Morison Load on AQWA

Discussion in 'Software' started by raysugar, Oct 27, 2014.

  1. raysugar
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    raysugar Junior Member

    Hello to everybody,

    I'm trying to calculate the morison load on a group of tubes in the vicinity of the hull. I would like to take into account the effect of diffraction from the hull on these slender member.

    I've created a simple model: a wall fixed and near it a group of tubes.
    It seems that when I'm doing a time response analysis to evaluate morison load, the software doesn't take into account the wall where waves should be diffracted. I say this because each times i change the position of the tubes the morison force acting is still the same.

    For the moment I'm doing a regular waves analysis, with 2nd order stokes.

    Thank you for your help.
     
  2. jehardiman
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    jehardiman Senior Member

    Don't use AQWA for that, use WAMIT, because the problem you are looking at is what WAMIT was developed to solve.
     
  3. raysugar
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    raysugar Junior Member

    Thanks for your reply. So you think that I can't solve the problem with Aqwa. Unfortunately I don't have the license for Wamit.
     
  4. raysugar
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    raysugar Junior Member

    I got this error when i try to run the analysis for two interacting strucutre :
    READING DECK 2: NON-DIFFRACTING STRUCTURE #2 CANNOT BE IN AN INTERACTING SETS

    what should i do ?
     
  5. CWTeebs
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    CWTeebs AnomalyGenerator

    Ray,

    I trust I answered your question in e-mail. If not, I can elaborate here. For those reading, you cannot have morison elements as interacting bodies in Aqwa.

    Does WAMIT implement morison's equation in the frequency domain including interactions between bodies? How does it account for the viscous term in morison's equation?

    In Aqwa the morison element is essentially a time domain phenomena although there's functionality to linearize the drag term in Aqwa-Line. As far as I know there is no way to get the morison element wave interaction. You could, however, mesh the tubes and import them as diffracting bodies and supply empirical drag coefficients for the time domain response.
     
  6. jehardiman
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    jehardiman Senior Member

    Morison's viscous (Cd) term is the easy part, the interference and radiation (i.e. the damping and added mass) is the hard part. WAMIT solves the multi-body radiation problem and returns fluid pressure and velocity components on the panels (i.e. the fluid force and potential) as well as the added mass and damping matrices for single period waves or across a given frequency range. From that point it is straightforward to include a viscous or rigid body component via Morison's equation (which is semi-emperical anyway) and spectral response with determinate confidence.

    FWIW, time domain sucks for design problems like this. It tells you what did happen in the specific case as opposed to what will happen in the general case with a specific confidence level. If what you are using AQWA for is to get the total (viscous and pressure) Cd for a time domain Morison's solution, then there will be issues in closing the instantious velocity loop. See the following paper chapter 2. http://www.nrel.gov/docs/fy08osti/41958.pdf If however you are using AQWA to try to get the vortex shedding, you would be better off to go to Sarpkaya's work.
     
  7. CWTeebs
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    CWTeebs AnomalyGenerator

    As I understand it, a big part of the reason morison elements were introduced into Aqwa was so you don't need to resolve the radiation and diffraction problem for slender tube elements each time you use them. The viscous term is built in.

    Aqwa-Line does this for diffracting bodies...

    EDIT:
    Another obvious reason for not doing radiation and diffraction on morison elements is that you typically see large motion installations made of tube elements simulated for the offshore industry. One common example is jacket installations that are pushed off the back of a barge to check for loads and stability. Radiation and diffraction is only done on the mean waterline and linearized to get the RAOs; the large motion simulations just can't be done with radiation and diffraction theory, period, although I'm unsure of the efficacy of using the morison's equation for large motions also.
     
  8. jehardiman
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    jehardiman Senior Member

    I'm sorry, I thought the OP wanted to solve the interaction problem, not just get a gross load estimate. I work with large bodies next to large bodies, so I was thinking that he was looking at two or more bodies that would influence each other. If all you are looking for are pipes with a diameter << the nearby body or wave length, then yes, just apply a constant conserative Ca and Cd and be done with it, which is apparently his initial complaint and your comment about AQWA above noting that in AQWA-WAVE, Morison viscous forces are added to the AQWA-LINE incident and diffracted wave forces. However if the bodies are of near size or near wavelength, then all sorts of affects based upon geometry and geometry effected potential flow may occur, such as the well known drillstring riser and jacket crossbrace VIV issues. While there is some discussion about what are "large" motions (i.e. more or less than D/2?), it will come down to either wanting a bounding load and frequency for design....or climbing into that barrel of monkeys that is flexible cable VIV chaos theory.

    Edit to Add;

    raysugar, have you looked in Sarpkaya & Isaacson, Mechanics of Wave Forces on Offshore Structures yet? Go look a para 5.3.8 for a bounding analysis of pipes near a wall.
     
    Last edited: Oct 31, 2014
  9. CWTeebs
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    CWTeebs AnomalyGenerator

    Morison elements in Aqwa are not the appropriate implementation. You create diffracting elements and solve the radiation and diffraction problem using Aqwa-Line. You add the viscous drag term later on.

    Aqwa-Wave is actually a load-mapping program. I've used it for Aqwa-to-FEA. While very interesting and useful, I believe it is unrelated to the discussion.

    Another program, called Aqwa-Flow, extracts the particle velocity amplitude and phase at points in the fluid domain from an Aqwa-Line run, which the OP I believe expressed interest in the original post.

    jehardiman, I think some of the confusion stems from the fact that you're unfamiliar with Aqwa and more of a WAMIT user. In Aqwa you can add either a special morison element, which implements the morison equation and summarily cannot take into account wave interactions, or you can mesh tube elements and solve the full radiation and diffraction problem as you would any other vessel. You can then add the drag coefficient later on. In fact Aqwa has a 'morison drag' card you can add to implement such functionality (MDIN).

    I do not know if WAMIT has a special morison element. In Aqwa it is typically used in the time domain but there is functionality to linearize its drag force in the frequency domain.
     
  10. raysugar
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    raysugar Junior Member

    Thank you all for your very interesting replies. Regarding my original post, I'm interested in evaluate the load in pipes (boat landing) situated closed to ship. This type of structure (see picture) usually are designed without taking into account any interaction, applying Morison load in each tube and regular wave theory (stokes or airy), Cd and Ca from literature or code (DNV API ec.)

    My intention is to take into account the wave diffraction mainly, now I'm doing some simple analysis considering a wall fixed behind 2 tubes fixed as well. The clearance between the side ship and tubes is 1 m.

    jehardiman.
    Yes I've read the chapter that you have mentioned. Of course to take in to account the real phenomena I can't neglect viscous effects. On the other hand if I start considering them I would have to develop another thesis, which is not my scope . My main issue now is to develop a tool to understand and evaluate which is the velocity (and accelerations) field in the vicinity of the ship and then see which is the difference between the typical approach.

    I'm interested in both aqwa flow and the function for aqwa line for linearized tube drag.

    Some questions. Does Aqwa flow evaluate the velocity along z or just in the surface ?
    I've read in the aqwa manual that I'll be able to check " linearized tube drag" in the analysis setting table. I can't find it in my table. Should I launch the simulation from aqwa GS?
     

    Attached Files:

  11. CWTeebs
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    CWTeebs AnomalyGenerator

    My recollection from Aqwa-Flow is it prints the X Y and Z components of velocity, in amplitude and phase form, for each wave frequency component, at the location specified in the text file with the .cor extension with the same basename as the aqwa-line run. You may also have to supply the wave amplitude although I may be confusing that now with Aqwa-Wave, a program typically used for mapping loads to FEA.

    As for the second question, I'd *really* need to go back and look at the syntax because I've not used Aqwa's approach to solving this problem before having always written my own in MatLab, but no, you can't run it from Workbench, you have to edit the *.dat files manually.

    I believe Aqwa's approach is as follows: you supply an input spectrum, and it linearizes the V**2 term of the morison elements to that spectrum, the results being manifest in an altered set of linear RAOs giving an equivalent linear response in that spectrum, the results of which can be use in Aqwa-Fer's spectral response. Morison elements are otherwise not represented in Aqwa-Fer (I dont' think). Morison elements are typically a time domain thing in Aqwa.

    I suggest you don't bother with this functionality for a while (e.g., you're very comfortable with Aqwa's batch mode syntax) or until you absolutely need it. I've been using Aqwa for years and it'd take a hefty time investment on my part to get this foolishness working. I'm not even sure I've relayed the procedure properly.

    You could create a bunch the morison elements connected by a hinge to the ship and do a time domain response, measuring the reaction at the hinge. Once that is done, convert the morison elements to diffracting bodies (by creating an equivalent geometry, meshing the panels, basically the same procedure you did for the ship), re-run Aqwa-Line and the time domain response, and see by how much the response and forces on the hinge differs.
     
  12. raysugar
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    raysugar Junior Member

    I've tried to use Aqwa-Flow.

    CWTeebs you are right, you need to create one ["same name of .dat file".cor] file with the notepad, in which you write an integer followed by the coordinates of the point where you want to calculate the velocity (1 x1 y1 z1). Then run aqwa flow directly from the same folder where you have all .RES .DAT file (I've copied the aqwaflow.exe ,it's in ANSYS/.../aqwa/bin, directly in the folder), specifying the wave amplitude.
    I've got this outpout:

    VELOCITY,POTENTIAL,PRESSURE,WAVE ELEVATION AT SPECIFIED POSITIONS

    Pot: Total potential
    Pre: Total pressure (excluding hydrostatic)
    Wav: Total wave surface elevation

    WAVE AMPLITUDE = 3.00000
    =================================================================

    WAVE DIRECTION =-180.0000
    WAVE FREQUENCY = 0.7854(PERIOD= 8.00SEC)
    -------------------------------------------------------------------
    NODE NO./COOR 1 0.0000 0.5000 0.0000
    Vx/Vy/Vz (Amp.) 2.3466 0.0180 2.5389
    Vx/Vy/Vz (Phase) -179.1065 -59.1412 -100.2502
    Ax/Ay/Az (Amp.) 1.8430 0.0141 1.9940
    Ax/Ay/Az (Phase) 90.8935 -149.1412 169.7498
    Pot/Pre/Wav (Amp.) 40.3605 32491.5762 3.2324
    Pot/Pre/Wav (Phase) -100.2512 -10.2512 -10.2512

    -------------------------------------------------------------------
    NODE NO./COOR 2 0.0000 1.2000 0.0000
    Vx/Vy/Vz (Amp.) 2.3466 0.0409 2.5397
    Vx/Vy/Vz (Phase) -179.1073 -58.5979 -100.2319
    Ax/Ay/Az (Amp.) 1.8430 0.0321 1.9947
    Ax/Ay/Az (Phase) 90.8927 -148.5979 169.7681
    Pot/Pre/Wav (Amp.) 40.3759 32503.9492 3.2336
    Pot/Pre/Wav (Phase) -100.2319 -10.2319 -10.2319


    How could I evaluate the velocity in a different phase of the wave ? Should I change the .dat file input for aqwa line ?

    I've tried to put a Z value >0 and it gives me and error, why ? Cause the solution is given only in the mean water level ?

    Thank you a lot
     

  13. raysugar
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    raysugar Junior Member

    Does exist a way to change the phase of the wave in a aqwa line analysis ? The flow velocity in a point depends on the wave phase as well, and i would like to find it at different wave phase.
     
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