Offshore and Structural Mechanics

Coupled Surge-Heave-Pitch Dynamic Modeling of Spar-Moonpool-Riser Interaction

[+] Author and Article Information
Pol D. Spanos

 Rice University, Houston, TX 77005-1827

Vincenzo Nava, Felice Arena

 Mediterranean University of Reggio Calabria, Via San Marco 3, Reggio Calabria, Italy

J. Offshore Mech. Arct. Eng 133(2), 021301 (Dec 06, 2010) (9 pages) doi:10.1115/1.4001956 History: Received April 02, 2009; Revised February 16, 2010; Published December 06, 2010; Online December 06, 2010

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.

Copyright © 2011 by American Society of Mechanical Engineers
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Figure 1

Model of a Truss Spar platform with four heave plates

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Figure 2

Model and reference frame of the hull of the Spar platform with the moonpool

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Figure 3

Power spectral density (PSD) of force in the surge direction

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Figure 4

PSD of the force in the heave direction

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Figure 5

PSD of the moment in the pitch direction

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Figure 6

PDF of the heave displacement (moonpool closed)

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Figure 7

PDF of the heave displacement (moonpool open)

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Figure 8

PSD of the surge displacement

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Figure 9

PSD of the heave displacement (moonpool open)

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Figure 10

PSD of the pitch rotation

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Figure 11

PSD of the heave displacement obtained by the nonlinear model in the time domain (moonpool open and closed)




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