'Tis only a guess, I'm afraid, but this kind of 'tapping' is often 'piquage' — though I did also wonder about other words for 'transferring from one container to another'; but here, I'm assuming the emphasis is being palced on the drilling / cutting of the holes into a tank full of something that is going to spurt out.

Sounds like the Costa Concordia sinking ;-)

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Note added at 28 mins (2012-04-14 19:55:10 GMT)
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Here's an earlier KudoZ wherein the term appears:

http://www.proz.com/kudoz/english_to_french/mechanics_mech_e...

And here's a Wiki article about it, used in a more general context:

Piquage en charge - Wikipédia
fr.wikipedia.org/wiki/Piquage_en_charge

De l'anglais « Hot Tapping » , le « piquage en charge », est la méthode de connexion à des tuyaux existants ou des récipients sous pression, sans l'interruption ...

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Note added at 30 mins (2012-04-14 19:56:55 GMT)
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Oh and look, here's the reverse question in FR > EN:

http://fra.proz.com/kudoz/french_to_english/mechanics_mech_e...

That should have shown up, Marion, if you had tried a term search with the 'also search reverse language pair' option checked.

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Note added at 11 hrs (2012-04-15 07:19:26 GMT)
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'hot' just means 'while in operation / not empty', and is an expression used in lots of other fields, e.g. IT; we also have related expressions like 'hot spare' and 'hot swap'.

The fact that in some contexts the idea of 'hot' also happens to imply 'under pressure' is purely fortuitous, and specific to the field concerned. Note that 'charge' here has the sense of 'load', the opposite of 'sous vide' = 'off load / unloaded'