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New Methodology for the Determination of the Vertical Center of Gravity of In-service Semisubmersibles: Proposal and Numerical Assessment

[+] Author and Article Information
Ivan N. Porciuncula

Petróleo Brasileiro S.A. UO-BC/IPP/EN – Naval Engineering CEP 27.923-510, Macaé, RJ, Brazil
ivann@petrobras.com.br

Claudio A. Rodríguez

LabOceano, COPPE – Federal University of Rio de Janeiro Department of Naval Architecture and Ocean Engineering Caixa Postal No 68.508, CEP 21.945-970, Rio de Janeiro, RJ, Brazil
claudiorc@laboceano.coppe.ufrj.br

Paulo T. T. Esperança

LabOceano, COPPE – Federal University of Rio de Janeiro Department of Naval Architecture and Ocean Engineering Caixa Postal No 68.508, CEP 21.945-970, Rio de Janeiro, RJ, Brazil
ptarso@laboceano.coppe.ufrj.br

1Corresponding author.

ASME doi:10.1115/1.4035770 History: Received March 29, 2016; Revised December 24, 2016

Abstract

Along its lifetime, an offshore unit is subjected to several equipment interventions. These modifications may include large conversions in loco that usually are not adequately documented. Hence, the accurate determination of the platform's KG is not possible. For vessels with low GM, such as semisubmersibles, Classification Societies penalize the platform's KG, inhibiting the installation of new equipment until an accurate measurement of KG is provided, i.e., until an updated inclining test is performed. For an operating semisubmersible, the execution of this type of test is not an alternative because it implies in removing the vessel from its in-service location to sheltered waters. Relatively recently, some methods have been proposed for the estimation of KG for in-service vessels. However, as all of the methods depend on accurate measurements of inclination angles and, eventually, on numerical tools for the simulation of vessel dynamics onboard, they are not straightforward for practical implementation. The objective of the paper is to present a practical methodology for the experimental determination of KG, without the need of accurate measurements of inclinations and/or complex numerical simulations, but based on actual operations that can be performed onboard. Indeed, the proposed methodology relies on the search, identification and execution of a neutral equilibrium condition where, for instance, KG = KM. The method is exemplified using actual data of a typical semisubmersible. The paper also numerically explores and discusses the stability of the platform under various conditions with unstable initial GM, as well as the effect of mooring and risers.

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