In this paper, a Discrete Event Simulation (DES) model for FPSO operations over its field lifetime is presented as a design tool for probability based risk assessment on occurrence of extreme events such as green water, wave slamming etc. The model simulates the cascading effect of realistic combinations of FPSO loading conditions and its dependence on shuttle tanker operations.

An FPSO engineering project normally includes a model test campaign to identify probability of occurrence and consequences of extreme events, such as green water or wave slam for a limited number of governing sea state with the FPSO in a vulnerable laden condition. At this stage, the risk assessment is limited to the fact that the FPSO experiences a governing sea state at a specified laden condition. The DES model presented in this paper addresses ways to remove the overtly conservatism in this risk assessment.

The DES model aims to characterise the FPSO loading patterns between successive offloading cycles. The model includes delays in offloading cycle due to shuttle tanker arrival and readiness at FPSO, scheduled maintenance on the FPSO process systems, etc. The results from DES iterations help to quantify the occurrence probability for any specified loading pattern in the FPSO over its lifetime. Together with the metocean scatter diagram which defines number of occurrences for each sea state, a conditional probability of FPSO experiencing a loading condition at governing sea state is calculated. This conditional probability can be applied to unbiased risk assessment of extreme events occurring in a governing seastate when the FPSO is at a specified loading condition.

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