The paper is concerned with the dynamic positioning control of a moored floating platform model. Moorings are used to maintain the position of the platform against quasi-constant external forces induced by current and wind. The control purpose is to suppress the slow drift oscillation due to drifting force in irregular waves using thrusters so that they do not respond to the liner exciting force in the wave frequency range. Control of motions in the wave frequency region has to be ignored because the amplitude of those motions is smaller than that of the slow drift oscillation and enormous thruster power is needed to control them. The problem was formulated in the framework of Hoo control, and was solved using the LMI (linear matrix inequalities) Control Toolbox on Matlab. Model experiments in oblique incident waves were carried out and some successful results are shown.
Floating offshore platforms are widely used in the exploration and development of ocean resources. They are required to maintain a given position under the external forces of ocean current, wind and waves. A mooring system or a dynamic positioning system using thrusters (Triantafyllou, 1979, Pinkster et al., 1986; Davison et al., 1987) is generally employed to maintain a position, however, the former system cannot completely prevent slow drift oscillation and the latter requires a great deal of energy because the thrusters must continually work against current and wind. A thruster assisted mooring system is needed which prevents the drift induced by these two quasi-constant external forces, and in which thrusters counteract the slow drift oscillation. Our concern here is the design of such a thruster controller. The PID controllers used to date tend to respond to the motion in the wave frequency range (see Fig.12).