BP's total cost of its response to the massive oil spill in the Gulf of Mexico has climbed to 8 billion U.S. dollars as the oil giant undertakes measures in order to resume work on its relief well and seal the damaged oil well for good.

BP said in a statement released on Friday the total costs include cost of the spill response, containment, relief well drilling, static kill and cementing, grants to the Gulf states, claims paid and federal costs. It also had made 127,000 claims payments totaling about 399 million dollars.

The company previously agreed to create a 20-billion-dollar third-party account to cover some of the costs and obligations arising from the spill triggered by the explosion of BP-leased Deepwater Horizon drilling rig off the Louisiana coast in late April. It had made an initial payment of 3 billion U.S. dollars into the fund.

Also on Friday, BP made progress in its final attempts to seal the ruptured underwater well as crews successfully removed a failed blowout preventer (BOP) from the well, a process intended to enable the company to resume work to complete the relief well.

"The Deepwater Horizon BOP stack was successfully detached" from the ruptured Macondo well at 1:20 p.m., said BP spokesman Daren Beaudo, adding the removal "appears to have gone very smoothly and as planned."

The BOP is designed to close off a well if oil or gas uncontrollably rushes to the top. But the Deepwater Horizon BOP failed to do so in the case of the April 20 rig explosion in the Gulf.

The company is planning to attach a replacement blowout preventer on the well.

The replacement of the BOP is to "allow operations to complete the relief well to resume," the company said in an earlier statement.

The removal of the BOP was in question earlier this week because of the unknown status of thousands of feet of drill pipe believed to be suspended below the device.

The detaching process was monitored by officials, said Thad Allen, the top U.S. official tasked to oversee the oil spill response.

"This procedure was undertaken in accordance with specific condition I set forth last week in a directive authorizing the capping stack removal, which was completed yesterday," said Allen. BP removed the capping stack, a key piece of equipment that stopped the oil leak on July 15, on Thursday afternoon.

After a new BOP is installed, BP plans to resume work on its relief well and then, after the relief well intercepts the damaged well, the company will execute a "bottom kill," a measure that fills the well from the bottom with cement and mud.

Now the relief well, at a measured depth of 17,909 feet and progressing to intersect with the ruptured well, "will involve drilling, alternating with ranging runs to confirm proximity to the well," the company said.

"Depending upon weather conditions, mid-September is the current estimate of the most likely date by which the relief well will intercept the MC252 well annulus," it added.

The Deepwater Horizon rig, leased by BP, exploded on April 20 off the Louisiana coast, killing 11 workers and unleashing the worst spill in U.S. history.