The format of the LNDW data record is the same as that for a LINE mooring line but with three additional parameters.
2 5 7 11 16 21 26 31 41 51 61 71 81
- --- -- ---- ----- ----- ----- ----- --------- --------- --------- --------- ---------
X| | |LNDW| | | | | | | | | |
- --- -- ---- ----- ----- ----- ----- --------- --------- --------- --------- ---------
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| | | | | | | | | | | |_(10)T_Max. Maximum Tension (F10.0)
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| | | | | | | | | | |_(9) Speed. Non-zero positive
| | | | | | | | | | or negative winch speed (F10.0)
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| | | | | | | | | |_(8) L_End. Final Length (F10.0)
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| | | | | | | | |_(7) L_Beg. Initial Length (F10.0)
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| | | | | | | |_(6) K_Beg. Initial Stiffness (E10.0)
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| | | | | | |_(5) 2nd Node Number (I5)
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| | | | | |_(4) 2nd Structure Number (I5)
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| | | | |_(3) 1st Node Number (I5)
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| | | |_(2) 1st Structure Number (I5)
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| | |_(1) Compulsory Data Record Keyword (A4)
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| |_Optional User Identifier (A2)
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|_Compulsory END on last data record in Data Category (A3)
(6) K_Beg: stiffness at the beginning of the winch action.
This is equal to EA/L_Beg for the line (E=Youngs' modulus, A = 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 L_Beg to L_End, will be EA/L_Beg to EA/L_End; where EA = K0*L_Beg.
(7) L_Beg: unstretched length at the beginning of the winching action.
This can also be considered as the initial length and will be the length used if there is no winch action. This value must greater than 0.1m (in S.I. units) or 0.3ft (in Imperial units). Values less than this will be set to the minimum values.
(8) L_End: unstretched length at the end of the winch action.
If the drum speed is positive (i.e. paying out) this is the maximum length of the line and can at no time be exceeded. If this length is reached, or L_Beg is greater than L_End, then all winching action will cease for the rest of the simulation.
If the drum speed is negative (i.e. winding in) this is the minimum length of the line and it can at no time be less than this. If this length is reached, or L_Beg is less than L_End, all winching action will cease for the rest of the simulation.
(9) Speed: paying out (positive speed) or winding in (negative) speed.
In mathematical terms the speed is dl/dt, where l is the unstretched length of line and t is time. For lines which have significant strain(ε) (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 (1+ε). 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 1/(1+ε), where ε 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 (1+ε/2). 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 1/(1+ε/2), where ε is the average strain.
(10) T_Max:The maximum tension causing the winch to lock.
If at any time the tension exceeds this value, all winching action of the drum will cease until the tension reduces below this value.
CAVEATS/ASSUMPTIONS
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 time steps.
Drum winches are not available with PULY lines.