Post mortem into rig disaster begins

The sinking of the platform surprised experts

By Tom Gibb in Sao Paulo

As the biggest floating oil platform in the world slipped beneath the waves of the Atlantic, questions were already being asked as to how it was possible for it to sink.

The platform, known as the P-36, went 125km off the coast of Brazil in waters 1300m deep.

Rio's Guanabara Bay was the scene of a major spill last year

This will make the investigation announced by the rig's owners, the Brazilian oil company Petrobras, that much more difficult.

Salvage experts say there is now no possibility of raising the platform - and it will be extremely difficult to recover the bodies of nine workers believed to have gone down with it.

Salvage battle

The five day saga started with a series of gas explosions on the platform in the early hours of last Thursday morning. Workers described these as coming from inside the structure of the platform itself.

The most likely explanation is a build-up of gas within one of the massive hollow pillars which hold the platform upright.

Water then flooded in through the ruptured pillar into the buoyancy tanks beneath the surface, which keep the platform afloat. The structure started to sink and listed over badly as the damaged pillar went completely under water.

Salvage teams finally lost the battle to save the platform

Salvage teams tried over the weekend to save the platform by pumping in nitrogen and compressed air into the tanks to expel the water.

It was a highly complex operation, with divers having to go down fifty meters to drill holes in the submerged tanks. For a while the operation stopped the platform from sinking further.

But after bad weather hampered operations, the salvage teams finally lost the battle.

Engineers were asking why the platform sank - when only one corner was damaged.

"Even if a pillar or another fundamental part is damaged, one would expect the rest of the structure would continue to float," said oil industry engineer Dr Sergen Esefen.

After the platform sank, Brazilian congressmen called for a wider investigation into the causes of the accident.

History of disasters

This is just the latest spill involving Petrobras.

In January last year it dumped as much oil and fuel as is on the platform into the bay of Rio de Janeiro, causing widespread environmental damage.

Six months later it was responsible for dumping four times as much crude oil into one of Brazil's main rivers.

The company say the latest spill is much less serious as it is a long way out to sea so that the ocean currents will break the oil up. They also say the quantity of fuel and oil - equivalent to half an Olympic swimming pool was much less than such spills and the Exxon Valdes.

Safety compromised?

Iguacu River: scene of another Petrobras spill

Family members of oil workers on the P-36 say they believe pressure by the company for increased production may have led to corners being cut on safety procedures.

The oil worker's union says 81 people have died in accidents in the Brazilian oil industry in the last three years.

The P-36 was built for Petrobras in Italy in 1994. It was recently refitted in Canada. There are many other floating oil platforms like it around the world.

The results of the pending investigation into its sinking could have implications for the oil industry not just in Brazil, but around the world.