The 2010 Deepwater Horizon explosion, catastrophic fire, and subsequent leak of millions of gallons of crude oil into the Gulf of Mexico 80 km from the Louisiana coast was just the most spectacular and disastrous of recent offshore oil rig accidents. There have been many more. The table below lists some of the more recent accidents .
On April 20, 2010, operations were underway on the Deepwater Horizon drilling rig which was stationed about 80 km from the coast of Louisiana in the Gulf of Mexico. BP was the main operator and lease holder responsible for the design of the well; Transocean was the drilling contractor that owned and operated the platform. On the day of the accident, the crew was completing temporary abandonment of the well so it could be left in a safe condition until a production unit could return later to extract the oil and gas.
The abandonment work was supposed to plug the well. But a cement barrier intended to seal the oil and gas had not been correctly installed at the bottom of the well. Personnel misinterpreted the results of a test to assess the integrity of the cement plug, leading them to believe that the well had been properly sealed. It was not.
The crew started to remove drilling mud from the well in preparation for the installation of an additional cement barrier. At the base of the well on the sea bed was a blowout preventer, called the BOP. The BOP was designed to shut and close the well automatically in the event of a blowout.
Removing the drilling mud after the pressure test allowed oil and gas to flow past the failed cement barrier up to the rig. This flow continued for almost an hour and was not immediately detected by the crew. The BOP did not activate. Eventually, the oil and gas blew out onto the rig. The crew acted immediately to manually close the blowout preventer. But oil and gas had already flowed past the BOP and continued to gush onto the rig, finally igniting and then exploding. An automatic emergency response system designed to shear drill pipe passing through the BOP and seal the well did not activate successfully.
Eleven crewmembers were killed by the explosions and 17 more were seriously injured. The fire was catastrophic, and after burning fiercely for 36 hours the drilling rig toppled over and sank. The image below captures the catastrophic scale of the explosion and fire .
The oil spill was the largest offshore spill in US history. Unconstrained by the blowout protector, crude oil continued to flow from the well for 87 days before the well was finally closed. Over 130 million gallons of crude oil spewed into the waters of the Gulf of Mexico fouling 1,300 miles of shoreline along five states  The northern Gulf coast is home to 22 species of marine mammals, including manatees in coastal seagrasses, and dolphins and whales in estuarine, nearshore, and offshore habitats—all of which were contaminated with oil. The disaster contributed to the largest and longest marine mammal unusual mortality event ever recorded in the Gulf of Mexico. For instance, bottlenose dolphins exhibited reduced survival and reproductive success in the years following the spill, leading to a 50 percent decline in the population. 
For more information check out these sources:
 See: The world’s worst offshore oil rig disasters, at //www.offshore-technology.com/features/feature-the-worlds-deadliest-offshore-oil-rig-disasters-4149812/, and Major offshore accidents of the 20th and 21st Century, accessed at : //www.arnolditkin.com/practice-areas/oil-rig-explosions/major-oil-rig-diasassters
 The description of the incident and the image are taken from the Investigation report and executive summary of the incident issued by the US Chemical Safety and Hazard Investigation Board. Available at: //www.csb.gov/assets/1/7/Overview_-_Final.pdf
 See the National Ocean Service report: Deepwater Horizon oil spill. Accessed at: //oceanservice.noaa.gov/news/apr17/dwh-protected-species.html
 See the National Ocean Report, ibid.