Mexico’s state-owned oil company Pemex was drilling a 3 km deep oil well when on 3 June 1979 the drilling rig Sedco 135 lost drilling mud circulation.
Sedco 135 was drilling at a depth of about 3,600 meters 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 hydro static 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 135, the drilling mud suddenly began to flow up towards the surface; by removing the drill-string the well was swabbed leading to a kick.
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 135 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.