<p>In order to predict ship motion in the wave, strip methods predict pitch, heave, sway, surge, and yaw motions in fairly good accuracy. In roll, however, the strip methods do not work well due to the significant viscous effect on roll damping. Therefore, the most challenging task in ship roll motion prediction in the ocean is an accurate estimation of nonlinear ship roll motion. Numerous issues regarding dangerously high roll motions have been reported in cases of a severe condition, having consequences ranging from cases such as extreme crew discomfort, downtime of process plants or impossibility of offloading operations, and degradation of the structural integrity of topside supports, hull, bilge keels, and risers. The mentioned differences between FPSOs and conventional ones may cause the roll damping of FPSO to become a highly nonlinear phenomenon and in-comparable with conventional ships. Many methods have been proposed by researchers those mostly limited in forced oscillation (fixed rotation center) or free decay techniques, generally, isolated the roll into one degree of freedom in absence of wave. Ikeda empirical method may be one of the most used approaches in order to predict roll damping.</p><p>According to inadequate roll response analysis due to the unsatisfactory nonlinear roll damping prediction provided from existing methods for FPSOs, international maritime organizations seek to provide improvements or new prediction methods. The second generation of the intact stability of IMO is emerged due to improve the accuracy of available methods or propose a new method. Ikeda method is extracted from an extensive series of forced roll experiments assuming a fixed rotation center in calm water which is a completely different phenomenon from a floating body in sea condition. Two important items were neglected by the Ikeda method, the interaction of the wave with the body and bilge keels and coupling effect on roll damping. Understanding these items can help us to evaluate better the nonlinearity of roll motion. Perhaps, previous works of the authors brought the new insights into the topic by showing the importance of coupled-mode into roll motion (especially sway) via investigation of asymmetric roll motion and propose a third-order system identification to obtain roll damping with coupled effects. The paper presents an experimental and numerical roll damping assessment of the intact 2D section of an FPSO zero speed exposed to regular beam waves. The experimental part provided from a reduced mode in the laboratory of wave and current of Federal University of Rio de Janeiro (LOC/UFRJ). This paper will investigate and discuss coupling effects on roll damping, analyzing instantaneous rotation center and asymmetric rotation for different wave periods and wave amplitudes. Finally, an alternative method to estimate the roll damping will be introduced by this paper.</p>

Nonlinear Roll Damping With Coupling Effect in Regular Beam-Wave

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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