Accident
Mexico's government-owned oil company Pemex (Petróleos Mexicanos) was drilling a 3 km (1.9 mi) deep oil well when the drilling rig Sedco 135F lost drilling mud circulation.
In modern rotary drilling, mud is circulated down the drill pipe and back up the well bore to the surface. The goal is to equalize the pressure through the shaft and to monitor the returning mud for gas. Without the counter-pressure provided by the circulating mud, the pressure in the formation allowed oil to fill the well column, blowing out the well. The oil caught fire, and Sedco 135F burned and collapsed into the sea.
At the time of the accident Sedco 135F was drilling at a depth of about 3,600 metres (11,800 ft) below the seafloor. The day before Ixtoc suffered the blowout and resulting fire that caused her to sink, the drill bit hit a region of soft strata. Subsequently, the circulation of drilling mud was lost resulting in a loss of hydrostatic pressure. Rather than returning to the surface, the drilling mud was escaping into fractures that had formed in the rock at the bottom of the hole. Pemex officials decided to remove the bit, run the drill pipe back into the hole and pump materials down this open-ended drill pipe in an effort to seal off the fractures that were causing the loss of circulation.
During the removal of the pipe on Sedco 135F, the drilling mud suddenly began to flow up towards the surface; by removing the drill-string the well was swabbed leading to a kick. Normally, this flow can be stopped by activating shear rams contained in the blowout preventer (BOP). 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 and the BOP rams were not able to sever the thick steel walls of the drill collars leading to a catastrophic blowout.
The drilling mud was followed by a large quantity of oil and gas at an increasing flow rate. The oil and gas fumes exploded on contact with the operating pump motors, starting a fire which led to the collapse of the Sedco 135F drilling tower. The collapse caused damage to underlying well structures. The damage to the well structures led to the release of significant quantities of oil into the Gulf.
Read more about this topic: Ixtoc I Oil Spill
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