Shortly after midnight on October 23, operators at the Caribbean Petroleum fuel terminal near San Juan, Puerto Rico, observed a foggy apparition spreading northwest through a 2.2 million barrel tank farm that included at least 10 percent of the gasoline immediately available to the tropical U.S. commonwealth.
That observation, rather than any electronic warning, was the first indication of a critical situation, said U.S. Chemical Safety Board (CSB) investigator Jeff Wanko.
"It was a warm and humid evening," he said. "The cool gasoline vapor condensed moisture in the air just like morning dew."
Within 20 minutes of being discovered, the vapor cloud ignited in a massive explosion comparable to the December 2005 Buncefield oil storage depot blast in the UK. The explosion almost simultaneously touched off 17 petroleum storage tanks averaging between 100,000 and 120,000 barrels of gasoline, diesel, jet fuel or fuel oil.
Registering 2.8 on the Richter scale, the blast in the San Juan suburb of Bayamon did extensive damage to the terminal which measures one-third of a mile square. Homes and businesses more than a mile away, including an adjacent Army base, exhibited heavy damage as well.
Miraculously, only minor injuries were reported.
Eventually, the 72-hour fire engulfed 21 of the 30 petroleum storage tanks on site. Firefighters were limited to a defensive posture, attempting to protect as much of the remaining terminal as possible.
Investigators determined that the storage tank that released the vapor cloud had one key difference from most of the other tanks in service at the terminal. Operators could not monitor it by computer from their control room, Wanko said.
"This particular tank that was being filled registered no alarm state in their computer at the time of the incident," he said.
ISLAND OF ENCHANTMENT
In 1955, the northern coastal village of Bayamon became the center of the Puerto Rican oil industry. A refinery that would eventually expand to a capacity of 48,000 barrels a day was built there to supply San Juan's chief electrical power plant.
Economic changes forced the closure of the refinery in 2000. However, the adjoining tank farm continued to operate as part of the only privately owned dock and terminal facility in San Juan Harbor, supplying a chain of more than 200 service stations across the island.
On the night of October 22, staffing at the terminal consisted of two guards, three Caribbean Petroleum employees and two contract workers whose job involved confirming fuel shipments received.
"These guys serve as intermediaries representing other parties so that they know commerce is going well," Wanko said.
Among that commerce was a storage tank being filled with gasoline from a ship docked in San Juan Harbor. Investigators determined that a likely scenario leading to the vapor release was an accidental overfilling of the tank, Wanko said.
"The tank overflow is not our only potential scenario for getting fuel out of containment," he said. "We are testing other theories. Until we can either confirm or deny other scenarios, there is not a lot to say."
A physical check of the tank at 11:20 p.m. October 22 revealed no problems, Wanko said. The tank overflowed sometime between then and discovery of the vapor cloud shortly after midnight.
"We're unsure right now how long the tank overfilled if that is indeed the scenario that occurred," Wanko said.
Gasoline spilled from the tank without detection. With regard to this particular tank, the liquid level could not be monitored by operators because the computerized system was not receiving a signal from the level gauge, Wanko said.
"They could not detect levels on this tank on their computer," he said. "Not only could they not see the level, alarms for this particular tank were disabled."
At this point, investigators have not determined the reason for the failure, Wanko said.
"That's the big step," he said. "When we can understand what the operators could and could not see, that will give us an understanding of the systemic issue."
Operators were aware that at the time of the transfer that gauging information was not available via computer. The actual overfill was uncovered during a required hourly gauging in which an employee physically inspected each tank, Wanko said.
The ship was notified to immediately shut down the transfer of gasoline once the vapor cloud was discovered.
"At the time of ignition, no gasoline was being transferred," Wanko said. "The tanker ship was in the process of disconnecting. It eventually left port after the incident."
As the gasoline spilled, it vaporized. CSB investigators estimated that the vapor cloud spread to a 2,000 foot diameter until it reached an ignition source in the northwest section of the facility.
"We know the general area where ignition took place thanks to surveillance video," Wanko said. "The northwestern corner of the facility is where their wastewater treatment plant is located. The ignition was somewhere in that area, perhaps in an office or some electrical switch gear based on the damage exhibited."
The wastewater treatment plant is located nearly a quarter mile away from the overflowing tank. With only a light breeze that night, spread of the vapor was mainly by disfusion and gravity, Wanko said.
"With the delay in ignition, we had a lot of vapor created," he said. "That's obviously one of the reasons this incident was so violent."
An industrial park borders the terminal on the west while, on the east, is the Army's Fort Buchanan, a 750-acre post that is the Army's only active installation in Puerto Rico and the Antilles. The installation bore the brunt of the pressure wave from the blast with significant damage to windows and doors, Wanko said.
Residential neighborhoods stand as close as half a mile from the tank that overfilled. Damage to at least 200 of the 450 homes was reported.
"We are aware of only three minor injuries in the residential area," Wanko said. "These involved secondary missiles such as things coming off roofs or falling light fixtures."
Nearly 350 residents from the area were evacuated to a nearby sports stadium.
Witnesses arriving at the terminal before 1:30 a.m. on October 23 reported that 17 storage tanks were burning simultaneously. Another four tanks ignited before the flames were brought under control.
"To really understand the fire's behavior is going to take some sophisticated analysis," Wanko said.
Firefighers with the Puerto Rico Fire Department (Cuerpo de Bom-beros de Puerto Rico) adopted a defensive stance, attempting to keep the fire from spreading to the remaining tanks. Beyond the 30 operational tanks, another 10 storage tanks at the terminal were out of service. The mothballed refinery, damaged by the blast, was not in danger from the fire.
CNN reported that Lt. Angel Crespo of the Puerto Rico Fire Department led a team of five firefighters who shut down ruptured pipeline valves that were spilling gasoline that fed the flames. The team used two 2?-inch hoses to protect themselves while working to reach the valves.
Still, even with the valves closed, it took another 24 hours before the flames were out, the report states.
Water supply on site after the blast was apparently adequate, Wanko said. A salt water line established from the nearby bay by National Guard personnel was not used during the emergency. A National Guard airplane was used to bring additional fire fighting foam supplies from the U.S. Virgin Islands.
According to Wanko, the Puerto Rico explosion and fire is already generating "a very large buzz" in the petroleum refining world because of comparisons to the 2005 Buncefield blast.
"At this point, this seems to fit the Buncefield model," he said.
Reported to be the largest explosions in the UK since World War II, the Buncefield blast centered on a storage tank overfill at an oil storage depot north of London that created an immense vapor cloud. That cloud flowed off site, causing extensive damage to unrelated businesses nearby.
Because the blast occurred early on a Sunday morning, injuries were limited to 43 people. Had the blast occurred on a weekday, the surrounding business would have been filled with employees.
As if to drive home the inherent danger of vapor cloud explosions, India reported a comparable incident only six days after the Puerto Rico explosion. A pipe rupture at a fuel depot on the outskirts of Jaipur caused a spill that created a massive vapor cloud. As with Buncefield, there was extensive off-site damage to adjoining industrial units from the ensuing blast.
What separates the Jaipur incident from Buncefield and Puerto Rico is the death toll. Despite the Jaipur incident occurring in the evening when most of the depot employees were gone, the explosion and fire killed 11 people. Smoke continued to pour from the depot nearly two weeks after the blast.
"So there are three incidents (involving vapor cloud explosion) now that we are aware of and looking at," Wanko said. "We are working with the investigation teams at Buncefield and Jaipur to really understand these incidents and make recommendations to prevent it from recurring."