Gulf Leak:

Here is a photo of the smaller top hat, already lowered to the sea-bed, seen from the control room on the Viking Poseidon.

Note the top had is rigged to inject methanol to counter the methane hydrates that form when the methane mixes with seawater at this depth. Those hydrates float, clogging the riser and creating buoyancy. A smaller top hat will allow less water in to minimise this effect. There is also a rubber gasket for the same purpose.

I believe this is the top hat that would go on top of the BOP stack should they decide to cut the riser and cap it. This will be tried should the mud top kill fail.