Feed-forward control for dynamic positioning in random waves based on quadratic impulse function and real-time wave measurement

Abstract The real-time estimation of second-order difference-frequency wave forces using real-time random-wave measurement is developed for the FF (feed-forward) control based dynamic positioning of floating offshore vessels and platforms. The efficacy of the developed FF control scheme is validated by using the in-house hull-mooring-riser-thruster fully coupled time-domain computer simulation program through comparisons with the results by the conventional feedback-control-only case. The feedback (FB) control intends to reduce the accumulated position-excursion error, meanwhile the proposed feed-forward control compensates the controllable slowly-varying wave loads by activating thrusters in advance based on the real-time estimation of the second-order difference-frequency wave loadings using the real-time signal of random incident wave. The real-time estimation of the second-order difference-frequency wave loads is done by using the double-convolution integral with pre-calculated QIF (quadratic impulse function). The numerical DP system is successfully implemented with the FF control algorithm in the vessel-thruster fully coupled time-domain simulation program. The developed schemes are applied to a turret-moored FPSO (floating production storage offloading) with six dynamic-positioning (DP) azimuth thrusters in two non-collinear storm conditions. It is clearly demonstrated that the developed FF control scheme performs much better than the conventional feedback-control-only case. The corresponding reductions in horizontal offsets, motions, mooring tensions, and fuel consumptions by using the developed FF control scheme are underscored.