FTA of subsea BOP control and safety system

MAR Consulting developed a Fault Tree (FT) and Analysis (FTA) on failures of subsea Blow-Out Preventer (BOP) components, equipment, and systems. The probabilistic study made use of the developed fault tree and industry reported failure data (JIP RapidS53). The failure data used was collected in 2017-2018 from drilling rigs with BOP systems operating in the Gulf of Mexico.

The main objective of the FTA of the subsea BOP was to develop a qualitative fault tree and use it to estimate the probability of the BOP system being unable to secure and seal the well (top event).

The output of the analysis may enable:

  • Troubleshooting BOP systems based on reported failures and determination of possible outcomes associated with that failure;
  • Model possible event progression paths and estimated likelihood from a reported failure to an undesirable top event;
  • Benchmark quantitative FTA results based on operational scenarios or other sensitivity analysis;
  • Potential improvements in equipment maintenance, operation, and design;
  • A potential extension of test periods;
  • Live probabilistic calculation of the probability of achieving the top event using the produced fault trees.

The scope of the Fault Tree development study included hydraulic controls of the subsea equipment and the BOP. The boundaries for the detailed control system hydraulic model were the subsea solenoid valves and respective hydraulic components downstream, including all hydraulic connections and interface seals. The remainder of the Multiplex subsea drilling controls system (MUX software and electronic hardware) and electrical failures were only modeled on a system level. Excluded from this study were the surface equipment including hydraulic power systems and other auxiliary surface systems.

The estimated probability of the top event relies on the accuracy of the input failure data. Therefore, it should be preferably used for comparison between systems and scenarios (such as failures of components, sensitivity analysis, etc.) other than for its absolute value.

The analysis calculated the probability of failure of manual operation of the BOP, manual activation of the Emergency Disconnect Sequence (EDS), and automated. These are the BOP barriers to prevent the top event. The analysis further included:

  • main contributors to the probability of the top event;
  • sensitivity analysis:
    • pressure test interval;
    • function test interval;
    • DMAS function test interval;
    • common cause failures;
    • operator reliability to operate BOP and initiate EDS.

The FT will be used during in-service operations to calculate the effect of a BOP failure, or multiple failures, on the probability of the BOP being unable to secure and seal the well.

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