faethor said:
Deep Water drilling is perfectly safe. The industrial nature of design has proven this. Clearly lots of money is spent ensuring that leaks and mishaps don't occur else lots of product will be lost and wated.
Deep water drilling in inherently risky. :roll: Deep-water drilling is a high-stakes game. There's geologic risk, technical risk, engineering risk, environmental risk, capital risk and market risk. The well is designed with triple fail safes & redundancies.
Of 18 other offshore oil well blowouts in the Gulf of Mexico the cause of all 18 oil well blowouts was gas cut (lightened / thinned) drilling mud while drilling. When drilling through an oil & gas bearing zone, the drilling mud becomes gas cut (mud weight reduced) and thinned (mud viscosity reduced). Defoamers are added to the drilling mud and then new mud is circulated into the well as the gas cut mud is circulated out the wellbore.
In this case the drilling was completed and the production casing was being cemented in place, and plugs were being set, to temporally abandon the well and move the drill ship away.
This well had been giving major lost circulation problems drilling all the way down. Heavy weight mud was required to contain bottom hole pressure, (16ppg+ mud weight). BP was cementing in place a long string of 7" production casing, run on drill pipe and hung off on the wellhead on the sea floor, like a "liner". They cemented this casing with a special lightweight foam cement containing nitrogen because they had lost circulation several times during drilling operations. The execution of a foam cement job are very complex, in order that you neither let the well flow from too little hydrostatic pressure nor break it down and lose the fluid and cement from too much hydrostatic. On the outside of the top joint of casing is a seal assembly - "packoff" - that sets inside the subsea wellhead and seals. This was set and tested to 10,000 psi, then a temporary "bridge plug" was run in on drill pipe to set somewhere near the top of the well bore and below the sea floor. This was the second redundant barrier. It is not know if this was actually set or not. At the same time they displaced the 16+ ppg mud from the the riser and replaced it with sea water so that they could pull the riser, off the BOP, lay it down, and move the drill ship off. When they did this, they dropped the hydrostatic pressure on the well. This was the plan since the well was plugged both on the inside with the casing and on the outside with the tested packoff. But something broke loose, gas and oil rushed up the riser; there was little wind, and a gas cloud hung over the rig. When the air intakes of the diesel engines sucked in the natural gas, they revved up until they exploded. This set everything on fire. Another engine explosion in the mud pit / mud pump room blew the mud pumps overboard. Another in the mud sack storage room, next to the living quarters, took out all the interior walls where the drilling crew was hanging out having a party to celebrate 7 years of accident free work on this rig.
In this case either Haliburton pumped too much salt water at the end of the cement job, so there was no bottom plug at the end of the casing, a miscalculation of fluid displacement volume by the operator of the pump or the foam cement failed. It will never be known for sure, the relief well will bury everything in concrete.
