5. Conclusion

The tandem offloading operation is a complex and difficult marine operation. It may range from once every 3 to 5 days, depending on the production rate, storage capacity of FPSO, and shuttle tanker size. The duration of the operation takes about 24 hours based on FPSO storage capacity and oil transfer rate. Meanwhile, a suitable environmental condition is required. Shuttle tanker loss of position in powered condition and subsequently collision with FPSO is the most significant risk.

The proposed method for risk assessment seems to be suitable for complex systems since it allows not only the identification of critical consequences to analyze this kind system but also is a tool to make decision because it allows a quantitative evaluation of accident progression in system that change its operational condition during the time.

The development of the fault tree and event tree is important for the understanding of the functional relation between system components and the relationship with accident progression. Based on the modeling of each accident scenario, the Bayesian analysis is performed considering the evidence of database and knowledge of offloading operation. The objective of Bayesian estimation was to develop a posterior distribution for a set of uncertain parameters allowing estimating a probability for several consequence categories as an integral part of current theories on decision-making under uncertainty.

Based on results of a ship dynamics simulator, the method allows to carry out the probability that the shuttle tanker was in a given position, indicating the variation of the position of the tanker in relation to the FPSO due to environmental conditions.

For the case under analysis, which considered the position between FPSO and shuttle tanker during offloading operation, defined by two operational states, the probability that a failure scenario is true given the system is in a specific operational state is obtained. Both states have the distribution of positions represented by a Weibull probability function.

The method is a proactive methodology to prevent accidents through risk assessment aiming at identifying and depicting a system, to reduce failures and to minimize consequences of the hazardous events. The results of the analysis support the development of mitigating scenarios for the causes of hazardous events and contingency scenarios for the consequences of hazardous events.
