Page 192 - Practical Design Ships and Floating Structures

Practical Design of Ships and Other Floating Structures 167
You-Sheng Wu, Wei-Cheng Cui and Guo-Jun Zhou (Eds)
0 2001 Elsevier Science Ltd. All rights reserved

DESIGN OF FPSO's BASED ON MANEUVERING STABTLITY

G.B. Matter', J.S. Sales Jr. ' and S.H. Sphaier'

I Department of Ocean Engineering, COPPE, Federal University
of Rio de Janeiro, Brazil

ABSTRACT
Ships have been used as floating units to store, process and transfer oil to shuttle ships. In currents
their movements in the horizontal plane may present an unstable behavior. Based on linear stability
analysis, a procedure is presented here to help the engineer to account for the phenomena in the design
of new units and in the offloading operation. Three examples of application are presented.

KEYWORDS

Stability. Maneuvering, OMoading, FPSO, DICAS, Spread-mooring, Turret.

1 INTRODUCTION
An economic and practical solution to store and to process oil exploited in deep water is the use of
large ships as stationary units, provided with large tanks and a processing plant. However, ships
moored to a single point or connected to the sea floor by a mooring system in presence of a current
may adopt an unstable behavior of the motion in the horizontal plane. This is not a new problem, once
the unstable behavior of ships when towed or moving in a channel or even in open seas motivated
many research for a long time (Abkowitz. 1964,1967, Eda and Crane, 1965).
Bernitsas and Krekides (1989, Schellin, Jiang and Sharma (1987) and Papoulias and Bernitsas (1 988)
brought many concepts of the theory of dynamic systems to the analysis of the behavior of moored
ship in currents. After that, many other substantive contributions appear in the literature. More recently
Garza-Rios and Bernitsas (1996, 1998 and 1999) developed some analytical expressions to study the
stability of floating systems based on the Routh-Hurwitz conditions.

Associated to the use of converted VLLC to operate as FPSOs in Brazil by Petrobras, Sphaier.
Fernandes, Correa and Pontes (1996, 1997, 1998, 1999, 2000a, 2000b) developed a maneuvering
model, based on experimental tests carried out in Brazil, and a time domain code to simulate the
dynamics of floating systems. Further, a procedure to analyze the stability of those kind of systems
were developed, consisting on the calculation and the analysis of the eigenvalues. Later on, Matter,
Sales and Sphaier (2001) and Matter, et a1 (2001b) extended the procedure by developing analytical
expressions for the eigenvalues and their derivatives for SPM (single point mooring), TMS (turret