## 3 Proposed Methodology
### FMEA Steps
Table 7 demonstrates this for a cross section of components and reveals that control pods may require more attention after blind shear rams as opposed to pipe rams in the traditional FMEA ranking system. Figure 4 provides a clear comparison between the traditional RPN values and the weighted RPN values. Though some components such as the kill valves still remain at top priority in RPN rankings with the proposed technique, a clear disparity in rankings and hence in the criticality of components is depicted in others such as the connectors (wellhead and LMRP). Choke valves are ranked at the same level with casing shear rams and are higher than blind shear rams and accumulators. \begin{table}
\begin{tabular}{c c c} \hline
**Detection Rating** & **Detection** & **Likelihood of Detection** \\ \hline \multirow{3}{*}{1} & \multirow{3}{*}{Almost certain} & Very high probability of detection (?90\% probability of detection) by design controls (redundant or independent self-diagnostic capability, independent alarms) will certainly detect failures \\ \hline \multirow{3}{*}{2} & \multirow{3}{*}{Very high} & High probability of detection (50 to 90\% of detection) by design controls (single device self-diagnostic capability, single alarms, visual monitoring, leak monitoring, loss of fluid etc.) will certainly detect failures \\ \hline \multirow{3}{*}{3} & \multirow{3}{*}{High} & Probability of detection via weekly on-stream tests/inspections will provide immediate detection of the failure \\ \hline \multirow{3}{*}{4} & \multirow{3}{*}{Moderately high} & Probability of detection via monthly on-stream tests/inspections will provide immediate detection of the failure \\ \cline{1-1}  & & \\ \cline{1-1}  & & \\ \cline{1-1}  & & \\ \hline \multirow{3}{*}{5} & \multirow{3}{*}{Moderate} & Probability of detection via quarterly on-stream tests/inspections will provide immediate detection of the failure \\ \hline \multirow{3}{*}{6} & \multirow{3}{*}{Low} & Can only be detected during routine inspections/tests while the BOP is pulled from the well \\ \hline \multirow{3}{*}{7-8} & \multirow{3}{*}{Very low} & Can only be detected during major PMs while the BOP is pulled from the well \\ \hline \multirow{3}{*}{9} & \multirow{3}{*}{Remote} & Can only be detected and/or corrected during major overhaul or rebuilding-type activities \\ \hline \multirow{3}{*}{10} & \multirow{3}{*}{Absolute uncertainty} & Currently no design controls or maintenance techniques in place \\ \hline \end{tabular}
\end{table} 
Table 6: Detectability ratings of a failure for BOP. \begin{table}
\begin{tabular}{c c c} \hline
**Component** & **RPN from Traditional FMEA** & **Weighted RPN from FTA-FMEA Approach** \\ \hline Wellhead connector & 40 & 400 \\ Blue Pod & 35 & 280 \\ Yellow pod & 35 & 280 \\ Lower pipe ram & 70 & 140 \\ Upper pipe ram & 76 & 152 \\ Blind shear ram & 80 & 640 \\ \hline \end{tabular}
\end{table} 
Table 7: A comparison between the results obtained from the traditional FMEA technique with those obtained by the proposed method. Comprehensive analyses of the system produce results that may significantly alter corrective measures and criticality priorities for the BOP and any complex system in general. Whilst FMEA focuses on causal effects, taking into consideration all possible failures but not necessarily component interdependencies, FTA considers the latter but avoids minute components and their effects on overall system failure. The integration of the techniques is proven to produce more robust prioritization of system risks as greater failure severity may not necessarily mean greater criticality. This phenomenon becomes explicit when weighted RPNs for BOP under two different operating conditions, namely deep-water and shallow-water, are compared. Six experts are consulted to provide their opinion on the severity, occurrence and detectability of various failure modes, and a Multi-Criteria Decision Analysis (MCDA) method based on Weighted Sum Model (WSM) is used to aggregate the experts' opinions (see Tables 8-10).