Reliability of subsea blowout preventer systems - A study of the emergency disconnect function
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The Deepwater Horizon accident in 2010 emphasizes the significance of safety in offshore drilling. The drilling rig was considered to be reliable and safe, but the blowout in the Gulf of Mexico costed eleven lives and the most terrible environmental disaster ever in the US. Reliability of subsea blowout preventer systems (BOP) is therefore a very important subject in drilling operation. The Deepwater Horizon accident also highlight the ability to safely close the BOP and disconnect the riser from the BOP in time. None of the emergency plans for closing rams and release the BOP were not satisfied.Various barriers failed during the operation, namely that the shear ram did not cull the drill pipe and isolate the wellbore. Hence, when the emergency disconnect system did not activate in time, the hydrocarbons were likely to ascend through the riser and to the drilling rig, where probability of explosion escalated.When drilling in deepwater, the vessel is often operated with dynamic position system, whichmeans that if the thrusters fail, the rig will drift away. Therefore emergency disconnect system is developed to let the rig move away to a safe place. The emergency disconnect system shall close the blind shear ram and release the LMRP from the BOP, giving the rig opportunity to escape from the scene.In retrospect view it is easy to point out that in the Deepwater Horizon accident, the Emergency Disconnect System (EDS) should have been activated earlier. But the rig-crew misinterpreted the negative pressure test, and they were not aware of the influx of hydrocarbons until it reached the riser. Further more, in this perspective, human factors has a huge impact, regarding when to activate the emergency disconnect function. It is discussed how rig-workers can be reluctant pushing the EDS-button.A Fault tree analysis is carried out to explain which component must fail resulting in a blowout after emergency disconnect system is activated. In the cut sets, it was found four cut sets with only one component, seven cut sets with two components, 22 cut sets with three components and finally 40 cut sets with five components.Further more the sequence of an emergency disconnect system was shown to highlight thedifferent functions that shall be initiated. This list is included to point out the complexity of the disconnect function.Manual and automatic EDS is discussed, where the advantage with manual is one mightavoid false alarms and it is possible to activate in an early stage. On the other hand, automatic do not consider human error, which might be more reliable.An Event tree analysis was added to created to identify the different outputs when differentbarriers acting or not.Finally a Failure Mode and Effects Criticality Analysis (FMECA) was carried out to highlightthe causes and the effect of failures. Results from this analysis were low hydrostatic pressure, corrosion and failure on control system.