Petrobras P-36

Rig:

P-36 Semi-Sub

Date:

15-20 March 2001

Location:

Roncador Field, Campos Basin, Brazil

Operator:

Petrobras

Introduction

The P-36 was originally named the 'Spirit of Columbus' and was constructed between 1984 and 1994 in Italy. Designed as a floating production unit, the platform was based on a conversion of the Friede & Goldman L-1020 Trendsetter-type semi-submersible. It was redesigned for Petrobras between 1997 and 1999 and brought into operation in the Roncador Field off the coast of Brazil in May 2000. The unit was capable of processing 180,000 bopd and 7.2 million cubic meters of gas per day. In May 2001, the P-36 was producing around 84,000 barrels of oil and 1.3 million cubic metres of gas per day when it became destabilised by two explosions and subsequently sank.

Platform Loss

At around 2221 hours on the evening of 14 March 2001, drainage operations began on the portside emergency drain tank (EDT), one of two 450 cubic metre tanks (one port, one starboard), which were used for the storage of oil and water during maintenance or during an emergency involving the process plant. At 0022 hours on 15 March 2001, an explosion was recorded in the starboard aft column, thought to have been the mechanical rupturing of the starboard EDT. This caused the release of gas-saturated water and oil into the aft starboard column and caused the platform to list 2 degrees by 0027 hours.

This was followed by a second larger gas explosion which killed 10 members and fatally injured one member of the attending fire-fighting crew. The resulting platform damage caused further flooding in the aft starboard column compartments and pontoon tanks, with further sea water entry through the open sea chest valves. By 0815 hours on 15 March 2001, the platform had assumed a 16 degree list, which submerged the openings of the chain lockers on the main deck level and caused a progressive list that led to the subsequent loss of the platform.

There were 175 people on board, of whom 138 were evacuated by crane to boat between 0144 to 0420 hours on 15 March 2001. The remaining crew were evacuated by helicopter at 0603 the same morning as the platform's stability deteriorated. Over the following days, attempts were made to stabilise the platform by injecting nitrogen into a vent line next to the damaged column, but bad weather disrupted rescue operations. The platform eventually capsized at around 1140 hours on 20 March 2001 before sinking in 1300m of water, making salvage of the unit impossible.

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 minimise the damage from the spill.

The Petrobras Inquiry into the P-36 sinking - independently verified by Det Norske Veritas - summarised 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 overpressure 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 downflooding 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 PDF document
Agência Nacional do Petróleo, Gás Natural e Biocombustíveis (ANP): P-36 accident considerations
BBC News 'Post mortem into rig disaster begins'
Det Norske Veritas P-36 Verification Report PDF document

Photos
1. Industries Davie
2. Reformatorisch Dagblad
3. Robin Perry and Associates
4, 5 & 6. UNEP-WCMC