Desirable roll period for buoy

Discussion in 'Stability' started by floating, Jul 29, 2013.

  1. floating
    Joined: Jun 2009
    Posts: 65
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: NJ

    floating Junior Member

    Please forgive a beginner's question. For a moored buoy, I am looking at its targeted roll period, which should be short enough that it can recover to a vertical position between waves, but not so short that it is in resonance or moves violently in typical seas (there are no passengers who will get sick so that is not an issue). What is a typical range of roll periods that would be acceptable?
     
  2. TeddyDiver
    Joined: Dec 2007
    Posts: 2,569
    Likes: 117, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    It's moored... to the Earth me thinks so the roll period should be about 24h.
     
  3. floating
    Joined: Jun 2009
    Posts: 65
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: NJ

    floating Junior Member

    Sorry, I should have been more clear. The buoy is axisymmetric, so roll period is the same thing as pitch period. Picture tilting the buoy from the vertical in flat water, then releasing it so it pitches back and forth. The period of this pitch, i.e. how long it takes for it to rock back and forth, is what I was calling roll period and an example value might be 15 seconds. Anyway I am seeking suggested ranges for this pitch period, say 15-25 seconds, so that it can recover between waves but doesn't resonate with typical ocean waves.
     
  4. TeddyDiver
    Joined: Dec 2007
    Posts: 2,569
    Likes: 117, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    If it's moored with chain there's so much damping involved there's no issue about roll period. Long vertical buoyo shouldn't have any problems..
    BR Teddy
     
  5. floating
    Joined: Jun 2009
    Posts: 65
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: NJ

    floating Junior Member

    The mooring weight is suspended from a subsurface float (this is not a typically buoy for mooring, but rather a specialty spar buoy which can pitch a lot in waves). So, the mooring should not be affecting the natural period of the buoy. I am supposed to recommend an acceptable range for the natural period of the buoy, and was wondering if 15-25 seconds is OK.
     
  6. Ad Hoc
    Joined: Oct 2008
    Posts: 6,267
    Likes: 441, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Is there a problem if the buoy's period is 5-10 seconds or 15-20 second or 25-30 seconds? What is the reason for setting a limit as such? It suggests that there is some reason behind the question, since a buoy's only requirement is to remain floating!
     
  7. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,131
    Likes: 393, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    If it starts to exhibit violent sustained motion, let me know so we can go halves in a wave energy perpetual motion machine project.
     
  8. John Perry
    Joined: Nov 2003
    Posts: 272
    Likes: 25, Points: 28, Legacy Rep: 129
    Location: UK

    John Perry Senior Member

    I assume that you are asking about navigation marks, not ordinary mooring bouys?

    At one point during the 1980's I was briefly employed (only a matter of weeks) by Trinity House, the organisation that looks after most of the UK navigaion bouys and lighthouses. I recall that at that time there was discussion in the office about whether navigation bouys should be designed to have a short or long roll period. UK policy for offshore bouys was generally to go for the longest possible roll period by having a small circular water plane and also to damp the motion by extending the structue down into relatively still water below the bouy. This keeps the light steadier but draft of the bouy makes it harder to crane it onto a support vessel for maintenance. My understaning was that in the US navigation bouys were made 'wave following' by giving them shallow draft and larger waterplane area/diameter. Trinity house had recently commissioned the first of their 'Lanby' bouys, which is a large wave following bouy intended to replace light vessels. Staff who had been sent out to do maintenance work on the new Lanby bouys reported that they had a very umpleasant sick making motion and that the traditional Trinity house bouys were much better in that respect.
     
  9. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,131
    Likes: 393, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    And I suppose if there is a bell on board it may be important as to what the motion is.
     
  10. floating
    Joined: Jun 2009
    Posts: 65
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: NJ

    floating Junior Member

    Ad Hoc - The motivation for setting an acceptable range for the buoy's natural period (say 15-25 seconds, or whatever this group would recommend) is that it is desirable for the buoy to remain reasonably upright. If the natural period is 'too long' then I would imagine that the buoy would not recover to the vertical between waves. If the natural period is 'too close' to typical wave periods then I would imagine the buoy could resonate and pitch excessively. If these guesses are wrong, or if you know of a good natural period range to target, please let me know.
     
  11. Ad Hoc
    Joined: Oct 2008
    Posts: 6,267
    Likes: 441, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    As a rough rule, make the natural period 1.6 times the prevailing/dominant wave period were the buoy is situated. So, if the dominant wave preiod is 10sec, make the bouy's 16 sec.
     

  12. floating
    Joined: Jun 2009
    Posts: 65
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: NJ

    floating Junior Member

    Great, thanks a million.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.