Position-keeping of a floating body is a very important matter in a production system in the deep sea. How a floating body can be kept stationary is a key problem in this study. Conventionally, the mooring system is adopted to position a floating body under disturbance of wind, wave, and current; but, in general, it is difficult to survive storm disturbance in the deep sea. The dynamic positioning system can solve this problem, but thrusters must be operated at all times against random drift force of wave, wind, and current, and a great deal of energy is required. So a composite system of single-point mooring system and fuzzy control dynamic positioning (FDP) system was studied in a previous paper (Inoue, 1994). However, up to now in almost all kinds of dynamic positioning systems, control strategy must be decided in advance. This is unreasonable under a random condition. In this paper, a self-tuning fuzzy control system is put forward. By this approach, the control precision of the previous fuzzy dynamic positioning is fairly improved. The main concept of this approach is that at first a feasible control strategy is decided in advance, then during the motion of the system, the control strategy is improved automatically according to the sea conditions of operation. This system is more reasonable than the previous system because, in fact, the disturbance of wave, current, and wind cannot be predicted.

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