An Experimental Investigation of Roll Motions of an FPSO

FPSO roll motions can be major contributor to riser fatigue. This is especially true in regions where wind, waves and currents are non-collinear. Roll motions as high as 23 degrees have been reported in the Campos Basin. The most common roll mitigation strategy consists of adding bilge keels to the FPSO. Motivation for this work came from a need to develop a better understanding of roll motions as a function of bilge keel width. In addition to roll motions, the hydrodynamic forces on the bilge keels were measured. A series of tests were conducted at the LabOceano offshore basin. This new facility has a length of 4 0 m, a width of 30 m, a depth of 15 m and is equipped with a multi-flap wave generator on one side. A ship-shaped FPSO design with sponsons for a deepwater offshore development in Brazil was tested. It has a length of 316 m, a breadth of 57.2 m and a draft of 28.3 m. A 1:70 scale model was constructed. A horizontal soft mooring system consisting of four lines with springs was used. Regular waves of different amplitudes as well as random waves were generated in the basin. Two different loading conditions, ballast (draft = 6.7 m) and loaded (draft = 21.7 m), as well as three wave headings, beam seas (90°), and quartering seas (22.5°, 45°) were considered. Tests were undertaken for four bilge keel configurations, corresponding to a case without bilge keels, as well as bilge keels of 3 different widths (1 m, 2 m and 3 m). In all cases, the bilge keels had a length of 200 m. An optical system was used to measure ship motions in all six degrees of freedom. The hydrodynamic loads on the bilge keels were measured using strain gages.