Learning Forum

[pc]

how to get restoring force in a mooring line vs displacement in a particular DOF graph in AQWA?

[Mike Pettit]

Hello,

To make sure I understand the question - are you trying to plot a force-extension curve for a mooring line from the time domain results? If this is the case, you will need to create 2 graphs in the Aqwa Workbench editor:

• in the first graph, output the cable tension:
  • right-click on Solution, then Insert Result > Connections > Cable Forces > Whole Cable Forces;
  • select the appropriate cable;
  • set the 'Component' as 'Tension'.
• in the second graph, output the displacement between the mooring line attachments:
  • right-click on Solution, then Insert Result > Structure Motions > Structure Position > Actual Response;
  • select the appropriate structure;
  • set the 'Component' as the DOF you are interested in, or set as 'Distance';
  • select the 'Reference Point' as the Connection Point at the vessel end of the mooring line;
  • set 'Motion Relative To' as the Fixed Point or Connection Point at the other end of the mooring line.

Once you have evaluated these results, in each graph you will find an 'Export CSV File' option. Save each graph as a separate CSV file, then open these in Excel, and use this to plot the tension values against the displacements.

I hope this helps!

Mike

[pc]

 

not specifically from time domain results, I want restoring force vs static offset in a specific DOF to understand the restoring propetires of the mooring line. I want to understand the amount of restoring force per specific displacement
The method you have told, I would have to apply a regular wave and that would give me a periodic displacement

 

[Mike Pettit]

Thanks, I see what you mean.

You can do this in Aqwa Workbench using a stability analysis Hydrodynamic Response system, and the parameter study capability of Workbench. On the Workbench Project Schematic, if you do not already have one, connect a Hydrodynamic Response (HR) system to the Hydrodynamic Diffraction (HD) system, with the HD Solution cell linked to the HR Setup cell.

In the Aqwa Workbench editor, check in the HR Analysis Settings that the Computation Type is set as Stability Analysis (this is the default for the first HR system connected to HD). Right-click on the analysis and Insert > Starting Position.

In the Starting Position object, set the 'Structure' field as appropriate and select the 'Reference Point' as the Connection Point of the mooring line on the structure. Change the 'Definition' from 'Absolute Position' to 'Relative to Geometric Position'. Click the small box next to 'Relative Position X' so that a 'P' shows in the box - this defines the field as an input parameter. (It may make sense to set a different field as your input, depending on the model.)

On the HR Solution, right-click and Insert > Connections > Cable Forces > Whole Cable Forces. In the graph input (named as 'Cable Forces, Whole Cable Forces') select the 'Structure' and 'Connection' as appropriate, and set the 'Component' to 'Tension'.

In the parent graph object (named as 'Parameters vs Iteration Step') click the small box next to 'Initial Value' so that a 'P' shows in the box - this defines the field as an output parameter.

Go back to the Workbench Project Schematic page, and you will see a Parameter Set which links into the Parameters cell of the HR system.

Double-click on this to open the Parameter Set tab. In the Table of Design Points (top-right panel), you can define a series of displacement offsets for the structure position. Once these are defined, click 'Update All Design Points' (top-left). Do not try to interact with Workbench or the Aqwa Workbench editor until the parameter study has finished - the Aqwa editor will open and close itself several times.

Once the design points are updated, the Table of Design Points should show the tensions corresponding to each offset:

You can copy and paste this data for plotting in another program, or you can use the chart function in Workbench: in the Outline of All Parameters table (top-left panel), click Charts. In the Toolbox panel (left-hand side) double-click on Parameters Chart. In the Properties table (bottom-left panel) select the input parameter as the X-Axis, and the output parameter as the Y-Axis. The chart should appear in the bottom-right panel:

I hope this helps - please let me know if you have any other questions.

Mike

[pc]

thanks mike, this is exactly what I was asking for
I have another question, I have three mooring lines, so instead of using tension in one cable, if I use the initial value of the result 'mooring sum only' will I get the restoring force of the whole mooring system in specified DOF

[Mike Pettit]

Hello,

That's correct, if you use a Structure Forces > Mooring Sum Only result you will get the total mooring force on the structure due to all 3 lines.

Cheers, Mike