Session: 16-10-01 Marine Hydrodynamics and Subsea Technology

Submission Number: 180863

Assessing the Performance of a Frequency-Domain Numerical Model for the Prediction of 2nd Order Roll Motions of an FPSO: Comparison With a Set of Bichromatic Wave Tests in a Model Basin 

With the increase in size and height of the top sides of new FPSOs, the dynamics of these floating units is now characterized by higher natural periods of Roll motion, typically ranging from 20 to 30 seconds, depending on the loading conditions. One of the consequences of the higher resonant periods is that the units are now more prone to resonant Roll motions induced by low-frequency second-order effects, an issue that is the focus of attention in recent literature.

As part of the efforts to define expedite procedures capable of providing reasonable estimates of the second-order roll amplitudes in early stages of the design of new FPSOs, a series of model tests were recently performed in the USP wave basin taking into consideration the geometrical and inertial features of a new generation of FPSOs that are currently under design for future operation offshore Brazil. In this paper, focus is directed to a set of decay tests, performed as a means to estimate the model-scale Roll damping levels, and on a series of regular wave groups (bichromatic waves), whose results are used to confront the QTF predictions obtained through the numerical modelling of the second-order Roll motions in frequency-domain.

The numerical estimations were performed in WAMIT^®, and comprised different approaches, such as the Roll QTF being computed with and without considering the free surface discretization, in addition to the direct use of the second-order Roll RAO provided by the software. In order to compare the numerical simulations with the experimental results, uncertainties regarding Roll damping were tackled through a procedure based on the model decay tests.

Results show a fair agreement between the experimental results and the different numerical approaches, indicating that the simplification derived from disregarding the free-surface forcing effects in the solution of the 2^nd order BVP is indeed acceptable for the case conditions of the study. In addition, it is also demonstrated that, for this new generation of FPSOs, dynamic coupling effects are very important and must be taken into consideration in the numerical simulations, advising against simplified procedures based on the analysis of one single DOF.

Presenting Author: Alexandre Nicolaos Simos University of São Paulo / Numerical Offshore Tank Laboratory (TPN-USP)

Presenting Author Biography: Alexandre Simos is an associate professor in the Department of Ocean Engineering at the University of São Paulo. With over 20 years of experience in Marine Hydrodynamics, he has coordinated several research projects funded by industry and government agencies.

Authors:

Pedro Cardozo De Mello University of São Paulo / Numerical Offshore Tank Laboratory (TPN-USP)
Lariuss Zago University of São Paulo / Numerical Offshore Tank Laboratory (TPN-USP)
João Vicente Sparano University of São Paulo / Numerical Offshore Tank Laboratory (TPN-USP)
Jordi Mas-Soler 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