Their new plan sounds a little bit like something I thought of immediately after the pipe broke. I never mentioned it anyone, assuming it was too naive, and knowing I had zero experience in this sort of thing.
I was thinking of attaching a large collar at the base of the BOP (two pieces bolted together), with heavy duty steel rods bolted to it and going up to and through holes in a thick steel plate. The rods would be threaded, with nuts above the plate to hold them in place. The holes in the collar and plate would be large enough to allow some play. Once the whole contraption was in place, with the edge of the steel plate resting against the side of the riser pipe, I hoped the submersibles would have some means of sawing off the pipe (it appears they do). If they cut it just below the plate they should then be able to push the plate in place over it and tighten the nuts.
If the submersibles aren't capable of tightening the nuts, a turnbuckle could be built into each rod. Surely they could twist those some way.
This might leak a bit if the end of the sawed off pipe wasn't exactly flat, and the leak might get worse over time if the collar or plate warped, and of course given long enough the whole thing would rust enough that the nuts would slip off. But the idea would be to stop most of the flow until the relief drilling could be completed.
