Ixtoc I oil spill (1979)

About 100 kilometers (62 miles) northwest of Ciudad del Carmen, Campeche, in seas 50 meters (164 feet) deep, the semi-submersible drilling rig Sedco 135 was drilling the exploratory oil well known as Ixtoc I in the Bay of Campeche in the Gulf of Mexico. The well blew up on June 3, 1979, causing one of the worst oil disasters in history. Sedco 135 was drilling at a depth of around 3,600 meters (11,800 feet) below the seafloor at the time of the disaster. The drill bit had struck a zone of soft stratum the day before Ixtoc had the blowout and subsequent fire that led to her sinking. Hydrostatic pressure was then lost as a result of the lack of drilling mud circulation. The drilling mud was leaking through fissures that had developed in the rock at the bottom of the hole rather than rising to the surface. In order to seal off the cracks that were causing the loss of circulation, Pemex executives decided to remove the bit, run the drill pipe back into the hole, and pump materials down this open-ended drill pipe.

The well was swabbed when the drillstring was removed, which is an effect seen when mud must flow down the annulus to replace displaced drill pipe volume below the bit, resulting in a kick. This happened while the pipe on Sedco 135 was being removed. Normally, the blowout preventer's shear rams can be activated to block this flow (BOP). However, in this instance, the drill collars had been brought in line with the BOP and the BOP rams were unable to sever the thick steel walls of the drill collars, resulting in a catastrophic blowout. These rams are designed to sever and seal off the well on the ocean floor; however, in this case, the drill collars had been brought in line with the BOP.

After the drilling mud, there was a significant amount of oil and gas flowing at a pace that was still rising. The Sedco 135 drilling rig riser was destroyed when the oil and gas vapors ignited when they came into touch with the running pump motors. The BOP stack at the seabed was harmed by the collapse. Large amounts of oil were released into the Gulf as a result of the BOP's destruction.

When: 1979
Where: Gulf of Mexico
Oil spilt: 140 million gallons