This work presents complementary control of Roll excitationby bilge keels in order to increase the global performance of BONGA FPSO in West African deep waters. Five (5) different sizes and configurations of bilge keels defined by the drag diameters were attached to the FPSO and the roll response analyzed using time domain technique in ORCAFLEX. The decay simulation for 60s and sustained dynamic simulation for 1800s were carried out for the sizes using the 100yr return period data of FPSO location, its principal dimensions and roll motion characteristics. Results showed that the roll amplitude decayed linearly with increase in the bilge keel drag diameter for the same environmental and hull parameters for both cases. An average roll amplitude reduction of about 2.06o signifying 38.3% was obtained for the largest bilge keel with a drag diameter of 2.5m when compared to the bare hull or base case (i.e. 0.0 m drag diameter). For the bilge keel with a drag diameter of 1.5m, a 1.42o reduction in roll amplitude was observed; which is an improvement of about 0.865o for an increase of 1m in drag diameter from 0.5m. However, a marginal reduction of roll amplitude analysis reveals that at a predicted drag diameter of about 4.6m, the percentage gain in roll reduction varnishes to zero showing that there is always an optimum size of bilge keel for targeted roll response reduction. Evidently however, the observed reductions are enough to cause significant increase in the global performance of the FPSO.
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