RIO DE JANEIRO, Brazil (Reuters) - The world's largest offshore oil rig sank on Tuesday despite desperate rescue efforts and began spewing oil off the coast of Brazil five days after powerful blasts ripped through the platform killing 10 people.

Rough seas forced Brazil's state oil giant Petrobras to abandon salvage operations early on Tuesday.

If the rig sinks there is the distinct possibility that some or all of the 21 pipelines could rupture. It could be a catastrophe

Argemio Pertence, director of the Brazilian Association of Engineers

Less than an hour after divers had fled the scene, the 40-story structure tipped sideways and sank below the waterline within 10 minutes.

Diesel oil bubbled to the surface creating an extensive but very fine slick and Petrobras said all 395,000 gallons (1.5 million liters) of crude and diesel stored on and under the rig are likely to spill into the ocean as pipelines rupture and tanks explode.

``We lost the platform, it sank,'' said Petrobras president Henri Philippe Reichstul, visibly worn-out after five tense days of rescue work. ``Without a doubt, the sea conditions accelerated the process.''

Television showed survivors working on the rescue boats break down in tears as they watched their rig quietly slip into the turquoise sea. The giant green helicopter pad was the last part of the platform to disappear.

Three explosions ripped through the rig off the coast of Rio de Janeiro state last week, killing 10 of the 175 workers aboard. One worker remains in critical condition with burns on 98 percent of his body.

On Tuesday, 1,580 gallons (6,000 liters) of diesel stored in tanks leaked out of the rig as it headed to the ocean floor almost a mile (1.6 km) below.

``It is inevitable that almost all of the crude that is in the pipelines and the diesel in the tanks will leak,'' said Carlos Eduardo Bellot, a local Petrobras manager.

The company said it is applying chemicals to break up the slicks and lining up absorption barriers. More barriers and boats are standing by to contain bigger spills, though high seas have made it difficult to use them.

``A Sad Day For Brazil''

Some 79,000 gallons (300,000 liters) of crude in underwater pipelines and 316,000 gallons (1.2 million liters) of diesel could burst into the sea due to water pressure. But Petrobras reiterated that deep-ocean well heads were well sealed, thus preventing a larger disaster.

The potential damage would just be a fraction of the world's worst oil environmental disaster when the Exxon Valdez supertanker dumped 11 million gallons (41.6 million liters) of oil into the Alaskan seas in 1989.

Non-governmental environmentalists were also not overly alarmed. ``It is not a biodiverse area, it is almost off the continental shelf,'' said Garo Batmanian, secretary general of World Wildlife Fund for Nature in Brazil. ``The current is also not bringing it to the coast.''

If the winds changed direction, however, oil could reach the Rio coast and contaminate precious mangroves.

``It's a sad day for Brazil,'' Energy and Mines Minister Jose Jorge Vasconcelos said in a television interview. ``All Brazilians are sad about what happened, about the human impact and about the environmental losses.''

Rio's vice-governor, who was with the families of Petrobras workers when the news came out, declared two days of official mourning. ``It was a moment of desperation for the wives, mothers and children who were there,'' Benedita da Silva said.

Engineers have determined it would be virtually impossible to salvage the rig or the nine missing bodies if it sinks to the ocean floor. It will also be difficult to determine the cause of the blasts.

``Since last Thursday I knew in my heart that that iron box would be my husband's coffin,'' said Ivani Peixoto, a distraught widow who has been waiting for her husband's body in Macae, the headquarters for oil operations off Rio's coast.

Oil union leaders have called for a work slowdown on Thursday to protest poor working conditions and to remember the accident. Including the 10 latest victims, 91 Petrobras workers have died over the last three years in accidents.

Petrobras is creating a commission to investigate the cause of the accident and publish results in 30 days. An independent expert will be on the board to ensure transparency. The platform was in the Roncador oil field 78 miles off the coast of Rio de Janeiro state in the Campos Basin, which provides 80 percent of Brazil's total oil.

The rig, which started operating last year, was pumping out 80,000 barrels per day of crude, or 5 percent of Brazil's total, and was expected to be producing at full capacity of 180,000 bpd by 2004. It was the biggest offshore rig in the world in terms of capacity.

Reichstul said that Petrobras would still manage to boost total output this year as it works to become self-sufficient in oil by 2005, but that it would fall short of initial targets of 1.42 million bpd.

The company said its fiscal 2001 earnings could be hurt by as much as $450 million because of the rig's sinking.

Petrobras shares closed down 2.3 percent at 49.90 reais ($23.80) after the company announced the rig had sunk. The P-36 rig cost $350 million and is insured for $500 million. The firm should receive the insurance payment in the next six months.

Website about this oil rig

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