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When under influence of an incident wave system, any floating body presents a general motion with all six degrees of freedom, unless it presents some kind of restrains on it. For a free moving body, the center of rotation will depend on the force distribution and might not coincide with its center of gravity. For long and slender floating structures, such as FPSO platforms, a small change in the center of Pitch rotation would result in significant change in the overall motions in its fore and aft regions. Therefore, it is of high importance to obtain a better understating of the instantaneous position of the body center of rotation in Heave and Pitch response. This paper investigates the position of the Instantaneous Center of Rotation in Pitch Response of a scaled down model of a FPSO platform under different regular wave conditions. The investigation uses basic kinematics equations for rigid body, defining the 6 degrees of freedom of the rigid body motion from a finite number of markers installed in the model. A high quality tracking system captures the markers positions in order to define the rigid body at each instant of time. For an initial approach, the study considers the response due to head waves seas with experimental validation.

The so-called roll center is not a concept well defined for a rolling ship or platform when submitted to a wave field. The present paper discusses and proposes a clear definition. The paper shows a methodology to assess this point and shows that, for regular beam wave incidence on a symmetric body, the roll center is not necessarily located at the line of symmetry of the symmetric bodies. Also shows that the locus of the roll center is frequency dependent. Finally, the paper discusses the limits for low and high frequencies. This investigation uses basic equations of the rigid body kinematics and information for better understanding the complicated roll center. To validate the proposed methodology the paper reports model tests and frequency domain calculations regarding the behavior of the vessel in regular beam wave. A closed form equation for the calculation of the roll center is also proposed. All these results match very well. This is so, despite the very complicated phase behavior with frequency. The paper also addresses the question whether the bilge keels at each board should always be symmetric for a platform that will always operate in beams seas.

Despite of numbers of method to estimate and predict the nonlinear roll damping, it is the mode least understood and the most difficult to determine so far. Reviewing the existing methods reveals that the coupling effects of other modes on roll motion are ignored by assuming just one degree of freedom (1DOF) roll in experiments. The new concept of Most Often Instantaneous Rotation Center - MOIRC proposed by Fernandes and Asgari has brough other parameters, which can help us to improve the roll damping analysis by including the coupling (roll-sway) that results in asymmetric roll responses. This paper, by describing experiments, aims to confirm this roll-sway effect on roll damping coefficient by taking a well posed 3DOF, which allows to follow the instantaneous rotation centers - IRCs. The regular beam-waves experiments were conducted for different frequencies and wave amplitudes. A 3DOF (sway, heave and roll) system identification is used to extract roll damping from the model test. It is shown that the locus of the IRCs follows a straight line and it has a statistical behavior whose probability density function of IRCs with a Cauchy probability density function. For the first time this characteristic is provided experimentally, well matching with the analytical Cauchy distribution.