Thrusters Dimensioning for Vessel’s Stationkeeping Close to Other Floating Structures: Two Case Studies

Abstract

Brazilian offshore scenario requires innovative solutions in order to increase the efficiency of operations, while still keeping the safety and the economical constraints. The global market and naval industry cannot meet the demand of the growing exploration activities, related to the recent discovery of new oil fields. Furthermore, they are located at a large distance from the shore (approximately 300km) and the environmental conditions are severe. These facts impose more logistics problems for materials and persons transportations. The retrofitting of vessels is an alternative for increasing their operability and this option has been studied by Transpetro, subsidiary of Petrobras responsible for the Ducts, Terminals, Maritime Transport and Natural Gas segments. This paper presents two real cases of retrofitting related to the application of new technologies for positioning, including Dynamic Positioning (DP) Systems or manually operated thrusters. In the first case, a conventional tanker will be converted to a DP2 tanker, in order to operate close to drill ships and to transfer fluids during the drilling operation. The second case is related to an offshore delivering vessel used for bunkering operation. This vessel is moored at a monobuoy, and up to 2 receiving vessels may be connected to it. An aft-azimuth thruster will be installed in the vessel, increasing its capacity to change heading in harsh environmental conditions and to keep a safe distance from the monobuoy during fast variations of environmental conditions. Both cases require the application of a comprehensive design methodology for thruster dimensioning. The retrofitting imposes tight constraints to the layout and positioning of the thrusters. Also, the proximity to others vessels and the harsh environmental conditions in the offshore fields requires a combination of static and dynamic calculation in order to evaluate power consumption and drift motions of the vessels.