A missing pressure cap on the West Atlas oil rig has been revealed as one of the likely causes of one of Australia's biggest environmental disasters.
PTTEP Australasia and the company it contracted to drill wells off WA have admitted they were aware a pressure containment cap was not fitted at the time the rig began spewing oil into the Timor Sea in August.
In submissions to the Montara Commission of Inquiry, PTTEP and Atlas Drilling said the missing cap was ordered to be installed in March when work on the well had stopped. Both companies falsely believed it had been, but it was never put in place because the cap was corroded.
An email sent from the drilling supervisor to the drilling superintendent in March advised that the corrosion cap was in place.
But PTTEP said that when work recommenced on the wells in August, before the disastrous leak, the company discovered "that the 340mm pressure-containing corrosion cap required by the drilling program had not been installed".
In its submission, Atlas listed the missing cap as one of the "most significant" issues in determining the cause of the spill, asking whether the failure to install it contributed to the disaster. It detailed the missing pressure cap under the heading "likely causes".
Atlas has also asked the inquiry to determine why the installation of the cap had not taken place.
The pressure containment cap is a plug that is run into the top of the well to stop any fluids moving in or out of it and protect the well from corrosion.
The submissions also revealed that cementing done at the bottom of the well was not set properly and caused a leak path through the cement.
PTTEP said the success of the relief well operation confirmed the source of the flow was "in the 244mmm casing in the H1 Well and the most likely cause of that was a channel in the cement".For more than 10 weeks from August 21, the Montara Oil well gushed an estimated 3500 tonnes of oil and gas condensate into the Timor Sea.
Before the spill was stopped on November 3, a fire engulfed the West Atlas rig, further postponing the capping of the leak after repeated attempts were made to stop it.
The inquiry into the oil spill will have the powers of a royal commission but potentially no legal repercussions for the company behind the 74-day environmental disaster.
The process, to be headed by a former top bureaucrat, will hear evidence on a "no blame" basis. Federal Resources Minister Martin Ferguson said at the time it was announced that other probes by government agencies into the oil and gas leak could lead to legal action.
A submission to the inquiry from the National Offshore Petroleum Safety Authority stated "a failure in the integrity" of the well caused the disaster and that the Northern Territory Government had responsibility for regulating the integrity of the well.
A spokesman for the Northern Territory Government said yesterday that the Commonwealth Government was the regulatory authority of the Montara well area and delegated some administrative functions to the Territory.
He said the Northern Territory had no responsibility for occupational, health and safety matters in Commonwealth waters.
On Wednesday, the oil well was finally secured almost five months after the beginning of the 10-week oil spill. PTTEP said the H1 well on the Montara wellhead platform had been plugged and sealed with a series of cement and mechanical barriers.