Thanks Will, Paq and Mark.

This is the control pod which would convert the command signal from the rig via fiber optic umbilical to hydraulic force which should have closed the blind shear rams and isolated the wellbore. You have a 18-3/4" wellbore with two pistons opposing each other perpendicular to the wellbore. These two pistons move a set of ram blocks with seal assemblies over the wellbore and isolate the wellbore from anything else. Blind Shear rams are designed to seal off against open hole (no tubulars..drill pipe etc) or to cut the tubular in half and then seal against the now open hole. It's a force calculation if there is enough piston area in relation to the wall thickness and tensile value of the tubular, blind shears will not cut everything....that is why in operations you do not place certain tubulars across the BOP stack.

I am a subsea engineer from another company similar to the one that owned the Deepwater Horizon. There was either a deficiency in the control circuit (something did not operate as designed) or they could not cut the tubular they had across the BOP's.