OrcaFlex Manual - Orcina

System Modelling: Data and Results, Winches
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� If the duty is such that the winch drive will give near to perfect constant tension or constant speed performance.
� If the winch drive data are not available.
Winch Control
OrcaFlex winches allow quite complex offshore operations to be modelled. The winch drive can be operated in
either of two modes:
Length Control Mode
For modelling constant speed winches. The winch wire is paid out or hauled in at a velocity specified in the data.
Force Control Mode
For modelling tension controlled winches. Since such winches are usually hydraulic devices whose performance
deviates quite seriously from the target tension ideal, OrcaFlex Winches provides facilities for modelling winch
deadband, damping and drag forces (force decrements proportional to velocity and velocity 2 respectively) and
winch stiffness effects such as those caused by hydraulic accumulators.
The winch can be switched between these two modes at predetermined times during the simulation and the
constant velocity or target tension can also be varied.
6.11.1 Data
Name
Used to refer to the Winch.
Type
May be either Simple or Detailed. See Winches.
Connect to Object and Object Relative Position
The (mass-less) winch wire connects at least two objects, one at each end of the winch wire.
If more than 2 are specified then the winch wire passes from the first connection point to the last via the
intermediate points specified. When intermediate connections are specified, the winch wire slides freely through
these intermediate points as if passing via small friction-less pulleys mounted there. The winch wire tension on
either side then pulls on the intermediate points, so applying forces and moments (if the points are offset) to the
objects concerned.
Each connection is defined by specifying the object connected and the object-relative position of the connection
point.
For connecting to a Line, the object-relative z coordinate specifies the arc length to the connection point. The z
coordinate specifies the arc length along the Line and this arc length may be measured relative to either End A or
End B as specified by the user. The connection point is attached to the nearest node. If torsion is not modelled then
the x,y coordinates are ignored and the connection point is at the centreline of the Line. If torsion is modelled then
the x,y coordinates allow you to offset the connection from the centreline.
For Fixed connections the object-relative coordinates given are the global coordinates of the point.
For connecting to an Anchor, the object-relative x,y coordinates given are the global X,Y coordinates of the anchor
point, and the z-coordinate is the distance of the anchor above (positive) or below (negative) the seabed at that X,Y
position.
For connecting to other objects, the coordinates of the connection point are given relative to the object local frame
of reference.
Release at Start of Stage
The winch wire can be released at the start of a given stage of the simulation, by setting this number to the stage
number required. Once released the winch no longer applies any forces to the objects it connects. If no release is
required, then set this item to '~'.
6.11.2 Wire Properties
Wire Stiffness
The elastic stiffness, K, of the winch wire. The winch tension contribution from wire stiffness is given by