Learning Forum

[mdti]

Hi,
I am working with the hydrodynamic simulation of a floating platform using Ansys Aqwa 2023 R1. The following query is specific to the "connection data" input of a catenary mooring line. 

Regarding Catenary mooring, lines have one ends attached to a point (fairlead) on the floating platform, and another end to a fixed point usually, at the bottom of water. A typical figure with a catenary mooring line is attached here from the Sea Loads book by O. M. Faltinsen.

Now, if we see the Aqwa input detail, I could not find a way to control the suspended line length (l_s) or (X-x) from the above figure. Instead it takes an input, maximum expected tension. May I know how to select a suitable value of this input? 

On the other hand, the weight of suspend length (l_s) of the mooring controls the vertical equilibrium position of the body, and tensions in the lines as well as stiffnesses. Could you please let me know how to find the suspended length of the mooring lines in Ansys Aqwa? 

Cheers!

[Shuangxing Du]

Please read Aqwa User's Manual, Section 5.8.1.4.

For nonlinear catenary cables attached between a fixed point and a structure (Connectivity set to Fixed Point to Structure), internally Aqwa converts them to the multi-dimensional load/extension databases.  The range of the possible fairlead locations of the cable in the database is determined by Negative dZ Range of Expected Connection Point Vertical Motion (measured from the lowest anticipated database point to the connection point in the definition position) and Positive dZ Range of Expected Connection Point Vertical Motion (measured from the connection point in the definition position to the highest anticipated database point) along with the slack and maximum expected tension positions (including the effects of the global or cable-specific seabed slope). This area is then divided up to form a database of cable end positions and corresponding tensions which is used in the analysis. It is recommended that the full size of this grid is used, of 600 points, which is formed by the multiplication of Number of Vertical Partitions and Number of X Coordinates.

If you have known the maximum tension at the fairlead of the nonlinear catenary mooring for the structure under the analysed environment conditions, you can directly input it as the maximum expected tension. If you do not know this value, it could be the tension value at the fairlead when the cable may be broken if the actual tension at this point exceeds that value.