Failures in efforts to control offshore oil well blowouts have raised questions as to whether offshore drilling can be made safe. An understanding of the technologies related to this problem should help answer these questions.
Background information edit
To understand various solutions to the blowout problem, some basic understanding of deepwater drilling and relevant safety technologies will be helpful.
Deepwater drilling - information on the well depths, pressures, etc., for the Deepwater Horizon blowout in the Gulf of Mexico, April 2010.
BP diagram of well - showing also the relief wells being drilled alongside.
Pore pressure gradient - basic facts about "pore pressure" of oil in rock formations. Static pressure could be as high as 1000psi for every 1000 feet of well depth. Such enormous forces can make can make "killing" the well from the top very difficult.
DOE data - data made available through the U.S. Department of Energy's Open Government initiative.
Blowout preventers have been used on land for decades. These devices may need to be modified for deepwater, where access and repair is more difficult.
Blowout_preventer - good general description of Cameron ram-type BOPs.
Deepwater Horizon BOP - detailed drawing posted on DOE website.
Design evolution of a subsea BOP, Melvin Whitby, Drilling Contractor, May07 pp.36-8. - lots of detail on a complex control mechanism.
MMS Project 455 - "Review of Shear Ram Capabilities", West Engineering, Dec. 2002. The unreliability of shear rams is well known to drilling engineers.
Surface containment technologies are effective with surface spills and calm water. The Deepwater Horizon blowout may have much of the oil not reaching the surface.[1] It is not clear whether this oil lingering at great depths is due to the use of dispersants, or some other condition in the deep water.
Floating booms - photos showing typical booms of various sizes.
Operational failures are as important as the design of equipment, and must be anticipated in any design relating to safety.
WSJ article "BP Decisions Set Stage for Disaster", Ben Casselman, Russel Gold (Wall Street Journal, 5/27/2010) - Good discussion of rushed safety checks. Good diagrams of well casing and cement.
Deepwater blowouts - how could one happen Larry Flak, Offshore Magazine, Jan 1997. Article by an oil-industry expert, describing four likely scenarios.
Relevant technologies edit
A safe design does not depend on any one device or procedure being perfect. Failures in operations and oversight, and even sabotage, will always be a risk. A well-designed system anticipates almost every failure and provides more than one way to deal with it.
- Note: The list here is not exhaustive, but should include the most important measures that are likely to be used in future deepwater wells.
- NPOV: We need to make sure this list represents a consensus, not just one person's opinion of what technologies are most important.
1) Have several flow regulators (downhole safety valves) at various depths in the well casing. These should be of such simple design that failure is almost unthinkable, and they should be tested periodically to ensure that they will work.
2) Have multiple wellheads already in place in case everything goes wrong with the active wellhead. This does not require drilling multiple wells, just extra piping to the alternate wellheads.
3) Use a blowout preventer that can be quickly repaired or replaced. Have spare parts, especially an extra riser pipe, ready in a nearby warehouse, to connect an alternate wellhead, and relieve the pressure while replacing damaged parts.
4) Provide a way to quickly insert mud deep in the well, without having to drill a relief well.
5) Practice emergency procedures, so we can be confident that getting the well under control will take no more than 24 hours.
6) Have floating booms ready to deploy, and sufficient pumping capacity to collect all the oil that may come to the surface. The barriers may need a special design to tolerate ocean waves.
7) Have independent oversight of all operations to make sure safety remains the first priority, regardless of business pressures.
Fluid flow regulators edit
Wellheads edit
Well casings edit
Casing_(borehole) - basic explanation, good diagram
Example well] - plan drawing for the Deepwater Horizon well
- article in The Oil Drum - good explanation of the above drawing
Casing a Well - good article from The Oil Drum
- still need discussion re blowout prevention
What are the modes of failure?
- burst from well bore pressure Barlow's formula
- break bond between casing and cement or between cement and rock
- expulsion of casing from well due to extreme pressure at wellhead ???
- Tudor Pickering slides - showing possible failures in Deepwater Horizon well casing.
How can the casing and cement be tested, and can testing ensure no failure?
Blowout preventers edit
Blowout preventers ( sandbox ) have been used for nearly a century in control of "gushers" on land. The same technology is now being used in deepwater offshore wells, where inspection and repairs are much more costly, and the consequences of failure much worse. Their are two variations in use, the surface blowout preventer, which sits between the riser pipe and the drilling platform, and the sub-sea blowout preventer which sits on the ocean floor. The surface unit is smaller, lighter, less costly, and more easily accessed for routine tests and maintenance. However, it does not prevent blowouts involving a broken riser pipe.[2]
Blowout preventers often contain a stack of independently-operated cutoff mechanisms, so there is redundancy in case of failure, and the ability to work in all normal circumstances with the drill pipe in or out of the wellbore. The BOP used in the Deepwater Horizon, for example, had five "rams" and two "annular" blowout preventers.[3] The rams were of two types: "pipe rams" and "shear rams". If the drill pipe is in the well, the pipe rams slide perpendicular to the pipe, closing around it to form a tight seal. The annular preventers also close around the pipe, but have more of a vertical motion, so they loosen slightly if the drill pipe is being pushed downward, as might be necessary in a snubbing or well kill operation.[4] Shear rams are used as a last resort to cut through the drill pipe and shut off everything, including whatever might be coming up inside the drill pipe.
Shear rams have well-documented reliability problems. Figure 1 shows the result of a 2002 study done on offshore oil rigs for the Minerals Management Service. This study was designed to answer the question “Can a given rig’s BOP equipment shear the pipe to be used in a given drilling program at the most demanding condition to be expected?” Seven of the fourteen cases in this study opted not to test, another had insufficient data to draw a definitive conclusion, and three failed to shear the pipe under realistic conditions of expected well bore pressure. In each case of failure, increasing the pressure on the rams above its design value, successfully sheared the pipe.[5]
A followup study in 2004 confirmed these results with a much larger sample of drill pipes and typical blowout preventers from three different manufacturers. This second report is far more detailed than the first, with many pages of tables and charts and equations, but in case anyone would miss the significance of the result, the first paragraph of the Executive Summary states "Shear rams may be a drilling operation’s last line of defense for safety and environmental protection."[6]
In addition to insufficient ram pressure, an investigation[7] of the [Deepwater Horizon oil spill] has revealed other areas where the design of blowout preventers could be improved for deepwater drilling. Two shear rams are essential, to avoid what the investigators call "single-point failure". These rams should have enough space between them that the threaded joint between two pipe sections cannot jam both of them. Shear rams should be designed so it is possible to force them closed in an emergency involving leaks or stuck valves in the hydraulic system.
Other deficiencies needing a better source than [3]
- Quick disconnect flanges, in case we need to replace a bent riser pipe or other damaged equipment.
- Planned breakage points. In the future, risers will never fall, but just in case, it would be helpful to relieve the excess forces in a controlled way.
- Alternate outlets to relieve pressure from the well while repairing a damaged BOP.
Surface collection technologies edit
Dutch skimmers - Claim that 4 Dutch skimmers could collect 150,000 barrels per day.
Dispersants edit
See also edit
- Oil spill#Prevention - discussion of spills from oil tankers and other surface accidents
- Blowout (well drilling)
- Deepwater Horizon oil spill
- Blowout_preventer
- Pore pressure gradient - basic facts about "pore pressure" of oil in rock formations.
References edit
- ^ Gillis, Justin (2010-05-18). "Giant Plumes of Oil Forming Under the Gulf". The New York Times. Retrieved 2010-05-18.
- ^ Risk Analysis of Using a Surface Blow Out Preventer (BOP), Marine Computation Services, Inc., April 2010, project 640 for U.S. Minerals Management Service.
- ^ a b Diagram of BOP used in Deepwater Horizon well from U.S. Dept. of Energy, Open Government program.
- ^ Schlumberger Oilfield Glossary for terms such as "snubbing" and "well kill".
- ^ Review of Shear Ram Capabilities, West Engineering Services, Dec. 2002, Project 455 for U.S. Minerals Management Service.
- ^ Shear Ram Capabilites Study, West Engineering Services, Sept. 2004, Project 463 for U.S. Minerals Management Service.
- ^ Barstow, David; Laura Dodd; James Glanz; Stephanie Saul; Ian Urbina (20 June 2010). "Regulators Failed to Address Risks in Oil Rig Fail-Safe Device". The New York Times. Retrieved 2010-08-15.
External links edit
- BP diagram of well showing the well depths, pressures, etc., for the Deepwater Horizon blowout in the Gulf of Mexico, April 2010.
- DOE website - oil spill data, BOP drawing, well casings, etc.
- Design evolution of a subsea BOP, Melvin Whitby, Drilling Contractor, May07 pp.36-8. - lots of detail on a complex control mechanism.
- Floating booms - photos showing typical booms of various sizes.
- Ben Casselman, Russel Gold (Wall Street Journal, 5/27/2010) [1] "BP Decisions Set Stage for Disaster" - good diagrams of well casing and cement, discussion of rushed safety checks.
[[Category:Energy]] [[Category:Engineering articles needing expert attention]]