Full welding of compartment plates 1

[farzam]

Hi All

Regarding API650 paragraph c.3.6, full welding of compartment plates along all edges is mandatory, I need to know that if welding of top edge of compartment plate is waived, capsizing of roof will be happend.(roof is double deck with ID=72m and area=4030m2,max. compartment area=56.5m2,roof liquid level=0.146m,total of compartment=50,Min compartment elevation=0.5m)

Thank you very much

 

[LeonEarle]

The intent of this requirement is to provide liquid tight compartments. In the design condition where two adjacent compartments are flooded, you could see sufficient sag of the roof to allow flooding of adjacent compartments across the top of the compartment bulkhead. The specific design would have to be reviewed since it is possible to provide sufficient depth to the double deck arrangement that even in the flooded condition, liquid could not overtop the compartment bulkhead.

 

[JoeTank]

farzam,
I have never heard of a double deck capsizing, but I have heard of progressive sinking of roofs due to lack of welding along the top of a compartment plate. API-650 was revised about 15 or 20 years ago to require full welding of all compartment plates. If this is for a new tank you have to weld them fully. If it is existing roof you can evaluate the condition and decide a course of action. API-653 does not require upgrading of an exsiting roof to meet cureent requirements of API-650

Joe Tank

 

[farzam]

Thank you my friends for your responds,


I have a question:If leakage is happend at one compartment,and the compartment is flooded, height of liquid will never reached to upper deck and flooding of adjacent compartments across the top of compartment bulkheah will never be happend. so how the tank will be sinked?

Thank you again

 

[JoeTank]

farzam,
For shallow compartments it is possible for the compartment plate to be over-topped. Sometimes designers forget about heavy reactions due to rolling ladders. This causes further depression and may contribute to progressive flooding.

Joe Tank

 

[farzam]

Dear Joe Tank,

Actually I dont get why you mentioned about rolling ladder,theres only on stairway outside of tank, but I dont understand the relation between compartments and stirway.
would you please explain me about this matter?

Thanks a lot

 

[IFRs]

A rolling ladder is a long ladder with rails that is pivot mounted on the top of the tank with wheels on the other end that ride on a track on the floating roof. As the roof rises and sinks with the liquid, the wheels move back and forth on the track. By its nature, the rolling ladder is heavy and can press fairly hard on the floating roof. Not all external floating roofs have rolling ladders - some have vertical ladders and some have no ladder. Also, there are other loads on the floating roof to be taken into account - rain, live, etc. The code says seal weld the compartments for a good reason - experience and good engineering judgment.

 

[farzam]

Dear IFRs

Thanks for your explanations,
Let me ask you something,we have four floating roof tanks,and our contractor has been finished erection of one roof without full welding of compartment, but I pushed them to perform it and they did it for 2 compartments and performing leaktest,I was there inside the compartment and we found theres leakage between upper deck plates that they are overlap.If we have to weld overlap plates to each other(inside lines)theres too much welding works, and maybe distortion is happend for upper deck(thk.=5mm), whats your opinion?

Thanks in advance

 

[IFRs]

The ideas is to make a buoyant compartment that has no leaks. I see your problem though - each compartment can leak into the adjacent ones through the lap joint, unless the lap joint is welded on the underside. You may have to cut the lap away where it crosses the compartment side so it is a single (butt welded) thickness in order to seal it up.

 

[farzam]

Dear IFRs

Thank you very much for your comments