Oil offloading from Spread Mooring System (SMS) FPSO is usually done by means of a shuttle tanker (ST) in tandem configuration. The ST receives the oil pumped by the FPSO from a bow or stern offloading station, and the operation may take up to 3 days. In order to minimize the risks associated with the operation, the shuttle tanker (ST) should be kept within a safety zone with respect to the FPSO, which is usually given as a minimum distance between the two ships and an aperture angle from the FPSO centerline. In order to guarantee the tanker position during the whole operation, the operation is usually assisted by tug boats or performed with tankers provided with DP (dynamic positioning) systems. Since SMS FPSOs may be not aligned to the environmental forces, keeping the shuttle tanker in position may be a hard task either for the DP system or for the tug boats, depending on the environmental conditions. There are non-rare situations in which the ST must be disconnected and the operation interrupted. The present paper presents a methodology based on fully-nonlinear dynamic simulations for evaluating the downtime of such offloading operations, considering both DP and conventional non-DP tankers, in the later case assisted by a tugboat. Typical Brazilian waters environmental conditions (current, wind and waves) are considered for each seasonal period of the year. The main objective is to provide a quantitative comparison between both cases in each season. The results may also be used for strategic definition concerning logistics of oil production. The procedure is based on exhaustive dynamic simulations, and a statistical analysis of the oceanographic data was used to generate a representative set of environmental conditions together with the corresponding probability of occurrence for each season. A full time-domain dynamic simulation is then carried out for each case, considering detailed models for the dynamics of each floating structure, mooring lines and DP system. The vessel excursion, hawser tension and other operational parameters are then evaluated for each case, and the occurrence of unsafe conditions may be properly evaluated. A statistical procedure is finally applied to compute the associated downtime. Illustrative results for two different classes of ST (Aframax and Suezmax) are presented and discussed, and the advantages of employing DP assisted vessels become clear.

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