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.


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 its position under external forces such as ocean current, wind and waves. A dynamic positioning system using thrusters is generally employed to assure position maintenance. In dynamic positioning control, it should be noted that motions caused by the linear wave exciting force are added to drifting of the platform, the enormous amount of power needed makes it impossible to resist the wave frequency motion using thrusters and, in any event, this motion need not be controlled because of its small amplitude. Therefore, we need 0nly to counteract the drifting force. Model experiments in oblique incident waves were carried out and successful results were obtained using the H controller.

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