In 1979, the Sedco 135F was drilling the IXTOC I well for PEMEX, the state-owned Mexican petroleum company when the well suffered a blowout. The well had been drilled to 3657m with the 9-5/8" casing set at 3627m. Reports then state that mud circulation was lost (mud is, in essence, a densely weighted drilling fluid used to lubricate the drill bit, clean the drilled rock from the hole and provide a column of hydrostatic pressure to prevent influxes) so the decision was made to pull the drill string and plug the well. Without the hydrostatic pressure of the mud column, oil and gas were able to flow unrestricted to the surface, which is what happened as the crew were working on the lower part of the drillstring. The BOP was closed on the pipe but could not cut the thicker drill collars, allowing oil and gas to flow to surface where it ignited and engulfed the Sedco 135F in flames. The rig collapsed and sank onto the wellhead area on the seabed, littering the seabed with large debris such as the rig’s derrick and 3000m of pipe.
The well was initially flowing at a rate of 30,000 barrels per day (1 barrel = 42 US gallons = 159 litres), which was reduced to around 10,000 bpd by attempts to plug the well. Two relief wells were drilled to relieve pressure and the well was eventually killed nine months later on 23 March 1980. Due to the massive contamination caused by the spill from the blowout (by 12 June, the oil slick measured 180km by 80km), nearly 500 aerial missions were flown, spraying dispersants over the water. Prevailing winds caused extensive damage along the US coast with the Texas coast suffering the greatest. The IXTOC I accident was the biggest single spill ever, with an estimated 3.5 million barrels of oil released.
Office of Response and Restoration: IXTOC I
The Royal Society of Canada: Report on Science Issues Related to Oil and Gas Activities