Learning Forum

[rohitshivalli22]

respected sir/madam,

i have created a spar torus combination model in ansys and want to simulate in hydrodynamic diffraction and hydrodynamic response, i have generated connection stiffness and fenders (and i am not sure wheather the procedure i followed for fenders is right or wrong,) between spar and torus which has gap of 0.75m. when i run the analysis i get "HYDRODYNAMICALLY INTERACTING STRUCTURES HAVE MOVED TOO FAR FROM ANALYST POSITION MOVEMENT EXCEEDS 75% STOPPED AT TIME = 4.00." this error and tried to search solution related to this and didnt got any solution with respect to this error, it would be a great help from your side if you guide me to clear this problem. some pictures related to the simulation and details related to the model, have been attached, please find the attachment.

[Mike Pettit]

Hello,

Please take a look at the last reply in this thread: https://innovationspace.ansys.com/forum/forums/topic/how-to-set-fender-condition-for-move-only-vertical-direction/

Let me know if you have any more questions.

Mike

[rohitshivalli22]

Dear sir, i thank you for your reply 

I have been gone through this discussion but cannot get it properly, i have created fenders between both structures, and settedd the fender axis and contact axis with the trial and error process until i get the graphics of fender changed to the red. now the fenders are displaying in red color, i am not sure whether they have connected in perfect, please guide me how to set the fender and contact axis. Also in the thread above you mentioned about to set some non zero stiffness coefficient (A-E) for the fender stiffness definition, i have set the stiffness coefficient to 5000N/m. but still I am getting "HYDRODYNAMICALLY INTERACTING STRUCTURES HAVE MOVED TOO FAR FROM ANALYST POSITION MOVEMENT EXCEEDS 75% STOPPED AT TIME = 4.00." so can you guide me to clear thhis error, it would be a great help from your side.   

[Mike Pettit]

Hello,

The fender definition may be correct in your model. The problem you are encountering now is related to the hydrodynamic interaction between the Spar and Torus 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 torus generates a radiated wave that affects the forces acting on the spar, while the diffraction effect of the spar will affect the wave forces on the torus. 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. In your case the solver probably stops when the torus vertical position changes due to the passing incident wave.

To avoid this problem, you could consider turning off the interaction effect. This may not be unreasonable, as the spar motion is dampened (small 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

[rohitshivalli22]

 

Thanks for the solution sir,
now the analysis is working, can i restrict the heave motion of spar for some extent like 2m in both +ve and -ve heave direction , 
the inner diameter of torus is 8m and the diameter of spar is tapering from 6.5m to 9.5m from 4m depth to 12m depth from SWL. a image of the model has been attached to the message, by visualizing it i feel both the structures are merging were in reality it is not possible.

 

[Mike Pettit]

Hello,

To restrict the heave motion of the spar you could also use a pair of fenders, with each fender attached to the spar and the contact planes on Fixed Points positioned +/- 2 m vertically from the initial fender positions. You should change the Fender Axes and Contact Axes Alignment Method option to 'Direction Entry', then set the Rotation About Local Y so that the X directions (red arrows) of each axis system are pointing towards each other. Then the Fender Initial Attachment Point Separation should show '2 m (Point to Plane)'.

Regarding the structures merging, if I understand correctly: Aqwa is a rigid body solver, and it does not detect contact between structures - only the fenders can detect contact. So, it is possible that two structures can pass through each other, if there are no fenders to prevent it.

Mike

[rohitshivalli22]

 

Thanks for the solution and for your time, 
When the structures are made non interacting structures I am getting two different RAO’s for heave, is there any option to get the merged heave RAO like single graph which describes the motion of both spar and torus. A sample graph has been attached to the message. I want merged graph of these two strands, is there any option for that.

 

 

 

[Mike Pettit]

Hello,

Sorry, I'm not sure I understand your question well - do you mean the heave RAOs of the two structures as if they are rigidly connected? You could do this by increasing the Connection Stiffness Z-Z values, so that the vertical linear connection stiffness between the structures is large.

Mike

[rohitshivalli22]

Thanks for the solution sir,
I need to add the end stop systems to limit the motion of structures, I have used the fender option for connection between both the structures, is there any option that i can adopt to limit the excess motion. 

 

[Mike Pettit]

Hello,

You can model the end stops with fenders, but you cannot plot (linear) RAOs of the (nonlinear) structure motions including end stops. For this, you should run a time domain analysis. Then you can plot the instantaneous structure motions of the spar and torus, as well as the Fender Forces result (Elastic Force component) which will indicate when the torus makes contact with the end stops.

Mike