Blowout Preventer #9

During the drilling of areas where are oil, water, or gas, there is a possibility this pressure may escape out through the well.  When this happens, it can blow out.
To control these formations of the pressure, a large underwater control valve called a Blowout Preventer (BOP) is initiated on top of the wellhead.  This is why its very important to have run the two sections of casing and properly cemented them in place. If not for the blow out preventer, oil and gas would escape directly to the sea causing tremendous damage to the environment.  Ensuring the BOPs are properly maintained and tested should be one of the highest priorities of the drilling companies. 

It is crucial to prevent the blow-out not only it damages the drilling rig, but it may cause enormous damages for both the employees and the environment as well. Many oil rig workers have lost their lives to explosions and fires when uncontrolled gases from blow-out. Drilling fluid is twice as heavy as sea water. This is helpful during the drilling, because it’s weight creates enough pressure to keep any pressure in the oil or gas formation from escaping back up through the well.
On the other hand, if the drilling fluid you are drilling with is too heavy, you run the risk of breaking or cracking the well.  When this happens, your drilling fluid begins leaking out into the underground formation.  This is also very bad, because without being able to circulate the mud back up through the marine riser, you will be unable to drill any deeper.
According to the BOP, tremendous problems can be avoided.

Source: http://www.treesfullofmoney.com/?p=1610