Oakes, the annular is at the top of the BOP. If there is a gas bubble under it you no longer have a BOP once it is unleashed into the riser. At that point your diverter must be in good shape and ready to go and the crew must be able to react on their own.
It only takes several BBLs from that depth, with the huge expansion that takes place as it rises, to cause mayhem if it finds ignition source. If this happens and it doesnt find ignition source you will never hear of it.
When it evacuates the mud/water from the riser it removes the hydrostatic head that is helping to keep the rest of the gas in the well at bay and pulls in a secondary, bigger flow from the formation.
When the Lexington burned the gas pushed all the drilling mud out of the riser and the hydrostatic pressure of the seawater at bottom collapsed the two bottom joints of riser. They are making riser stronger these days.
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