Model Test Investigation of LNG Tandem Offloading with Dynamic Positioned Shuttle Tankers

LNG FPSOs are being developed for production and processing of gas in remote offshore locations. The floating production unit is positioned over the reservoir and replaces the offshore platform, the pipeline to shore, the onshore LNG plant and the jetty. Alternatively, the LNG FPSO can be utilized to liquefy and export the associated gas, produced by one or several production units.
Side-by-side offloading and stern-to-bow (tandem) offloading are the main options for the direct transfer of the cryogenic product from the FPSO to the LNG shuttle tanker. For stern-to-bow we can further distinguish between a hawser mooring (passive) or a dynamic positioned shuttle tanker (active). Experience suggests that the side-by-side operation is limited to relative benign metocean conditions, whereas a stern-to-bow arrangement allows for offloading in more severe sea states in which larger maneuverability area and large capacity mooring equipments are required.
This paper discusses a first investigation of LNG stern-to-bow offloading with dynamic positioned shuttle tankers, based on a basin model test program. The shuttle tanker was controlled by a full closed loop DP system which is largely identical to real DP systems, including extended Kalman filtering, PID control and thruster allocation. The modeling of the azimuthing thrusters, rudder and main propeller is discussed in the paper, as well as the modeling of the relative position between the two ships. Some results of the model test program are presented.