The strain of mounting costs from the Gulf oil spill has finally shattered the bonds between BP and its main partner in the leaking well.

Late last Friday Anadarko Corp., which holds a 25% stake in the leaking Macondo well, accused BP of "gross negligence or willful misconduct" leading up to the explosion aboard the Deepwater Horizon drilling rig that triggered the oil spill. Those words are chosen carefully--under the terms of the operating agreement between the Macondo partners, which also includes Japan's Mitsui, if BP was guilty of this charge it would be liable for 100% of the costs.

Anadarko Chairman and Chief Executive Jim Hackett said his sudden outburst was motivated by his dismay over what he has heard in public testimony about BP's safety decision aboard the rig, rather than his desire to dodge the billions of dollars in liability his company is facing. Although surely his mind was also focused by Moody's decision to cut his company's debt rating to junk Friday.

Moody's decision may seem harsh. Anadarko has come in for nothing like the public mauling that has so diminished confidence in BP. So far, it hasn't had to pay anything towards the cost of the cleanup. But while BP takes the flak, it is far better placed to survive the crisis.

For Anadarko, paying its quarter of the $2 billion BP has spent on the spill so far, plus its $5 billion share into the $20 billion compensation fund BP agreed last week, would be extremely painful, if not impossible.

The mid-sized oil explorer generated $3.9 billion in cash flow in 2009, compared with BP's $27 billion. Anadarko was planning to spend this hard-earned cash developing a string of recent big discoveries and exploring some very promising new areas in
Africa . Its ability to retain these new assets would be questionable if all its cash were being sucked away by the Gulf cleanup.

Hackett is only serving the best interests of his shareholders by trying to pass this huge liability back to BP. Whether he succeeds will no doubt be decided in a long, costly and acrimonious legal battle--BP said it strongly disagrees with his view of events and intends all partners to pay their full share of the oil spill costs.