36 (P-36) was a semi-submersible oil platform. Prior to its sinking on 20 March 2001, it was the largest in the world. It was operated by Petrobras, a semi-public Brazilian oil company headquartered in Rio de Janeiro.
The proximate cause for the sinking was a series of explosions that killed 11 crew. In terms of lives lost, it was the worst offshore oil and gas accident in Brazil since 1984, when a rig blowout and explosion caused 36 fatalities, and the worst worldwide since the explosion of a platform off Nigeria in January 1995, which killed 13.
Vessel The platform was built at the Fincantieri shipyard in Genoa, Italy between 1984 and 1994. It was an oil production unit integrated with a drilling rig. The project – initially named SANA 15,000 – was speculative, as a lessee or a buyer had not been identified. It was a Friede & Goldman L-1020 Trendsetter semi-submersible, designed to operate at depths of 500 m. It had been ordered and was owned by Società Armamento Navi Appoggio S.p.A. (SANA). In 1990 Midland and Scottish Resources (MSR), acquired SANA's parent company, effectively becoming the owners of the rig, which was then renamed Spirit of Columbus. However, the planned development of the Emerald field in the North Sea proved to be commercially unviable. Petrobras selected the unit for the development of their oil resources. After being considered for the Marlim Sul field, the platform was assigned to the Roncador oil field in the Campos Basin.
A conversion project became necessary to adapt the platform to the new field. The project, commenced in March 1997 (merely five months after the discovery of the oil field), was assigned to contractor , a company owned by Bolivian entrepreneur Germán Efromovich – who was personally connected to a former president and financial director of Petrobras – and with little experience in projects the size and complexity of P-36. The contractor was selected by Petrobras without open competitive bidding. selected subcontractors Noble Denton for design of the hull, Amec for the design of the topsides, and Davie Shipbuilding for construction. Petrobras was directly in charge of the mooring system, the flexible marine risers, and the subsea infrastructure. The decision was taken to keep the original Italian flag, which entitled the owners to receive subsidies from the Italian government. The associated agreement obliged the project to keep RINA as the classification society. The American Bureau of Shipping (ABS) was selected to verify the conformity of topsides equipment.
The rig was converted into a 31,400-tonnes oil production platform (with no drilling capacity) in Lévis, Quebec, starting in September 1997. With construction activities ongoing, Davie went bankrupt, which obliged Petrobras to conclude construction in the Mauá shipyard in Niterói, state of Rio de Janeiro.
P-36 left the Canadian yard in late October 1999. It first had to navigate the St. Lawrence River to Sept-Îles at the mouth of the river. This passage was carried out using three tugboats and took four days. The inclining test was executed at Sept-Îles. At the St. Lawrence estuary, the semi-sub was loaded onto the faster Mighty Servant 1, a heavy-lift ship. The 32,700-ton lift was the heaviest ever executed up to that time. The Mighty Servant 1 had to undergo dedicated structural works to accommodate the large rig. The dry tow to Niterói took nineteen days at an average speed of 12 knots (whereas a wet tow by way of tugboats would have required 50 days).
The change of shipyard pushed the cost of the project from US$354 million to over US$500 million (from US$ to US$ considering inflation). P-36 went on to operate for Petrobras on the Roncador oil field, off the Brazilian coast. Production started on 16 May 2000.
P-36 produced oil from 21 wells and injected water into the reservoir through five wells. A total of 90 marine risers were connected to the platform. As part of the conversion project, the original process plant on the topsides was completely replaced with new production equipment and utilities. The platform's nominal oil production capacity was per day, which at the time of construction was nearly twice the average production of the largest floating facility in operation. It also had gas production capacity of per day. By the time of the accident, six out of 21 wells were producing. Production had reached about of crude – about 5–6% of the total oil produced by Petrobras – and of gas per day. The rest of the wells were scheduled to be connected to the platform by the end of 2005.
Oil produced by P-36 was exported to turret-moored FSO P-47 via subsea pipelines for later transportation to shore. P-47 was a floating storage and offloading facility converted from VLCC oil tanker Eastern Strength at the AESA shipyard in Cádiz, Spain. A part of the fuel gas produced by P-36 was also sent to the FSO for power generation. The semi-submersible also exported produced gas through a subsea pipeline connected to a platform manifold and from there through a pipeline to fixed platform PNA-1, from where the gas was sent to shore.
The unit had two pontoons, four columns, a large central caisson, and three decks. In order to accommodate the larger production facilities, a 13-m long cantilevered extensions was added to the aft main deck, spanning its entire breadth. Each column was retrofitted with a "stability box" that provided extra buoyancy. Additional stability boxes were installed on the pontoons.
P-36 was attached to the seabed with mooring lines using polyester ropes and vertically loaded anchors. This was the result of un upgrade of the mooring system of Spirit of Columbus, which had twelve lines reaching depths of approximately 500 m, whereas P-36 had sixteen, 1800 m long and designed for depths between 1200 m and 1520 m and minimum breaking load of 1000 tons (9.8 MN).
Technical innovations introduced in the project garnered acclaim in the industry. The Offshore Technology Conference (OTC) awarded Petrobras a distinguished achievement award in 2001 "[f]or outstanding advancements to deepwater technology and economics in the development of the Roncador Field; a timeline of 27 months from discovery to first oil production in a water depth of more than 1800 meters; made possible by the use of a dynamically positioned early production system, and a dedicated production system using steel catenary exporting risers, taut-leg polyester mooring, and subsea production hardware."
Columns
The starboard column at aft of the hull housed an emergency drain tank (EDT), ballast tanks, a waste oil tank, a deareator for pretreatment of injected water, a fresh water tank, and associated pumps and piping. The tanks were not standalone but integrated in the column outer shell. The spaces were ventilated through a dedicated set of ducts. A dedicated space within the starboard pontoon housed seawater pumps that supplied the plant with cooling water and firewater. The pumps were also serving as secondary source of ballast water.
The emergency drain system also included another EDT, housed in the port aft column. The tanks were connected to the production facilities on the topsides and between each other. Fluid transfer capacity to and from the tanks was provided by dedicated pumps, one in each column. Both tanks had a capacity of 450m3. Their purpose was to provide temporary storage for water and oil from the topsides process equipment during certain maintenance activities.
Accident
EDT burst and gas explosion
In the early hours of 15 March 2001 there were two explosions in the starboard aft column. The first explosion was caused by an overpressure event, the second by the ignition of leaking hydrocarbon vapors.
The first event was the burst of the starboard aft emergency drain tank (EDT). This had been mechanically isolated because its pump had been removed for maintenance. The atmospheric vent pipe of the tank was isolated with a blind flange to prevent water intake from other equipment on upper decks. However, a valve exposed to backflow from the production header was leaking. This allowed oil from the process plant on the topsides to flow into the starboard aft EDT. The leaking valve was also exposed to the discharge side of the port aft emergency drain pump. When this pump was started, water from the port aft emergency drain tank flowed through the valve into the starboard aft EDT (although this resulted in a decreased inflow to the tank). The pump had been started in the late evening of 14 March. It took about two hours for the pressure in the tank to reach about 10 bar(g). The EDT was not a pressure vessel. Being continuously open to the atmosphere through a vent pipe (which had been temporarily isolated), it was designed to withstand pressures barely higher than the atmospheric. As a result, the EDT walls started to deform. At 12:22 am the tank shell gave way. At this moment, 50% of the starboard aft EDT was filled with water, with a 4-meters oil layer on top of it. Water and oil spilled into the column from the tank. The seawater line that run along its wall failed, resulting in flooding of the column space. Additionally, the firewater system lost pressure due to the pipe failure. Because of the logic programmed in the control and safety system, this resulted in the platform shutdown and the activation of the firewater pump in the starboard aft column, which was trying to compensate for the loss of pressure. This further exacerbated the flooding. The passage of water between different compartments within the column was facilitated by the ventilation dampers failing to close automatically. This allowed a path for the flooding to extend to other levels. Some manholes in the lower part of the column had been left open. This furthered the flooding in lower spaces including one of the stability boxes and a ballast tank. The seawater pump short-circuited because it became submerged; however, flooding continued because the sea chest valves allowing inflow of seawater from the ocean were designed to fail in their set position and therefore they remained open. The flammable vapors from the EDT dispersed and climbed to the decks above the column through watertight doors and ventilation ducts that had been left open in preparation for unrelated maintenance activities. The presence of the vapors was revealed by the gas detectors on the main deck.
At 12:39 am the vapors ignited. The ensuing gas explosion killed eleven people – all Petrobras employees – who were in the area taking part in the emergency response. Of the 11, ten perished immediately and one died a week later.
Sinking Within five minutes of the gas explosion, the rig started developing a list. Operators tried to contain it by filling the port forward ballast tank, i.e., the one diagonally opposite the column affected by the explosions. However, this operation was unsuccessful and probably contributed to make the situation even worse. At the time of the accident, there were 175 people on the rig, of whom 85 were crew members. Evacuation of non-essential personnel started at 1:44 am. In about two and a half hours 138 non-essential workers were evacuated using a crane and a personnel transfer basket. The evacuees were taken onboard FSO P-47, which was 12 km away. When all options to level the platform seemed exhausted, it was decided to evacuate all remaining personnel on board. This was completed by 6:03 am using helicopters, with the platform tilting at an angle of 6°. The list increased to about 20° by 8:15 am. This was sufficient to allow further down-flooding through the hawseholes (or openings to the anchor chain lockers). This event made the later sinking of the platform inevitable. Later, the starboard aft pontoon tanks also began to flood through their vent ducts.