Parametric Assessment of the Buoy for Supporting Lines BSL Applied to Large FPSOs

Abstract

An innovative Oil Loading Terminal (OLT) system was recently developed to attend FPSOs (Floating, Production, Storage and Offloading) units up to 250.000 barrels/ day of oil production and operating in deep waters. This OLT allows offloading operations from the FPSOs onto Very Large Crude Carriers (VLCC) shuttle tankers moored in a Single Point Mooring Buoy type usually called CALM Buoy. The heart of this OLT concept is a tether anchored subsurface buoy - named BSL (Buoy for Supporting Lines) - application to support the submerged Oil Offloading Lines OOL segments between the FPSO and CALM Buoy. The BSL presence de-couples the motions of the two floating bodies. As a result, the OOLs loads and fatigue efforts also decrease on all connections. The fact is the submerged BSL will decrease the wave's impact on the design. It is important to recognize that all components are field proven and installable as described in [4]. The scope of this paper is to evaluate some different arrangement of the OLT submerged components to assess the level of influence of each one. This work will perform a parametric assessment of the main geometric and inertial characteristics of each component seeking consequences on the static and dynamic tensions in hot spots and the fatigue life on the main components. The first aspect to watch is related to the relative and total horizontal distances between the FPSO, BSL and CALM Buoy. The other is the depth of the BSL, followed by the floater length in both branches of the flexible lines. Geometries were tested in Santos basin, Brazil (2200m water depth). For each case an economic quantification is performed, since the best economic result may not coincide with the best tension and fatigue life. The proposed OLT should allow the employment of conventional tankers, either Suezmaxes or VLCCs, connected onto the FPSO via a CB and a BSL plus OOLs, by placing the largest fraction of the OOLs weight supported by the FPSO and BSL. The sections of the OOLs between the BSL and the FPSO can be of either steel or flexible; or even a combination of both. The use of BSL conception for the Brazilian offshore pre-salt area is in fact a reliable, safe and robust system when compared with the FPSO tandem offloading or the complementary Ship-to-Ship oil transfers.