Following the explosion and fire on the rig, the blow-out preventer on the sea bed should have automatically sealed the well, but it failed, resulting in the massive oil spill.

The well is still gushing, however. BP's technicians have been unable to activate the blow-out preventer, essentially a massive valve that must be operated remotely by robot submarines.

The new sealing cap - similar to a smaller version of the damaged blow-out preventer - has three devices that are designed to shut the flow.