The Caribbean Petroleum Corporation (CPC) oil refinery is located near San Juan in Puerto Rico, and occupies a third of a square mile. It includes a tank farm comprising 30 operational aboveground storage tanks, and a refinery which was shutdown in 2000. Prior to the 23rd of October 2009, the tank farm stored gasoline, diesel fuel, jet fuel, and fuel oil.



On the 23rd of October 2009, shortly after midnight, operators at the Caribbean Petroleum Refining facility observed the apparition of fog spreading northwest of the site. It was a warm and humid evening and according to the accident investigation being currently carried out, such fog was likely due the condensing of moisture in the air by the cool gasoline vapour.

At 12:23am, a large vapour cloud ignited in a massive explosion which affected 17 petroleum storage tanks. The size of the vapour cloud before ignition has been estimated to 2000ft in diameter by the accident investigation team. A 72 hour fire which eventually engulfed 21 of the 30 petroleum storage tanks on site then followed the explosion.

Although the results of the accident investigation have not been released to date, the most likely cause of the accident is the overfilling of one of the tanks.

At the time of the accident, a tank was indeed being filled with gasoline from a ship docked in San Juan harbour. The accident investigation has revealed that the level of liquid hydrocarbon in that tank could not be remotely monitored on the evening of the accident because the facility’s computerised monitoring system was not fully operational. That night, a mechanical gauge on the exterior wall of the tank was therefore used by operators to monitor the level of hydrocarbons in the tank. As a result, employees located in the facility’s control room were unable to monitor the gasoline level in the tank and detect any overfilling. In addition, alarms for this particular tank were disabled. A physical check of the level of Gasoline within the tank was carried out at 11.20pm on the night of the accident without any problems being found. Should the overfilling hypothesis be verified, the tank would have overflowed sometime between then and the discovery of the vapour cloud shortly after midnight. The Gasoline would have spilled from the tank without being detected and would have vaporised, therefore generating a very large gas cloud which spread across the facility.

The accident investigation team has identified the area where ignition took place thanks to video surveillance recordings; ignition is believed to have occurred in the north western corner of the facility, where a wastewater treatment plant is located.

The offsite damage caused by the blast was significant; numerous damages to windows and doors of buildings located in the areas bordering the facility were reported as well as damages to at least 200 of the 450 homes located half a mile away and beyond.