natural frequencies and periods of a spar structure in all six DOFs in Ansys Aqwa

Discussion in 'Software' started by mildteal, Jul 1, 2016.

  1. mildteal
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    mildteal Junior Member

    I am trying to study the natural frequencies and periods of a spar structure in all six DOFs in Ansys Aqwa.
    After modelling and hydrodynamic diffraction analysis when I open the .LIS file I find that natural frequencies and periods of surge, sway and yaw are all zero(See attached picture). The natural periods in roll, pitch and heave are matching with the literature I am referring to for my model(Chen et al). As per the literature the other three degrees of freedom are not matching.
    Why am I getting such a value of natural frequencies in surge,sway and yaw? Please help.
     

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  2. b1ck0
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    b1ck0 Senior Member

    You get this problem because there is not stiffness in (x,y,Rz) direction by default. You get the the stiffness when you add mooring lines. You can't have an oscillation in x direction if there is nothing to bring you back ...

    To find those you have to make a hydrodynamic response analysis (stability) where you can include the mooring lines, seabed friction and etc. and of course see what the natural periods in surge, sway and yaw will be.
     
  3. mildteal
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    mildteal Junior Member

    natural frequencies

    Thank you so much for replying. I ran the hydrodynamic response analysis (stability) as you mentioned but even then I get an equilibrium.lis file which shows the same natural frequencies(without considering mooring lines).
    I have modelled the mooring line through the Ansys workbench interface.
     

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  4. b1ck0
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    b1ck0 Senior Member

    They are not written in lis file. You need to start AQWA GS and check them inside. Or ... if you have ansys v17 and above you can see the frequencies in Workbench itself (check the result objects)
     
  5. mildteal
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    mildteal Junior Member

    Thank you so much for the inputs. I have been using Ansys 16.2. I am not too familiar with Aqwa GS but I am upgrading to ansys v17 soon.
     
  6. b1ck0
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    b1ck0 Senior Member

    What can I say ... you can download AQWA tutorials (including AQWA GS) from the ANSYS customer portal
     
  7. mildteal
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    mildteal Junior Member

    Hello, I am having one more doubt and hoping you could help me out with it. While modelling the mooring lines in Aqwa, I am calculating the anchor point co-ordinates using static analysis of catenary cables. (I am using non-linear catenary cable). Also I am selecting the diameter of cable from floating condition as I know the draft of spar and density of water. Is this approach correct?
     
  8. b1ck0
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    b1ck0 Senior Member

    I don't see the logic of this. You choose the diameter of the rope so it has sufficient strength for holding your spar in place in some environment (same applies for the coordinates of the anchors). The diameter of the rope gives you the material stiffness and the coordinates of the anchors are tweaking your geometrical stiffness of the mooring line.

    if you are afraid that your spar will sink too much because of the vertical load from the moorings lines it's another issue. You first have to know the exact length and diameter of the lines and then you make another analysis where you will take this extra force into account and modify the draft of the floater.
     
  9. mildteal
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    mildteal Junior Member

    How can we obtain surge-static offset curve or mooring line tension- offset curve in ANSYS aqwa?
     
  10. b1ck0
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    b1ck0 Senior Member

    It seems that you are trying to cheat your homework by using ANSYS AQWA ? Who and for what reason will be interested in this curve if he already has AQWA ?! And of course you first need to have this curve before running the analysis ... because this is how you design the mooring system ... then you use AQWA to verify it (dynamically) ... ;)

    I would advise you to spend 20 mins with a textbook and calculate the damn thing
     

  11. Salman82
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    Salman82 New Member

    Hi everyone!
    I am modeling a Spar Structure and did everything like ANSYS tutorials!
    have 2 questions if anyone kindly can help me on them!

    1. In ANSYS AQWA, for Point Mass Input, I saw for a regular ship there is a formula for Kxx,Kyy and Kzz calculation, 0.34*beam,0.25 Length and 0.26 Length respectively. I am just wondering to know is there any specific or rough idea that I can use for Spar Structures?

    2. when I modeled my spar structure, in Analysis of the Time Domain Response when I consider the use cable dynamic as yes I encounter with an error such as bellow,
    " CABIN4:CONV. FAILED STAGE#6 - ERRN=2.49E-10 LINE#4" is there any idea how can I fix this error. if you can help me it would be highly appreciated.

    kind regards to everyone! and thanks in advance for reading my questions!
    cheers, Sal
     
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