[ZenGum]

Guessing at the physics here. A small pipe inserted inside the main pipe with a tight seal would maintain the pressure, allowing extraction. A big containment dome allows the spill to burst from the pipe. The spill is part natural gas. As the gas gets released from the pipe, it undergoes pressure drop, thus expanding and cooling substantially. This cooling causes some hydrocarbons in the spill to freeze. These frozen bits clog up the containment dome, so additional oil or gas is forced out into the open sea again. I think that was what went wrong with the Top Hat and similar strategies.