Sampling requirements for accelerometer to detect slamming

Discussion in 'Boat Design' started by floating, Aug 14, 2015.

  1. floating
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    floating Junior Member

    We have a buoy that experiences slamming, and we would like to put an accelerometer on it to detect slamming events. I would like to know sampling requirements for the accelerometer. DNV 'Hull Monitoring Systems' suggests a sampling frequency at least 500 Hz for bow accelerometers to detect slamming (applicable for ship sizes including light craft). However I am wondering if this applies because the buoy is smaller than most ships: the area vulnerable to slamming is only 1.5m wide. Two questions:

    1) There is a 400 Hz off-the-shelf accelerometer we are considering. Is this sampling frequency sufficient to resolve slamming events?

    2) Does slamming follow Froude scaling, i.e. would slam duration be shorter for a smaller body hitting the water? Perhaps it doesn't since it's driven by the initial impact, but set me straight if that's wrong.
     
  2. NavalSArtichoke
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    NavalSArtichoke Senior Member

    Slamming is manifested by vibrations set up in the structure of the vessel by wave impacts or by the vessel impacting the water while it is at speed. Think of it like ringing a bell: all it takes is one hit to set the bell vibrating.

    The period of the vibration is going to depend on the structure and its response to an impulsive application of force.

    It's not clear why you want to instrument your buoy. You know it's slamming; is the buoy getting damaged, or do you just want to collect statistical data on the occurrence of slamming conditions for the buoy's location?
     
  3. floating
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    floating Junior Member

    "all it takes is one hit to set the bell vibrating": I am looking to detect the hit, rather than the structural response (ringing). We got some unexpected machinery damage during a deployment, and one possible cause is high acceleration during slams. So for future test deployments, we want to be able to detect if slams are occurring when failures occur.
     
  4. NavalSArtichoke
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    NavalSArtichoke Senior Member

    I don't know right off hand of a way to detect the hit independent of the structural response.

    Maybe you need to short-circuit the process here. If the method of deployment of the buoy is suspected of leading to the slam leading to the damage, perhaps a different method of deployment is required.

    It's just speculation without knowing more about the design of the buoy, the method of deployment, and the character of the damage occurring.
     
  5. floating
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    floating Junior Member

    The slams occur after the buoy is deployed, in storms. Inside the buoy is machinery that operates over the deployment. In the past sometimes this machinery has been damaged. On future deployments we'd like to see if machinery damage occurs when slams are occuring.
     
  6. Remmlinger
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    Remmlinger engineer

    6 years ago I conducted some droptests with a model-yacht that might be of interest to you, if you want to detect the hit.
    here is the report: http://www.remmlinger.com/droptest.pdf
    Uli
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Picking up on what NSA is indicating, i would suggest that the design of the buoy might also be an issue.

    Without know what size shape etc your buoy is (fixed, tethered, free floating), a "normal" buoy floats like a cork. Therefore it experiences just pure heave. And since the buoy is so small simply rises and falls with each passing wave. If the buoy has been designed correctly the main body of the buoy should not be "lifted" off the local water surface to create a slam. It should also have a very long period of heave motion to prevent such as well from any resonance with the wave frequency - or period of encounter.

    Perhaps you mean a large wave simply engulfs the whole bouy and slams onto the buoy?
     
  8. floating
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    floating Junior Member

    Our buoy is atypical in shape (spar buoy) and could slam in storms. So, what I would like to do is to detect high impact events using the accelerometer. I would like to know if 400 Hz is sufficient to detect slams.
     
  9. floating
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    floating Junior Member

    Thanks for this article! In your study, I think your 1kHz sampling rate works out to 333Hz at full scale. If that is correct then a 400Hz sample rate should be OK.
     
  10. Remmlinger
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    Remmlinger engineer

    That is the correct scaling, if the full size craft is a sailing yacht of 10 meters.
    Your buoy will most likely have a lower inertia than the yacht and will therefore exhibit faster movements if caught in a breaking wave. You can tell from the g-load-spikes in the time function if your sampling rate is high enough. If the spikes are truncated or if they consist of only 1 or 2 points, your sampling rate is too low.
    Uli
     
  11. BMcF
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    BMcF Senior Member

    For whatever it is worth...the many tests that we have conducted to measure structural slam responses on various craft over the years were conducted, all, using 1 Khz as the sampling rate. The craft ranged in size from 6 meters to 74 meters.

    We simply decided on that sampling rate, to try and standardize our data reduction processes, and figured we could always decimate the data after the fact if necessary. It has been my experience that you can never go wrong over-sampling..but can often get left with unanswered questions otherwise.
     
  12. CDBarry
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    CDBarry Senior Member

    Look at the papers at the Chesapeake Power Boat Symposia. There is a lot of material on filtering an so on for the problem of slam measurement on planing boats.
     
  13. Alik
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    Alik Senior Member

    If the purpose is slamming event not structural vibration or human body vibration, the frequency of 40Hz is more than sufficient. This value is also recommended by HSC Code.
     
  14. floating
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    floating Junior Member

    If anyone is looking for these presentations, they are recorded here. If I figure out where the papers can be obtained from, I will post that.
     

  15. jehardiman
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    jehardiman Senior Member

    Floating;

    The issue here is why the damage is occurring. I have dealt with a slamming issue where the body accelerations were ~ 1.2g, but the structural plate accelerations were +10,000g. First you need to figure out what and where you need to measure. That will then set the frequency of sampling to try to stochastically determine a maximum acceleration/load.
    If failure is occurring because the equipment foundations are failing under "relatively static" loads (1/10th second normally ductile tension failure) then your foundation is under designed and needs to be improved. If however the equipment foundations are failing under "shock" loads (1/10,000th second generally non-ductile brittle fracture) then a completely different approach is needed.
     
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