How'heavy is 88 tons of steel at 5000 ft?

jeremyshaw

jeremyshaw

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I see that the attempt to cap the oil leak in the Gulf of Mexico has failed due to ice crystals making the steel funnel BP lowered down too buoyant.

We are familiar with the fact that stuff weighs less under water, and thus you need much bigger concrete blocks etc. for mooring than you might think. I've even noticed the difference wandering around under water looking for a better spot to park my anchor. But I can't find a formula for it. I was wondering if at 5000ft 88 tons of steel might be too light. You'd think they would have thought of that (but then thought the bankers knew...).

Anyone know the answer? Thanks...
 
It's not ice, it's hydrates. Hydrates are a peculiar mixture of water and mainly methane which under certain pressure and temperature conditions solidify into something similar to ice. This in turn blocks the passage for oil and gas coming from the the well causing them both to accumulate under the structure rather then to flow to the surface. The depth of 5000ft or any other depth makes no difference to the weight of the steel as water in incompressible. As soon as the steel is immersed however its weight is reduced by the weight of the volume of water it displaces (its mass remains the same of course).
 
Cant remember the formula offhand, but think its 65.44-8.34 divided by 65.44, gives bouyancy factor in fresh water, for steel. So salt water would be a bit denser, approx 1.0256 SG.
which would mean, 88 tons would be 75 tons approx, but then pressure effects, not sure, but its still effin eavy' Guv!
But just thought, some of it is concrete, which is considerablky lighter then steel, but still effin heavy!
 
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Steel has a density of a bit less than 8 tonnes per cubic metre.
Sea Water has a density of a bit more than 1 tonne per cubic metre.
So a lump of steel that weighs eight tonnes in air weighs a bit less than 7 tonnes in sea water -- i.e. it "loses" about 13% of its weight.
So I would say the answer to the original question is about 75 tons.

Incidentally, for those who think aluminium anchors are a good idea: most aluminium alloys "lose" about 40% of their weight in sea water.
 
Malo37 said:
It's not ice, it's hydrates. e. The depth of 5000ft or any other depth makes no difference to the weight of the steel as water in incompressible. As soon as the steel is immersed however its weight is reduced by the weight of the volume of water it displaces (its mass remains the same of course).
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Thanks for that - two very interesting facts.
 
There is quite a good account of the whole story, with lots of pictures, on the official BP website.
 
yes, and the internal briefings to staff have been quite astonishingly open as well ! The new boss is very well advised in this matter ...
 
I wonder..

If the thing ways anything like 88 tons in the first place.

I can't even imagine what kind of equipment you would need to lower a mass of 88 Tons 5,000ft.
 
activesail said:
If the thing ways anything like 88 tons in the first place.

I can't even imagine what kind of equipment you would need to lower a mass of 88 Tons 5,000ft.
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Of course, you have to lower rather more than 75/88 tons: you need to add on 5000ft (or 1500m in real money) of, say, 150 ton breaking strain cable. I can see that coming in at another 50 tons.

<edit> Oops. 150 ton breaking strain cable would be a bit risky once you add the cable weight to the payload, but of course 200 ton cable would weigh more, as so on...</edit?
 
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Lady Campanula said:
Huh! On that sort of misleadin' argument, you'll be sayin' next that anchors should be made out of lead....!

Or depleted uranium....!

:eek:
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Or you could "save weight" and make it "easy to handle" by making it out of some kind of plastic.
At least that way, you wouldn't need to worry about whether there was enough weight for the flukes to penetrate the sea bed ... you'd be more concerned about how to get it to go down in the first place (and stay down once it was there) Maybe you could use some kind of weight? :D
 
We can quite easily manage to lower much heavier weights than that, the riser weighs considerably more, but if you use bouyancy inteligently, you can manage heavy weights.
 
Buoyancy Factor - Quick Method of Calculating Weight In A Fluid From Air Weight

A simple way of calculating weight reduction from air weight due to fluids is to use buoyancy factor (BF).

BF = 1-(Fluid Density / Steel (or other material) Density)

For typical 4130 steels in sea water you can multiply the air weight by a BF of 0.869 to get the weight in sea water, or by a BF of 0.873 to get a weight in fresh water.
 
BlowingOldBoots said:
A simple way of calculating weight reduction from air weight due to fluids is to use buoyancy factor (BF).

BF = 1-(Fluid Density / Steel (or other material) Density)

For typical 4130 steels in sea water you can multiply the air weight by a BF of 0.869 to get the weight in sea water, or by a BF of 0.873 to get a weight in fresh water.
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Yep, correct, the formula I gave is just that. Used to use it all the time for bottom hole assembly calculations, but the confuser does all that nowadays. What with SG and kilos per litre, etc, it all goes tits up! I liked the old lbs/gal calcs, I could remember those.
 
jeremyshaw said:
I was wondering if at 5000ft 88 tons of steel might be too light. You'd think they would have thought of that (but then thought the bankers knew...).

Anyone know the answer? Thanks...
Click to expand...
the 5000ft doesn't make much difference (maybe 1%) as water is almost completely incompressible. salinity and temperature are more important.
 
The mind boggles at the logistics and the lack of knowledge of the public (why don't they just plug the gap). I think it would be easier to put a cap on a well on the surface of the moon than in that spot.
What are the underwater currents like in that area? Imagine sending a boat along on a 1.500m long tether, and steering it to a spot the size of a football goal.
 
activesail said:
If the thing ways anything like 88 tons in the first place.

I can't even imagine what kind of equipment you would need to lower a mass of 88 Tons 5,000ft.
Click to expand...
I was on Lombo East platform off Angole, McDermotts were doing an add on, they had a flippin big barge alongside, the add on bit weighed a few hundred tons. The crane was like a giant giraffe, enormous. The swells coming in off the atlantic (about 15 ft in 180ft of water) were regular and the crane driver didnt have anything fancy to help him adjust for the swell. The plan was to hoist it along side the platform, attach chain blocks, come alongs etc, sweat it in to place, then start welding. Basically a horizontal platform welded on the side at right angles.
Anyway off they went, they got it along side, the jib head was level with me, going up and down like a demented nodding donkey, the McD crew were trying to snag the hooks of the come alongs etc without much luck, the next thing was that the Nigerian bossman launched himself off the platform on to the piece (they were about 100ft up!) and tried to pass the hooks back to the main platform, the chains were snapping like cotton and me laddo now was stuck on the piece wich was going up and down like a yo yo, we all thought next thing that is going to go is one of the suspending chains and sure enough one went. The whole lot tipped, at this point they admitted defeat and they swung the piece back to the barge with the bossman hanging on for his life!
They did it a few days later when the swells moderated!
Stu
 
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