Rank: Forum user 
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Thoughts please - feel free to ask additional questions.
Welding onto a back deck on a ship, weld is about 8cm deep. Fuel tank directly underneath, the weld covers a small corner where the fuel tank is located. Tank is 22cm from the deck, no void space. Tank is 75% full of fuel oil. Is there an explosion risk?
Any pointers to experts in this area would also be welcome.
Thanks.
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Rank: Forum user
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Sorry, anticipated ignition point for fuel oil is probably >45C, not sure of tank dimensions.
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Rank: Super forum user
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Yes -very high risk of explosion. Tk should be m/t and certified gas free. In my day it was a job for the shipyard welder.
Have it p/p out or transferred to another f/o tk.
"Back deck on a ship" - not exactly marine speak . Can you offer further and better particulars?
Jon
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Rank: Forum user 
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Wasn't there a thread about using abbreviations?
We weld oil filled tanks and either fill them right up or drain them completely. (Impractical on a ship I imagine!)
Either way we then fill the void with Nitrogen to ensure a non explosive atmosphere.
You would though have to ensure the tank will not leak the gas back out while working or ensure over pressurising with the gas so that oxygen cannot re enter.
However the tanks we work on are only up to approx 50,000 litres.
Don't know how practical this method is in your case but maybe worth considering.
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Rank: Super forum user
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Safetygirl,
I'd suggest the only route you should take is to get someone competent to evaluate the task, look at the drawings, visit the site and advise properly. Anything less would not be robust!
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Rank: Forum user
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Should have clarified guys, this has already occurred and I'm looking to reclassifying the incident from a Near Miss to a High Potential but need to substantiate it.
Thanks JohnM for the text speak, made my eyes bleed a bit. Not sure how long you've been out of the industry, or whether its my industry specific, but "back deck of a ship" is pretty specific marine speak in my world of masters, offshore construction managers, engineers etc - wold have preferred using "vessel" instad of ship, but that may have caused confusion.
Thanks guys for your contribution, as always, its appreciated.
Emma
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Rank: Super forum user
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Emma
Just to clarify: Was the welding carried out on the body of the fuel tank itself or on something else above the tank? If it was the tank itself, then appropriate stringent precautions would have been needed to prevent the risk of an explosion through heating of the air and oil inside the tank. However, if it was the latter scenario, the crucial question was surely whether or not the extent and duration of the welding process could have imparted a sufficient amount of heat onto and through the tank body to pose the explosion risk.
Some years ago someone told me that the Royal Navy would carry out repairs to the outsides of fuel oil tanks PROVIDED that the tanks were full of oil or water so as to totally exclude air. However, perhaps this only applied to urgent repairs needed at sea and during wartime conditions. Can anyone expand on this?
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Rank: Forum user
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Hi Graham
It was a winch which was seafastened onto the deck of the vessel. The depth of the deck is 22cm which sits onto the tank, the depth of the weld was approximately 8mm. I don't know how long it took to do this weld as the subbies onboard changed out. Whether this was long enough/deep enough to create enough heat generation, that, indeed is the question.
I remember something from a HAZOP study I did which identified that a tank full with Methanol was at less risk of explosion risk if impacted than a half full tank due to the air/vapour content - perhaps similar applies to fuel oil.
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Rank: Forum user
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It does yes. It's all about reaching flammable limits which depend on the fuel / oxygen / heat balance.
It's how you can extinguish a fire by putting excess fuel on it. So too much fuel means it can't enter flammable limits.
But I wouldn't recommend it as a sole method of control!
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