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Blow Out Preventers

  1. Sep 11, 2010 #1
    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?
     
  2. jcsd
  3. Sep 11, 2010 #2

    russ_watters

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    Staff: Mentor

    It may just be that the force required makes a "normally closed" type difficult to do. It could also be that the damage caused by an accidental closing makes you want to try to avoid that.
     
  4. Sep 11, 2010 #3
    russ_watters is right. The BOP knives must have the force to cut through the drilling column.

    Any accidental closing is always undesirable, because going back to drilling means running lengthy systems and equipments checkups and also one would have to face the problem of trying to fish the drilling column out of the well, which causes further delays.

    Any subsea equipment design is based at least on legislation, working depth and probability of failure. Let's say you got a powerfull normally closed BOP, will it be approved ? Probably. What is the probability of failure ? The same as a conventional BOP, because you will be required to make it redundant, as redundant as the system is today. Will this redundancy be enough ? No, you cannot make anything perfectly fail proof, only fail-safe enough to mitigate the probability of catastrophic failure.

    You should treat Deepwater Horizon disaster as an aircraft accident, where it usually takes one or more years to gather enough information and build the whole picture, which is commonly related to a sequence of mistakes spaced in time and low probability events that when combined resulted in a tragedy, rather than the failure of a single component.
     
  5. Sep 13, 2010 #4

    stewartcs

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    Science Advisor

    The size of the spring would be unpractical for starters. The shearing force required is in the 1 to 2 million pound range (typically) depending on the tubular that is being sheared.

    Plus spring breaks would be a hidden failure mode since the only way to detect it would be to function the BOP. Granted this would be discovered during routine testing, but the period between such testing would be a problem.

    CS
     
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