This work addresses the dynamic positioning response with respect to the environmental disturbances caused by wave, wind, and current effects. Concerning the first and second order wave effects, only the latter may be rejected by the closed loop system. On the other hand, the rejection of first order oscillatory motion causes the so called propeller control signal modulation, which means an earlier and more intensive mechanical wear and tear and higher fuel consumption. The fundamental issue in the dynamic positioning design consists of minimizing the propulsion modulation by filtering out this first order wave effect, which contaminates the controller input measurement. It is worth to mention that the wave spectrum is time varying, with an autocorrelation time of about three hours. This non-stationary behavior has led the work to develop a filtering algorithm which is adaptive with respect to the time varying sea state condition. Therefore, it is highly desirable that the dynamic positioning control system be robust, by having an adaptive behavior with respect to the environmental disturbances and by tracking the wave spectrum parameters. This paper presents a novel algorithm for tracking the modal frequency of the surge, sway, and yaw response spectra, which automatically tunes the first order wave effect filter of a DP system. The proposed name for the technique is modal identification applied to dynamic positioning.

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