The recent Pre-Salt oil fields discovered in Brazil are imposing some new logistical challenges concerning distribution in the internal and external markets. In order to reduce the demand in the on-shore terminals, the export oil could be directly transferred from the platforms to the export tankers. However, internal regulations define that the offloading operation must be carried out by DP Shuttle Tankers (ST). Due to this definition, since 1994, the DP-ST fleet has been increased, and the safety and uptime of the offloading operations are improving. The utilization of a DP-ST to navigate across the oceans for exportation is not economically advantageous, since the day-rates of those vessels are significantly higher than conventional (non-DP) STs. Hence, a possible solution is to transfer the oil from the DP-ST to the conventional ST in the offshore waters. Several alternatives were proposed, considering stationary tandem configuration, ship-to-ship or tandem with advance speed.
This paper is focused on the stationary tandem configurations, considering the aid of offshore tugboats and/or monobuoys. The alternatives comprehend the conventional ST connected to the DP-ST or vice-versa, with the position being held by the DP system, a tugboat or a monobuoy. The numerical time-domain simulations were used for predicting the safety under typical environmental conditions, with crossed-bimodal sea states and sudden wave-wind changes. The relative motions of the vessels, hawser force, DP or tugboat utilization and loads on the vessels mooring equipment were verified in order to define the limiting environmental window for a safe operation, for each one of the alternatives. This comparative analysis was used to support the decision about this topic, since the advantages and problems of each solution could be identified.