Blow Out Preventers: Addressing Design Shortfalls and Improving Safety

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Discussion Overview

The discussion revolves around the design of Blow Out Preventers (BOPs), particularly in the context of the Deepwater Horizon incident. Participants explore the mechanisms of BOPs, the implications of design choices, and the challenges associated with ensuring safety and reliability in high-pressure environments.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why BOPs are not designed to default to an "on" state, requiring hydraulic pressure to keep the valve open, suggesting a spring mechanism that would close the valve if pressure is lost.
  • Another participant proposes that the force required for a "normally closed" design may be impractical and that accidental closing could lead to significant operational issues.
  • A third participant emphasizes that the shearing force needed to cut through the drilling column is substantial, complicating the feasibility of a spring-based mechanism.
  • This participant also notes that any design must consider redundancy and the probability of failure, indicating that a more powerful BOP would still require redundancy to be approved.
  • Concerns are raised about hidden failure modes, such as spring breaks, which could go undetected until routine testing, highlighting the challenges in ensuring reliability over time.

Areas of Agreement / Disagreement

Participants express differing views on the practicality and safety implications of various BOP designs. There is no consensus on the best approach or the feasibility of proposed mechanisms.

Contextual Notes

Participants acknowledge that the design of BOPs is influenced by legislation, operational depth, and the need for redundancy, but the discussion remains focused on theoretical design considerations rather than established practices.

talk2glenn
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?
 
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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.
 
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.
 
talk2glenn said:
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?

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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