Orcaflex - Cable lay

Discussion in 'Software' started by c.jack.sparrow, Aug 29, 2012.

  1. c.jack.sparrow
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    c.jack.sparrow Junior Member

    Hey guys,

    I would be glad for some help regarding the software Orcaflex. I am a fairly new user to Orcaflex but I have a good understanding of FEM.

    Basically I am trying to model a subsea cable and find the tension and other values of the cable. Now first I set my cable to follow a spline and over the chute which is on my aft deck. I connect one end of the cable to a winch and the other anchored with enough slack in it.
    I want to vary the tension in the winch and check the corresponding tensions and touchdown. Hence the winch either pulls in cable or pays out according to tension.

    What happens is initial condition my model converges and the cable find the nodes perfectly over the chute.
    when i start varying the tension all the problems start. The cable starts to find nodes within the vessel and under the chute and other places. I have tried using links to hold the cable above the chute but it ends up falling through the vessel or under the chute.
    If anyone could help me please do let me know. I have also tried all possible methods and tips from the user manual as well.

    thanks in advance.
     
  2. Tim B
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    Tim B Senior Member

    Can you get a sufficiently good answer by setting the tension from the outlet of the chute? Obviously, the friction of the chute will make a difference, and you'll have to think of some way of accounting for that, but it should be small by comparison to the laying forces.

    I don't know how good Orcaflex is at modelling cables running around structures. I seem to remember that it's clash-modelling was a bit basic and needed careful setup to get convincing answers. I would have thought that this would be something that Orcaflex support could help you on.

    Tim B.
     
  3. c.jack.sparrow
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    c.jack.sparrow Junior Member

    Hi Tim,

    Thanks for your reply.
    And your right, it needs a lots of careful setup :/
    I did get in touch with the support and they gave me quite a good explanation about my problem. But at the end to keep my cable(line in orcaflex) above the chute, they just used temporary links to hold it over the chute and at the end of the simulation I have to make sure the links have 0 tension in them.

    Well this works fine for one condition. My cable is fixed to a winch (all on my vessel). If I vary the tension in the winch, I have to change the values of the Links as well, which is trial and error method and its quite laborious. So if I want to get results for 10 conditions it will take quite a long time. So I was wondering if there was an easier way to just tell my line to follow a specified path or close to that specified path.

    But it works some times and other times it just finds coordinates on the under side of my chute or just through the vessel. So was wondering if someone knew some tips or tricks around this.

    Thanks again.
     
  4. CWTeebs
    Joined: Apr 2011
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    CWTeebs AnomalyGenerator

    Could you post a screenshot of your setup? I don't use OrcaFlex but I may be able to see if I can replicate this in AQWA.
     
  5. c.jack.sparrow
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    c.jack.sparrow Junior Member

    Hey CW,
    thanks for your response and i'm sorry for the late reply. I was away for a bit.
    ok here are three screen shots. one is before statics and another after.
    I have attached them here.

    in the first screen shot you can see a spline shape. which in this case is my desired shape after statics. the second is after statics are done but i have a 'temporary link' attached in order to keep it over the chute.

    problem is are the end of the simulation the link has some tension on it. the support told me i should keep the tension at 0.
    I want to vary the tension on top, for eg; 20KN, 30KN etc. each time if i have to find a 0 tension on the link its a long process. and if i dont use the link it just falls under the chute (blue cylinder).

    basically all i want to do is. get the cable over the chute, vary the tension to certain values. check the touchdown tension, layback etc...

    Hope you can help.
    Thanks in advance.

    Regards

    12-09-2012 15-31-18.jpg

    12-09-2012 15-32-02.jpg

    12-09-2012 15-40-35.jpg
     
  6. c.jack.sparrow
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    c.jack.sparrow Junior Member

    Hello everyone!

    I am still sort of tweaking with this laying analysis. I had my hand full with other things so this was sort of in the back seat, now its coming up again.

    Does anyone have easier ways to do this? Its basically the statics I have problem with!
    If I can get the cable to go over the cylinder I would be able to do the rest...

    those who use Orcaflex will know this; i have also tried to use 'Links' and 'Line setup wizard' in orcaflex to get the anchor positions and lengths but no real use!

    Thanks guys
    Regards
     
  7. troglodyte
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    troglodyte Guest

    Hi
    I am not sure if you are still up with this analysis. I am new to this website and this is going to be my first post in a thread!

    During the modeling stage, you may try setting the EndA coordinates of the cable (i.e. the cable end you have on the vessel deck) a few meters away from the chute, say 2m aft and 5m above the chute surface. Now, when you run the static analysis, the winch will pull the free end (i.e. EndA) toward the vessel and the cable will "fall" on top of the chute rather than coming from the bottom. In this approach, the links won't be necessary. This is one method.

    Coming to your method, while using temporary links, it is worth checking the stiffness defined for the links and the chute. If the stiffness of the chute is set too high (say 100000 kN) and if your link has just 100 kN, then it is no way going to help in lifting the cable (through the chute) in line to the predefined spline. Plus, if your cable gets too noisy during the static run, you may then apply a small damping value to keep the cable robust.

    As you might be aware of, there are still many more ways in which a cable lay analysis could be modeled in OrcaFlex. If your aim is just to determine the layback and touchdown tension, then you may even try the simple option of modeling EndA of the cable as "fixed." This eliminates the need for the winch too as the required tension can be adjusted by increasing/decreasing the length of the catenary.

    Hope my explanation above is clear!

    Cheers
     
  8. c.jack.sparrow
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    c.jack.sparrow Junior Member

    Hi torglodyte,

    Thanks for your reply, I will try the method u mentioned soon, currently doing something else :)

    If I remember I think i did try the winch and End A a bit further the chute. Not 100% though, will have a look again.

    One another subject related to Orcaflex, I want to do a small analysis related with a crane.

    I have modeled a crane and some steel wires. Now I need a weight at the end of the wire to study the behavior.

    I cannot use shapes as they cannot be connected to lines!!
    So i guess Bouy's are my option. But I cant seem to make the bouy sink!
    I know I need to modify some setting not able to get a convergence.

    Thanks once again for your reply.
    Will check it out soon

    Cheers
     
  9. troglodyte
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    troglodyte Guest

    Hi
    If your crane study is to just observe the impact of shock load on the steel wire, then you can simply model the weight as a "clump." This is available under "attachment types" within the lines. A simple option!

    For the buoy (3D/6D) to sink, you have to define both the mass and the volume. The software uses these inputs to derive the submerged weight.

    Cheers
     
  10. c.jack.sparrow
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    c.jack.sparrow Junior Member

    Thanks again for the quick reply.
    ahh why dint i think of clumps!!

    Not the best start of the day! need to wake up soon i guess!

    Thanks
     
  11. c.jack.sparrow
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    c.jack.sparrow Junior Member

    Hi Troglodyte,

    Sorry took a bit time to get back. Was a bit held up.

    OK I tried placing End A a bit further that the chute and then applying a tension using the winch but the winch just falls straight through the vessel ( check the pictures i uploaded earlier, pic 3).
    I tried adjusting stiffness, damping, give support structures etc.
    I feel its quite simple, but i'm not getting my head around it yet!
    I also tried changing starting shape. Spline, quick , catenary etc.

    Does the Line setup wizard help or make things easier here?? even that seems to be a problem at the moment! :(

    Thanks in advance!
     
  12. troglodyte
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    troglodyte Guest

    Hi
    Been away for sometime.

    The vessel object in OrcaFlex has zero stiffness. So, that would explain why the winch or line passes through the vessel. If you wish to retain the winch/line over the vessel, then you may consider modeling a shape flushed with the main deck level. This way you will have contact between them.

    I took a quick look at the 3rd picture. The link connections seem to have considerable offset from the line (why??). The link connection point should sit on the line rather than on fixed space. In your model, though I assume you have referenced the link to the line, it is still "not on the line."

    I just managed to pull out a knowledge base article from Orcina website. It has numerous pointers for achieving static convergence. Please do read the attached article and see if any of these points help you in zeroing in on your issue.

    Cheers!
     

    Attached Files:

  13. c.jack.sparrow
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    c.jack.sparrow Junior Member

    Hi,

    Thanks for the reply and the attachement. will have a look at it. I had read this guide a while ago, will try the tips mentioned on this again.

    Regarding vessel object seems to have stiffness, I imported hydrodynamic data from an existing model. Will check that again just in case.

    The third picture was only an expample of what happens when I dont use any links to hold (cable and winch goes through vessel).

    When I use links I can see the line almost converges and then fails at one pirticulat node! I am trying to make small changes in order to converge.

    Is it always a problem to get the solution?
    back to my initial question: if I need to vary tension between 10 values for e.g., each time I need to play with the model??

    Also when it does give me a solution the bottom tension take a long time to settle down, I think usually a few meters and then it should be close to zeor? or am I wrong?

    But here If top tension is 20Te, it shows bottom tension of about 16Te and then reduces slowly till the end of cable to 8Te. I would think it should be close to zero after a few meters?

    Cable length: about 400m.
    water depth: 28m.
    Please find attached the screen shot.

    Change seabed stiffness, friction to get a relaxed cable on the seabed.

    Thanks once again.

    Cheers
     

    Attached Files:

  14. troglodyte
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    troglodyte Guest

    Hope you are aware of the excel file that comes with OrcaFlex. It allows you to define multiple load cases with varying inputs. I recommend you try it. In addition to preprocessing, it also has great features where you could automatically postprocess the results for summary preparation.

    For varying laybacks, there is also an option in one of the menu bar commands where you can specify the layback you require. Don't exactly remember where it comes though.

    Looking at the graph, I see the tensions are actually in kN and not in te. The touchdown tensions get lesser with increase in water depth. In your case, the water depth is just 28 m. Following the touchdown point, the cable will be subjected to the residual tension after the seabed friction is taken into account. Hope this helps.

    Cheers.
     

  15. c.jack.sparrow
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    c.jack.sparrow Junior Member

    Hello.
    Sorry for a bit later reply, was travelling.
    And also sorry I meant kN and not Te! ;) Typo error.

    Regarding the excel script file your talking about 'Automation' right?
    Chapter 4 in the user manual? or something else?

    I am having a look at it to get various cases. I only was using Batch procssing for calculations.

    Please let me know in case it strikes you. I will look around as well.

    I have started getting some decent results for cable lay with additional links to keep the cable over the chute.

    just want to know if the links can be avoided? any tip for that? will try that as my next step.

    Once again thanks a lot for the tips
    Much apprciated.

    Cheers
     
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