Learning Forum

[yoonho choi]

I tried to analysis the box moving only vertical direction following pile structure.

But I can not make fender condition move only vertical direction.

I refer to an another same question(/forum/forums/topic/ansys-aqwa-how-to-set-only-a-vertical-movement/)

but I could not find how to set fender condition.

I set fender point around box and pile with omni-directioal and set the size 0.5m with is same with gap of box and pile.

 

[Mike Pettit]

Hello,

Referring to the Fender details, please confirm that you have set at least one non-zero stiffness coefficient (A-E) for the fender stiffness definition.

You also need to set the Fender Axes and Contact Axes so that they have the correct orientations. When the Fender Action is Omni-Directional, you need to rotate the axes so that the X directions of each axis system (red arrows) are pointing towards each other. Click on Fender Axes, and change the Alignment Method to Direction Entry:

For your case, you probably need to set Rotation About Global Z for each set of axes, with a difference of 180 degrees between the Fender Axes and Contact Axes Z rotations.

Once the fender alignment is correct, the fender graphic should change from black (no possible contact) to red (fender in contact) or orange (not in contact). You can also check the Initial Attachment Point Separation in the Fender details, where the distance shown should be a positive, non-zero number.

I hope this helps!

Mike

[yoonho choi]

 

Hi, Mike

 

Thank you for reply. I tried to apply your coments and I have rotated each fender X direction axis are pointing towards each other and of coursely set the stiffness coefficient and distance between of fender and contact is not zero. But the same error still showed up. If you can review my modeling file I can send to you vis email.

Do I have to set the another joint for restrict move X or Y direction?

I just set the fender and fixed joint for pile base(seabed).

 

 

[Mike Pettit]

Hello,

Your fender arrangement looks perfect to me, and I don't think that you need another joint in the model.

The problem you are encountering now is related to the hydrodynamic interaction between the pile and box structures. In the Hydrodynamic Diffraction analysis, the Aqwa solver calculates the radiation and diffraction effects for each structure, but also the interaction effect: the heave motion of the box generates a radiated wave that affects the forces acting on the pile, while the diffraction effect of the pile will affect the wave forces on the box. In other words, the frequency-dependent added mass and radiation damping matrices calculated by Aqwa are extended to include the degrees of freedom of both structures (i.e. 12x12 instead of 6x6).

However, the hydrodynamic interaction effect is only calculated for the structures in their definition positions in the Hydrodynamic Diffraction analysis. In the time domain, as the structures move relative to one another, the interaction effects between them are not recalculated. Therefore the Aqwa solver applies a limit to the relative motion that it will allow in the time domain, because the results become unreliable when the relative motion is too large. Also, this limit is more restrictive when the structures are close together in the Hydrodynamic Diffraction analysis.

To avoid this problem, you could consider turning off the interaction effect. This may not be unreasonable, as the pile is fixed in position (zero radiation term) and is quite small in diameter (small diffraction term). To turn off hydrodynamic interaction, click on the Hydrodynamic Diffraction Structure Selection, then click on Interacting Structure Group 1, click on some empty space in the Graphical Window (to deselect the structures) and then click Apply. The field should change from 'All' to 'None'. You will need to re-run the Hydrodynamic Diffraction analysis, but this should allow you to run the time domain analysis without any problems.

I hope this helps!

Mike