Session: 06-05-03 Marine Hydrodynamics - III

Submission Number: 180835

Computing the Roll QTF of a New-Generation FPSO: A Sensitivity Analysis With the Discretization of the Free Surface Mesh 

The evaluation of the second-order Roll motion of a FPSO is conditioned to the numerical calculation of the Quadratic Transfer Functions (QTFs) of the ship, in particular the Roll QTF.

In the frequency domain, using a Boundary Element Method (BEM) like WAMIT®, this QTF calculation poses a very high computational cost issue, not only because of the many combinations of frequencies involved to get the “Full-QTF” matrix, but also because the free surface must be modeled by panels (or patches), just like the submerged hull surface.

In order to establish a “trade off” between total runtime and the accuracy of the results of the second-order Roll motion of a new-generation FPSOs in WAMIT®, this paper investigates the impact of the discretization of the free surface on the total CPU runtime and on the second-order Roll significant height of the FPSO, for a centenary sea condition. This is done by means of a sensitivity analysis: the results from user-defined free surface meshes are confronted with those from WAMIT®’s automatic free surface algorithm and also with those from the “No Free Surface Forcing” feature of the software.

The results show that, for a first estimate of the second-order Roll motion of a FPSO using WAMIT®, the “No Free Surface Forcing” feature of the software provides very acceptable results at a much lower CPU runtime. Moreover, if the option of modelling the free surface by panels has been decided, the same provides very good results with a total number of panels between 10000 to 20000.

Other aspects of WAMIT® second-order run are addressed in the paper, such as the dimensions of the free surface paneled zone, quadrature zone and asymptotic zone - depending of the wave periods range selected - and the impact of these parameters in the calculation of the Roll QTF and, subsequently, in the second-order Roll Moment and Motion Spectra.

Presenting Author: João Vicente Sparano University of São Paulo / Numerical Offshore Tank Laboratory (TPN-USP)

Presenting Author Biography: João V. Sparano is a Marine Engineer from the Polytechnic School of the University of São Paulo, with a Master's, Doctorate, and Post-Doctorate in Maritime Hydrodynamics from the same institution. He has experience in Naval and Ocean Engineering, with an emphasis on Hydrodynamics of Ships and Ocean Systems, working mainly on the following topics: Seakeeping, Hydrodynamic Resistance, Design of Offshore Systems, Mooring Systems and Signal Analysis. Recently he has also been working with Floating Offshore Wind Turbines (FOWT) research projects.

Authors:

João Vicente Sparano University of São Paulo / Numerical Offshore Tank Laboratory (TPN-USP)
Alexandre Nicolaos Simos University of São Paulo / Numerical Offshore Tank Laboratory (TPN-USP)
Allan Carre De Oliveira PETROBRAS Research Center
Marcos Donato Auler Da Silva Ferreira PETROBRAS Research Center