external floating roof API650

[dorios]

hi,
I have an external floating roof 60m dia tank experiencing problems with deformations possibly because of vapour bubbles underneath. Water accumulation on top exacerbates the deformation but as soon as the vents are open, the water comes back towards the central area which to me suggests that the internal vapour pressures are responsible for this deformation. This is a 5mm pontoon roof, oil tank to API 650 (designed to 0.7 specific gravity).no product on the tank, seals working perfectly.
Q: what other factors can be responsible for slightly tilting the roof (pontoon corrosion?, flooded pontoon?)
some brainstorming would help.
thanks

 

[IFRs]

As for tilting the roof, the seal(s) may appear OK but are getting hang up on a tank shell bulge. What type and manufacturer and year is the seal? It could be that the floating roof is horizontal but the tank is leaning? Perhaps a roof drain swivel is frozen? Or an anti-rotation roller getting bound up on a gauge pole? Or a floating suction line swivel joint bound up? Exactly how much is it out-of-level? Does the floating roof change levelness?

 

[dorios]

hi,
shell bulge and differential settlement are now being investigated. no frozen swivel as the tanks are in a tropical zone.
how much is out of level is difficul to say: we see this when the rain accumulates on one side (I have to ask the maint. people if it is all the time on the same side?)but we know that when the bleeders are open the roof assumes initial position.
we assume it is a ballooning effect of the central area where gas bubbles are buit up. I could not find anything in the Codes that tells me how much deflection of the floating roof I can have. that is another matter but still of interest. thanks

 

[JoeTank]

Roof tilt may also be due to a flooded compartment. Have you checked them to see if they are dry? What is tank diameter? How long are the deck leg sleeves below the deck?

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[dorios]

we are currently investigating the pontoon and look for flooded compartments. It is a 60 m diam tank. We discarded the shell bulging, diff settlement and it seems that the slight tild is due to gass pressure (probably over 10 psi)... we'll do some FEA to see if this pressure can cause central ballooning although the roof should be designed to withstand 11 psi according to API650.

The sequence in my mind is:
- large gas bubble (5-6 dia)due to unstable components forms in the centre
- roof is slightly balooning in the centre
- rain (+ the poss effect of wind ) accumulates on the roof and is pushed sideway as the centre deck is already slightly ballooned
- tilt (bear in mind: this is a sl;ight tilt) occurs


we opened the bleeder vents 1 first-> water tends to come to the centre then the second bleeder vent no 2 opened which gave gas release (+ some crude) following which water runs towards the centre which suggests a) the central deck was ballooned and b) the cause was the formation of gas