Model experiments on dynamic positioning system using gain scheduled controller

The paper is concerned with a dynamic position control for a semi-submersible platform model using a gain scheduled H_∞ controlled. Offshore platforms are required to control a given position against external forces of ocean current, wind, and waves. A dynamic positioning system using thrusters is generally employed for this, but it is impossible for thrusters to resist wave frequency motion because the linear exciting force is very strong. The specifications of the proposed controller are that it maintain a given position and to change its position without offset using thrusters that do not respond to this force in the wave frequency range. Based on a rotation matrix in yaw, a linear model with four vertices was introduced. The problem was formulated in a framework of multi-model based design of H_∞ control law with pole region constraint. LMI (linear matrix inequality) based methodology was used to solve the problem. The gain scheduled H_∞ controller was implemented by interpolation of the four vertex controllers. Model experiments were carried out and some successful results are shown.