Lifting and Rotating Ship Blocks/Mega Blocks

Discussion in 'Metal Boat Building' started by Furkan, Oct 24, 2021.

  1. Furkan
    Joined: Nov 2020
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    Furkan Junior Member

    Hello all,

    I hope my post is not somewhere unrelated to the topic.

    Anyways, if you ever spent some time and watched the ongoing operations in shipyards, you might have seen that 'huge' steel ship blocks and megablocks( joining of 2 or more blocks) are lifted and rotated with the help of small structure called 'pad eyes' (or eyeplates, lifting eye etc.) that welded directly on ships contstruction. Steel rope of winch's is connected to the padeye's hole with shackles. In that way, mega structures of 300 tonnes are lifted and rotated, they also transferred with vehicles, being supported with brackets welded shellplate and whole block stans on pipes or stakes.

    My question is how they can be so sure that nothing will break and block won't fall down with a great crashing noise? What calculations they do and how? How come they can be so sure about the stakes will support the weight and pad eyes are strong enough?
    [​IMG]
     
  2. TANSL
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    TANSL Senior Member

    Total safety never exists, there may be hidden defects in materials or welding, but high safety coefficients are taken. The most difficult and delicate thing is to calculate the total weight and the center of gravity of each block.
     
  3. Furkan
    Joined: Nov 2020
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    Furkan Junior Member

    Modelling software made it easier. It defines all geometries under parts and can give center of the modelled volumes. After that, you get the COG with excel.
     
  4. TANSL
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    TANSL Senior Member

    Well, the geometry of the parts is defined by yourself but then, yes, the total volume and the CoG are calculated by the software and the calculation will be as good as the model created by the user is good.
     
  5. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    As noted above, the most important thing to do first is ensure that they have an accurate weight estimate of the component and also where the centre of gravity (COG) is.
    I am just assuming now, but maybe the next stage in the process would be to determine how many lifting eyes will be needed for the structure, and where to locate them.
    Re the sizes of these lifting eyes, and the amount of weld area required to attach them to the structure, these can be calculated.
    No doubt the shipyards have programs for doing this, but I think you could simplify it to what is the maximum allowable stress that each lifting eye can take?
    Where stress = Force / Area
    And throw in a healthy Factor of Safety to these calculations to be on the safe side and ensure that the risk of anything breaking is minimal.
     
    Ad Hoc likes this.
  6. TANSL
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    TANSL Senior Member

    Its not that easy. The weld is calculated as you say but there are other joints that are more complicated to calculate (shackle bolt vs eye bolt, for example). On the other hand, it is normal to use several cranes, with different lifting powers, so that not all the eyebolts will support the same weight. There are lateral forces, which are not in the plane of the eyebolts and that must be taken into account. The forces vary in magnitude and position throughout the maneuver and the most unfavorable condition of each element involved in the maneuver must be detected. In short, it is not only a question of safety factor * Force / area.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Calculations.

    As Bajansailor notes above, it all starts by knowing the total weight of the unit to be lifted. How you establish this value is up to you. You can take estimates from data during the build, you can get estimates from CAD etc etc... then, from this, you locate the most appropriate location for the lift. In order to do this, you need the LCG of the unit you are lifting. Thus, again, you can establish this value by any means you wish, hand calcs, CAD etc.... so the whole process begins with knowing the weight and LCG of the unit being lifted.

    So, now you know the location of the LCG and the weight, you identify structure that can support large loads and transfer the load to surrounding structure. These locations are to be, where possible, at locations that are equi-distant from the LCG. The reason for this is that you want to lift the unit without the unit rotating. So the load on each lifting eye is approximately the same. And by structure, i mean are there frames at the location you have identified for the lift positions? If there is, great, if there isn't then you add structure.

    And since the calculation is a simple force/area this tells you that the first load path will be in shear. So, you need to make sure that the location has sufficient shear area, in the structure, and then the surrounding structure, which then forms the secondary load paths.

    So this is a simply Area = Force x Factor of safety/shear stress value. The factor of safety depends upon how many times the unit shall be lifted, also you need to add a %'age (to the load being lifted) for any possible wind loads - or simply state not to be lifted if the wind exceeds 5knots etc etc. This then gives you the shear area required for the lift.

    Like this one we did on lifting the whole vessel:

    upload_2021-10-25_7-12-14.png

    You can see the ever decreasing thickness away from the lifting eye. So transferring the applied load to surrounding structure and at each junction you calculate the shear area required. And then internal structure as well, doing the same.

    The other part is selecting the shackle that is required.

    Since you have identified the amount of load required, you select a shackle that is rated to this load, like so:

    upload_2021-10-25_7-14-23.png

    Very simply, that's it. No magic..

    It is all about understand how to calculate the loads and how to distribute them safely.

    During the lift, depending upon the size of the unit, you would have either 1 or 2 cranes.
    If you have one, then the lift would be either via a simple frame above that connects to one single location onto the crane, or the position of the lifting point where the 4 slings meet is very high, so the angle (and hence increase in load) on the lifting eye is within your calculations (and also does not impart a bending moment). Otherwise, you would have 2 cranes working together for the lift of large units. This is more tricky and must be done slowly and safely under strict conditions.

    Some times you are not in a position to add lifting pads to the unit to lift - for whatever reason, so you make a beam that can do it. yet the lifting pad calculation is exactly the same, and then each pad looks the same.
    This is another one I did, as noted here:

    upload_2021-10-25_7-23-33.png

    So you can lift the whole boat:
    upload_2021-10-25_7-24-47.png

    That's it... simple.
     
    Last edited: Oct 24, 2021
    hoytedow, Sam C and bajansailor like this.

  8. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

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