System Identification of a Coupled Two DOF Moored Floating Body in Random Ocean Waves

[+] Author and Article Information
R. Panneer Selvam

Wind Engineering Laboratory, Structural Engineering Research Center, CSIR Campus Chennai, Indiapselvam@sercm.org

S. K. Bhattacharyya

Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai, India

J. Offshore Mech. Arct. Eng 128(3), 191-202 (Dec 23, 2005) (12 pages) doi:10.1115/1.2199557 History: Received July 27, 2005; Revised December 23, 2005

Dynamics of a large moored floating body in ocean waves involves frequency dependent added mass and radiation damping as well as the linear and nonlinear mooring line characteristics. Usually, the added mass and radiation damping matrices can be estimated either by potential theory-based calculations or by experiments. The nonlinear mooring line properties are usually quantified by experimental methods. In this paper, we attempt to use a nonlinear system identification approach, specifically the reverse multiple input-single output (R-MISO) method, to coupled surge-pitch response (two-degrees-of-freedom) of a large floating system in random ocean waves with linear and cubic nonlinear mooring line stiffnesses. The system mass matrix has both frequency independent and frequency dependent components whereas its damping matrix has only frequency dependent components. The excitation force and moment due to linear monochromatic waves which act on the system are assumed to be known that can either be calculated or obtained from experiments. For numerical illustration, a floating half-spheroid is adopted. The motion as well as the loading are simulated assuming Pierson-Moskowitz (PM) spectrum and these results have been analyzed by the R-MISO method yielding frequency dependent coupled added mass and radiation damping coefficients, as well as linear and nonlinear stiffness coefficients of mooring lines satisfactorily.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

Schematic view of a multi-point moored floating body

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Figure 2

Three input-single output model for Eq. 7

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Figure 3

Two input-single output model for Eq. 7

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Figure 4

Added mass curves

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Figure 6

Magnitude and phase plots of wave excitation force. (a), (b) Surge direction—wave angle 0deg. (c), (d) Pitch direction—wave angle 0deg.

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Figure 7

(a) PM spectrum (b) surge force spectrum, and (c) pitch moment spectrum

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Figure 8

Retardation functions

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Figure 9

Added masses at infinite frequency

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Figure 10

Time series of water surface elevation, excitation surge force, surge motion, excitation pitch moment, and pitch motion

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Figure 11

Surge and pitch response spectra

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Figure 12

Comparison of restoring force in surge with and without K11

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Figure 13

Magnitude and phase plots of BL1, BL2, BNL

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Figure 14

Magnitude and phase plots of CL1 and CL2

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Figure 15

Estimated and reference added mass curves

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Figure 16

Estimated and reference damping curves

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Figure 17

Coherence plots of surge and pitch equation



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