‘Deepwater Horizon,’ an Explainer: How a Faulty Plug Caused a Deadly Environmental Disaster

Peter Berg’s fact-based drama lands in theaters on Friday

On April 20, 2010, the Deepwater Horizon oil rig exploded, killing 11 people and leaking millions of barrels of oil into the Gulf of Mexico.

The first film about the incident, Lionsgate/Summit’s “Deepwater Horizon,” starring Mark Wahlberg, hits theaters this weekend. While there’s always a certain amount of Hollywood burnishing on any film based on a true story, it’s helpful to know what really happened — and how a piece of cement played such a crucial role in the tragedy.

According to the U.S. Chemical Safety Board’s final report dated April 12, 2016, which builds on four earlier volumes, one of the biggest disasters in environmental history could have been prevented by nothing more than a properly installed cement plug.

The problem began when the crew of the Deepwater Horizon — an offshore drilling rig owned by Transocean and leased to BP — was in the final stages of abandoning the well so that oil and gas could be extracted later by a different type of rig.

That involved testing a cement barrier that was intended to cap the well, followed by removing some of the drilling mud — a viscous fluid used to drill for oil and gas — in the well. It didn’t go as planned.

“BP and Transocean personnel misinterpreted a test to assess cement barrier integrity, leading them to erroneously believe that the hydrocarbon bearing zone at the bottom of the well had been sealed,” the investigators wrote in the chemical safety board’s report.

When the rig’s crew removed the drilling mud so that they could install an additional cement barrier, the open blowout preventer — a large, complex valve intended to do exactly what its name indicates — was the only thing standing between the oil and gas beneath the seafloor and the rig. The preventer would only work if humans detected the “kick” — oil and/or gas flowing upward through the bore, and manually closed it.

It stayed open for nearly an hour without anyone detecting the problem, as oil and gas flowed past the faulty cement barrier and up to the rig. Workers then manually activated it and closed a pipe ram, temporarily sealing the well. While the pressure below the ram continued to increase, it fell above the ram as mud, hydrocarbons and seawater dumped onto the rig.

As a result, investigators believe that the differential pressure caused the pipe to buckle and move off-center within the preventer.

Worse yet, a backup or “deadman” system activated — despite multiple electrical problems — but failed to stop the flow. “The safety critical systems responsible for shearing drillpipe in emergency situations had performance deficiencies even before the [open blowout preventer] was deployed,” the report said.

Because the pipe was not centered, the ram punctured the pipe but did not seal the well. So oil and gas continued to flow out — eventually adding up to more than 4.9 million barrels until it was finally cut off after 85 days.

All of that ecological and economic devastation might not have happened if that first cement barrier had been installed properly.

“If either the bottom hole cement job had been successful or the [blowout preventer] had functioned that day, the blowout could have been avoided,” the report concluded.