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.

Effects of the asymmetric riser and bilge keel arrangements on FPSO green water assessment

“…For an FPSO with both truncated bilge keel and asymmetric risers, the two asymmetric effects can be at opposite side, acting against each other. Studies by Tom et al [6], Seah et al [7] and Bigot et al [8] suggested that the effect of asymmetric bilge keel is mainly due to quadratic damping. Rezende et al [9] claimed that the inertia effect of asymmetric risers is insignificant in their study, and simplified the riser effect using Morrison element.…”

“…In the present work, the method similar to Bigot et al [8] is applied to simulate bilge keel in frequency domain, with constant drag coefficient. The drag force on a bilge keel, bk F , which is normal to the bilge keel, is given by the Morrison drag formula, 0.5…”

“…Seah et al [7] performed the simulation in time domain, using the improved formulation of Morrison drag coefficient characterized by the Keulegan-Carpenter number (KC). The KC-dependent drag coefficient was also implemented by Bigot et al [8] in the frequency domain simulation, and results comparison indicated that the constant drag coefficient is already performing very well. Rezende et al [9] illustrated significant heave-roll coupling effect on FPSO caused by the damping effect from asymmetric risers, by using a simplified frequency-domain approach following the method of Bigot et.al [8].…”

“…The KC-dependent drag coefficient was also implemented by Bigot et al [8] in the frequency domain simulation, and results comparison indicated that the constant drag coefficient is already performing very well. Rezende et al [9] illustrated significant heave-roll coupling effect on FPSO caused by the damping effect from asymmetric risers, by using a simplified frequency-domain approach following the method of Bigot et.al [8]. By conducting the numerical analysis of the forced oscillation, they concluded that the inertial effect of the riser is negligible.…”