OrcaFlex Manual - Orcina

Theory, Winch Theory
where
CdX and AX are the Drag Coefficient and Drag Area, respectively, for the X direction
Vr is the velocity vector of the fluid relative to the buoy and |Vr| is its absolute magnitude
VrX is the component of Vr in the X direction.
Fluid Inertia Effects
Fluid acceleration force = (1+Ca) . ρ . PW . Volume . A
in each of the global axes directions, where
Ca is the Added Mass Coefficient for that direction
A is the acceleration of the fluid in that direction.
This force is often considered as being made up of two parts:
1. The Froude-Krylov force = ρ . PW . Volume . A
2. The added inertia force = Ca . ρ . PW . Volume . A.
In addition, the inertia of the buoy in each of the global axes directions is increased by:
where
Added mass = ρ . Ca . PW . Volume
Ca is the Added Mass Coefficient for that direction.
Contact Forces
206
w
Finally, 3D Buoys are also subjected to a reaction force from the seabed and any elastic solid with which they come
into contact, given by:
where
Reaction = KAd
K is the Stiffness of the seabed or elastic solid
A is the contact area
d is the depth of penetration of the buoy origin B.
In addition to this reaction force, 3D Buoys receive a contact damping force. For details see Seabed Theory and
Solids Theory.
Finally, friction forces can also be included.
5.15 WINCH THEORY
Static Analysis
If the Statics winch control mode is set to Specified Length then for the static analysis the unstretched length of
wire paid out, L0, is set to the Value specified, and the wire tension t and winch drive force f are both set to equal:
where:
t = f = K . ε (1)
ε = Wire Strain = [ L - L0 ] / L0
K = Wire Stiffness data value, specified on the winch data form
L = total length of the winch wire path
L0 = unstretched length of winch wire paid out
Alternatively, if the Statics winch control mode is set to Specified Tension, then for the static analysis the winch
drive force f and the wire tension t are both set to the Value specified, and the unstretched length paid out, L0, is
then set to match this wire tension according to equation (1) above.