Tandem Mooring LNG Offloading System

Abstract

Abstract The attractiveness of placing a natural gas liquefaction plant on a vessel moored at or close to an offshore field is known. This vessel can either be part of an oil development in which it liquefies associated gas or a stand alone development of a gas field. In the associated gas case, the Floating LNG (FLNG) unit makes certain oil developments feasible by turning the associated gas disposal into a moneymaking opportunity. In the gas field case, the FLNG makes remote offshore field production economic. The safe reliable transfer of this LNG in open sea, possibly harsh, environment has however not been possible. The proposed LNG Offloading Arm (LOA) effectively combines proven technology and operating experience to effect the safe and reliable transfer of LNG between tandem moored vessels or between a fixed tower and LNG carrier in up to 5 meter significant seas. The concept is based on the experience of mooring systems surviving 100 years storms, dis-connectable mooring systems in typhoon areas and work performed on cryogenic swivels. The LOA structure consists of "vertical" and "horizontal" arm section with swivels to provide articulations required to follow the relative motions between the FLNG and the tandem moored LNG carrier. The vertical section is supported from a cantilevered rotating structure on the back of the FLNG vessel or on a fixed tower. The horizontal section connects the LNG carrier to the lower end of the vertical section. Each section is composed of a central pipe for LNG transfer and a large diameter outer pipe structure also used as the vapor return from the LNG carrier. The LNG carrier can be passivelymoored using gravity restoration by weights located in the horizontal section of the loading arm or DP assisted using both the arm and the LNG carrier thruster system. Introduction The technology for cryogenic liquefaction and storage of LNG on an offshore floating vessel is known. The safe reliable ship to ship transfer of this LNG in an open sea, possibly harsh, environment has however not been possible. SBM having experience in both the supply and operation of mooring systems put this experience to work in the development of a Tandem LNG Offloading system. The philosophy adopted for this development was to keep the design simple and whenever possible stick to components that had a known track record. Having over 40 years experience in the design, supply, and operation of Single Point Mooring (SPM) systems and Floating Production Storage and Offloading (FPSO) systems the value of simple robust design was well understood and applied to this LNG Arm development. Known technology and operational experience either directly or indirectly applied to the Arm include soft yoke or gravity moorings (Fig. l), articulated fluid transfer arms (Fig. 2), cryogenic swivels (Fig. 3a and b) and tandem mooring systems (Fig. 4).