PETROBRAS' P36 Semi-submersible Disaster

"On March 20, 2001 the P-36, stationed over Roncador, sank after three explosions. At the time, the world's biggest semi-submersible had been producing 84,000 b/d and 1.3 million cu m/d of gas. The P-36 had been projected to produce 90,000 b/d of crude by the end of the year and to peak at its capacity of 180,000 b/d by 2004.

Cause of the explosions were unknown although Petrobras said there had been a gas leak that escaped into the column where the blasts took place.

Water flooded the platform's pontoons, causing the platform to list at a 25° angle. The platform commenced sinking, despite 4,100 tons of nitrogen being pumped into the flooded compartments at a rate of 200 liters/hour in an attempt to discharge about 15% of the estimated 7 million liters".

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Aftermath and Conclusions

The platform sank with an estimated 9500 bbls of oil on board, of which around 2000 bbls
leaked from the rig in the first 24 hours. Operations to disperse the oil with chemicals and
to recover the oil were undertaken in an effort to minimize the damage from the spill.

The Petrobras Inquiry into the P-36 sinking - independently verified by Det Norske Veritas -
summarized the chain of events leading to the loss of the P-36 and proposed the following
as the most likely causes of the accident:

• the alignment of the port EDT to the Production Header instead of to the Production Caisson, permitting the entry of hydrocarbons into the starboard EDT;
• the unexpected flow of oil, gas then water under pressure through the entry valve of starboard EDT, causing over-pressure inside the tank;
• the mechanical rupture of the starboard EDT, which released oil, water and gas and initiated flooding in the starboard column;
• the rupturing of the service sea water pipe in the starboard column, causing further flooding of the starboard column;
• gas migration to upper areas of the starboard column via open doors and ventilation hatches;
• an ignition source causing the gas cloud to explode, resulting in major platform damage;
• the activation of two fire-fighting pumps, which caused further flooding via the ruptured sea water pipe;
• the failure of watertight dampers, which allowed water to invade all aft starboard pontoon rooms, including the pump room, propulsion room, water injection room and access tunnel;
• the ingress of water causing the failure of the seawater pump, with the intake valves to the sea-chest remaining open;
• further flooding via the open sea-chest valves causing a progressive platform list; the submersion of the chain locker pipes at the main deck level, which caused down flooding in the platform;
• the continued slow flooding of the starboard aft pontoon tanks and deck box compartments, until water eventually flooded the central caisson and caused the platform to capsize then sink.

The main causal factors were listed as:

• the alignment of the port EDT to the Production Header instead of to the Production Caisson, permitting the entry of hydrocarbons into the starboard EDT;
• the delay in the activation of the port EDT drainage pump, allowing the reverse flow of hydrocarbons for about one hour;
• the failure of activators to close ventilation dampers, allowing water to flood the starboard column and pontoon compartments;
• two sea water pumps being under repair without measures in place in case of emergency;
• inadequate contingency plans and inadequate training for dealing with emergency ballast and stability control situations.

Sources:
Petrobras Inquiry Commission: P-36 Accident Final Report 
Brazilian NAVY | DPC - Directorate of Ports and Coasts and ANP - National Petroleum Agency: Platform P-36 Explosion and Sinking ANP/DPC Investigation Comission
BBC News: 'Post mortem into rig disaster begins'
Det Norske Veritas: P-36 Accident - Lessons learnt