Leak in Double deck floating roof compartment

[shyamponnada]

Hi, We have a problem recently noticed in Two of the external double deck floating roof Naphtha / reformate storage tanks, which were commissioned in 1999.

During recent periodic checks, one of the circumferential compartment of the double deck floating roof storage tank was observed with product to a height of approximately 6”, This is the 3rd circumferential compartment out of total 7, (observed from the deck manholes).
In the other tank 2 of 6 compartments were observed to have compartment leaks (3&4th from outer)
These leaking deck compartments are planned for repair during an outage, however till the opportunity arises, these were under close monitoring.

In this context I would seek technical clarifications by design experts on,
• The load bearing capacities of the floating roofs with liquid filled and their stability.
• The probable reasons for liquid filling into the compartments all around (the middle compartments are circumferentially connected, suspected corrosion of the under deck plate)
• Are there any online leak repair methods recommended for double deck floating roof compartments to prevent further damages.

Appendix C of API 650 requires that the design of the roof be such that it can withstand a leak in any two adjacent pontoons, but in this case the leak is in the circumferential compartments of the DDFR's

I shall provide detailed information and required details for the technical support either in forum, (or) to inbox.

Thank you.

 

[brpillai]

shyamponnada
I have attended many tank situations like, long back when I was in charge of inpsection and maintenance of tank farms and pipelines.

The tanks properly designed could withstand two adjacent compartments having leaks.
There is no such load bearing capacity as scuch, as the roof is floating in liquid and is fully supported.
As some of the pontoons are having leaks, the roof will get tilted.
The more it gets tilted, the more fuid will enter the compartments and further it will pull down the roof and this effect will be cumulative.
Slowly, the whole roof could sink. I had witnessed such occurances. One incident is that against my report for de commissioning the tank, the tank was operated and become a casuality.

The probable reasons could be
i) corrosion leakes of the bottom deck pates/ weld joints.
ii) some times the vertical plates dividing compartments are not tested for leaks and hence leak in one of teh compartments will enter the other, even if that compartment is not having any leak.
iii) no leak repair of the bottom plate will be effective as it is always wetted with liquid.

I suggest to have the leak closely monitored and take out the tank for early repair, beofre something serios happens. It is to be very seriously taken. Once the roof sinks, you will have a more serious problem.

 

[shyamponnada]

Thank you brpillai.

The design of the roof is double deck floating roof with multiple circumferential ring compartments, The leaks are not in the pontoons / bulk heads. In our case the 3rd and 4th circumferential rings are filled with product reformate out of 6 rings. Inner rings are circumferential compartments and outer is like pontoon types.

At present the liquid is at uniform level of 6" height for the entire circumferential rings of 3rd & 4th, attached a drawing for reference.

I am also trying to locate a source where we can do any online repair of the floating roof, such as filling the compartments with foam or service suitable compound to extend until we have an opportunity for repair.

Thanks

 

[NBrink]

Yeah, tilting isn't a big concern with a roof like that.

That said, you clearly have something going on, and things happen. Someone forgets to open the roof drain during a big storm, and bam, you've got all sorts of problems...

Depends on where you are located, but I've had foaming quoted against buoyancy balls before. We ended up going with buoyancy balls, mostly so we didn't have to hydroblast out 16,000 cubic feet of hydrocarbon soaked foam and then dispose of it. The balls are easy when you come down for turnaround, just land the roof and cut a hole in the bottom deck. There are folks that claim you can get bladders built to match the inside of a pontoon, and displace product that way, but I've never heard anything good from anyone that's used them.



I plead being rudely shoved to night-shift for any spelling errors...

 

[MAshok]

It will be advisable to go for immediate repair of the roof after decommissioning the tank due to following reasons:
1) The bottom deck plate is stressed due to the buoyancy force applied on it by the product in tank. The stresses are usually near the allowable stress under the design conditions (which are when the roof is floating with two compartments punctured or with the design rain water accumulation). So with two compartments already leaking, the roof is on its design limit. Any additional load on roof (due to rain water accumulation or additional leaking compartments) could cause a cascading effect of more ruptures (in buoyant compartments) and eventual collapse.
2) The cost of repairing the tank & floating roof and the time required for the repair will be much higher if the roof sinks, as such an event will cause additional damage to stilling wells, tank shell, tank bottom and the roof itself. You may have to replace the entire roof, part of shell and part of bottom under such circumstances, whereas you may get away with just replacing the bottom deck plate at the most, if repair is attempted before failure.
3) The cost of product lost (product escaping through the roof drain) and the consequential damage to the environment will be much higher than the cost of repair. .