The Deepwater Horizon sunk yesterday due to a fire apparently caused by a blow out that occured while cementing production casing. The precise cause is unknown but what is indisputable is that the well control system failed. A rig like the Deepwater Horizon has millions of dollars worth of well control equipment on board. The systems are redundant and extensively tested all during operations.
Drill crews receive intensive training in well control procedures which they practice regularly. The failure could be a systems failure or it could be a consequence of human error or both.
The Deepwater Horizon is a semi-submersible floating drilling vessel designed to operate in deep water for the purpose of drilling oil and gas wells and installing subsea production systems.
In water depths over 1000 feet or so bottom supported structures are prohibitively costly and not technologically the best solution. In deep water, most of the well control system is installed at the sea floor. Since divers cannot reach these water depths all equipment must be installed remotely using electro-hydraulic controls and Remote Operating Vehicles (ROVs) with video cameras.
Oil and gas wells are drilled in stages to maintain structural integrity of the well. Several strings of pipe called casing are installed concentrically as the well is drilled to prevent the hole from collapsing and to seal the wellbore off from geological formations through which the well has been drilled.
Casing is cemented into the well by pumping a cement slurry down the casing. It exits the bottom of the casing and fills up the space between the drilled hole an the outside of the casing (annulus). Production casing is the last string run during drilling, and is usually 7 to 10-3/4 inches in diameter. It extends from the wellhead all the way into the target geologic formation. It is suspended in the subsea wellhead before cementing.
The subsea wellhead is positioned at the top of the wellbore 10-20 feet above the sea floor. At the top it features a profile that provides an interface for the subsea blow-out preventer (BOP) to be hydraulically latched and sealed onto it.
The rig must be able to circulate drilling fluids into and out of the well. A large diameter steel tube is installed from the subsea BOP to the rig called a drilling riser. The drilling riser is tensioned and supported at the rig.
At the top of the drilling riser is a well control device attached to the rig called a diverter. Unlike the BOP the drilling riser is not designed to seal off the well bore. The design includes outlets that divert the flow of any gas or oil the enters the drilling riser outside the perimeter of the rig where it is less likely to ignite.
The well casing and BOP are rated to withstand high internal pressure (10,000 to 20,000 pounds per square inch). The drilling riser is only rated to about 500 psi internal pressure. For this reason high well pressures must be contained below the BOP.
During a cementing operation like the one in which the Deepwater Horizon was engaged, the casing string is assembled section by section (joint in oil field jargon) and lowered into the well bore through the BOP, the wellhead and the previously installed casing into the section of hole that has no casing (open hole).
Since the top of the casing will be suspended in the wellhead, when the last casing joint is threaded onto the string the susea casing hanger and installation tool is installed on top of it.
To lower the casing into the subsea wellhead the drill crew attaches drill pipe to the casing hanger and continues to lower the casing through the drilling riser supported on the drill pipe.
Drill pipe is the pipe used to carry the drill bit and tools into and out of the well. It comes in 30 foot long joints but when drilling, the drill crew handles three joints at a time to save time. When they are pulled out of the well they are stood back in a rack in the drilling mast.
When the casing is landed and sealed in the wellhead, the crew connects a special cement pumping system to the top of the drill pipe that extends up to the drilling platform on the rig (drill floor). Rig engineers calculate how much cement is required to fill up the casing annulus and begin pumping it down the drill pipe. When they pump enough cement they stop pumping cement and follow it with drilling mud to force all of the cement out the bottom of the casing into the annulus. They then wait for it to set.
Given that process, when the rig is engaged in cementing the casing, drill pipe extends from the subsea wellhead through the BOP and riser to the drill floor.
The first and most important method of well control is a special fluid called drilling mud. This mud has specific chemical and physical properties. The well must be kept full of mud at all times. The weight of the mud creates a hydrostatic head of pressure which offsets the geologic formation pressures encountered in the strata through which the well extends. Ideally the weight of the mud balances the formation pressures when the hole is full.
Many things can go wrong that would cause the pressures in the geologic formations to force gas, oil and water into the wellbore. All of them involve problems with the mud. It might leak into a formation, it might not be heavy enough, it might not have the needed viscosity. If the mud fails to hold back the formation pressures the well begins to flow. This is referred to as a kick in oilfied jargon.
If the well begins to kick, the first sign is that the crew sees mud flowing back at their mud tanks where the drilling mud is mixed and stored.
If they cannot stop the flow by pumping in a special mixture of mud, the next alternative is to close the BOP.
The BOP has four to five hydraulic rams which are like large valves that can close over and seal off the wellbore. An electronic signal is sent down to the BOP control system from the surface to intiate a hydraulic pump to close a ram. There are several types of rams since there is no way to predict the position of pipe in the hole when a well kicks. Pipe rams seal around the outside of the pipe. Shear rams shear through the drill pipe. Blind rams seal over an open hole if no pipe is in the wellbore.
There is also an annular BOP which is like a big rubber washer that can seal around any shape that may be in the BOP.
BOPs also have an ROV interface that enables an ROV to operate the BOP if the surface control system fails. If the BOP fails the next alternative is to let the well flow but divert the flow out the diverter outlets.
Since they are now attempting to send an ROV to operate the BOP, I presume the BOP controls failed to operate properly or were not actuated in time. The well unloaded the mud and then gas made it to the surface.
Apparently, the diverter didn't function properly either or wind brought the venting gas back to the rig. Hence an explosion.
By the time a well is kicking and the crew recognizes they have a problem, they should be evacuating the rig. The rig has escape capsules. Rig crews are trained on evacuations and practice the procedure on a regular basis. Every place on the rig has a designated escape capsule. In the case of an evacuation the drill crew on duty (on tour in oilfield jargon) would typically stay until last trying to get things under control. I suspect those lost were on duty at the time the explosion occurred.
Oil and gas is volatile. Oil and gas wells can experience extremely high pressures. Those two basic facts makes the enterprise risky and complex. Given the number of wells drilled and the number of rigs operating, they actually have a remarkably good safety record. There is just a lot of things that can go wrong.