shape (i.e. when Vn is positive).
Trapped Water
Inside a trapped water shape the fluid motion is modified as follows:
� The fluid translational velocity and acceleration are calculated on the assumption that the trapped water moves
and rotates with the shape. So if the trapped water shape is Fixed or Anchored then no fluid motion occurs
inside the shape. But if the shape is connected to a moving vessel, for example, then the trapped water is
assumed to move and rotate with the vessel.
� The fluid angular velocity and acceleration of the local water isobar are both taken to be zero. (These angular
motions are only used for calculating moments on 6D buoys.)
Notes: If the shape intersects the water surface then the surface is assumed to pass through the shape
unaltered. Thus a wave in the open sea also appears inside the shape. We make this assumption
because of the difficulty in predicting, for realistic cases, how the surface will behave inside the
trapped water volume.
For example, a moonpool with an open connection at the bottom will suppress most of the wave
and current action. However there will be some flow in and out of the moonpool, depending on the
size of the opening to the sea, pressure difference effects and the local geometry. The surface
elevation in the moonpool therefore does respond to the wave outside, but it is attenuated to some
extent and lags behind the surface outside.