Most often instantaneous rotation center (MOIRC) for roll damping assessment in the free decay test of a FPSO

Abstract

The previous study of Fernandes et al. [1] showed that rather than a fixed rotation center, the phenomenology can be better represented by the concept of a MOIRC. This study contributes to understanding Instantaneous Rotation Centers (IRC) and Most Often Instantaneous Rotation Centers (MOIRC) behavior during the free roll decay tests of a FPSO by following the time series of the IRC. It is important from a practical standpoint to understand that IRC behavior leads to distinct damping values. The work shows that the time series trace of the vertical position of the IRC resembles a tangent-form for lower damping. But when the damping increases, the time series trace changes to double parabolic type (one parabola with concavity downwards and another with concavity upwards). Consequently, the MOIRC changes its positions. The effect of the MOIRC on roll damping as a consequence of vortex generation on bilge keels has for half cycles. It explains why the damping is different from the clockwise to the counterclockwise oscillation, finally a point of finding out that considering the IRC is important for roll damping assessment. The linear and nonlinear analyses, also presented, have qualitatively confirmed the observations from the experimental analysis.