One of the remotely operated vehicles (ROV) is seen near the site of the Macondo well in the Gulf of Mexico, in this frame grab captured from a BP live video feed Monday. (Reuters)

The cap that ended BP's three-month oil leak in the Gulf of Mexico was set to come off Thursday as a prelude to raising a massive, failed piece of equipment and preparing for a final seal on the broken sea floor well.

Engineers and the government were not expecting crude to break out again when the cap is lifted, but the government wasn't offering any guarantees, and oil collection vessels were set to be on standby on the surface just in case.

The cap is an elongated, metal cylinder that was placed on top of the failed blowout preventer to finally stop the flow of oil and gas July 15. With the cap gone, the old blowout preventer can be removed and a new one put in place before engineers try to seal the well for good deep underground.

Once the cap and blowout preventer are removed, a lot will be riding on the stability of a plug that was created when mud and cement were pumped down into the well from the top. Essentially, the pressure exerted downward served to counter the pressure coming up.

The well blew out when the offshore drilling rig Deepwater Horizon exploded April 20, killing 11 workers and setting off the massive spill.