Learning Forum

[javier.medina.galera]

I'm working on my master thesis with Ansys Aqwa about mooring analysis of spar platforms. I was having some issues when performing some calculations.

In particular, I'm trying to add some additional stiffness to roll, pitch and yaw motions to decrease/increase the natural periods of the platform I'm studying. I'm using the additional hydrostatic stiffness matrix option but it doesn't change anything in the dynamics of the platform for the rotational degrees of freedom. If, for instance, the natural period of yaw is 30 seconds and then I add an additional stiffness component for yaw, then the natural period would stay the same. The latter happens for roll and pitch motions. I have tried from really small values to huge ones but nothing happens.

By contrast, if I want to modify the surge, sway or heave natural periods, adding additional hydrostatic stiffness for those degrees of freedom does change the platform dynamics. To get the natural periods I perform a decay test with a time response analysis.

I don't know what is happening as changing the stiffness should modify the natural periods also for rotational motions. I'm aware that the hydrostatic stiffness of yaw is negligible but not for roll and pitch.

I would like to be able to increase the global stiffness for roll, pitch and yaw but I don't know how to do it. I can't change the mooring lines as they are a design condition. I was wondering if there is an option to increase the stiffness in those degrees of freedom that I could use.

Thank you very much

[Yu Bi]

Hi, you can use connection stiffness (structural stiffness) matrix (right-click on Connection > Insert Connection > Connection Stiffness). However, you need to use a pre-solve script (if you work in Aqwa Workbench) to remove the NASF option in the *.dat file to enable the program to use the connection stiffness.

[javier.medina.galera]

Thank you very much for your help!! I have been triying to remove the NASF option with a pre-solve script of the *.dat file of my time response analysis but I am not sure what to include into the script, I've got no experience in AQWA scripting.

I would be very grateful if you could help me!

[Mike Pettit]

Hi Javier,

If you are using Release 2022 R1, you can turn off NASF by turning on Beta Options (from the Project Schematic, open Tools > Options, then under Appearance you should find the Beta Options checkbox - you will need to restart the Aqwa Workbench editor for this to take effect). Then there is an option 'Include Linear Connection Stiffness Matrices' in the Hydrodynamic Response Analysis Settings - setting this to Yes will turn off NASF.

I hope this helps.

Mike

[javier.medina.galera]

Hi Mike,

It's just what I needed ! Thank you very much for your help !!