Corrosion/abrasion FOWT mooring systems

Discussion in 'Class Societies' started by CWTeebs, Nov 25, 2013.

  1. CWTeebs
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    CWTeebs AnomalyGenerator

    Hi,

    I am looking for material related to the effects of corrosion and abrasion for floating offshore wind turbine mooring systems. Strictly speaking, it does not have to be classification society documentation.

    In general, I'm trying to determine how these factors contribute to the fatigue life of the FOWT's mooring.

    Thanks for your time!
     
  2. Stumble
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    Stumble Senior Member

    Cwt,

    There is no necessity that fatigue life play a role in the base or morring system. Simply switching to steel would eliminate the fatigue issues. It does raise corrosion issues, but protecting maritime steel structures is a very well understood issue. The experts are the oil and gass industry who deal with this in large offshore structures as a matter of course.

    What specific issues are you trying to address? I know a number of people who have advanced degrees as corrosion engineers that deal with this issue exclusively.
     
  3. CWTeebs
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    CWTeebs AnomalyGenerator

    Hi, thanks for the reply. The specific issues being looked at are corrosion and abrasion and how they impact fatigue life of FOWT moorings, specifically chain catenary and polyester mooring systems. I'm not looking at truss spar or other solid mount systems, even though they are more appropriate in certain circumstances.
     
  4. Stumble
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    Stumble Senior Member

    CWT,

    My concern is that fatigue is a specific type of material failure, which doesn't occur in a properly designed steel structure. Assuming the chain used is of appropriate size fatigue will never occur. It may fail from other processes, but not fatigue.

    See the F/N curve for whatever alloy you are considering, and ensure that the cycle load is below the knee for the alloy, and poof, no fatigue.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    CWT
    There is not a lot of data on the Windfarms per se, since these are "new" installations. Your best bet is to use FSPO and similar data.

    Whilst fatigue is material related, it is also dependent upon how the structure is designed too. The inherent structural stiffness and the modes of natural vibration play the major role in this, as I'm sure you know. Thus the design aspect to one side, for materials, either the FSPO/Off shore Oil & Gas sector, or just general data is your only guide.
     
  6. ruysg
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    ruysg Junior Member

    Hi CWT,

    I actually work with FPSO and Semi-Sub mooring design. Corrosing is ussually taken into account in the S-N curves. We ussually consider the S-N curve of a chain with a diameter reduction equal to half expected corrosion for the desing life of the morring.

    Hope it helps.

    Cheers
     
  7. CWTeebs
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    CWTeebs AnomalyGenerator

    All,

    I apologize for this latent reply, I hope it doesn't come across as ungracious, I do appreciate the responses.

    ruysg, the specifications I've read stipulate up to 0.8 mm reduction per year. 50% reduction for an FPSO or semi sub would be the ultra conservative design.

    For a chain catenary, do you expect to see the fastest corrosion at the active touchdown region on the seabed or at the splashdown region where the chain goes into the water?
     
  8. ruysg
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    ruysg Junior Member

    Hey CWT,

    Don't know if I was clear on my less message. We consider, for fatigue purpose, only half of the total expected corrosion, not half of the total diameter. For ultimate strength we consider the total corrosion.

    Usual design practice considers the top chain to be under more sever corrosion, about double the rate of the bottom chain. Actual data on the subject for tropical waters, were are designs are, is scarce. I believe that assumption comes from the fact that the warm water, sunlight and splash (air/water) are more sever on the material than the cold dark water at the bottom.
     
  9. ruysg
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    ruysg Junior Member

    By the way 0.8mm seems very severe. Half of that is a typical value over here.
     
  10. CWTeebs
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    CWTeebs AnomalyGenerator

    0.8mm was the high end, the low end was 0.2mm so half of that seems right. Also, I did mis-read your first post but some pictures I've seen show severe reduction in diameter, so it didn't seem like a totally crazy design parameter, just very, very conservative. Anyway, the tidbits of info you've mentioned are useful.
     
  11. gonzo
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    gonzo Senior Member

    Abrasion is directly related to the type of bottom. Sand will abrade a lot more than soft mud.
     

  12. Murat124
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    Murat124 Junior Member

    Do you mean that you will use multipoint mooring system for wind turbine a float??

    Or can you please describe mooring scheme for system ??
    for corrosion
    http://www.turkloydu.org/TurkLoydu/...hapter-70---Multi-Mooring-Systems-(2010).aspx

    take a look at Section 9 above classification document, if not satisfy, I can help by another way.
     
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