Tandem offl oading of LNG by means of DP


Recently, SBM commis- sioned MARIN to perform model tests to investigate the feasibility of tandem offloading of LNG from an


LNG FPSO to an LNG shut- tle tanker. The carrier was kept in position by a full


DP system and follows the weathervaning FPSO. Report explains the DP


tandem offloading method. Jorrit-Jan Serraris


j.j.serraris@marin.nl


LNG FPSOs are being developed for gas production and processing in remote offshore locations. The floating production unit is positioned over the reservoir and replaces the offshore platform, pipeline to shore, the onshore LNG plant and the jetty.


A typical LNG shuttle tanker was equipped with a single main propeller and one azimuthing thruster fore and one aft. The propellers of the LNG shuttle tanker were controlled by MARIN’s DP control program RUNSIM. This program contains all the elements present in real-life DP systems, including a position measurement system, a Kalman filter, a controller and an allocation algorithm. A typical aspect of DP tandem offloading is that the carrier has to follow the weath- ervaning motion of the FPSO, while in single body DP the vessel has to keep station at one single Control Point. During the tests, the DP system of the carrier was configured to keep the offloading point on the bow of the carrier 100 m behind the stern of the FPSO.


Multibody DP requires knowledge of both the FPSO and the LNGC motions as input for the DP control mechanism. Within the carrier’s DP control mechanism Kalman filtering is applied to filter off the wave frequent motions of the carrier. However, due to the FPSO’s stern motions wave frequent motions are induced in the Control Point of the carrier, which results in wave frequent thrust fluc- tuations and wear-and-tear on the propul- sion system. To exclude this wave frequent fluctuation of the Control Point a tolerance window is applied.


Offloading tests were performed in MARIN’s Offshore Basin, where wind, waves and current were applied. The station-keeping accuracy of the LNG shuttle tanker was monitored for the DP tandem-offloading configuration, as well as for offloading by means of a bow hawser and inactive thrusters. Results of the model test program showed that the station-keeping accuracy of the shuttle carrier on DP is more accurate than during tandem offloading by means of a hawser.


report 13


98476


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