8000 PSI or 8000 pounds per square inch of pressure IS the approximate pressure of the oil gushing up from the gulf of Mexico. To bring this into perspective. We all mostly live in about 15 PSI or 15 pounds per square inch of air pushing our bodies together. Without this 15 pounds per square inch of air pushing on our bodies we would literally explode like some fish do when brought up from the deepest of depths. However, if our air pressure was 8000 pounds per square inch each one of us would look something like a 1 inch spherical marble to give you an idea of what this much pressure would do to us in air pressure. So, 8000 PSI is literally unfathomable to air breathers like ourselves at sea level or about living in the air.
So when this pressure is reduced by valves closing it could cause many sorts of damage to the integrity of the oil pipe sunk deep into the ground under the ocean all the way to the oil pool.