Ship-to-ship (STS) operations have been widely applied for oil and cargo transfer in order to improve operational efficiency and reduce operational costs. Brazil has seen an increase in underway Ship-to-Ship operations due to the lack of available berths and terminals. The transfer operation is carried in sheltered locations in Brazil and abroad, but still has issues such as cable ruptures and high downtimes due to environmental conditions. This work proposes the development and validation of a control system for an innovative configuration for oil transfer between a DP shuttle and a conventional tanker, using a modified tandem configuration. The conventional vessel is underway and the DP vessel follows it with a lateral offset that provides a safe route for escape in case of any failure. The focus of the present paper is the control system design applied to the DP vessel that ensures the safe operation inside the offloading site. This control applies two individual sliding mode controllers for rudder and thruster control, with coefficients obtained from numerical simulations, associated with a line-of-sight strategy for course and speed over ground controllers. The set-point for the controller is obtained from the conventional vessel that is navigating ahead from the controlled vessel. Control performance and operation safety are evaluated through a set of real-time simulations of the transfer operation, where performance is evaluated through measurement of the Bow Loading System point position in both straight line navigation and in a 10km radius curve. The control robustness is evaluated by varying the environmental conditions in the simulations. Safety issues are also considered by evaluating the possibility of evasive maneuvers in case of component failure.