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.

References

References
1.
IMO
,
2008
, “
International Code on Intact Stability (IS Code)
,”
Maritime Safety Committee, International Maritime Organization (IMO)
,
London
, Chap. 3, 4.
2.
NMI
,
1985
, “
A Study of Methods of Measuring the Metacentric Heights of Semi-Submersibles
,” Report No. OTH 84211, Department of Energy, Offshore Technology, National Maritime Institute, London, UK.
3.
Bradley
,
M. S.
, and
MacFarlane
,
M. J.
,
1986
, “
Inclining Tests in Service
,”
Advances in Underwater Technology, Ocean Science and Offshore Engineering, Vol. 9: Stationing and Stability of Semisubmersibles
,
Society for Underwater Technology
,
Graham & Trotman, London
.
4.
HSE
,
2006
, “
Review of Issues Associated With the Stability of Semi-Submersibles
,” Health & Safety Executive, BMT Fluid Mechanics Limited, Teddington, UK, Research Report No. 473.
5.
Nogueira
,
S.
,
2009
, “
Development of an Inclining Test Procedure Applicable to Semi Floating Production Units Moored on Location
,”
ASME
Paper No. OMAE2009-79184.
6.
Nogueira
,
S.
,
2010
, “
Sistemática Para Executar Teste de Inclinação em Unidades Semissubmersíveis de Produção Operando na Locação
,” M.Sc. dissertation, COPPE – Ocean Engineering Department, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil (in Portuguese).
7.
Porciuncula
,
I. N.
,
Rodríguez
,
C. A.
, and
Esperança
,
P. T. T.
,
2016
, “
New Methodology for the KG Estimation of In-Service Semisubmersibles: Experimental Assessment
,”
ASME J. Offshore Mech. Arct. Eng.
,
139
(
2
), pp.
1
9
.
8.
Journée
,
J. M. J.
, and
Massie
,
W. W.
,
2001
,
Offshore Hydromechanics
,
Delft University of Technology
,
Delft, The Netherlands
, Chap. 2.
9.
Lewis
,
E.
,
1988
,
Principles of Naval Architecture—Vol. I: Stability and Strength
,
2nd ed.
, Vol.
1
,
SNAME
,
Jersey, NJ
, Chap. 2.
10.
Attwood
,
E. L.
, and
Pengelly
,
H. S.
,
1931
,
Theoretical Naval Architecture
,
8th ed.
,
Longmans
,
Green, London
, Chap. III.
11.
Rawson
,
K. J.
, and
Tupper
,
E. C.
,
2001
,
Basic Ship Theory—Vol. 1: Hydrostatics and Strength
,
5th ed.
,
Butterworth-Heinemann
,
Waltham, MA
, Chap. 4.
12.
Gudmunsson
,
A.
,
2009
, “
Safety Practices Related to Small Fishing Vessel Stability
,” Food and Agriculture Organization of the United Nations (FAO), Rome, Italy,
FAO Fisheries and Aquaculture Technical Paper, No. 517
.
13.
Barrass
,
C. B.
, and
Derrett
,
D. R.
,
2012
,
Ship Stability for Masters and Mates
,
7th ed.
,
Butterworth-Heinemann
,
Oxford, UK
, Chap. 16–20.
14.
IMO
,
1979
,
IMO Resolution A.414: Code for Construction and Equipment of Mobile Offshore Drilling Units
,
International Maritime Organization (IMO)
,
London, UK
.
15.
Huijs
,
F.
,
2015
, “
The Influence of the Mooring System on the Motions and Stability of a Semi-Submersible Floating Wind Turbine
,”
ASME
Paper No. OMAE2015-41947.
16.
Semyonov-Tyan-Shansky
,
V.
,
1960
,
Statics and Dynamics of the Ship
,
Peace Publishers
,
Moscow, Russia
, Chap. 3.
17.
Porciuncula
,
I. N.
,
2014
, “
Uma Nova Metodologia para Estimar o VCG de uma Unidade Semissubmersível na Locação considerando seu Comportamento Dinâmico em Ondas na Condição de Estabilidade Neutra
,” M.Sc. dissertation, COPPE – Ocean Engineering Department, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil (in Portuguese).
18.
Porciuncula
,
I. N.
,
Esperança
,
P. T. T.
, and
Rodríguez
,
C. A.
,
2015
, “
A New Methodology for the KG Estimation of a Semisubmersible in Service
,”
Offshore Technology Conference
(
OTC2015
), Rio de Janeiro, Brazil, Oct 27–29.
19.
Coelho
,
L. C. G.
, and
Nascimento
,
A. S. A.
,
2002
,
Manual do Programa SSTAB—Estabilidade e Controle de Lastro de Sistemas Flutuantes
,
CENPES-Petrobras/Tecgraf-PUC
,
Rio de Janeiro, Brazil
.
20.
NMD
,
1992
, “
Regulations for Mobile Offshore Units
,”
Norwegian Maritime Directorate
,
Oslo, Norway
.
21.
ABS
,
2001
, “
Rules for Building and Classing Mobile Offshore Drilling Units
,”
American Bureau of Shipping
,
Houston, TX
.
22.
DNV
,
2001
, “
Stability and Watertight Integrity
,”
Det Norske Veritas
,
Oslo, Norway
, Offshore Standard DNV-OS-C301.
23.
Coelho
,
L. C. G.
,
2001
,
DYNASIM System Manual—Análise Dinâmica de Sistemas Ancorados
,
CENPES-Petrobras/Tecgraf-PUC
,
Rio de Janeiro, Brazil
.
24.
Kobylinski
,
L. K.
, and
Kastner
,
S.
,
2003
,
Stability and Safety of Ships. Vol. I: Regulation and Operation
,
Elsevier Ocean Engineering Book Series Vol. 9, Elsevier
,
London
.
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