Experimental study on dynamic positioning control for semi-submersible platform

The paper is concerned with a dynamic positioning control for a semi-submersible platform model. An offshore platform is required to maintain a given position against external forces such as ocean current, wind, and waves. In such a control, motions caused by linear wave exciting force add to the drifting of the platform and it is impossible for thrusters to resist this wave frequency motion because the linear exciting force is very strong. Purpose of the control is to maintain a given position using thrusters that do not respond to this force in the wave frequency range. The problem was formulated in a framework of H∞ control, and a controller was designed using a linear mathematical model obtained from nonlinear motion equations of the platform. Model tests were carried out and the designed controller performed well. Model experiments were conducted in oblique incident waves and some successful results are shown.