Reply #17: It has semed to me this should have been part of the design of a blowout preventer

[muriel_volestrangler]

with the plate in a slot in the BOP, and you slide (probably rotate, around a vertical axis through a corner of the plate) the plate across the pipe if you need to. If the oil pressure is low, the weight of the plate (plus the water pressure at that depth) will hold the plate down on the bottom of the slot; if the oil pressure is high, it will force the plate against the top of the slot - but, either way, you should get a reasonable seal of the plate against a part of it.