Longitudinal or Transverse Stiffeners?

[DavidT03]

Well i have a tank that is 23m long and 4.77m wide and im wondering what type of stiffener i should use? Longitudinal or Transverse?

 

[MikeJohns]

You make the tank the main longitudinal structure and support it transversely. The plating may need further support with small longitudinals, but the main frames should be transvereses.

There are other issues like baffle arrangements if this is a tank in a ship.

 

[DavidT03]

Thanks for the advice. For baffle plating im going to add baffles longitudinally and transversly every 3 frame spaces (so about every 2.5m). Speaking of baffles what's a good thickness of plate to use? Im currently using 16mm thick plate for the tank shell but 16mm seems to thick for baffles.

Thanks.

 

[gonzo]

The baffles should double as stiffeners. That is the easy and economic way to do it.

 

[DavidT03]

Ok well ive added the baffle plates now as you can see below and Gonzo you say this should be doubled as the stiffeners? Id like to do that but il need to justify it to my lecturer. Oh and as before, would anyone reccommend a specific thickness for the baffle plates?

Thanks again for all the help

 

[Ad Hoc]

Oh and as before, would anyone reccommend a specific thickness for the baffle plates?

You don't need so many, you can easily take out every second baffle.

For scantlings, any Class rules such as DNV/LR will suffice. It is a 5min calculation.

 

[tom28571]

Not knowing any of the special uses or requirements for the tank, I would go to basic principles. First, I think 16mm is way overkill for this tank although that is just a guess. The baffle walls divide the tank into separate volumes that will reduce loading by the shorter spans. Look at the kinetic loading of the baffles of individual cells resulting from motion at the speeds expected. because there is fluid on the other side of the baffles, that will reduce the net loading. In other words, for dynamic loading forces, you don't have one big tank but many smaller ones.

 

[DavidT03]

You don't need so many, you can easily take out every second baffle.

For scantlings, any Class rules such as DNV/LR will suffice. It is a 5min calculation.

My lecturer told me to add a baffle every 4 frame spacings which that is in the drawing but im with you on this as i think there are to many baffles so il speak with him. Also i do appologize but i dont no what DNV/LR is, could you elaborate?

Thanks again.

Edit: I've researched baffle plate thickness' and i've hit the same common number of 6mm plate so il go with that.

 

[messabout]

Structural design will depend on what you put in the tank. Specific gravity of the contained mass is a principal consideration. For example you could fill it with water, wet sand, diesel fuel, or phosphoric acid. All with different specific gravities. Then there is the matter of projected lifespan and the particular material that you will use for the tank skin. If the tank is buried in earthen matter, or partially buried, or if the tank is to be submerged, perhaps in sea water, then the design numbers will be influenced.

If this is a classroom project you should gain some points by conspicuous and careful consideration of the variables.

 

[DavidT03]

Structural design will depend on what you put in the tank. Specific gravity of the contained mass is a principal consideration. For example you could fill it with water, wet sand, diesel fuel, or phosphoric acid. All with different specific gravities. Then there is the matter of projected lifespan and the particular material that you will use for the tank skin. If the tank is buried in earthen matter, or partially buried, or if the tank is to be submerged, perhaps in sea water, then the design numbers will be influenced.

If this is a classroom project you should gain some points by conspicuous and careful consideration of the variables.

Its a heavy fuel oil tank in a car carrier i was working on last year and ive been designated the task of modifing the tanks that dont comply with the new MARPOL regulation 12A.

 

[gonzo]

The baffles are effectively bulkheads with holes. They are, or can be, structural members.

 

[Ad Hoc]

Also i do appologize but i dont no what DNV/LR is, could you elaborate?

LR = Lloyd's Register and DNV = Det Norske Veritas.
These are Classification societies that you vessel shall be or should be designed to meet and satisfy.

My lecturer told me to add a baffle every 4 frame spacings which that is in the drawing but im with you on this as i think there are to many baffles so il speak with him.

I don't think your lecturer has designed a fuel tank before. Since to state every 4th frame sounds too abstract and without out logic. It is as if this is something he has been told, and no idea of the application and/or methodology.

It is like a boat builder saying, i know all about stability, the boat is fine..or structure, aahh, she'll need 4mm plate. But no idea how to demonstrate why. As a student you role is to demonstrate the "why's".

If this is a classroom project you should gain some points by conspicuous and careful consideration of the variables.

Agreed. Thus do some research into baffles.

Baffles, apart from helping to reduce sloshing as already noted help in the structural design.

However, the baffle is a large plate, which requires painting. It is also much much heavier than a frame. Thus too many baffles adds weight and costs. Since a simple transverse frame shall suffice structurally.

Therefore, as Tom notes, do some research into baffles, for this type of tank (there is plenty of data available). Then draw your own conclusion to the number of baffles. Then use Class rules to calculate your scantings. Class rules may also indicate the minimum number of baffles per unit length too.

 

[Dean Smith]

i do not think there are too many baffles, even as an rectangle cube you would have 50 tonnes slopping around between 5m baffle spacing. and that's why he has gone 16pl
It makes a tidier job than going 8pl and then having to use many longitudinal stiffeners
that's my take on the build
When designing a tank , one always has to consider the build of it If that tank if fuel oil fine, if it is water you have a big potential rust trap in that tight joint at the bottom and so on
So I would design it so that that acute angle was not there and fold the botton and side
The yard that would build such a tank would have the presses that could do this Keep welds to minimum, with such tight angles it makes it very hard to cut back a poor weld

 

[Dean Smith]

something like this rough sketch