Research Papers: Offshore Technology

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,
Macaé, RJ CEP 27.923-510, Brazil
e-mail: ivann@petrobras.com.br

Claudio A. Rodríguez

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

Paulo T. T. Esperança

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

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received March 29, 2016; final manuscript received December 24, 2016; published online May 5, 2017. Assoc. Editor: Qing Xiao.

J. Offshore Mech. Arct. Eng 139(4), 041301 (May 05, 2017) (10 pages) Paper No: OMAE-16-1031; doi: 10.1115/1.4035770 History: Received March 29, 2016; Revised December 24, 2016

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 center of gravity (KG) is not possible. For vessels with low metacentric height (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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Fig. 1

General definition of stable, neutral, and unstable equilibrium

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Fig. 2

Stable, neutral, and unstable equilibrium for a free-floating structure

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Fig. 3

GZ-curve for vessel at an unstable upright condition

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Fig. 4

Typical GZ-curve for two types of vessel

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Fig. 5

Three-dimensional view of the SS hull at operational draught

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Fig. 6

Layout of mooring lines and risers as modeled in DYNASIM

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Fig. 7

Static stability curve for the SS's operational draught

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Fig. 8

Paths to zero GM, by means of (a) pure deballast and (b) deballast and vertical shift

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Fig. 9

SS's stability curves leading to neutral equilibrium

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Fig. 10

SS's static stability curves for unstable conditions

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Fig. 11

Inclined GMs for the SS at unstable conditions

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Fig. 12

MRE on SS restoring moments




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