The commission, which had been monitored by independent Norwegian shipping classification group Den Norske Veritas, said that a valve of the emergency drain tank had apparently triggered a chain of accidents and a blast on the platform.
In its final report, the commission recommended that rig manufacturers and operators in the future abstain from putting any tanks or vessels linked to the production process inside the support columns of pontoons.
"The accident which took 11 lives and sank the rig had various causes and contributing factors. It would be unfair for the commission to choose one cause or one responsible person," said Carlos Heleno Barbosa, commission's top coordinator. He pointed out that after analyzing dozens of witness statements, hundreds of photos, thousands of computer records and tons of documentation, the body only came up with "the most probable hypothesis" of how the tragedy occurred.
According to the commission's report the problems at the platform started several hours before the explosions, when operators started emptying an emergency drain tank based in a support column of the rig and used to collect excess oil, gas and water from the upstream process.
Another tank, in the opposite column, had previously been blocked by a valve as its drain pump had been withdrawn for repairs. However, the valve let some oil, gas and water through for about an hour, overfilling the "blocked" tank.
"We have two witnesses saying the valve had been properly closed," Barbosa said. "But we cannot say with all certainty if it was a mechanical fault of the valve or if someone did not completely shut it."
As a result, the tank's hull did not support mounting pressure and broke, letting a large amount of gas out and crushing water pipes inside the column that are used for sea water intake. The water started flooding the column, causing the rig's listing.
Although there had been no explosion involving fire, the platform automatically entered in "firefighting mode", actioning water pumps that increased the flooding level.
A brigade of firefighters dispatched to the area opened hatches leading to the compartments near the tank, allowing gases to spread to higher levels of the column, where an ignition occurred, possibly from electric equipment.
A strong blast that followed apparently killed most brigade members on the spot and damaged equipment in the column. One of the firefighters had been rescued, but he died a week later from severe burns.
Meanwhile, the rig's ballast operators injected water into the opposite column to try to level the platform, which, according to some experts, contributed to its sinking.
Ventilation shafts took the water to other compartments and two watertight tanks were flooded through check doors that had been left open. The "progressive flooding" made the rig list further taking water via its other openings and finally sink to the bottom of the ocean, according to the report.
Stopping short of attributing any occurrence above to concrete human error, the commission also recommended that Petrobras review some of its procedures on the rigs. It recommended that the bureaucratic workload of managers and supervisors be reduced so they can concentrate on operating activities.
It also advised that personnel get better training and that more gas detecting devices be installed.
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