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Thread: Ship Draft Survey

  1. #1
    mall Guest

    ship draft survey

    hi to all

    normally bulk eg clinker gypsum coal is transport by ship to port.
    the cargo quantity is identified in our case only by using draft survey water level method.

    as customer, what things should we check when receive the draft survey report? eg water density what is the effect to the accuracy of survey etc.
    and how to ensure the accuracy is acceptable?

    any comment input pls post.

    tq

  2. #2
    Teus Tuinenburg

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    Dear mr Mall,

    The mass of a floating body (ship + bunkers + stores + consumables + cargo)
    equals the mass of the displaced volume. (Law of Archimedes)

    By measuring the draft, either from draft marks directly or through the distance between the water level and marks, and using the ships frame drawing (carene), converted into the draft table, it is possible to determine the displaced volume of water.
    Multiplying the displaced volume by the water density gives the total mass of the ship.
    (ship + bunkers + stores + consumables + cargo)

    The total weight of the ship (ship + bunkers + stores + consumables + cargo) is measured before loading + all readings of tanks, bunkers etc.

    After unloading, this procedure is executed again.

    The difference of those two measurements is the cargo + the changes in bunkers, consumables etc.

    Errors can occur in:
    - difference between draft table (based on drawings) and as built
    - draft reading due to f.i. waves
    Draft marks are normally 10 cm -> accuracy +/- 1 cm
    - accuracy in water density estimate ( saltiness and temperature)
    - accuracy in determining bunkers, stores etc.

    Due to the shape of the carene (underwater body), whereby forward- and aft ship are differently shaped, a trim correction on the draft measurement has to be done.

    Nowadays, the pressure at keel level is measured at 6 locations and those readings are processed in a computer.
    This method eliminates the necessity for a water density measurement.
    This automatic system is still calibrated to a manual draft survey.
    An eventual error in this calibration becomes a system error.
    The bunkers, stores etc still have to be monitored during loading and unloading.

    Official draft measurements should be executed by authorized pesons, agreed by both parties involved.

    In case a measuring installation on shore is available, the draft survey can be checked against this output.
    In case the sea going ship is divided over a number of barges, each barge can be measured
    separately and the sum of the barges should equal the ships reading.
    ( It normally does not, I experienced differences of 1%)

    take care
    teus
    Teus

  3. #3
    Join Date
    Feb 06
    Posts
    38
    Hi.

    Checking the draft survey report you should be careful with any figure in it. All data in this report we can divide onto three types: incoming, working, and outgoing information. The most important is incoming data. There are: draft readings, ballast soundings, density of sea (river) water, density of ballast water, and quantity of other liquids on board. The rest information you can recalculate by yourself having hydrostatic and sounding data from that ship.

    Important outgoing information is: ships displacement, weight of deductible liquids, and constant. The difference between these three figures taken from initial and final surveys gives you the weight of laden or discharged cargo.

    The accuracy of draft survey normally is 0,5%. But its very rough figure. From my experience, if draft survey was properly done, the difference between draft and shore-scale figures for handy-size (50000 MT DWT) vessel is about 50 MT in still water.

    The effect of density variation can be easily calculated. Almost all ships hydrostatic tables prepared for so-called sea density witch is 1.025 kg/L. So density change only for 1g/L for ship with 60000 MT displacement will give correction: 60000*(1-1.024/1.025) = 58.5 MT. If surveyor use certified hydrometer for determining the density of water, usually it doesnt cause any mistakes.

    How to ensure the accuracy of draft survey having only draft survey report in your hand? I think its most important for you but most difficult to answer question. There are hundred ways to make mistake in draft survey whether accidentally or intentionally. They can be made while draft reading during the big waves, ballast sounding when tanks overflowed, during density measuring, calculations etc. The first signal for surveyor that he probably mistaken it is a constant, if it differs from previous draft survey reports. To recap all lets say that you should pay attention if:
    1) Determined constant differs from normal constant for this vessel.
    2) Initial and final constants are different enough.
    3) Unusual sea or ballast water density.
    4) Big difference between declared and determined weight.
    But all of this is only reason to ask the questions and doesnt mean for 100% that any mistake has a place.

    Best regards,
    Vladimir.

  4. #4
    mall Guest
    tq for the replies.........

    in my practive i take draft survey inaccuracy plus minus 5%, is this acceptable?

    according to the replied above it is around 1% or <1% inaccuracy, which is very2 good.

    any study has been done to compare draft survey cargo mt with weighfeeder reading normally install at one of loading conveyor to storage location........

    how much is the different between draft survey and weighfeeder in total mt?

    tq again

  5. #5
    Join Date
    Feb 06
    Posts
    38
    5% inaccuracy?! no, definitely no. Its absolutely inaccurate draft survey. I can be agreed with Teus about 1% if sea is in worry condition, but 2500 MT mistake for 50000 MT of total weight its too much, pls believe.

    About the weighfeeder, balance, weighing scale or whatever you call it. Any weight measuring device can be of cyclic (interrupted) or continues weighing type. In first case it is more complicated, and more expensive but very accurate device. Our company and many others who works with the grains use such kind of weighing scales and we always use shore figure for bill of lading. Though, we do draft survey if receiver or ship owner has requested it. So, as I told in first message, the usual difference between measured weight and draft survey result for 50000 DWT vessel is less than 50 MT and abt 15-30 mt for the vessels with less than 10000 DWT, but it's for sheltered port. If you will load/discharge ship directly from/into the trucks or wagons using the trucks or railroads weighing scales, this difference will be greater. It is not a standard but more likes to an informal international agreement: if the difference between shore weight and draft survey result less than 0.5% - nobody blames, in other case master of ship can refuse shore figure or issue letter of protest with corresponding explanations.

    I know some terminals in our area that are working with continue type of weighing devices. In such case measuring unit under the belt of conveyor weighs the mass of cargo with belt in run. I dont know about others, but all that I know are very inaccurate (5-10% inaccuracy) and used only for internal mass estimation. They are using draft survey results for official documents too.

  6. #6
    mall Guest
    tq again........

    actually the inaccuracy of plus minus 5% was given by one of our draft surveyor........

    it's true vladimir and teus on the inaccuracy, we did it two times by using truck to carry load and get the net weight of load/cargo using weighbridge and unloaded to ship. upon completion of unloading, we did draft survey to the ship and compare survey vs weighbridge figures so the difference is very small around 2 mt.

    what i am not really sure is how they get the trim and correction to the water density? i know the surveyor uses table etc

    pls elaborate on the whole process

    tq

  7. #7
    Join Date
    Feb 06
    Posts
    38
    Hello, mall.

    The main steps of draft survey as follows:

    1. Reading the draft
    2. Correction to perpendiculars
    3. Meandraft calculation for hull deformation
    4. Calculation of corresponding displacement
    5. Displacement correction for the effects of trim and list (First, second trim correction, list correction, last if required)
    6. Displacement correction for water density
    7. Measurement of liquids on board. ie ballast sounding (measuring)
    8. Soundings trim correction
    9. Liquids volume calculation
    10. Measuring the density of sounded/measured liquids
    11. Calculation of weight of liquids in ships tanks
    11. Determination of constant: Const=Displacement-Light Ship-Weight of Liquids
    12. Weight of loaded/discharged cargo calculation as the difference between final and initial draft surveys.

    Its difficult to say abt trim, because I dont understand clear what trim correction you mean: correction for perpendiculars or displacement correction?

    The water density correction its very easy. When you have displacement corrected for trim (and list if you need), the density correction will as follows: Displacement corrected for density = (Displacement * Density of Surrounding Water) / Density Shown in Hydrostatic Tables. For example: Displacement = 10500 MT. Surrounding water density = 1.002 (river). Hydrostatic tables prepared for 1.025. Displacement corrected for density = 10500*1.002/1.025=10264 MT.

    I cant describe detailed all process because its a whole book. But you can try to found some literature. I can advice UN Standard Code of Uniform Standards and Procedures for the Performance of Draft Surveys of Coal Cargoes. Or maybe it would be better to find the GOOD prepared draft survey report from some vessel with all formulas and explanations. I can share it with you if you cant find it.

    But if you are talking about some certain report, you can send it (or part of it) to me and probably Ill able to give you a most comprehensive explanation.

    Best regards,

    Vladimir.

  8. #8
    Join Date
    Feb 06
    Posts
    38
    If it will help you, below you can find the formulas for trim corrections. They are for metric system.

    First trim correction:
    FTC = ( TRIM * LCF * TPC * 100 ) / LBP
    Correction has sign + if: Trim by head and LCF forward of midship or Trim by stern and LCF aft of midship. In other case -.

    Second trim correction:
    STC = ( TRIM * TRIM * 50 * dm/dz ) / LBP,
    dm/dz = ( MCT(at draft+0.5m) MCT(at draft-0.5m),
    the sign is always positive.


    Where:
    TRIM difference between drafts at fwd and aft perpendiculars
    LCF length to the center of flotation from midship
    TPC tones per centimeter
    LBP Length between perpendiculars
    MCT moment to change trim

    You should take TRIM from draft readings after perpendiculars correction, LBP from ships particulars, TPC, LCF and MCT from hydrostatic tables of the ship.

  9. tq

    I have a paper written by a Naval Architect consultancy on the accuracy of draft survey, which is public domain, which I have attached. We have permission to distribute the document since it is beleived that users should have no illusions about the accuracy of ships draft survey.

    It was commissioned by CETOA, the the Coal Export Terminal Operators Association. In the case of coal, it said for old ships expect about 1% systematic error +/- 1% random, for the newer automated ships, about 0.5% +/- 0.5%.

    A good belt weigher can do 0.25% pretty easily, draft survey is pretty rough really. Good belt weighers dont just happen however, many peoples/companies claim they can do them, few deliver.
    Attached Files Attached Files
    Regards
    L. Ian Burrell
    Managing Director
    CONTROL
    SYSTEMS TECHNOLOGY
    http://www.controlsystems.com.au
    ___________________________________________________
    ( +(61) 2 9584 2566 Mobile +(61) 419 292 604 +(61) 2 9584 8080
    + PO Box 249 Mortdale NSW, Mortdale NSW 2223, Australia
    Unit 9, 41-45 Lorraine St, Peakhurst NSW 2210, Australia
    : iburrell@controlsystems.com.au

  10. #10
    mall Guest
    tq for the infomation.........

    as client i just receive the report on the final draft survey only and not with the formula/table on how they derive it.........

    tq again for a very very good explaination and very useful literature and attachment............

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