Untitled in english

Author(s)

Marcelo Ramos Martins

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Escola Politécnica (EP/BC)
• Defense date: 1999-03-29

Examining board members

Hernani Luiz Brinati; Jose Augusto Penteado Aranha; Marcos Donato Auler da Silva Ferreira; Kazuo Nishimoto; Sergio Hamilton Sphaier

• Advisor: Hernani Luiz Brinati

Abstract

The work presents a procedure to calculate the wave drift damping matrix coefficients. The study of this damping component began in the early eighties when Wichers (1982) presented some experimental results, which showed that the damping increases when there is an incident wave on the unit. Afterwards several works, from different authors, have treated this question. Aranha (1996) presented a formulation, which caused polemic, to evaluate the elements of the wave drift damping matrix, corresponding to the surge and sway motion of the unit, based on the drift coefficients. The objectives of the present work are the verification of the adherence of the formulation proposed by Aranha (1996) to the experimental results obtained in reduced scale tests and, in a next step, extend this formulation to the yaw motion of the unit. Initially it is analyzed the load acting on the unit due to the incident wave, including the effect of the wave drift damping. To obtain the expressions of the wave drift damping matrix elements corresponding to the surge and sway motions of the unit, the work used the conservation laws for the energy and for the momentum transported by the waves. Numeric and experimental comparisons were performed in order to evaluate the results produced by the developed expressions. The numeric evaluation, which was carried out withtwo units operated by PETROBRAS, showed the importance of the wave drift damping effect on the composition of the total load acting on the unit. To validate the results obtained with the expressions used in the numeric evaluation, model tests were carried out in IPT towing tank with a reduced scale model of the Vidal de Negreiros tanker. These tests demonstrated that the results obtained with the proposed expressions are very accurate and illustrated the importance of the wave drift damping effect on the equilibrium position of the vessel when a turret configuration is used for mooring system. Once it was confirmed the importance of the wave drift damping effect and validated the good results obtained for the damping in surge and sway due to this effect, it is analyzed the influence of the yaw motion of the unit on the effect due to the incident waves. Due to the discrepancy of scales, i.e., the fact of the beam is much smaller than the length, the yaw motion in each section of the unit can be treated like a sway motion with amplitude equal to the product of the angular velocity by the horizontal distance of the section to the center of the angular motion.Based on this premise, this workevaluates the effects of the yaw motion on the drift forces acting upon the unit making use of the expressions derived for the sway motion, that has been previously evaluated. Therefore, the sectional drift force coefficients is necessary and to get these coefficients, a program based on the Slender Body Theory was elaborated, whose results were evaluated by the WAMIT program. To allow the utilization of the results provided by the commercial programs, for instance, the WAMIT one, heuristic expressions, based on verifications in high frequency and in discrepancy of the scales, are presented to yield an estimation of the effect of yaw motion of the unit on the potential damping. Finally, adopting the same geometry of the body used by Grue & Palm (1996), it is performed a comparison between the results publicized by those authors and the values obtained by the proposed methodology. It was found out a good agreement between these values, mainly for the elements of the wave drift damping matrix that produce a moment associated with the yaw motion of the unit. (AU)