Prediction of the accuracy of a dynamic positioning system of a vessel, The Solitaire

Abstract

This report presents the modeling of a pipelay vessel’s motions -the Solitaire- and its DP controller using the bond graph method. The governing equations are obtained with hydrodynamics considerations. Problems of finding good equations as well as realistic coefficients to describe forces, especially damping forces, are pointed out. The DP controller is modeled as described in manuals of such controller from Kongsberg. The controller is then tuned and the effects of each parameters is emphasized. Eventually several scenarios are modeled and then simulated to forecast the behavior of the DP system in such cases the final goal being to see how evolves its accuracy.

The results from those simulations show that waves significant height is the parameter which has the most influence on the accuracy of the DP system whereas current is is easily counteracted. Wind has very little influence on accuracy as its velocity and direction can be measured. This work also shows the effect of wake interactions in position keeping. Not forbidding thrusters to blow in the direction of another thruster diminishes the accuracy. Eventually the importance of having reliable reference systems is highlighted by simulating an increasing offset in the GPS output. In the latter case the DP system which relies only on the GPS output and an internal model of the vessel is unable to detect the drift of the GPS and therefore drifts along.