Reason behind Shape of Bulk Carrier Midship

Discussion in 'Boat Design' started by kiddo14, Jul 4, 2015.

  1. kiddo14
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    kiddo14 Junior Member

    Hye ! Everyone
    I am curious to know the reason why bulk carrier midship has top side hopper tank and Bottom hopper tank instead of conventional L-shaped or continuous wing tank? I have an initial guess it maybe because to for easy cargo handling i.e. the shape of tank allows the cargo to slide and come at the center portion of the tank and then it is easy to discharge. Another thing I can guess is that it maybe because to carry some additional cargo. Because in the middle portion of shell it is single skin. Whatever it is I want to know the actual reason behind this kind of midship section. I have gone through some books, there they only describe the section not the reason behind it.
     

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  2. NavalSArtichoke
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    NavalSArtichoke Senior Member

    Bulk carriers can either carry high-density cargoes like ore or low-density cargoes like grain. For either type of cargo, the sloping sheets in the top and bottom of the hold block off inaccessible corners in the cargo holds where the bulk cargoes might suddenly shift while the vessel is at sea. Such shifts in cargo can cause the vessel to develop large angles of heel, or even capsize.

    For grain carriers, special stability calculations are required to be performed to ensure that the vessel is capable of loading grain and to give the crew a means to assess the stability of the vessel when it is carrying a grain cargo.

    For vessels carrying ores in bulk, the high density of such products usually means that the holds may only be partially filled before the available deadweight of the vessel is used up. The saddle tanks can be filled with ballast water to reduce bending moments should a hold be left empty of cargo, for whatever reason.
     
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  3. jehardiman
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    jehardiman Senior Member

    The reasons are spelled out in SNAME's Ship Design and Construction (Chapter VI in the old single volume). As you have guessed, trimming the cargo and stability are behind it.
     
  4. kiddo14
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    kiddo14 Junior Member

    Thanks for your reply. If I am getting you correctly then you are saying the shape of the tank works as a cap or blocker that prevents sudden cargo movement so that we can avoid sudden large angle hill. But in this shape would not it be adverse for damage stability? Because after a sudden height from baseline the hull is basically a single skin so if any damage in that portion would eventually lead to flooding the entire compartment ?
     
  5. kiddo14
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    kiddo14 Junior Member

    Thanks for your reply. I will definitely try to read that chapter.
     
  6. NavalSArtichoke
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    NavalSArtichoke Senior Member

    The bulk carrier is designed first to ensure that it meets the applicable intact stability criteria for these vessels, which means that protecting against cargo shifts is a pretty important consideration.

    In the damage condition, double skinning or other longitudinal subdivision of the cargo holds would lead to off-center flooding, which means the vessel would heel to the damaged side, assuming equilibrium can be reached. Off-center flooding reduces the amount of residual righting energy for stability in the damaged condition. In other words, it's better to have the damaged vessel remain substantially upright, rather than heeled to one side. Also, if the damaged hold is loaded, the presence of cargo will limit the amount of flooding which occurs and reduce the amount of free surface due to water in the hold.

    Ship designers have to make various trade-offs in trying to produce a vessel which operates economically while still satisfying stability regulations.
     
  7. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I think it may have been included already in the answers, but part of the reason may also be that very heavy bulk like iron ore will only fill the bottom of the cargo space and make the ship too stable, and also a small radius of gyration, which combined causes dangerous accelerations particularly for the crew. So when carrying iron ore they could fill the upper ballast tanks to raise the center of gravity and increase the mass moment of inertia. I think other designs and regulations specifically for heavy bulk like iron ore might require some tanks bellow the bulk cargo space. Not sure. I am not a naval architect or marine engineer.

    I read it some place else, but Wikipedia mentions it...
    https://en.wikipedia.org/wiki/Metacentric_height

    A larger metacentric height on the other hand can cause a vessel to be too "stiff"; excessive stability is uncomfortable for passengers and crew. This is because the stiff vessel quickly responds to the sea as it attempts to assume the slope of the wave. An overly stiff vessel rolls with a short period and high amplitude which results in high angular acceleration. This increases the risk of damage to the ship and to cargo and may cause excessive roll in special circumstances where eigenperiod of wave coincide with eigenperiod of ship roll. Roll damping by bilge keels of sufficient size will reduce the hazard. Criteria for this dynamic stability effect remains to be developed. In contrast a "tender" ship lags behind the motion of the waves and tends to roll at lesser amplitudes. A passenger ship will typically have a long rolling period for comfort, perhaps 12 seconds while a tanker or freighter might have a rolling period of 6 to 8 seconds.

    The period of roll can be estimated from the following equation[2]

    T =\frac{2 \pi\, k}{\sqrt{g \overline{GM}}}\
    where g is the gravitational acceleration, k is the radius of gyration about the longitudinal axis through the centre of gravity and \overline{GM} is the stability index.
     
  8. NavalSArtichoke
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    NavalSArtichoke Senior Member

    While what you say may be true, a simpler explanation exists: when a bulk carrier travels in ballast without cargo, a certain amount of water ballast will be required to maintain trim and immersion of the propeller. Given the layout of the cargo hold, there is simply no other space in which to construct ballast tanks except in the bottom under the cargo holds or in the the saddle tanks running under the main deck.
     
  9. Jamie Kennedy
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    Jamie Kennedy Senior Member

    But the question was regarding the shape of the upper ballast tanks. Why that rather than conventional saddle tanks? Would you ever fill the upper ones and leave the lower ones empty to reduce the rolling period when carrying something like iron ore? I think swash plates in the bottom fuel and ballast tanks can also reduce the period, or at least dampen motion if you get into a harmonic situation with waves. But the upper side tanks will directly affect the rolling period by raising the center of gravity and increasing the moment of intertia if a heavy but partial load of cargo has made the ship too stiff.
     
  10. Jamie Kennedy
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    Jamie Kennedy Senior Member

    This amateur has related queery. When cargo ships load up is there any way for them to measure the rolling period before they leave dock, to confirm their calculations. I know when I did sea trial of a 40 foot fishing boat (best summer job ever) we just ran from side to side. Is there sensitive instrumentation that can detect more subtle movements while shifting a small amount of ballast back and forth or some such thing? Or does a captain just wait to see how it handles and recalculate and shift ballast if needed?
     
  11. NavalSArtichoke
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    NavalSArtichoke Senior Member

    It's not clear what you mean by "conventional saddle tanks".

    As was explained previously, the sloping plates in the cargo holds are there to prevent voids from forming in the hold when cargo is loaded, into which voids the cargo could settle and shift unexpectedly while the vessel is at sea.

    Swash plates in long tanks are installed primarily to prevent the contents of partially filled tanks from slamming into the tank boundaries, to prevent the contents of the tank from moving in synchronization with the vessel as a whole.

    Like I said before, maybe you can dampen motions somewhat with the upper saddle tanks, but that's not why they are there. With a bulk carrier, you want to get from A to B with the vessel upright and in one piece, i.e., stability and strength are the two most important considerations. A bulk carrier is not that prone to bobbing around in the ocean like a cork.
     

  12. NavalSArtichoke
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    NavalSArtichoke Senior Member

    In general, no. The crew is more concerned that when the vessel leaves port, it is properly trimmed and that the load line is not submerged.

    While one can easily get a small boat to start rolling, doing so with a 100,000 ton container ship is a bit harder. Also, ships are generally designed so that they trim out properly when fully loaded, so adding any ballast would probably cause the vessel to submerge her load line.

    Ships are fitted with inclinometers, to provide the crew with a visual gauge of how much roll is occurring at sea, but I think these have only crude precision of measurement.
     
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