Instantaneous Center of Rotation in Pitch Response of a FPSO Submitted to Head Waves

Abstract

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.

Rigid Body Dynamic Response of a Floating Offshore Wind Turbine to Waves: Identification of the Instantaneous Centre of Rotation Through Analytical And Numerical Analyses

“…The authors showed that the ICR locus of an FPSO vessel is a straight line not coinciding with the line of symmetry of the vessel, dependent on the frequency of the waves. Costa et al [20] tested the model in a range of regular head waves of different frequencies. Two position trackers allowed the calculation of the pitch angle by geometry and subsequent calculation of the ICR as the ratio between the displacement of a marker and the pitch angle.…”

“…In [21], the distributions of both the vertical and horizontal components of the ICR along the locus line were found to adhere to the Cauchy probability density function. In the latest article by Costa et al [22], the investigation was extended to the consideration of oblique seas and 6 DOF motions. Once again, the ICR's dependence on the frequency of the regular waves has been demonstrated through a numerical approach (potential flow theory).…”

“…The method presented here follows that previously introduced in [20,22,36]. The motion of a generic point 𝐴 of the floating body in 6 DOF can be represented as a combination of the point's translation with respect to a generic reference point 𝐶 (𝜟𝐒 𝐂 ), and its rotation by a small angle around the same reference point (𝜟𝜽).…”