We had a contingency plan for Louisiana spill, and it's working, BP chief tells angry senators

BP insisted last night that its contingency plan had worked, despite coming under fire in Congress for minimising the risks of offshore drilling and trying to shirk blame for the oil disaster in the Gulf of Mexico.

With an estimated 4m gallons of oil polluting the gulf from the ruptured well, Lamar McKay, the chief executive of BP America, said the company had adequately anticipated the potential scale of any spill and that its clean-up operation had gone according to plan.

"We had a very specific plan," he told the Senate. "It has actually worked." But he acknowledged the spill could grow to nearly 19m US gallons by the time a relief well – the only sure method of stopping the leak – is drilled. BP's defence came at the end of a testy day of hearings before two committees which saw the three oil titans connected to the disaster repeatedly accused of trying to slough off their financial and legal obligations.

Executives of BP America, which owned the well, Transocean, which owned the sunken Deepwater Horizon rig, and Halliburton, which cemented the wall, were repeatedly taken to task by senators for failing to put in place adquate safety regimes. BP was also pressed for specifics on its commitment to honour all " legitimate claims" for damages from the spill.

The senators' anger grew as all three admitted they could offer no guarantees against another calamity in the Gulf. "There is just nothing there underneath your statements," said Barbara Boxer, the chair of the Senate environment and public works committee. "If you look at what's happening it is very very disturbing."

But other senators acknowledged a failure of regulatory oversight. "We dropped the ball here," said Max Baucus, a Democrat from Montana.

The day got underway with BP America's chief executive, Lamar McKay, pointing to Transocean, the operator of the rig, and pinning a failed blowout preventer, a 450 tonne set of valves now lying on the ocean floor. "We have a blowout preventer that didn't work," McKay said.

Transocean's Steven Newman fired straight back. "Offshore oil and gas production projects begin and end with the operator, in this case BP," he said.

That left Halliburton. Its health, safety and environment officer, Tim Probert, started off by warning against a premature rush to judgment – then took his turn at assigning blame. Like Newman, he told the hearing that Halliburton had carried out its work according to BP's specifications.

Yesterday's hearings - on Capitol Hill and in Kenner, Louisiana - mark the first official efforts to unravel the causes of spill, and prevent future disasters.

BP and the others were put on notice the spill could well change the future of offshore drilling. "If you can't convince people that you can operate safely, not only will BP not be out there, but Transocean won't be out there to drill the rigs, and Halliburton won't be out there cementing," said Lisa Murkowski, a Republican from Alaska who supports drilling.

The first changes were set in motion yesterday, with the Obama administration proposing reform in the policing of offshore drilling following repeated charges that regulators were too cosy with oil companies. A proposed overhaul of the mineral management service would put a firewall between officials approving projects and those responsible for making operations meet safety and environmental standards.

The move comes amid repeated charges from senators yesterday that the mineral management service was "too cozy".