Development of Roll Suppression Device for LNG FPSO Application
Authors: Lee Jung Han , Kim YongSoo , Kim Dae-Woong 
Published: July 2010

Abstract
Vessel movement of the LNG FPSO (Floating Production Storage and Offloading) may affect the performance and availability of topside liquefaction and gas treatment systems. In order to reduce the vessel movement, " L?? shape roll suppression device (L-RSD) has been developed and evaluated by model basin tests and numerical analyses. The L-RSD adopts the large scale bilge keel type structure with removable connections to prevent any interruption during FPSO construction and transit stages. The performance comparisons between conventional design and the one with L-RSD have been made extensively by varying wave periods and directions. From the evaluations, the L-RSD has been proved its superiority of the roll motion reduction in offshore operation. Environmental conditions for specific operation site have been selected to evaluate the effects on LNG FPSO topside process performance, plant downtime, and accumulated LNG production. The application of the L-RSD to LNG FPSO may bring higher availability of the topside gas treatment system, natural gas liquid (NGL) extraction system, and LNG liquefaction system. It also contributes to higher availability of LNG offloading by reducing relative motions between the LNG FPSO and the shuttle LNG carrier. The reduced FPSO motion brings lower LNG sloshing loads in the cargo tanks. The quantitative test results and analyses may justify the application of the L-RSD by providing the economic benefits in the overall LNG FPSO operation. At the same time, the L-RSD will provide more comfortable environments to onboard crew members.

General

Background
With the expansion of natural gas exploration in remote and stranded areas, LNG FPSO has been emerged as an attractive option for the natural gas monetization. Many projects around the world are under developments or feasibility study stages. In the materialization of the LNG FPSO, one of the challenges is the proper functionality of the topside equipment in floating environment. Considered that the topside equipment used in LNG FPSO adopts the basic designs from land-based LNG plants and that no sufficient offshore operation records has been accumulated, the performance degradation possibility has to be verified before the commencement of detailed engineering. The vessel motion in floating environment may lead to reduction in LNG production or off-specification products, or even shutdown of the topside system. Therefore, efforts to reduce the vessel motion have to be made in parallel with the motion tolerant topside equipment design.
LNG FPSO hull design usually adopts traditional oceangoing ship type geometry. However, as it does not enter harbors or navigate, draft limitation and speed consideration need not be complied. In LNG FPSO, deep draft and large bilge keel may be applied provided it does not interfere with adjacent shuttle LNG carrier during side by side offloading. Since the roll motion is the governing factor in the LNG FPSO motion, priority should be given to the reduction of the roll motion.

Roll Suppression Device
Vertically and horizontally extruded large bilge keel type roll suppression device attached to the bottom sides of the LNG FPSO is suggested. The DSME proprietary " L?? shape " YongSoo?? roll suppression device (L-RSD) provides very big damping for roll motion, and it is designed to accommodate several removable segments to avoid interference during shipyard construction stage and ease for offshore installation. Refer to Figure 4 through Figure 7 for the conceptual scheme.