Deepwater Horizon BOP details from the NY Times. Recaps a lot of existing details and adds in many new parts of the story.
The area where my nose twitches is the line that “today’s drill pipes” are twice as strong as in the past”… that even if the shear rams had worked correctly, they might have been unable to cut the pipe. With a single ram BOP, there is a 10% chance that the ram will hit the joint between the 30 foot sections of pipe, which are purposely very thick and much too rigid for the ram to cut. One of the changes already suggested to force all rigs to have twin rams, so that one of them is guaranteed to not be at a joint at all times.
With 32 years of oil field experience, let me clarify somewhat. On the “joint of pipe”, this is where two joints of drill pipe or casing is threaded together, male thread into a female thread. On most land drilling rigs, thisdrill pipe tool joint is typically 6″ across and the drill pipe is 4″ across. Offshore rigs, at the depth they drill, they would run 5″ drill pipe for more tensile strength to support the increased weight of the drill string. Casing on land rigs can be 24″ for surface pipe, 12.25″ for wellbore to as small as 7″. Offshore, I’m not sure of the size they normally run.
To reduce the need for dual sets of rams for drill pipe, the driller could simply pick up the rig brake handle for the traveling blocks to lower the pipe two feet and then close the rams. If they are refering to casing shear rams, then two sets would be a great idea if the casing was already cemented in place. If not cemented, the casing could be lowered two feet just as the drill pipe to clear the shear rams.
From what I’ve read, the issue is raising the drill string to get the joint out of the way assumes you still have functional control of the drilling platform. And in an underwater rig, you can’t see the drill pipe coming out of the BOP.
One of the activation reasons for the shear rams to close is because of the loss of control by the rig and/or the rig attempting an emergency disconnect when the rig is in trouble (earthquake, tsunami, hurricane, explosion, etc). They did attempt an emergency disconnect (over the objections of the captain), but it didn’t work. A later story adds that a few days earlier, one of the two “pods” was malfunctioning, so they turned it off and were running without a backup control pod.
Even if both pods were functional, it was known the last chance safety system had a single point of failure – a single hydraulic shuttle valve shared by both pods. A second ram would eliminate that single point of failure.
The Deepwater Horizon was built right on the cusp of when folks got worried about a single ram. The BOP and the rig are built as a set… All of the control panels and infrastructure work together.
When they cut the riser, the drill string was inside – that’s why the diamond string saw was unable to cut the riser. From the first day, John Batchelor’s sources were reporting that the drill string was in the BOP and probably why it failed. Since they were just finishing up the cement job, that’s not a surprise. It takes a while to pull out 13,000 feet of drill pipe.