Root cause for Roof Deck rupture & Pontoon rupture in an EFRTFrom: SUKANT DEV 
Date: Friday, April 20, 2012
Time: 7:26:02 AM
Remote Name: 220.127.116.11
CommentsTank 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.
1 Thickness measurement of the shell plates does not reveal significant thickness loss.
2 Overall condition of the shell is satisfactory.
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.
1 The leg support is of NPS 2.5â€ & 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.