## 5 Conclusions  
This paper presented an integrated methodology of fault tree analysis (FTA) and failure mode and effects analysis (FMEA) in a manner that provides a modified, systematic and structured approach for identifying, evaluating and prioritizing the risks associated with different components in a complex system. The synergy of complementary techniques is harnessed by modifying criteria, hence increasing efficiency in analysis. By using the proposed technique, it is possible to gain insights about any complex system which otherwise might be overlooked. The basis for corrective action is improved in  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Expert** & **RPN** & **Weight Score** & **Weighting** \\ \hline
1 & 60 & 0.16 & 60 \(\times\) 0.16 = 9.6 \\
2 & 70 & 0.18 & 70 \(\times\) 0.18 = 12.6 \\
3 & 192 & 0.18 & 192 \(\times\) 0.18 = 34.56 \\
4 & 96 & 0.13 & 96 \(\times\) 0.13 = 12.48 \\
5 & 84 & 0.18 & 84 \(\times\) 0.18 = 15.12 \\
6 & 70 & 0.18 & 70 \(\times\) 0.18 = 12.6 \\  & & & **Total = 96.96 \(\approx\) 97** \\ \hline \hline \end{tabular}
\end{table}
Table 10: Final aggregation for RPNs (example of the blind shear ram component).  
Figure 5: Weighted RPN values for shallow-water (\(\blacksquare\)) and deep-water (\(\blacksquare\)) cases.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Expert** & **Occupation** & **Experience** & **Education** & **Weight Factor** & **Weight Score** \\ \hline
1 & Engineer = 4 & [0–9] = 1 & PhD = 4 & 4 + 1 + 4 = 9 & 0.16 \\
3 & Engineer = 4 & [10–19] = 2 & PhD = 4 & 4 + 2 + 4 = 10 & 0.18 \\
4 & Junior academia = 2 & [10–19] = 2 & MSc = 3 & 2 + 2 + 3 = 7 & 0.13 \\
5 & Senior academia = 3 & [20–29] = 3 & PhD = 4 & 3 + 3 + 4 = 10 & 0.18 \\
6 & Senior academia = 3 & [20–29] = 3 & PhD = 4 & 3 + 3 + 4 = 10 & 0.18 \\  & & & & **Total = 56** & **Total = 1** \\ \hline \hline \end{tabular}
\end{table}
Table 9: Expert weighting.  
a computationally sound manner. The application of the proposed methodology was demonstrated with a critical subsea oil and gas system known as Blowout Preventer (BOP).  
The results of the study proved the benefits of the proposed FTA-FMEA methodology and combinatorial risk analysis techniques in general. Components that may be overlooked based on the traditional FMEA technique were brought to the limelight for consideration. More so, the priority rankings were improved significantly. Minute changes in ranking order may have huge implications, particularly for a safety-critical system such as BOP. Aside being essential for further quantitative research, component priority can significantly alter maintenance and testing policies. As shown in Table 7 and Figure 4, some components such as the kill valves remained at top priority in RPN rankings with the proposed technique. However, a clear disparity in rankings was observed for some components such as the connectors (wellhead and LMRP). Choke valves were ranked at the same level with casing shear rams and were higher than blind shear rams and accumulators.  
Future research can be directed towards the effects of double and multiple failures, the comparative difference obtained when different importance measures are used, and linking the software interphases of the proposed integration scheme. Also, the normed design of subsea accumulators for actuation and control of BOPs may sometimes not be safe (see reference [35]). Thus, consideration of the actuation of the BOP and its design in reliability analysis can be an interesting subject for research.  
[PERSON] performed the analysis and wrote the initial manuscript draft. [PERSON] reviewed the initial manuscript, made contributions to its structure and responded the reviewers' comments. Both [PERSON] and [PERSON] supervised the work.  
This work was supported by Commonwealth Scholarship Commission.  
The authors declare no conflict of interest. This research received no external funding.