Due to the rather intense ongoing development of deep water gas and oil fields, the technical community has devoted considerable attention to the dynamic behavior of Spar floating systems. Spar dynamics exhibits a highly nonlinear behavior due to the presence of various components such as mooring lines, moonpool, and risers. Certain studies have focused on the reduction of the heave response of single-degree-of-freedom Spar models due to the oscillations of water entrapped in the moonpool through the partially closed bottom plates. In this paper, a novel coupled six-degree-of-freedom analytical model of a Spar system comprising top tensioned risers is proposed. The model accounts for the interactions among the Spar hull kinematics (heave, surge, and pitch), the riser kinematics (heave and surge), and the moonpool. This model involves six coupled differential equations comprising nonlinearities associated not only with stiffness and damping but also with inertia terms. A dynamic analysis is performed by subjecting the model to JONSWAP ocean wave spectrum compatible extreme forces (corresponding to the 100 year wave) and to moments applied to the center of gravity computed by means of a standard motion simulation program. Both numerical and analytical techniques (statistical linearization including inertia terms) are used for the determination of the response of the proposed dynamic model, both in the time and the frequency domains. Related parameter study results are reported, including ones pertaining to the dependence of the Spar system motion on the degree of opening of the bottom plates.

1.
Vardeman
,
R. D.
,
Richardson
,
S.
, and
McCandless
,
C. R.
, 1997, “
Neptune Project: Overview and Project Management
,”
Proceedings of the Offshore Technology Conference
, Houston, TX, Paper No. OTC 8381, Vol.
2
, pp.
227
235
.
2.
Ma
,
Q. W.
, and
Patel
,
M. H.
, 2001, “
On the Non-Linear Forces Acting on a Floating Spar Platform in Ocean Waves
,”
Appl. Ocean. Res.
0141-1187,
23
, pp.
29
40
.
3.
Colby
,
C.
,
Sodahl
,
N.
,
Katla
,
E.
, and
Okkenhaug
,
S.
, 2000, “
Coupling Effects for a Deep Water Spar
,”
Proceedings of the Offshore Technology Conference
, Houston, TX, Paper No. OTC 12083, Vol.
2
, pp.
639
646
.
4.
Ma
,
Q. W.
,
Lee
,
M. Y.
,
Zou
,
J.
, and
Huang
,
E. W.
, 2004, “
Deepwater Nonlinear Coupled Analysis Tool
,”
Proceedings of the Offshore Technology Conference
, Houston, TX, Paper No. OTC 12085.
5.
Ran
,
Z.
, and
Kim
,
M. H.
, 1997, “
Nonlinear Coupled Responses of a Tethered Spar Platform in Waves
,”
Int. J. Offshore Polar Eng.
1053-5381,
7
(
2
), pp.
111
118
.
6.
Spanos
,
P. D.
,
Ghosh
,
R.
,
Finn
,
L. D.
, and
Halkyard
,
J.
, 2005, “
Coupled Analysis of a Spar Structure: Monte Carlo and Statistical Linearization Solutions
,”
ASME J. Offshore Mech. Arct. Eng.
0892-7219,
127
, pp.
11
16
.
7.
Tasaka
,
E.
,
Kengaku
,
M.
, and
Koyanagui
,
M.
, 1965, “
Anti-Pitching Tank
,”
J. Soc. Nav. Archit. Jpn.
0514-8499,
117
, pp.
72
83
.
8.
Fukuda
,
K.
, (1977), “
Behaviour of Water in Vertical Well With Bottom Opening of Ship, and Its Effects on Ship-Motion
,”
J. Soc. Nav. Archit. Jpn.
0514-8499,
141
, pp.
107
122
.
9.
Aalbers
,
A. B.
, 1984, “
The Water Motions in a Moonpool
,”
Ocean Eng.
0029-8018,
11
(
6
), pp.
557
579
.
10.
Nishimoto
,
K.
,
Videiros
,
P. M.
,
Fucatu
,
C. H.
,
Matos
,
V.
,
Cueva
,
D.
, and
Cueva
,
M.
, 2001, “
A Study of Motion Minimization Devices of FPDSOs
,”
ASME
Paper No. Paper No. 1131.
11.
Cueva
,
M.
,
Malta
,
E. B.
,
Nishimoto
,
K.
, and
Costa
,
A. P.
, 2005, “
Estimation of Damping Coefficients of Moonpools for Monocolumn Type Units
,”
ASME
Paper No. 67332.
12.
Gupta
,
H.
,
Blevins
,
R.
, and
Banon
,
H.
, 2008, “
Effect of Moonpool Hydrodynamics on Spar Heave
,”
ASME
Paper No. 57264.
13.
Spanos
,
P. D.
, 1981, “
Stochastic Linearization in Structural Dynamics
,”
Appl. Mech. Rev.
0003-6900,
34
, pp.
1
8
.
14.
Roberts
,
J. B.
, and
Spanos
,
P. D.
, 2001,
Random Vibrations and Statistical Linearization
,
Dover
,
New York
.
15.
Gupta
,
H.
,
Nava
,
V.
,
Banon
,
H.
,
Gkaras
,
V.
, and
Spanos
,
P. D.
, 2008, “
Determination of Riser Tension Properties From Full-Scale Data
,”
ASME
Paper No. 57791.
16.
Streeter
,
V. L.
, and
Wylie
,
E. B.
, 1985,
Fluid Mechanics
,
8th ed.
,
McGraw-Hill
,
New York
.
You do not currently have access to this content.