shear the pipe under realistic conditions of expected well bore and seawater pressure. In each case of failure, increasing the pressure on the rams above its design value, successfully sheared the pipe. A follow-up study in 2004 confirmed these results with a much larger sample of drill pipes and typical blowout preventers from three different manufacturers. In addition to insufficient ram pressure, a New York Times investigation of the Deepwater Horizon oil spill listed other problem areas for deepwater blowout preventers. If one of the threaded joints between pipe sections is positioned within a shear ram, the ram would probably not cut through it, because the joints are "nearly indestructable". Requiring two shear rams in every blowout preventer may help to avoid this problem and to avoid some types of "single-point failure". Other technologies that might improve the reliability of BOPs include backup systems for sending commands to the BOP and more powerful submersibles that connect to the BOP's hydraulics system. Well casings Casing of offshore oil wells is done with a set of nested steel pipes, cemented to the rock walls of the borehole as in Figure 4. Each section is suspended by a threaded adapter inside the bottom end of the section above. Failure of either the casings or the cement can lead to injection of oil into groundwater layers, flow to the surface far from the well, or a blowout at the wellhead. In addition to casings, oil wells usually contain a "production liner" or "production tubing", which is another set of steel pipes suspended inside the casing. The "annulus" between the casing and the production liner is filled with "mud" of a specific density to "balance" the pressure inside the casing with the "pore pressure" of fluids in the surrounding rock "formations". To ensure that the cement forms a strong, continuous, 360-degree seal between the casing and the borehole, "centralizers" are placed around the casing sections before they are lowered into the borehole. Cement is then injected in the space between the bottom of the new casing section and the bottom of the borehole. The cement flows up around the outside of the casing, replacing the mud in that space with pure, uncontaminated cement. Then the cement is held perfectly still for several hours while it solidifies. Without centralizers, there is a high risk that a channel of drilling mud or contaminated cement will be left