The paper is concerned with a dynamic position control for a semi-submersible platform model using a gain scheduled H∞ controller. 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 Hnear exalting 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.
Semi-submersible platforms are widely used in the exploration and development of ocean resources and many such platforms are now in operation. They are required to maintain a given position and to rectify this position under external forces of ocean current, wind and waves. A dynamic positioning system using thrusters is generally employed for this. In this system, it should be noted that motions caused by the linear wave exciting force are added to the drifting of the platform, the enormous amount of power is needed to resist the wave frequency motion and, in any event, this motion need not be controlled.