I am new to ANSYS AQWA. I found out that the undamped natural periods (without the mooring cables) is shown in .LIS file. The structure is actually a TLP floater for wind turbine (ref. attached file).
Q.1> Why is the natural period of the system is shown zero always in surge, sway and yaw ? The depth of water for my model is 200m. The model is 32m deep inside water and 10m above water line. I am using non-linear steel wire as the mooring cable.
Q.2> I don't understand how to decide the values for unstretched length, asymptotic stiffness, asymptotic offset. Where can I find more details about definition of these terms ?
Based on time history motion simulation I found out arbitrarily if my unstretched length is around (200-32) = 168m with asymptotic stiffness = 10e6 N/m and asymptotic offset = 1m the motion is not completely haphazard but I get output info as "solution diverged after certain time" ( ref. attached file ). If I try to increase the stiffness I get time step error ( ref. attached file ) so I decrease the time step.
The user manual says : T = k (x - d (tanh(x/d))) where T = Tension in mooring line. x = extension of mooring line k = asymptotic stiffness (constant) d = asymptotic offset (constant) The names of the constants k and d arise from the fact that, at large values of extension, tanh(x/d) tends to unity and the equation tends to the asymptotic form: T = k (x - d). What do these term mean actually ?
Kindly help.
please find: OTC Paper Number 18287 Coupled Dynamic Modeling of Floating Wind Turbine Systems of E.N. Wayman and P.D. Sclavounos, Massachusetts Institute of Technology; S. Butterfield, J. Jonkman, and W. Musial, National Renewable Energy Laboratory. I believe that will solve many of your questions.
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