Two types of winch can be applied to a cable,
a winch that can wind in or pay out as required to maintain a constant tension in the cable
a winch that can wind in or pay out at a given rate to lengthen or shorten the line
Note: In a Hydrodynamic Response System where the Computation Type is set to Stability Analysis or Frequency Statistical Analysis, only the first of these options is available.
To add a Cable Winch:
Select the Hydrodynamic Response object in the tree view.
Right-click on the Hydrodynamic Response object and select Insert > Cable Winch, or click on the Cable Winch icon in the Analysis toolbar.
Select the Cable Winch object in the tree and configure it.
Cable winches that maintain constant tension can only be used with Linear Elastic or Nonlinear Polynomial cables.
In the case of a constant tension winch, friction coefficients
during the winding in (
) or paying out (
) process are required;
hence the tension in the winch mooring line when winding-in is given
by 
where 
is the winch
tension specified. When paying-out, the winch tension is given by 
. For example, if the tension specified is 1000 tonnes
and 
and 
are 0.3 and 0.1 respectively, then the
tensions will be 700 and 1100 tonnes respectively.
The initial tension is undefined. The default initial tension is the winding-in tension i.e. 700 tonnes in the example above. The winding-in friction coefficient should be specified as negative if the paying-out value of tension is required as the initial tension.
The tension will be varied according to whether the range (distance between the anchor and vessel attachment point) is increasing or decreasing. If the range is less than the initial length specified the line becomes slack and the tension is zero.
To set up the constant tension Cable Winch object, click on the object. In the Details panel:
Select from the Cable Type dropdown list
Select the cable to associate with the winch from the Cable Selection dropdown
Enter the value of the Winding In Friction
Enter the value of the Paying Out Friction
Enter the desired Winch Tension value
Cable Winches that change the cable length can only be used
with Linear Elastic (without Pulleys). In the case when the winch
changes the cable length, it can be made to start at the given speed
after the start time, and pauses if the maximum tension is exceeded.
The winch will stop paying out or winding in when the final length
is reached. An additional length, 
, can be entered to affect
the initial stiffness of the cable and the maximum stiffness (
).
The initial stiffness of the line, 
is used, which is equal
to 
for
the line (
= Young’s modulus, 
= cross
sectional area). The stiffness of the line during the winch action
will change as the length changes; i.e. the stiffness during the simulation,
assuming that the length varies from 
to the final length, 
, will be 
to 
; where 
.
The speed entered is positive for paying out and negative for
winding in. In mathematical terms the speed is 
, where 
is the unstretched length of line
and 
is time. For lines which have
significant strain (
) (this precludes steel which yields
at about 0.001), the user may wish to consider the speed of the drum
in terms of stretched length.
When winding in, the line wound onto the drum will have the
same tension as the free line itself at any particular time. This
means that in order to wind a length of unstretched line the effective
speed must be increased by a factor of 
. This is done automatically.
If the user wishes to simulate a stretched line speed for winding
in, then the speed specified should be input with a speed reduction
factor of 
, where 
is the average strain.
When paying out, the adjustment of speed is not straightforward.
The elastic energy of the line on the drum will depend on exactly
how the line was wound on the drum originally. This 'energy' stored
on the drum is unknown and is assumed to be zero. i.e. the line on
the drum when paying out is assumed to be unstretched. The effective
winch speed, (effectively with only 1 side of the line stretched)
is 
. If the user wishes to simulate
a stretched line speed for paying out, then the speed specified should
be input with a reduction factor of 
, where 
is
the average strain.
To set up the changing cable length Cable Winch object, click on the object. In the Details panel:
Select from the Cable Type dropdown list
Select the cable to associate with the winch from the Cable Selection dropdown
Enter the value of the Start Time
Enter the cable's Final Length at which winching will stop
Enter the value of the Speed of the winch
Enter the Max Tension at which winching will pause
Enter the Additional Length which affects the initial stiffness of the cable
Note:
The exact length of the line at any time will depend on the previous motions which have been encountered by the structure connected to this line. This in turn means that the length of the line has 'memory'. The implication of this is that in situations where initial or specific positions are used in Aqwa, the line length cannot be determined and will be assumed to be the initial length. An example of this is the hot start. A warning message to this effect will be issued in these cases.
The resolution of switching on and off the drum winch can only be the same as the time step. This means that the winch drum can only be switched at the beginning or the end of a time step and NOT in the middle. In order to conserve energy/momentum in the equations of motion, the length of the line can only be changed in steps of (time step)*speed of the winch. The tension resolution will therefore be stiffness*time step*speed. Large stiffnesses or drum speed should therefore be specified with appropriately small timesteps.
If the unstretched length of the cable is less than 0.1m, it will be defaulted to 0.1m.