Vanishing Angle for Bulk Carrier

Discussion in 'Stability' started by zaim_2102, Feb 8, 2008.

  1. zaim_2102
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    zaim_2102 Junior Member

    Hello,i'm designing the bulk carrier 187m in length,26.81m beam and 11.29m draft,but the vanishing angle of designed ship (at 77.2 deg.) insufficient to meet the requirement of MCA UK regulations at (90 deg.)... if any have an idea or suggestion how to overcome this matter???
     
  2. Guillermo
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    Guillermo Ingeniero Naval

    I'm not used to design bulkcarriers and, as I'm not at the office at this moment, I cannot consult my books. Are you sure about the 90º minimum AVS thing? Could you post the relevant MCA regulation on that, please?
    Cheers.
     
  3. zaim_2102
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    zaim_2102 Junior Member

    Code
    11.2.3.1 Monohulls

    Criteria
    11.2.3.1.2 Range of positive stability
    from the greater of
    spec. heel angle 0 deg.
    angle of equilibrium 0.2 deg. (my ship = 0.2)
    to the lesser of
    first downflooding angle
    angle of vanishing stability 77 deg. (my ship = 77 deg.)
    shall be greater than (>) 90 deg. (my ship = 76.8 deg.)


    Can u give me some explanation regarding to the 'Trim and Stability Booklet', about the content that be included? i'm sorry about my weakness...
     
  4. gwboats
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    gwboats Naval Architect

    Bulk carrier Stability

    zaim_2102

    I have been trying to find your MCA reference of 11.2.3.1
    My only source seems to be from the old 'Code of Practice for Safety of Large Commercial Vessels' which applies to vessels over 24 metres BUT does not apply to ships that carry cargo.
    The wording for your stability reminds me of that found on the 'Hydromax' list of criteria.

    It seems you may be:
    a) Using out of date/irrelevant requirements - The Code of Practice is now refered to as 'The Large Commercial Yacht Code'; a more meaningful title.
    b) In the UK a bulk carrier would come under a Statutory Instrument such as the 'Merchant Ship Construction Regulations'

    Stability information books have to be written in a standard format for presentation. You can obtain examples from the MCA, I believe they call them Model Books, which are relevant to your vessel.

    I hope this is of some use to you.

    Good Luck
    Graham Westbrook
    Naval Architect
     
  5. Guillermo
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    Guillermo Ingeniero Naval

    Regarding stability matters for cargo ships, the MCA refers to IMO instruments, specifically to the 'Code on Intact Stability for all types of ships covered by IMO Instruments'. I don't remember having seen the 90º AVS thing there, but I'll check tomorrow.

    Cheers.
     
  6. zaim_2102
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    zaim_2102 Junior Member

    thank you...

    thank you for the guidance. i'll check the updated requirements but sometime lack of sources.
     
  7. drshaddock
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    drshaddock Design Engineer

    Zaim, I'm curious what the freeboard is on this bulk carrier. And also, are you including cargo in your calculations? The angle seems quite low and makes me wonder if you're figuring the vanishing stability angle when the ship is empty and without ballast added to compensate. (The TP7301 states, I believe, that the owner and master are always responsible to ensure ballast is added if cargo is not enough to keep the ship within the test limits established when the stability booklet was prepared. I don't know how the MCA regards this issue, however.
     
  8. zaim_2102
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    zaim_2102 Junior Member

    Loading Condition

    can u tell me the correct procedure to test the transverse stability? is it the ship must be full load or without the cargoes inside holds (empty)? hope u can make me clear, i really want to know the correct procedures to analyse the stability of the vessel.
     
  9. drshaddock
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    drshaddock Design Engineer

    The correct procedure depends on the governing body that's going to decide whether the boat is suitable. If the ship is built, here's an example of various tests, which I found in a post by Guillermo: http://www.tc.gc.ca/marinesafety/tp/tp7301/part1.htm#STAB-2-APPENDIXB

    But since I believe you're in the design stage, and you didn't mention what the freeboard of the ship was, I put together a really quick exercise to discover what design might lead to an angle of vanishing stability of only 77.2 degrees. I attached a jpg file--very rough, but very simple--to illustrate. Let's say your bulk carrier is a very simple design with a flat bottom and vertical sides, just to make it easy. If I draw a section view of that using your draft and beam figures as shown on the left, then tilt it to 77.2 degrees, I can figure a center of gravity that would make it balance at that point by drawing a few perpendiculars, and then use that point to 'mirror' the hull which gives me a box with perfect balance. This reveals a freeboard of 17.3 meters--but it's for an empty box, and it's assuming that your deck plating would be exactly the same weight and thickness as the bottom of your hull. This is unlikely.

    A few easy ways to fix the vanishing angle problem and get you above 90 degrees would be to lower the freeboard, or increase the thickness of your bottom and below-waterline hull as compared to the topsides and deck, or to add internal ballast and/or cargo. It's a carrier, so you're going to be carrying cargo, right? You can assume the cargo is homogeneous but if you're depending on cargo or ballast to give you stability, obviously you're going to have to make arrangements to guarantee that it won't shift in a rolling sea, etc. and that it's onboard at sea. (As I mentioned before, that's the responsiblity, ultimately, of the master and owner, but still--you're the designer, and you want to sleep peacefully at night, right?)

    If you can provide more details, people can help out a lot more. Guillermo and others are much better qualified than I, certainly, but I've got two cents' here to throw in.
     

    Attached Files:

  10. drshaddock
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    drshaddock Design Engineer

    That was way too simplified--and I was wrong

    If you read through my simplified discussion from my last post, it was over-simplified and missed a point. It was nagging at me after I did it, and I woke up this morning thinking what it was! I had just rotated the model, ignored the waterline (it was only a 'box' that I was rotating, and I wasn't thinking of it as a ship) and ignored the water itself--kind of important, actually...

    What I missed was that the real key to the angle of vanishing stablity is the point where the center of gravity is directly above the center of buoyancy. This happens under three circumstances: one, which we like, is when there is no heel angle whatsover (assuming centralised weight and cargo distribution, etc.) and the CG is directly over or under the CB. As you start to heel, the CB moves away from the CG and that establishes a righting moment--the horizontal distance times the weight (displacement) of the boat. The further you heel, for a while, the further the CB moves from the CG, and the righting moment increases--the ship wants more than ever to return to level and make everything right again... But as you continue to heel, the CB starts to move back under the CG. And as the deck submerges, it starts moving back faster, which you'll see on a righting moment chart--and it's a point you need to indicate in a stability manual. Finally the CB will be back under or over the CG and that's your angle of vanishing stability, when your ship is teetering. And then the wind blows or a wave hits or cargo shifts or someone turns over in their berth and you possibly roll over to full capsize, where, again, the CG and CB are aligned over each other and the ship is stable again but unhappy. This is all elementary stuff, and I apologize for that.

    I thought it would be elementary to 'back into' your problem and determine a CB by just submerging my box model so it continued to displace the same area below the waterline, and calculate the CB where it would be directly below the CG, and arrive at your freebaord by the back door, so to speak. But it seems to turn into an iterative process and my mind isn't up to the task this morning as I've got other pressing work. And since it's your design, you already KNOW the freeboard; you just haven't shared it yet.

    Again, I apologize for misleading you by oversimplification in the first place. But if you provide a few more details, I'll try again to help out.
     
  11. Kerry Thomas
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    Kerry Thomas Junior Member

    A bulk carrier would come under IMO rules. Which are usually also a regulation in the flag state. They also state what has to be in a stability booklet. It also depends on the classification society rules you are building under.
    Simply put. To increase the angle of vanishing stability. Increase the freeboard and/or lower COG.
    However i believe you should already know this or i am a little concerned about the safety of the vessel!

    The stability booklet will have examples of lightship (which often cannot be allowed to occur because of instability),
    Ballasted, Full cargo of varying types, and part cargo. Part cargo, especially of grains, is the worst case because it may move.
     
    Last edited: Feb 14, 2008
  12. Guillermo
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    Guillermo Ingeniero Naval

    Zaim,
    Definitely I find nothing regarding the 90º exigency you mention, which probably is for another kind of vessels.
    What are applicable to bulk carriers are the IMO stability recommended general criteria and the weather criterion. When carrying grain, they shall comply also with the resolution MSC.23(59).

    Here a list of all MSC Resolutions:
    http://www.imo.org/InfoResource/mainframe.asp?topic_id=435&doc_id=3710
    (Several of them are applicable to bulk carriers, depending on type of cargo)

    Here the link to buy the IMO Code on Intact Stability:
    https://www2.imo.org/b2c_imo/b2c/init.do
    (Click on "Marine Technology" on the left side bar menu, and scroll for the book)

    Cheers.
     
  13. Kerry Thomas
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    Kerry Thomas Junior Member

    You will also need to look for requirements for bending and sheer stresses in IMO. and under Class. They are as critical for a vessel that size as stability

    Also
    Refer to Lloyds, DNV, Veritas etc under classification societies. If your vessel does not meet a class rules then it is uninsurable.
     

  14. zaim_2102
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    zaim_2102 Junior Member

    a lot of thanks for all of you...
     
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