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Root cause for Roof Deck rupture & Pontoon rupture in an EFRT - Friday, April 20, 2012 - SUKANT DEV [1326]
Tank details: TYPE- Single Deck External Floating roof tank Service- High Speed Diesel Capacity- 10,000 M3 Diameter- 37.5M Safe filling height-10.8M, Shell ThicknessesBottom to top course- 18mm/14mm/12mm/10mm/8mm/8mm Roof plate thickness-6mm, Bottom plate/annular plate Thickness- 8mm/14mm. Seal Type- Mechanical Pantograph Type seal, Roof drain- 4 NPS pipe 01 No.
Roof of Tank was observed to have sheared from one of the leg support assemblies.
Observation:
Shell:
1 Thickness measurement of the shell plates does not reveal significant thickness loss.
2 Overall condition of the shell is satisfactory.
Bottom plate:
1 Heavy patch work 40% area approx.
Roof & Pontoon:
1 Heavy undulations were observed on the roof deck & pontoon boxes.
2 Roof was observed tilted on the north east direction.
3 Heavy dust & sludge deposit was found on the roof deck.
4 Seepage was observed at many locations between roof deck plate & inner rim joints.
5 All the leg support assembly of the roof deck & pontoon box were found in seized condition. Legs-2.5 size, Leg sleeve-3
6 Several leg supports were found in broken & buckled condition & crack in pontoon parent metal was observed in 02 pontoon boxes & 01 no. location on roof deck due to failure of support legs.
7 7 out of 10 pontoon boxes were observed to be in punctured condition pontoon No. 1, 2, 3, 4, 5, 8 & 10 & 3 were observed to have seepage pontoon No. 6, 7 & 10.
8 Structures inside the pontoon boxes were found damaged & broken inside most of the pontoon boxes.
9 The lap given for the lap joint on the roof deck ranges from 5mm to 1400mm. At the point of rupture on the roof deck, the lap observed was 8mm.
Salient Observations:
1 The leg support is of NPS 2.5†& sleeve of NPS 23â€. The required radial gap of 4mm as per API 650 not present.
2 The sleeves of the leg support are projected more than man height 1700mm approx. from the roof deck.
3 There is no sump for the emergency roof drains in the underside of the roof deck API 650 Prohibits emergency roof drains in single deck floating roof tanks, as per 2007 edition.
4 There is 01 rim vents provided in the tank. at about 120 degrees to the inlet nozzle. Diagonally opposite to the pontoon 1,2& 3
5 Very few numbers of weep holes are provided in the bottom portion of the foam dam.
6 02 Nos. of bleeder vents are provided at the centre of the roof deck.
7 There is only 01 No. manhole provided on the roof deck.
8 The lap given for the lap joint on the roof deck ranges from 5mm to 1400mm. At the point of rupture on the roof deck, the lap observed was 8mm.
Please share your experience & expert comments on what may be the root cause of the failure. Probable causes that I have in my mind are:
1 Both bleeder vents are located at the centre of the deck alongwith roof drain sump.
2 Pipe schedule of the leg support & leg support sleeves are not proper thereby jamming the leg support assembly.
3 Only 01 No. rim vent is provided in the tank & major pontoon rupture has occured diagonally opposite to the rim vent.
4 API 650 says the minimum lap to be given for lap joint is 5 times the plate thickness. What is the maximum value that is permitted?
5 What is the maximum leg sleeve projection from the deck that is practiced?
Attached are some photographs of the failure. May please give your valuable advice on probable root cause.Post a Reply...
Click on picture for full size view or to view PDF!
Re: Root cause for Roof Deck rupture & Pontoon rupture in an EFRT - Friday, April 20, 2012 - DavidR@Fast.net [39]
This looks like a classic case of one leg left in the high position and all the others in the low position.
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Re: Root cause for Roof Deck rupture & Pontoon rupture in an EFRT - Friday, April 20, 2012 - BABRTs 973 [53]Concur.
Also emergency drains are prohibited on single deck or commonly known as a pan floating roof. This tank is a pontoon roof and emergency drains are allowed.
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Re: Root cause for Roof Deck rupture & Pontoon rupture in an EFRT - Friday, April 20, 2012 - CNT [492]
I thought that at first too, but in the middle photo it looks like the pin has been properly pulled. I have seen some pretty severe corrosion of roof legs that sometimes goes un-noticed where the legs are covered by the sleeves. Im thinking maybe the leg was really corroded where the leg comes out of the sleeve at the bottom and when the roof came down, it got cocked a little bit. If the leg was corroded right at the sleeve, there would be a weak spot so instead of the leg just breaking free of any corrosion and pusing up through the sleeve it crimped over. Once it bent enough that it wouldnt go up the sleeve, something had to give and the decking ripped out.
Similar results of a pin not being pulled, just a slightly different cause.
I dont think it was just because there was some corrosion gunk build-up in the sleeve making for a tight fit. Even if the leg was junked up in the sleeve where it would be difficult for a person to pull the leg up which isnt unusual, I think the weight of the roof coming down on the leg would break the leg free and push it up.
I think the tell-tale is in the bent over leg on the underside. If the leg was as strong as it should have been, I think it would have remained mostly straight and the deck would have ripped before the leg bent over if it was just a pin or stuck in the sleeve issue.
I recommend you check the thicknesses and conditions of your other legs. Make sure you have cribbing or something under the roof incase you get more leg failures while the tank is out of service.
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Re: Root cause for Roof Deck rupture & Pontoon rupture in an EFRT - Friday, April 20, 2012 - BABRTs 973 [53]Looking at the second picture the leg is not pined. Looks like when the legs on the roof were pined in the low position and your observation there was corrosion preventing the legs from operating,the pin was removed with the thought being the weight of the roof would push the leg up through the sleeve.
They were wrong.
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Re: Root cause for Roof Deck rupture & Pontoon rupture in an EFRT - Friday, April 20, 2012 - DavidR@Fast.net [39]The pin may be gone now but it looks like the leg is up in the sleeve.
If the leg was up in the sleeve, this will take more thought.
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Re: Root cause for Roof Deck rupture & Pontoon rupture in an EFRT - Friday, April 20, 2012 - DavidR@Fast.net [39]
Re: Root cause for Roof Deck rupture & Pontoon rupture in an EFRT - Thursday, April 26, 2012 - Raymand [1324]I think that the cause has been addressed. Are they asking for repair procedure and a recommendation for steps to take to prevent this from happening again?
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Re: Root cause for Roof Deck rupture & Pontoon rupture in an EFRT - Thursday, August 16, 2012 - Sukant Dev [1326]Thank you BABRTS, DAVIDR & CNT sirs for your knowledge sharing.
Upon further investigation it was revealed exactly what all of you told.
It is a classic case of some legs left in high leg position while other legs in low leg position.
Of course the leg support & leg sleeve assembly was jammed because there wasnt sufficient radial gap between leg support & leg sleeve. When the jammed leg assemblies could not be moved, the pin was taken out thinking that the weight of roof would enable the legs to operate. This negligence leg to the tank failure.
We are in the process of reconstructing the tank. So far there has been no hassles.
We are replacing all the leg supports with proper radial gap between leg supports & sleeve, brought the roof deck down & reconstructing the failed pontoons after dismantling.
Hoping valuable inputs from you in the future too.
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