Our Capt. Tom, an ex-Schlumberger oil rig engineer, offers this explanation in reaction to our photo of a blowout preventer:
Just in case anybody is interested, a blow out preventer of the type used on Deepwater Horizon relies on three major methods of stopping the flow - two hydraulic and one manual which, in theory, is supposed to be done with ROVs. The reason is that oil is under pressure at all times from either natural gas or pressure from the surrounding rock or salt strata (normal depth pressures). Pressure on land can reach as high as 6,000 psi, under water (at the depth of Deepwater Horizon's well head) it can reach as much as 3,800 psi. Remember the old films of "oil gushers"? That's why the oil was spraying everywhere.
There are two basic types of valves - one called a ram valve and the other is called an annular valve. The ram valve is exactly that - a ram powered by hydraulics or mechanically that shuts off the flow of oil. Think of these as "pinchers". The annular valve is a rubber/steel device that is actuated in the same fashion as the ram valve, but creates a better seal. They can be set for different operating pressures.
In the case of the Deepwater Horizon's well head, the preventer worked partially meaning that the ram/annular system hydrauics worked, but some as yet unknown reason, didn't close all the way. The manual systems were rendered inoperative when the rig collapsed.
There is a fourth way to close a preventer which is usually required on all deep water projects - an acoustic actuator on one of the mechanical valves. When this preventer was built, the Mineral Materials Service waived the requirement for an acoustic actuator which it has for quite a while under pressure from the oil industry. The acoustic actuator is required by most other oil producing countries.
In the picture of the preventer, what you are seeing with all those pipes and valves are the various types of ram/annular actuators.
The problem with Deepwater Horizon is that the pipe leading to the surface collapsed when the rig collapsed. On the videos where you see the leak, that is actually at the top of the preventer - the pipe that leads to the surface.