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Safety and Reliability

Reliability of Switched Model-Based Controller for Vessel Dynamic Positioning With Switching Under Estimated Motion Frequency

[+] Author and Article Information
Dong T. Nguyen

Center for Ships and Ocean Structures (CeSOS), Norwegian University of Science and Technology, Otto Nielsens vei 10, N-7491 Trondheim, Norwayntd@marinecyb.com

Ser Tong Quek

Department of Civil Engineering, National University of Singapore, 1 Engineering Drive 2, E1A 07-03, Singapore 117576cveqst@nus.edu.sg

J. Offshore Mech. Arct. Eng 132(2), 021601 (Mar 01, 2010) (9 pages) doi:10.1115/1.4000392 History: Received December 05, 2007; Revised June 16, 2009; Published March 01, 2010; Online March 01, 2010

In switched-control methodology, which integrates several controllers handling different environmental conditions for dynamic positioning (DP) systems in marine vessels, the supervisor uses the estimated wave peak frequency (WPF) to classify the existing environmental condition so as to switch to the appropriate controller. The WPF is assumed to be the same as the peak frequency of the vessel’s response spectrum at wave frequency estimated by fast Fourier transform (FFT) of the measured position and heading. Using a short window width leads to error in the WPF estimation whereas a longer width causes a significant time lag in control activation. This paper addresses the effect of uncertainty in the estimation of WPF by FFT on the reliability of the switched-controller based DP vessel. The probability of the supervisor switching to a wrong controller due to WPF estimation error is first evaluated for four different environmental conditions. The failure probabilities of the system with and without WPF uncertainty are then evaluated and compared. Numerical simulation based on a supply vessel shows that the increase in failure probability of the switched-controller DP system due to the uncertainty in the estimated WPF by FFT is not significant compared with the failure probability of a perfect switched-controller DP system.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Concept of switched-controller DP system using discrete switching signal

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Figure 2

Scale-independent hysteresis switching logic (10)

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Figure 3

Event tree for DP operating with controllers under four different environmental conditions

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Figure 4

Power spectrum of control forces under calm seas with correct controller

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Figure 5

Histogram of vessel’s position and heading (left) and control forces (right) under calm seas with correct controller

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