The paper is concerned with a dynamic positioning control of a moored floating platform model. 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 linear exciting forces in wave frequency range. The problem is formulated in the frameWork of H∞ control, and is solved using LMI(Linear Matrix Inequalities) Control Toolbox on Matlab; some successful results are shown.
Floating offshore platforms are widely used in 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 aI., 1986; Davison et aI., 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 in which the mooring system prevents the drift induced by these two quasi-constant external forces, and thrusters counteract the slow drift oscillation is needed. Our concern here is the design of such a thruster controller. Slow drift oscillation appears as a result of resonance between the mooring system and the drifting force, but it can be counteracted using thrusters because the exciting force is small. The PID controllers used to date tend to respond to the motion in the wave frequency range (sec Fig. 9). It should be noted that it is impossible to resist the wave frequency motion using thrusters because of the enormous amount of power needed. Our control aim is to suppress the slow drift oscillation using thruster activities that do not respond to wave frequency motion.