This may be a silly question, but... I recall one of the points of failure in the Deepwater Horizon's BOP was the loss of power when an explosion severed the hydraulic and electric cables. Why don't they design these things to default to on, so that a flow of hydraulic fluid is needed to keep the valve open during normal operation, rather than to close it when there's a problem? Is there some practical reason inherent to BOP design generally, or was it a design shortfall of this particular model? I'm envisaging some kind of high-pressure spring mechanism kept primed by hydraulic or mechanical pressure. If pressure is lost, intentionally or otherwise, the spring releases, forcing the valve closed. Practical?