An explosion on the Deepwater Horizon oil rig that killed 11 workers and ruptured a pipeline that spewed tens of thousands of barrels of oil a day into the Gulf of Mexico left BP P.L.C. with a huge self-insured loss caused by a massive risk management failure.

The oil spill, triggered by the April explosion that two days later sank the rig owned by Transocean Ltd., was the worst in U.S. history. A blowout preventer on the pipeline failed, allowing oil to flow into the Gulf for months.

BP agreed to pay the majority of costs related to the cleanup and losses businesses suffered due to the spill, which BP estimated would cost it about $40 billion.

To pay the claims, BP established the Gulf Coast Claims Facility, which had paid nearly $4 billion as of early December.

Fitch Ratings Ltd. said the disaster may cost insurers as much as $6 billion. Liability rates for energy risks spiked as much as 100% after the disaster.

In July, BP finally shut in the well and stopped the flow of oil into the Gulf.

In a report, BP acknowledged that decisions by “multiple companies and work teams,” including misreading pressure data that indicated a blowout was imminent, among other mistakes, cleared the way for work on the well to continue and led to the fatal explosion and fire.

The well spewed as much as 40,000 barrels of crude oil into the water each day it was uncapped, some experts said, making it five to six times worse than the 1989 Exxon Valdez spill in Alaska.

More recently, however, BP's lawyers reportedly asserted that estimates of the spill were vastly overstated.

The spill imperiled at least $1.6 billion in tourism, recreational and commercial fishing, and economic benefits from coastal wetlands, according to the Corpus Christi, Texas-based Harte Research Institute for Gulf of Mexico Studies at Texas A&M University.

The disaster prompted Torus Insurance Holdings Ltd. to set up an excess liability facility known as excEED, for excess energy exploration and development. The facility plans to offer up to $1 billion in excess casualty and pollution capacity in tranches of $250 million through a consortium of reinsurers and insurers if demand for the coverage is sufficient.

Separately, Munich Reinsurance Co. said the Deepwater incident led to its plans to offer as much as $20 billion in excess casualty limits if U.S. lawmakers raise the $75 million liability cap set by the Oil Pollution Act.