"Ask the Experts" is moderated by Béla Lipták, process control consultant and editor of the Instrument Engineer's Handbook (IEH). The 4th edition of Volume 3, Process Software and Networks, is in progress. If you are qualified to contribute to this volume, or if you are qualified to answer questions in this column or want to ask a question, write to firstname.lastname@example.org.
Q: I've received a number of questions about the oil drilling accident in the Gulf of Mexico: What caused it? If properly designed process control systems were used, could the blowout have been prevented? What contributions could process control have made to stop the flow after the blowout?
A: To answer these questions, we must understand the drilling process, the causes of the BP accident and the kinds of automatic controls necessary for both the normal and the emergency drilling operation.
Drilling a test well on land is as simple as digging a hole. Drilling deep under the sea increases both the risks and the costs because of the high pressures and hostile environment at the ocean bottom. As the depth increases, the weight of the connecting piping between the sea floor and the platform above also increases. To reduce this weight and the associated cost, the pipe thicknesses had to be reduced, which could only be achieved if some of the drilling equipment was lowered down to the ocean floor where the pressure is high (2200 PSI in case of BP), the temperature is near freezing, and the environment is corrosive. Furthermore, the lowered equipment is inaccessible and has to be operated by remote-operated vehicles (ROV) or subsea robots.
Under these conditions, state-of-the-art technology, including automatic safety controls, trips and redundant equipment, should have been used, potentially making the drilling of deep sea wells uneconomical and in some cases consuming more energy than the wells produced. The industry and its regulator, the Mineral Management Service, in the absence of standards requiring such controls, concentrated on cutting costs.
The Drilling Process
A simplified sketch of the drilling process is shown in Figure 1. At BP installation, the well was 18,300 feet deep (36 in. diameter at the top; about 10 in. at the bottom) and included a number of casings, conduits, seals, spacers, snubbers and burst disks that are not shown. The casings are inserted into the well bore at various depths and held in place by cement slurry injected between them and the well bore. They should protect against cave-ins, provide a foundation for the drilling fluid (mud) and seal off high-pressure zones.
During drilling, mud is circulated down the drill pipe and up through the annulus between the well bore and the drill pipe. The mud carries the rock fragments produced by the drilling. This circulating mud also serves to prevent the oil and gas in the deposits from entering the well, because the pressure of the mud inside the well bore is higher than that of the oil outside. If for any reason this pressure difference (ΔP = PMUD – POIL) starts dropping, the mud pressure has to be increased to keep this ΔP positive. Otherwise the oil or gas will enter the well bore.
Once the oil pressure exceeds the mud pressure, it can lift the fluid in the well, and a blowout can occur. In case of the BP installation, four shut-off devices—placed one on top of the other—were provided in the blowout preventer (BOP) in Figure 1. These are shut-off valves that close the well in case of evolving blowout conditions. The first three are pipe rams that close only the annulus. The last one, the shear ram, also cuts the drill pipe and completely closes the well bore.
Causes of the BP Accident
Now we must understand the reasons why the pressure outside the well can suddenly rise and how gas "kicks" can develop. The cause is methane hydrate or methane ice (MI). The MI crystal is a solid similar to ice, except that it traps large amounts of methane within its crystal structure. The extreme cold and crushing pressure (2200 PSI at 5000 feet at the ocean bottom and about 8000 PSI in the oil deposits at 15,000 feet) keeps this crystal in the solid state. If conditions drop below the point of phase transition (PhT), because the pressure drops or the temperature rises, PhT is triggered, and the MI vaporizes. Each cubic foot of MI crystals explodes into 164 cubic feet of gas. Therefore, it is wise to avoid drilling through MI deposits and, if it is done accidentally, to keep the pressure inside the well above and the temperature below the PhT point.
The phase change can also occur in the reverse direction. If methane gas is exposed to water under such conditions that exceed the PhT point, the gas and the water will "freeze" into MI crystals that will plug the piping and other equipment. This can occur if the mud pressure drops below the methane pressure in the deposits, and methane enters the well bore while the pressure and temperature conditions are above the PhT point.
Under these conditions the MI crystals that are formed can also be carried up by the mud flow or by the other fluid that is circulated in the well. As they rise, the pressure in the well decreases, and the MI crystals dissociate into methane gas and water. The rapid gas expansion ejects the circulating fluid from the well, further reducing the pressure, which leads to more hydrate dissociation and further fluid ejection. This violent expulsion of fluid is referred to as a "kick," which can cause blowouts. Once the mud is blown out and methane escapes, all that is needed is a spark to ignite it.