Prediction of Asymmetric FPSO Roll Using a Frequency Domain Method

Abstract

FPSO roll prediction has traditionally been performed assuming symmetric roll damping resulting in identical roll responses from portside and starboard waves. Recent interest in the industry to predict asymmetric roll response, either due to asymmetric mooring and riser configurations or damping devices, has led to the development of time domain models utilizing asymmetric Morison drag elements. Here, a frequency domain methodology has been developed to account for asymmetric bilge keels leading to differing port versus starboard wave roll response. A nonlinear bilge keel drag formulation, that includes the effects of radiation velocity, is used, coupled with linearization techniques, to predict the difference in roll RAO from port versus starboard waves. The drag formulation is initially calibrated against FPSO decay tests before the model is validated against measured model test motions. Thus we show that the methodology proposed is capable of predicting the motions from an asymmetric configuration efficiently, such that it can be utilized in design projects requiring FPSO motions analysis.