The CEO of the company that owned the Deepwater Horizon oil drilling rig that exploded in the Gulf of Mexico said Tuesday the accident was caused by a failure in the way cementing was done to seal the well.

Transocean Ltd. CEO Steven Newman told a U.S. senate hearing that the April 20 explosion at the rig was unusual because it happened after construction of the well was essentially complete.

He said the explosion that killed 11 workers could not have happened unless there was a failure either in the cementing of the space between the well casing and the walls of the hole through the rock or of the cement plug used to seal the inside of the casing.

The cementing work was done by Halliburton Inc., which had been subcontracted by BP to encase the well pipe in cement before plugging it and had finished the work shortly before the explosion.

A Halliburton executive, Tim Probert, planned in prepared remarks to assert that the company's work was finished "in accordance with the requirements" set out by BP and with accepted industry practices.

Newman's testimony was a continuation of efforts by the leading companies involved in the spill to point the finger at each other during testimony before the committee.

The operator of the well, BP, said the spill was caused by the failure of a key safety device, called a blowout preventer, which was owned by Transocean.

"That was to be the fail-safe in case of an accident," Lamar McKay, chairman of BP America, testified.

The blowout preventer was supposed to shut off oil flow on the ocean floor but "failed to operate," McKay said.

Transocean CEO Steven Newman, in testimony before a U.S. senate committee on Tuesday, said the disaster was a result of a failure to seal the well properly. (Pablo Martinez Monsivais/Associated Press)

But Transocean said BP was in charge.

"Offshore oil and gas production projects begin and end with the operator, in this case BP," said Newman.

Newman said it was BP that prepared the drilling plan and was in charge when the drilling concluded and the crew was preparing to cap the well 1,500 metres underwater.

To blame the blowout protectors "simply makes no sense," because there is "no reason to believe" that the equipment was not operational, Newman argued.

Reverse order

A report in the Wall Street Journal Tuesday suggested two tasks in the process of finishing the well were done in reverse order.

Common practice is to first pour wet cement down the inside of the pipe, which then hardens into a plug and prevents the escape of explosive gas.

Then drilling mud, which has been in the pipe and holding back the petroleum, is displaced by seawater in what's called the riser, a buoyant pipe that connects the wellhead on the sea floor with the rig.

The seawater, according to the Wall Street Journal report, was half the weight of the drilling mud. The newspaper report quoted a rig worker as saying once the drilling mud was displaced, "that's when the well came at us, basically."

The Senate hearing is the first into the spill, which has been contaminating water in the Gulf for three weeks and continues unabated at a rate of 5,000 barrels of oil a day. The rig was 80 kilometres off the coast of Louisiana when it exploded and sank but the resulting spill has since spread to the coast and threatens sensitive marshes and marine life from Louisiana to Texas.

"I hear one message: don't blame me," said Republican Senator John Barrasso of Wyoming.

"Shifting the blame game doesn't get us very far."

With files from The Associated Press