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Since I first heard how a BOP works I’ve been amazed it didn’t have a spring return to a failed closed position. Why the hydraulic system was blown up, the valve would not work. That’s HAZOP step 1.

There is no 'failed close' position as it is not an ordinary valve. It is a device that pinches off the pipe to stop the flow.

http://www.huffingtonpost.com/dr-philip-neches/gulf-oil-spill-meet-the-b_b_589852.html

In 1922, two men named Abercrombie and Cameron created a device to eliminate gushers, called a blow out preventer, or BOP. Basically, it is a big valve that sits on the top of the well hole. When the pressure or flow from within the well is too great, the valve closes, stopping the flow. There are two different kinds of valves: ram and annular. A ram valve uses a number of pistons or rams that close around the drill pipe (called a "drill string" in the trade), shear through the pipe, and then seal the well hole. The annular design, created in 1946 by a man named Knox, uses a hemisphere of rubber reinforced by steel ribs for the same purpose. The annular valve operates more smoothly, requiring less hydraulic pressure, resulting in longer service life.