Risk assessment of surface vs subsea blowout preventers (bops) on mobile offshore drilling units focusing on riser failure and the use of subsea shear rams

The use of a slim, high-pressure drilling riser for surface blowout preventer operations in the deepwater Gulf of Mexico was assessed as an alternative to conventional drilling procedures from floating units. Comparison of the low- and high-pressure system was accomplished through a detailed qualitative (assigned frequency) and quantitative (reported incidents) risk analysis using generic fault tree models to statistically determine the reliability of the system based on metocean conditions from the Gulf of Mexico. It is hoped that this investigation will serve as a starting point for drilling companies and regulatory agencies to understand the risk of implementing a high-pressure riser for surface blowout preventer applications in the Gulf of Mexico, because specific failure events and conditions of the area were considered. Despite the generic description of the drilling riser and pressure control system, the models are flexible enough to be modified and adapted to a specific rig configuration and location. Results from the qualitative comparison suggest an acceptable risk and high reliability for high-pressure riser systems and surface preventers. The quantitative portion of the study is influenced by the data quality of the high-pressure system, however it provides a range of possible reliability values with an acceptable overall risk.